Storm System to Bring Wintry Mix to CT 12/28/2015-12/29/2015

While several locations have witnessed snow flurries or snow showers a few times during the fall/early winter, many of us will see our first wintry precipitation of the 2015-2016 season late Monday night into Tuesday as the next storm system approaches.  Unfortunately (if you're a snow lover) for the state of CT we are not looking at much in the way of snowfall.  You would have to travel further northward into central and northern New England for that.

After a several day stretch of insanely warm temperatures, including numerous records being broken, a push of cold air is working towards and into the state, although its stay will be rather brief.  A shortwave trough moving through southeastern Canada will allow for a pretty strong cold front to push through the state during the late evening/overnight hours of Sunday into Monday.  While the passage of this front will yield to clear skies, temperatures will be much colder than they had been the past several days.  In fact, temperatures will be as much as 15-20F colder with high temperatures barely above the 40F mark!

The next storm system is expected to pass well to our north and west.  As typical with these sort of storm tracks warmer air will eventually overspread the state.  However, initially there will be enough cold air in place for precipitation to began as snow across much of the state with perhaps the exception of the immediate shoreline.  The one feature responsible for this will be the area of high pressure which will slide to the east well off to our north.  The flow around the high will transport and keep colder air locked in at the surface.  However, as the high departs off to the east, we will lose the effects from the high pressure and eventually warm air will also work in at the surface and this is when the transition to plain rain will occur.  While the colder air will be initially locked in at the surface initially, warm air advection will be occurring aloft in the lower levels of the atmosphere.  This will allow for snow to transition to sleet and perhaps even some freezing rain.

 
The end result will be very little in the way of snowfall across the state with the most occurring across the NW/NE Hills, however, this doesn't mean the Tuesday AM commute will be messy.  We will be dealing with slushy roads with sleet perhaps some freezing rain falling along with plain rain.  Expect a very slow AM commute.  Below is what I'm currently thinking as far as any snow goes:

2015-2016 Remainder of Winter Outlook

December 2015 is likely going to end up as the warmest December on record for numerous climate locations across the Northeast.  Through the first 23 days of the month, the entire Northeast has not only been slightly above-average but WAY above-average.  The graphic below depicts this very well:

Looking at the image and the color scale, you can see that temperatures through the first 23 days of the month have been running as much as 6-9F to 9-12F+ above-average!!  These numbers are nothing short of impressive and come months end numerous records will be broken for warmest December on record.  While winter only officially began just a few short days ago, the winter season has not gotten off to a great start for cold or snow lovers across the Northeast.  In addition to the record warmth, numerous climate locations may also break or tie some records for least snowiest December on record (we will have to watch Monday/Tuesday (28th/29th) time frame for the potential of some winter precipitation across portions of New England).

The Northeast hasn't been the only region of the United States to experience above-average warmth this month, in fact much of the United States has seen December temperatures running near or above-average.

What was/were the causes for this anomalous December warmth and what does this mean for winter moving forward?  These questions and more will be addressed below as we highlight how this winter may play out and what we can possibly expect.

EL-Nino Southern Oscillation (ENSO)

One of the biggest drivers of the global atmospheric weather pattern is that of the EL Nino-Southern Oscillation (ENSO).  ENSO is broken down into three distinct phases; La Nina, EL Nino, and neutral ENSO conditions.  Each state is defined based upon the sea-surface temperature anomalies (SSTAs) across the equatorial Pacific across a region from about 170E to 90W latitude and 5S to 5N in longitude.  For visual reference see graphic below:

 Since about mid-to-late winter/early spring the equatorial Pacific Ocean region has been suggestive of EL Nino conditions.  EL Nino has been all over the news as of late but what exactly is EL Nino?  An EL Nino episode occurs when SSTA's are running at least +0.5C above-normal for at least five consecutive trimonthly periods.  (A few examples of trimonthly period would be December-January-February, January-February-March, February-March-April, etc.).  La Nina and EL Nino episodes are also broken down into weak events, moderate events, and strong events.  One method used to indicate the SSTA's across the ENSO regions and also indicate the strength of the ENSO event is called the Oceanic-Nino Index (ONI).  For the purposes of EL Nino, a weak EL Nino occurs when the SSTA's are between +0.5C and +0.9C, a moderate EL Nino occurs when SSTA's are between +1.0 and +1.4, and a strong EL Nino occurs when SSTA's are at or greater than +1.5C.   Below is ONI data dating back to the start of 2015.  The red text indicates the beginning of the current EL Nino episode.  With the latest ONI reading of +2.0, this indicates that this EL Nino episode is a pretty strong EL Nino.  In fact, this will go down as one of the strongest EL Nino's on record when all said and done.  (Note:  The EL Nino was officially defined with the JJA reading).

Beyond ENSO classification and strength classification, ENSO can further broken down into where the ENSO event is 'based'.  The classifications for this are west-based, east-based, and basin wide.  Where each ENSO event is situated is just as, if not more important than the actual ENSO state itself.  When determining where the ENSO event is based we can look at both the SSTA configuration across the equatorial Pacific and look at where the focus for tropical forcing is located (we will touch upon this aspect next).  Below are the SSTA's as of December 24th, 2015:

Looking at the equatorial Pacific region (black highlighted circle) the waters are exceptionally warm across the region.  Given how widespread the well above-average SST's are this is indicative of a basin wide EL Nino event.

Tropical Forcing

The next atmospheric variable we will discuss is that of tropical forcing.  When talking about tropical forcing we are looking at convection (areas of showers and thunderstorms) which develop in and around the Indian Ocean and propagate eastward across the equatorial Pacific.  When water vapor condenses to form clouds, this process releases what is called latent heat (latent meaning "hidden").  The more convection you have and the stronger the convection, the more heat that is released into the atmosphere.  This is significant because this heat which is released (especially if there is a great deal of heat released) can have a major influence on the jet stream across North America.  The strongest influences of the latent heat and subsequent ridging typically (not always) takes place anywhere from 30° to 50° east of where the tropical forcing is occurring. 

When assessing tropical forcing, one thing we can look at are outgoing longwave radiation (OLR) anomalies across the Pacific Basin.  Outgoing longwave radiation is a measure of the amount of solar radiation which is emitted back into space from the Earth's surface.  The more convection and clouds, the less in the way of radiation emitted back into space.  Assessing tropical forcing, however, can be a little tricky as over the course of time the forcing can shift across the Pacific.  Looking at OLR anomalies from December 1st through the 22nd, we can see that the majority of the tropical forcing (enhanced convection has been located between about 170W-150W.

Stratosphere/Polar Vortex/Arctic Oscillation (AO)/ Quasi-Biennial Oscillation (QBO)

Outside of the strong basin-wide EL Nino other contributions to this anomalous warmth across the east has been due to the state of the Stratosphere, the strength of the polar vortex (which we can use the Arctic Oscillation (AO) to measure), and the phase of the Quasi-Biennial Oscillation (QBO).  Through much of the fall and into the month of December, the AO has been quite positive.  A positive AO is defined by below-average height anomalies in and around the Arctic region.  These below-average height anomalies indicate a stronger than average Arctic low pressure center (polar vortex).  This is enhanced by very strong westerly winds between about 30-50mb (lower stratosphere) which is indicated by the positive phase of the QBO.  Below are graphics of the 500mb and 50mb height anomalies (left and center images respectively) along with the 50mb zonal wind anomalies (right image) from December 1st through the 22nd.  (Note the below-average height anomalies are indicated by the darker blue/purple shadings and the positive wind anomalies (indicative of westerly winds are indicated by the brighter orange shadings).  

Looking at the highlighted area on the below graphic, you can see the AO has been predominately positive with the exception of the brief period around the 15th of the month where the AO dipped slightly negative.  However, since then the AO has been extremely positive.

Connecting the strong EL Nino, the placement of the strongest tropical forcing, and the strength of the polar vortex indicated by the very positive AO, the influences on the atmospheric pattern across the United States has been for a trough across Alaska, western Canada, and down into the western United States at times and for ridging across the eastern United States.  We can reference the 500mb height anomaly graphic to see this.

With December coming to a close what can we expect for the duration of winter moving forward?  Will a pattern favoring extreme warmth and no snow in the east continue or will winter finally show its true face?  Below we will explore the signals which are in place which may be suggestive of where winter is headed moving forward.  

We will begin by viewing the December 25th 12z 500mb height anomaly charts for the 8-10 day period from the American GFS forecast model (graphic on the left) and the European forecast model (graphic on the right).  When looking at the run (and this has actually been developing over the past few runs) we notice some substantial differences within the pattern compared to what we saw for December.

The major differences here are the big ridge developing across the western-tier of the United States extending northward through western Canada and towards Alaska and the deep trough across the eastern United States.  Below are observations and ensemble forecasts for both the AO as well as the Pacific-North American teleconnection pattern (PNA).  The PNA can hold a strong influence on the atmospheric pattern across the Northern Hemisphere mid-latitudes.  The state of the PNA is classified as being either positive, negative, or if there is no clear signal, neutral.  The state of the PNA is measured by analyzing geopotential height anomalies at the 500mb level.  The positive phase of the PNA is characterized by above-average height anomalies situated across the western-tier of Canada and the western-tier of the United States and below-average height anomalies situated across the southeastern United States.  The negative phase of the PNA is a mere opposite of the positive phase with below-average heights situated across the western-tier of Canada and the western-tier of the United States and above-average height anomalies situated across the southeastern United States.  With a positive phase, this leads to ridging out across the west and a trough at least across the southeast and with a negative phase, this leads to troughing across the west and ridging across the east.  

While the ensemble spread is a bit more chaotic for the AO as opposed to the PNA, the consensus within the guidance is not only for the AO to drop but perhaps drop significantly.  This would indicate a weakening polar vortex which favors a higher likelihood for colder air to spill southward into the central/eastern United States.  The consensus for the PNA, however, is much stronger with the ensemble mean all favoring a rapid rise in the PNA.  This would promote ridging developing into the western-tier of the United States northward into western Canada, Alaska and perhaps poking into portions of the Arctic.  These signals all point to the likelihood that as we at least move through the final week of December and into the first week of the New Year, a major pattern change may take shape.  However, will this new pattern change become a fixture for the duration of winter and what will come with the pattern change?  

When looking even further down the road than just the next two weeks, we must continue to keep in mind how strong the EL Nino is along with the state of the Pacific as a whole along with the state of the lower stratosphere and the polar vortex.  Throughout the month of December, the strong EL Nino has held its dominance throughout the Pacific and one of its fixtures has been the strong vortex in the Gulf of Alaska region.  When you combine these with the strong polar vortex the result is for major warmth across much of the United States, especially in the east.  In order to alter this pattern, you need the Gulf of Alaska vortex to weaken and retreat northward and the polar vortex to weaken.  Unless the polar vortex continues to be attacked by wave propagating in the lower stratosphere/upper troposphere which would further induce weakening and unless the zonal westerlies weaken in the lower stratosphere, it will be very hard pressed to think the upcoming pattern change becomes a fixture through the remainder of winter.  What this means is after a potential pattern change over the next 10-14 days, the pattern could once again begin a transition and we see more in the way of ridging once again building into the east, however, not likely to the extent we saw during the month of December.  

The other major interest of note is the lack of colder air which will arrive if this pattern flip does indeed occur.  For the most part, temperatures across much of Canada have been running above-average.  We can see this from the image below.

  

Looking at the GFS ensembles from Friday 12/24/2015 0z run, the forecast for the 11-15 day period is continued warmth relative to average for much of Canada.  

Unless the changes which are forecasted to happen within the lower stratosphere, the polar vortex, and Gulf of Alaska region with regards to the Gulf of Alaska vortex become a constant the pattern which looks to develop moving into January may not be a permanent fixture.  This means we could see ridging become established once again across the east, however, this should not be to the degree which occurred in December unless the polar vortex were to once again restrengthen to the levels we've seen this month.  Despite the potential for more troughing to occur across the east, the lack of cold in and across Canada should limit more in the way of cold working into the central and eastern United States.  Below is my forecast for the month of January split into two parts; the first half of the month to reflect the potential upcoming pattern change and the second half to reflect the relaxation of that pattern change.

Moving forward into February if the changes discussed above headed into January don't become a constant it will be hard-pressed to find a source for major Arctic air to really work into the United States.  In fact, it is very possible warmth (although not record warmth) may once again overspread a good portion of the United States.  Below is my forecast for February.

  This forecast does not mean we will not see periods of colder temperatures or even at time some really cold temperatures, however, when these episodes occur they should be rather brief, lasting no more than a couple of days at most. 

I also provide this forecast with a low confidence rating.  If the polar vortex does indeed continue weakening (this can be aided by a weakening of the westerly QBO) and the PNA can become positive, this would promote a higher likelihood for a pattern shift towards colder in the east and mild across the west.  

As for snow lovers across the east, each scenario does not necessarily mean a poor snow winter (although once you get to New York City and points south things certainly don't look good), however, we would like to see some troughing become more present in the east and also see a more active pattern/storm track.  Thus far, the strong ridging that has been in place across the east has worked to weaken storm systems moving through the jet stream to our west.  

The winter of 2015-2016 is certainly not lost, however, big changes must occur.  There are signs for these changes to take place but the million dollar question is, will they persist?  

Santa to Bring Record Warmth for Christmas

If you're a cold and/or snow lover and you live in the Northeast (well with the exception of northern Maine) this December has not been very kind to you as we have seen some rather impressive warmth this month.  Unfortunately for you snow/cold lovers there is no end in sight for this pattern to change.  In fact, as we move into the 3rd week of December all indications are we will not on;y see another surge of warmth but we will be looking at major record breaking warmth.

Below is a graphic showing the forecasting 500mb height means from two computer forecast models.  The image from the left is from the European model while the graphic on the right is from the GFS American forecast model.  This run is from today's (December 15th) 12z initialization and is for the 8-10 day period.  What we see on the two graphics below is a very deep trough (blue color shadings) digging into the western-tier of the United States.  The response to this deep trough in the west is for a massive ridge (orange/red color shadings) to develop across the eastern United States:

Both forecast models are indicating the 570 meter height lines to build northward just to the south of New England!  This in unheard of for what will be the later part of December.  What does this mean for temperatures?

Across southern New England we could be talking about temperatures not only well into the 50's for a few days towards the middle and later part of next week but we could be looking at temperatures well into the 60's!!  Further south across New York City and down into the mid-Atlantic temperatures will push into the 70's!!

After this surge of warmth a cold front is expected to move through sometime next weekend which will bring temps closer to more seasonal levels, however, there is no big cold or snow storms in sight and this may hold true for at least the first part of January.

First True Taste of Late Fall to Arrive This Weekend.

We will be experiencing a taste of late fall weather this weekend as an Arctic cold front pushes through the region and ushers in the coldest airmass of the fall season thus far.  The coldest of the airmass will move in place during the day on Saturday and we can expect nighttime lows across many locations reaching as low as the mid to upper 20's both Saturday night into Sunday and Sunday night into Monday.  Daytime highs on Sunday and Monday may also fail to reach 50F across many spots.  If you're not a fan of this cold, the good news is it's stay will be rather brief as by Tuesday a warm front swings north and temperatures return to more seasonal levels.

So what is the cause for the incoming cold?  Well a substantial ridge of high pressure has been building across the western-tier of the United States and extending into western Canada.  This ridge is in response to an increasing state of the Pacific-North American teleconnection pattern.  The response here across the northeastern United States will be for a very sharp trough and associated Arctic cold front to push through.  On the backside of this front, our airmass will be deriving straight from the Arctic, hence the early season, albeit brief, cold.  We can see below:

What does this translate to in the lower levels of the atmosphere and at the surface?  You get it...cold.  We are looking at temperatures between about 2500ft and 5000ft ASL as much as -3 to -4 standard deviations below-normal come Sunday morning!

Here is one computer model's forecast projection (The GFS model) for 2-meter temps both around 5 AM Sunday morning and 5 AM Monday morning.  The GFS shows temperatures about 2M off the ground well in the mid to upper 20's, which actually means some of the typically colder locations may even see temperatures each the lower 20's!

Fall is upon us and we will certainly get our true taste of it this weekend!  However, more seasonal temps are just right around the corner.

Low-topped thunderstorms Friday 10/0/2015?

A cold front will be approaching southern New England during the day on Friday and out ahead of this cold front scattered showers and perhaps even a few thunderstorms are expected to develop.  In fact, some potential may even exist for a few strong to severe t'storms, but the probability/confidence is on the lower side.  If the probability/confidence is on the low side than why mention it?  Well the setup that will be in place on Friday is termed a low cape/high shear setup.  This means that while appreciable surface instability will be lacking, winds as you advance upwards in the troposphere will be quite strong.  When dealing with the potential for strong to severe t'storms, one thing we like to see is higher amounts of instability, but that can be compensated for with stronger wind energy aloft.  While these low cape/high shear events don't always produce strong to severe t'storms, it is important to follow the setup closely because when they do produce, sometimes the storms can pack a punch!

A strengthening trough to our west associated with a vigorous piece of energy at 500mb will push a cold front towards southern New England.  Out ahead of the cold front, a warm front will push through southern New England and usher in juicer air.  This is evident by forecast surface dewpoint temperatures well into the lower 60's and Precipitable Water Values (PWATs) increasing towards 1.5'' to 1.8''.  In fact, these values indicate we could see some heavier downpours around on Friday.



While clouds will be a limiting factor as to how high temperatures will become, temperatures in the warm sector are expected to climb into the upper 60's to even lower 70's.  The combination of a moist low-level airmass, surface temps around or just north of 70F, and the presence of steepening 700-500mb lapse rates upwards of 6.5 C/KM (meaning for every 6.5 KM you ascend in the 700-500mb level, the temperature will decrease by as much as 6.5 C), will lead to the development of some weak surface-based instability across the region.  In fact, some computer forecast guidance develops anywhere between 250-700 J/KG of surface-based instability with lifted index values around -1C to -2C.  While these values aren't particularly high and certainly nothing that would make you think strong to severe thunderstorms, its when you combine these values with stronger wind shear that can lead to the potential.

Computer forecast models develop a 500mb mid-level jet streak of 50-60 knots pushing through New England between 18z-0z (2:00 PM to 8:00 PM EDT) Friday.  This is something that can really help to enhance the lifting process of air parcels and could enhance the potential for some t'storms
.
(500mb wind graphic from the GFS...notice the tan-ish color traversing through New England...this is an area of enhanced winds within the main jet stream and this is referred to as a jet streak)


With a sharpening trough and vigorous piece of shortwave energy moving through southeastern Canada, this will increase the pressure gradient across the region and in response we will see increasing winds throughout the troposphere.

The combination of strong winds aloft and weak surface-based instability could in fact yield to the potential of some low-topped thunderstorms forming ahead of the cold front on Friday.  The term :low-topped" just indicates the height of the cloud tops...with these setups the cloud heights will not very often exceed 20,000ft in height.  This is different than your typical summer t'storms where cloud tops can exceed 40,000 to even 50,000ft.  However, given how winds aloft will be very strong, if clouds can develop strong enough updrafts (this is where the presence of instability comes in) and tap into the strong shear, we would see a few strong to even severe t'storms around.

Outside of thunder potential, expect a few bands of heavier showers to move through as well which could lead to some localized spots of flooding, especially in the flood prone areas.

Hurricane Joaquin discussion

The prolonged stretch of dry and tranquil weather here across southern New England is about to come to an abrupt end as we enter the first week of October.  As we move towards the later half of the week and towards the weekend, our attention is strictly focused on what is now Hurricane Joaquin which is located just northeast of the Bahamas.  Several pieces of computer forecast guidance, including ensemble members all indicate that Hurricane Joaquin will make landfall somewhere along the east coast, with perhaps anywhere from VA to NJ receiving the highest likelihood of a direct hit.

The latest (2:00 PM EDT on September 30th, 2015) advisory from the National Hurricane Center (NHC) has Hurricane Joaquin as a category 1 hurricane with maximum sustained winds of 85 mph near the center and with hurricane force winds extending outward up to 35 miles from the center with tropical storm force winds extending outward up to 125 miles from the center.  The estimated central pressure is 968mb.  The NHC is also forecasting further strengthening of Joaquin over the next couple of days and aren't ruling out the possibility of Joaquin becoming a major hurricane (sustained winds of at least 115 mph).  Below is the latest forecasted track:



Notice the large shape of the cone in the projected track.  This cone is referred to as the "cone of uncertainty".  The larger the spread within the cone, the lower the confidence in the exact track of the storm.  The reasoning behind such a large spread is due to the fact that the atmosphere will be undergoing some incredibly complex changes over the next several days and how the pattern exactly evolves is going to vastly influence what happens with Joaquin's track.  We will explore this further below and get into the reasoning as to why the pattern will become so complex and why our computer forecast models are/will continue having a very difficult time.

One reason for the large spread in track is purely due to the large spread with computer forecast model ensembles.  Below is a graph composing of various computer forecast models and their mean projected track.  This chart is called a spaghetti chart because...well all the different tracks and such resemble a piece of cooked spaghetti.
 

Notice you have a cluster of models bringing Hurricane Joaquin into VA/NC , then another small cluster up around northern VA/MD and then a few tracks into portions of southern New England and even a couple straight out to sea.  From this we can gather a couple pieces of information:

1)  There is much spread within the models which means there is a very low confidence at this time as to exactly where the storm will track and where the storm will make landfall.  When these plots and lines are much closer together, this is when confidence is typically much higher.

2)  It does appear there is a stronger cluster of the data out across the mid-Atlantic coast (NC/VA/MA) area.  What we can decipher from this is that perhaps the likelihood of a landfall anywhere in this area may be more likely.  However, given the spread as mentioned above, we can't say this with the highest of confidence.

When it comes to the forecasting aspect, as mentioned a bit earlier, the atmospheric pattern configuration is going to be quite complex with many variables all tying into how the system evolves and where it tracks.  Depending on how each feature (which we will discuss below) evolves, that will impact what happens with the system.  We will take a look at some of the important features below which are labeled on the map:
 

 The above graphic is from one of our computer forecast models, the GFS and this is from the 12z run (8:00 AM EDT) from this morning projected out to 48-hours.  So we're looking at 8:00 AM EDT Friday morning.  This particular graphic was just chosen to provide a visual of the key features at play.

1) Trough - This trough will be a major focal point over these next several days.  Computer forecast models really amplify the trough as it moves into the southeast and digs it very far south.  The trough is also expected to become negatively tilted.  What computer forecast models do is as Hurricane Joaquin begins to turn more to the north and west, the trough will eventually capture Joaquin and bring the system right towards the East coast.  So the question here becomes, if/where this capture takes place.  If it takes places further south and earlier, the chances vastly increase for a landfall perhaps across VA/MA area.  If it's later and slower with the capture, then the chances for a landfall further north increase.

#2, #3, and #4 - I grouped all these together because they are will seemingly work in conjunction with one another...well at least with respect to what happens with Joaquin.  The strong high pressure to our north, and ridging off to our east will ultimately do its very best to keep Joaquin closer to the coast.  That cut-off low (#4) will also work to prevent the ridging (#3) from sliding east as well which would allow the opportunity for the Hurricane to slide east.

So with all this...what can we perhaps expect here in southern New England?  At this juncture it appears that the highest likelihood for the most significant impacts will be somewhere along the mid-Atlantic coast where substantial impacts may be possible.  Up here across southern New England we may get spared, however, we could still deal with the potential for a quite a bit of rain, some wind and maybe some flooding.

Confidence should continue to build over the next few days as hopefully our computer forecast models develop a better consensus.  

Cold Front to Bring Relief From Heat/Humidity and Much Needed Rain

The high heat and humidity is set to come to an end as a cold front slowly progresses towards New England late Wednesday and on Thursday.  As the cold front traverses New England, the front is expected to stall across the region.  As this occurs, weak waves of low pressure are expected to develop and rid up along the cold front.  The presence of the stalled front overhead along with areas of weak low pressure riding up along the front will set the stage for numerous periods of showers and thunderstorms and some of the rains will be quite heavy.  Once the cold front passes the region, we will see much more seasonable temperatures and humidity levels filter in.

Moving through the remainder of Wednesday, as of 4:00 PM EDT, the cold front was slowly pushing through western and central New York state where a broken line of showers and thunderstorms have developed, including a few strong to severe thunderstorms.  As this front continues to slowly slide south and east, the risk for showers and perhaps some thunderstorms will increase as we work through the evening and into the overnight hours.  With very weak shear aloft, poor mid-level lapse rates, and instability expected to wane with loss of dayitme heating, the risk for strong to severe thunderstorms will be very low.  The main threat would be for torrential downpours.  However, gusty winds can't be ruled out.

On Thursday, clouds will be present region wide, holding back temperatures more closer to 80F rather than 90F, however, the humidity will still be pretty high with dewpoints right around 70F.  While it will be cooler, it will still be quite humid.  Mid-level lapse rates will be continued weak with weak shear in place as well.  While thunderstorms will be possible given the presence of weak instability thanks to very moist low-level airmass, the above factors indicate the threat for strong to severe t'storms will be very low.

For widespread heavy rainfall, it would be beneficial to see a much stronger low-level jet forecasted, however, that is not the case here.  Computer forecast models though are indicating PWAT values as high as 2'' throughout the day on Thursday, a very strong indicator of how moist the airmass will be.  With multiple waves of low pressure developing, presence of weak instability, and moist airmass, we will see periods of showers and thunderstorms throughout the day tomorrow.

As far as rainfall totals go, computer forecast guidance is suggesting anywhere from 1''-2'' of rainfall would be possible.  Looking at the SREF forecast model below, (a compilation of numerous pieces of numerical guidance to generate an average) it seems like a good bet most folks can expect anywhere from 0.50'' to 0.75'' of rain.  However, there will be some locations who potentially exceed 1.50'' to 2'' of rainfall tomorrow.

Strong to Severe T'storms Possible Late Sunday Evening into Overnight Hours

A shortwave trough with a fairly strong piece of shortwave pushing east through the Great Lakes during the day tomorrow will allow for a pre-frontal trough and cold front to push east as well approaching southern New England late tomorrow evening.  Out ahead of the front a hot and extremely humid airmass will overspread the region with temperatures ranging from the mid-80's to the lower 90's and dewpoints into the lower 70's.  The combination of high heat/humidity will allow for an extremely unstable airmass to develop during the late morning and afternoon hours.

While typically we see the possibility of strong to severe t'storms decrease with the loss of daytime heating as this allows the atmosphere to stabilize, that won't be the case this time.  Computer forecast models have been rather consistent in an area of steep mid-level lapse rates associated with an elevated mixed-layer plume moving overhead on Sunday.  With the presence of steep mid-level lapse rates, combined with the hot/humid airmass, the atmosphere will remain quite unstable through the evening and into the overnight hours.

Below is the 18z run of the GFS from 7/18/15 showing surfaced-based cape values as high as 1500 J/KG valid for 11:00 PM Sunday evening:

Associated with the approaching shortwave trough will also be some pretty strong wind shear aloft.  Computer forecast models overspread as much as 40-50 knots of surface-500mb bulk shear values between 11:00 PM Sunday evening and 2:00 AM Monday morning:

With such an unstable airmass in place, strep lapse rates, increasing shear, and a source for lift, we will see showers and t'storms develop and move into the region during the evening hours tomorrow and lasting through the overnight.  A few of these storms will have the potential to become rather strong to severe and pose a threat for large hail and damaging winds.  In addition, storms will contain torrential downpours and vivid lightning.

The degree of the severe potential is still a bit in question as there are some uncertainties with regards to timing of all the features and whether or not all the best parameters align together.  The NAM computer forecast model also develops a quite a bit of activity during the afternoon on Sunday just off to our west and across New England and this would weaken lapse rates aloft reducing the severe threat after sunset.  Given how the atmosphere should be capped pretty well during the afternoon this scenario doesn't seem all that likely at this time.

The timeframe for t'storms and severe potential should be from 7:00 PM Sunday evening lasting through 3:00 to 4:00 AM Monday.  A second threat for strong to severe t'storms may be possible later Monday morning and early afternoon across southeastern New England as the cold front pushes through.

Wednesday, July 1st, 2015 storm potential

A potent trough and attendant shortwave will slowly be pushing eastward during the day on Wednesday allowing for a cold front to also slowly push east.  Ahead of the trough the surface and low-level flow will be out of the southwest allowing for a warm and moist low-level airmass to set in place.

As the shortwave moves just to our north it will be associated with some pretty cold temperatures in the mid-levels of the atmosphere for this time of year.  The GFS computer forecast model suggests temps at 700mb (~10,000ft ASL) as cold as +4 to +6C with temps at 500mb (~18,000ft ASL) between -10C and -12C and perhaps as cold as -12C to -14C.  This should yield to some fairly steep mid-level lapse rates (change of temperature with height).

With surface temperatures projected to warm into the lower 80's and dewpoints expected to climb into the mid to perhaps upper 60's, the combination of warm temps, high dews, and presence of steep lapse rates should yield to a modestly unstable airmass to develop in the afternoon/evening hours.

There will also be some decent wind shear associated with the trough with winds a few thousand feet up off the ground as strong as 25-35 knots with winds around 10,000ft as strong as 30-40 knots and winds around 18,000ft as strong as 40-45 knots.

What all these ingredients point to is the potential for showers and thunderstorms to develop and perhaps become quite numerous as the afternoon progresses.  While typically we see the threat for showers and t'storms diminish as the sun begins to set, colder air working in the mid-levels of the atmosphere may keep activity going well through the evening hours.  Given the degree of winds aloft and such cold mid-level temperatures leading to low freezing levels, the strongest t'storms will be capable of producing hail and strong winds.  Even coastal areas will have the potential despite a sea-breeze.  Typically in these setups the sea-breeze is a non-factor and if anything can sometimes act as an enhancer.  All t'storms will also produce torrential downpours given degree of low-level moisture.

One thing to watch, however, is the potential for some rising heights across portions of southern New England later in the afternoon into the evening.  The GFS forecast model shows heights rising a bit later in the afternoon.  If this occurs this could diminish activity as the afternoon progresses or prevent further activity developing.

Noon severe weather update

Showers and some embedded thunderstorms pushed through portions of New England this morning, particularly along and north of Windsor Locks, CT.  While clouds are still in place, satellite trends have been breaking up the clouds west of southern New England and the sun has been in and out.  Given recent satellite trends, more and more breaks of sun should occur across much of MA and south into CT and RI.  

The combination of steep mid-level lapse rates, in excess of 6.5 C/KM, rich low-level moisture characterized by dewpoints climbing into the lower 70's, and surface temps rising into the 80's will all lead to quite an unstable airmass this afternoon.  It's still a bit unclear as to how unstable we become due to the presence of clouds but some areas could end up with over 1500-2500 J/KG of Cape...which would be quite significant given the degree of wind shear aloft.  

Some of the latest computer forecast models, more particularly the higher resolution models, develop what appears to be a broken line of supercells moving through MA and CT later this afternoon.  At this time the potential for severe weather is still certainly in place along with the possibility for some significant severe weather including damaging wind gusts in excess of 70 mph, hail as large as golf balls, and a few isolated tornadoes.  A strong tornado can't be ruled out either.  

The best timeframe appears to be between 2-6 PM (as late as 8 PM for RI and SE MA if activity sustains) for strong to severe t'storms capable of the hazards described above.  

The situation will continue being monitored with updates provided as needed.  

Below is the latest satellite image from noon showing breaks of clouds moving east:

Update on today's severe potential

Forecasting severe weather across New England can be quite challenging at times...in fact it can be challenging most of the time.  There are just so many mesoscale factors and features that need to come together perfectly and you really need to just watch things evolve pretty much as they are.  This situation is no different.

For the past few days computer forecast guidance has really been hammering the potential for a significant severe weather event to occur across portions of New England with the potential for widespread damaging winds, large hail, and even a few tornadoes all possible.  They key to realizing the highest potential is to get all the main ingredients to fall in place at the same time.

While all these hazards are still on the table, as the morning progresses confidence has decreased a bit and confidence is on the lower side as to exactly what will happen today and exactly how big the event will be.

Currently there is a large complex of t'storms just to the west of New England called a mesoscale convective complex (MCS) that is racing towards CT/MA/VT/NH.  This complex is associated with a great deal of cloud debris and if this complex does not fall apart quickly that could vastly reduce the degree of solar heating we achieve today which means less in the way of instability.  It could also mean for an earlier show of action which also means less instability and less potential for a major severe weather event.

While some early indications projected areas along and north of the MA Pike for the highest severe weather potential, this area may have shifted southward to the MA Pike including interior NE MA, and down into northern CT and northwestern RI.

Here are a few scenarios which are still on the table for today

1)  If the complex of t'storms to the west erodes soon and clouds begin to break up this would allow stronger surface heating to occur over New England.  Given the atmospheric environment that will be in place, this scenario would be the most likely to produce a major severe weather event with damaging winds which could be as strong as 70 mph in the strongest storms, large hail of golf ball or even bigger in the strongest storms, and perhaps even a few tornadoes.  In this scenario a strong tornado couldn't be ruled out somewhere.

2)  If the complex of t'storms to the west does not weaken much and thicker cloud cover remains, the degree of surface heating will be much lessened across New England and the degree of instability will be much less.  This will mean a reduced threat for a major severe weather event and while some strong to severe t'storms will still be possible the threat would be much more isolated.  Some pockets of damaging winds would be possible, along with hail up to quarters, and still perhaps a tornado.

3) The MCS weakens as it moves into New England but comes through prior to peak heating.  In this scenario the severe threat is vastly reduce.  A few strong gusts perhaps knocking down weakened trees would be possible and small hail.  The threat for a tornado would be very small.

At this juncture the next few hours are very crucial and all we can do is watch current radar and satellite trends along with latest hourly computer data observations and go from there.  It's important to keep an eye and ear to the skies along with all the latest forecasts and updates.  

Strong to Severe T'storms Possible Tuesday 6/23/2015

Several pieces of computer forecast guidance are showing the potential for showers and t'storms across the region during the afternoon and evening hours on Tuesday.  Several atmospheric ingredients may move in place and move together to allow for the potential for some very potent thunderstorms posing a threat for strong to damaging winds, large hail, and perhaps even a few tornadoes.

Typically setups like this are on the rare side, however, what is opening some eyes with this setup is the potential advection of an elevated mixed-layer plume working in from out west.  For more on elevated mixed-layers, their origin, and importance in higher-end severe weather events please read this more detailed information provided here:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

To explain EML's as simply as possible, they are capping inversions which prevent clouds, showers, and thunderstorms from developing too early and this allows for stronger heating and can lead to extreme amounts of instability developing below the capping inversion depending on how high the temperatures are and how high the dewpoints are.

Computer forecast models are showing a very strong piece of shortwave energy passing just to the north of the US/Canadien border.  This track may favor the greatest risk of severe storms being further north across New England, (central/northern MA into portions of VT/NH and into ME so the track of the s/w will have to be watched closely:

Computer forecast guidance also develops some extreme instability across the region with Cape perhaps as high as 2500-3000 J/KG and very strong wind shear, both speed shear (increase of winds with height) and directional shear (change of wind direction with height).  This combination could lead to the potential for any thunderstorms that develop to become quite severe and have the potential to produce damaging winds, large hail, and perhaps even a tornado.

Below is the 12z NAM bufkit for Windsor Locks, CT late Tuesday afternoon.  This is a pretty alarming sounding with extreme instability, high shear, and very high helicity being forecasted.  This is something seen more typically in the plains than here in CT.

This definitely needs to be watched!

What is an Elevated Mixed-Layer?

The origin of the EML

An elevated mixed-layer (EML) is a region of extremely warm and dry air typically located between about 800mb and 600mb (~6,500ft to ~16,000ft ASL) in the atmosphere.  In the United States, EML's originate and develop across the Rocky Mountains region near the desert southwest and across the high plains.  Air moving up and over the Rocky mountains warms and dries significantly.  This allows for very steep lapse rates (change of temperature with height) to exist between the surface and 600mb in the atmosphere.  

Importance of the EML and severe thunderstorms 

The EML is also known as a capping inversion because of the warming and drying of the atmosphere where the EML begins.  Remember, air will continue to rise as long as the surrounding air is cooler than the temperature of the parcel.  Once the air begins to warm, this process is no longer occurring, thus the air parcel no longer rises.  This process inhibits clouds, showers, and thunderstorms from developing which allows for the sun to continue shining strong.  If the low level airmass is warm to hot and there is a rich presence of moisture (high dewpoints) the combination of the very steep lapse rates, high surface temps, and high dews leads to the development of extreme instability, a key ingredient in the development and strength of thunderstorms.  

If some sort of lifting mechanism, such as a cold front, potent shortwave, etc, pushes in and provides enough lift to break through the cap, thunderstorms would begin to initiate and become very strong very quickly.  If the capping inversion is too strong then one of two things can happen; 1) No activity develops and all the instability goes to waste or 2) Only a few storms develop.  This can actually enhance the severity of the storms that develop because with only a few storms around the updrafts are not fighting each other.  

How can EML's be observed?  

An EML is easily observed on a skew-t forecast sounding which gives you a look at the entire atmospheric profile.  Below is an example of what a true EML looks like on a forecast sounding:

In the above sounding you can see the region of extremely warm (where the black temperature line jets off to the right) and extremely dry (where the green dewpoint line shoots off to the left) air.  

EML advection into the Northeast

While EML's don't often survive the journey into the Northeast, every now and then a plume of EML air can break off from the source region and ridge up over ridging at 700mb and remain in tack all the way into the Northeastern United States.  However, ridging at 700mb does not always guarantee EML advection into the Northeast.  In a research paper published by meteorologists Michael Ekster and Peter Bannacos, both identified 36 EML events and the corresponding 700mb pattern in place 36 hours prior to the event, 24 hours prior to the event and the observed day of the event.  As you can see ridging at 700mb appears dominant across the southeastern United States extending northward into the mid-At;antic and into the Northeast: 

Potent Storm System To Bring Region Wide Impacts Saturday Through Sunday

A clipper system diving southeast out of Canada is going to rapidly intensify into a very powerful storm just off the coast.  This system will be will bring a slew of impacts to southern New England including; moderate to heavy snowfall, the potential for very strong damaging winds, brutally cold temperatures, and dangerous wind chills.  All the latest trends indicate that the potential for the highest snowfall amounts will be confined to eastern MA once again.  This system is expected to have significant impacts and we will break everything down.  First, we will take a brief look into the setup:

Brief Meteorological Look

An innocent clipper system will be tracking southeast from central Canada and move right over southern New England.  As this clipper system moves off-shore, rapid cyclogenesis (strengthening) is expected to occur.  As the system begins to rapidly intensify, moisture will get drawn into the system and this will eventually lead to snowfall breaking out across the region.  In response to the system rapidly strengthening, this will increase the pressure gradient across the region and this will yield to very strong to perhaps damaging winds developing, especially across eastern MA and the Cape area.  With the wind component mainly from the north and northeast, this will reinforce brutal Arctic cold into the region with temperatures mainly in the single digits and teens throughout the duration of the storm.  This coupled with the potential for very strong winds will yield to some dangerous wind chill values.

We will now do a breakdown of each associated hazard expected with this system and what we can expect with each associated hazard:

Snow

We will first look at the snowfall potential associated with this system.  This will be an all snow event...no worries about any sleet, freezing rain, or rain mixing in as temperatures throughout the atmospheric column are more than supportive for snow.  One thing with this system is, the precipitation really doesn't get cranking until the system is strengthening off-shore.  This will lead to reduced snowfall totals the further west you go, however, the further east you go, this could yield to some significant snowfall accumulations (>12'').

During the day on Saturday we will see periods of light snow region wide, these snows will be associated with the clipper as it moves overhead towards the coast.  Snowfall accumulations during the afternoon hours should be on the light side, perhaps a few inches in spots but the periods of heavier snow don't begin until we near the evening hours.

As we approach the evening hours and the clipper and energy aloft associated with the clipper begin to move off-shore, the strengthening process will rapidly begin to occur.  As this happens stronger lift will develop across the region and more moisture will work into the system.  This is when we will begin to see periods of moderate to heavier snowfall break out across some spots.

Due to the fact that this system will be strengthening off to our east instead of off to our southwest, this means the majority of the heaviest snows will be on the backside of the system associated with what is called the cold-conveyor belt (or CCB).  It is this which will be key as to where the heaviest snowfall sets-up and where the highest snowfall totals occur.  The big question with regards to the CCB is when does it begin to develop and take shape.  This is something that needs to be watched, especially for CT and central MA as if this develops later as opposed to earlier it will mean reduced snowfall totals, however, if it develops earlier, that would lead to increased snowfall totals so this will be closely monitored over the next 24-hours.

We are looking at the time period from late overnight Saturday (roughly 2-3 AM) through late morning to about noon time on Sunday as the timeframe for the heaviest snows to occur.  Across western and central MA down to western and central CT, snows should begin to taper off mid-Sunday AM.

Under the heaviest banding of snows, snowfall rates could approach as much as 4-5''/HR but this will not be the norm region wide...these totals will only be confined to the heaviest banding and where thundersnow is possible which is more likely to occur out across eastern MA.  Elsewhere expect snowfall rates of up to 1''/HR at times.

Here are my initial snowfall projections but this will be further fine tuned tomorrow:

Wind

Wind will be a major issue with this system, especially out across eastern MA thanks to a very tight pressure gradient that will be in place with the near 970mb low just to our east.  The combination of very strong winds and snow will yield to blizzard conditions at times, especially out across eastern MA and will also lead to very dangerous wind chill values.  Winds out across eastern MA towards the Cape could gust upwards of 70-80 mph!  These are gusts which will rival category 1 hurricane strength and would lead to widespread power outages along with big time drifting blowing and drifting of snow.  As you move further west across southern New England the magnitude of the winds won't be as severe, however, we are still looking at winds gusting potentially upwards of 65 mph.  These values are still strong enough to yield scattered power outages along with blowing and drifting snow.  We will actually see winds becoming gusty during the day today and continue right through Monday with the peak intensity of the winds from tomorrow night through much of Sunday.  If the higher end of these values are realized across eastern MA not only would we be looking at widespread power outages but outages lasting several days or longer.  Here is a map showing the potential wind gusts:

 
Temperatures

Temperatures will be down right COLD.  We are only looking at temperatures to be in the single digits to perhaps the low teens throughout the duration of this event as brutally cold Arctic air is transported southward thanks to strong northerly to northeasterly winds.

Wind Chill Values

The combination of temperatures only in the single digits to perhaps low teens and winds sustained between 20-30 mph (higher further east) with potential for gusts 60+ mph will yield very dangerous values at times.  We could see wind chills range anywhere from -15F to as low as -25F during the storm.  Those are very dangerous values and pose a high risk for frostbite in a very short amount of time.  It is this reason, along with the accumulating snows that travel should not be done unless absolutely necessary.  If you are out traveling and get caught this will pose a life threatening situation given the degree of cold we are dealing with.  

Snow...Snow...And More Snow

     The weather pattern over the next several days looks to remain quite active as multiple pieces of energy moving through the jet stream aloft will yield the potential for not only snow every day between tomorrow and Tuesday but the potential for accumulating snows each and every day.  It is EXTREMELY IMPORTANT to note that it will not be snowing this entire time and not everyone may see accumulating snows or even snow each and every single day.  This is quite the complex situation and while this will be quite a lengthy post, this post will go into great depth and detail regarding what to expect and what we are looking at.

     Below is a graphic displaying 1000mb to 850mb thicknesses.  I have highlighted several key areas which will be of extreme importance as we move through the weekend and into the beginning of the work week.  The explanation of the highlighted areas and their importance will be discussed below the image:

     The red circled areas are areas of high pressure.  Notice how there is a big area of high pressure just to the north of the United States across central Canada and a huge area of high pressure across the southern portion of the United States.  In between these areas of high pressure we have our frontal boundary, an Arctic cold front and this, along with the area of high pressure across Canada is allowing for brutal Arctic cold to work here in to New England and the flow around the high pressure is allowing for this cold to be maintained.  The area of high pressure across the southern-tier of the United States is allowing for this front to stall across the extreme northern-tier of the country and just south of New England.

     The blue circled area is where not only our frontal boundary is positioned but this is where the mid-level jet stream is located as well (although not pictured here).  Notice the green shadings within this circled zone?  This is moisture associated with the boundary and this moisture is streaming across this portion of the country all the way from the Pacific!  As long long as this pattern configuration is in place Pacific moisture will continue being streamed all the way here into New England.  As we eventually move into Sunday the pattern will also begin to allow more in the way of Atlantic moisture to work into the picture as well and this is when some areas could receive some significant snows.  What our focus will be on over these next several days is pieces of energy developing and riding along the stalled out boundary in this highlighted area.  Lift + moisture = precipitation and we will have plenty of moisture to work with so add in any extra lift and we will be looking at the potential for quite a bit of precipitation...but the million dollar question is, where will the axis of highest amounts occur?  

     Now we will take a look at each of the upcoming days with a bit of detail and what we could potentially expect.

Saturday

     For the most part, Saturday and Saturday night is looking to be the "quieter" of the days between tomorrow and Tuesday.  Computer forecast models bring a weak piece of energy through the jet stream tomorrow and have some light moisture associated with the energy.  As of right now it appears the track of this system will be more through central and northern New England, meaning the highest degree of lift will be across these areas.  If this track verifies it will be areas north of the MA Pike up through central VT, central NH and into southern ME which see the periods of the heavier snows.  Across these areas 1-3'' of snow will certainly be possible within this area.

     South of the MA pike across southern MA into CT and RI, the degree of lift is not expected to be as strong with this track so these areas should remain outside of the heavier snows.  Still, some locations may pick up an inch or so of snow, especially across the higher terrain.  

Sunday

     As we move into the overnight hours of Saturday into Sunday, this is when things will begin becoming more interesting.  By this time more in the way of Pacific moisture will have spread into the region and computer forecast models are hinting at some stronger pieces of energy moving through aloft.  Higher moisture content and stronger lift indicate periods of moderate to heavy snows will become much more common overnight Saturday through much of the day on Sunday.  While it will not be snowing this entire time everywhere, we will see multiple periods of moderate to heavy snows throughout this time.  We will not be seeing a widespread precipitation shield which encompasses a large area (like we see with coastal storms), rather we will see numerous bands of snowfall which will likely be more narrow in nature.  This means that you can have an area where snow is coming down extremely heavy, however, you go just several miles away and nothing is occurring.  It is a situation like this which will make forecasting snowfall totals EXTREMELY difficult.  Between overnight Saturday into the overnight hours of Sunday some of the hardest hit areas could receive as much as 6-10'' of snowfall and perhaps even some higher amounts in some spots.  On the other hand, areas who don't see much action may be luck to get 2-4'' and the axis between these high and low amounts could end up being only a matter of miles!

Monday

     Moving overnight Sunday into Monday this is where the forecast becomes even more complex and that is because we are still a little far out.  However, computer forecast models develop a stronger area of low pressure right along the stalled out front.  This is where we will begin to see more in the way of Atlantic moisture become involved as well and this is when we could see even higher snowfall totals.  The track of this low and the strongest lift and moisture will all depend on the exact positioning of the stalled out boundary.  This is something which will be sorted out over the next 24-48 hours.  With this system we may also be looking at a more widespread precipitation shield meaning a much more widespread area is hit harder...as opposed to only a few select locations.  This system will have the potential to yield upwards of 9-15'' of snow with lesser amounts of 3-6'' across the areas outside of the heaviest precipitation.  This portion of the forecast though is much more tricky at this time.  

Summary

     Between tomorrow and Tuesday it is not out of the question that some spots could pick up anywhere from 12'' to even 20'' of snowfall depending on how everything aligns and comes together.  Just keep in mind this is not going to be a uniform system where we are looking at uniform totals across a widespread area.  The difference of even 5-7 miles could be the difference between 6''+ of snowfall totals.  This can't be stressed enough.  With this in mind, I feel creating a snowfall map is not the right thing to do at this time as amounts may vary widely across such a small area.  

How to Prepare?

     Just keep listening to all forecasts and all the updates provided.  Just keep in mind that on the higher end of the scale you could see as much as a foot-and-a-half of snow and on the lower end you may only see a half-a-foot of snow.  Always prepare for what the higher end of the scale may bring as it's better to be over-prepared than under-prepared.  

Beyond Tuesday

     Looking ahead to the end of the week and next weekend we could be looking at some of the coldest temperatures we have seen in quite some time as computer forecast models indicate Arctic air plunging directly into New England but we will worry about this as we get closer.  




 

Active pattern continues; Fast moving system late Sunday through first part of Tuesday

The active weather pattern looks to continue as we move into the beginning of the work week as a fast moving storm system is expected to slide just south of the state.  Unlike the big blizzard several days ago, this system will be moving at a much quicker rate with the duration expected to be less than 12-hours in total.  In fact, the heaviest if the activity will only occur in a 6-8 hour window. 

Computer forecast guidance suggests as this system slides south of the state it will slowly begin to intensify.  With winds in the lower levels of the atmosphere coming from the southeast, this will contribute quite a bit of Atlantic moisture into the system.  With sufficient lift in place, we will see a fairly large precipitation shield develop and with temperatures at all levels cold enough, at least initially, we will see snow break out. 

The first flakes could begin between the hours of 11:00 PM Sunday night and 1:00 AM Monday morning.  By 3:00 AM to 4:00 AM, periods of moderate to heavy snowfall will be occurring across the state and snowfall rates in the heaviest echos will approach 1’’ to 1.5’’ per hour.  We will continue to see moderate to heavy bursts of snow continue up until about 10:00 to 11:00 AM before the snow begins to taper down.  Across southern CT, computer forecast hints at warmer air working in the lower levels of the atmosphere.  This will allow for a transition from snow to sleet and perhaps some freezing rain or even plain rain during the early morning hours, thus cutting down snowfall totals across this portion of the state. 

Due to the timing of the system and the heaviest of the precipitation rates, morning travel will be extremely messy with significant impacts.  Expect numerous delays on the roads and very slow travel.  School closings are highly likely across many towns and cities.

While my snowfall forecast is a bit less than what many others are suggesting, to me there are a few flags which is inhibiting me from going any higher;

1)       The speed of the system.  When looking for snowfall totals approaching amounts such as 8’’…12’’ or higher, one of the main factors to take into account is the speed of the storm.  If you have a storm system moving as fast as this one, you have to ask yourself, how long is the heaviest precipitation expected to last?  What are the projected snowfall rates during this time?  What kind of snow growth are we looking at?  What type of lift are we dealing with? 

2)       There is no established mid-level circulation.  Below is a graphic from today’s run of the 12z GFS.  While I am using the GFS to give a visual, all other significant models are similar.  Having no established mid-level low can make for some not so great lift, especially in the mid-levels.  One way to get excellent snow growth and subsequent ratios is to have the presence of very strong mid-level lift. 

3)       Looking at bufkit profile for Windsor Locks, CT there are some definite flags to me here.  There isn’t a ton of omega (the red contours) and much of the highest values remain outside of the area of best snow growth zone (area highlighted by the yellow contours).  What this says to me is that much of the best lift is all mainly in the lower-levels of the atmosphere and not within the area of best snow growth.  This indicates that overall we will be looking at fairly poor snow growth and snowflakes that won’t be very large in nature…small flakes take longer to accumulate.  This will also lead to some poor snowfall ratios.  However, with this said, some of the low-level lift is rather impressive and this will lead to a very narrow zone of extremely heavy snowfall with much better snow growth and ratios.  Within this zone snowfall totals up to 10’’ will be possible and a 12’’ amount can’t be ruled out. 

With this said, below is my snowfall forecast: