Storm System to Impact southern New England 1/16/2016

If you're a huge snow fan living here in southern New England the winter of 2015-2016 has been nothing sort of depressing.  After the warmest December on record across many locations we finally have more frequent cold outbreaks transpire across the region, however, the cold has come without any meaningful snow.  Unfortunately this trend will continue this weekend as the next major storm system is set to bring predominately rain to much of the region with perhaps some snow mixed across higher elevations and the further north into New England you go.

When looking at the track of Saturday's storm system we see a track which brings the surface low pressure just to the south and east of southern New England.  Typically, especially this time of year a storm track like this would be yielding not only snows but some hefty snowfall totals for someone:

If the storm track is favorable for snow then what's the problem?

Re-posting the same graphic above, however, looking at the continental United States as a whole, there are three key pieces to note:

1) The primary low pressure system moving northward through the Great Lakes.  When certain conditions are in place, these primary areas of low pressures can "transfer" their energy to just off the east coast of the United States.  When this process occurs and when the primary low pressure weakens, the secondary low begins to take dominance and this can help to bring down colder air from Canada (the flow around the low coming from the N to NE brings down colder air).

2) The secondary low pressure development and track just to the south and east of southern New England.  Unfortunately with this weekend, the secondary low pressure development is just going to occur too late and the primary low stays way too strong.  The end result is warmer air advecting into southern New England from the south and southwest.

3) The lack of a high pressure system to our north.  A good chunk of our snowstorms, especially bigger snow producing snow events all have an area of high pressure to our north.  What this does is reinforce colder air into the region from Canada.  Unfortunately, with a lack of high pressure to our north, there is nothing to reinforce cold air and nothing to prevent warmer air from working northward.

Unfortunately this system is going to produce precipitation mainly in the liquid form, however, higher elevations areas and central/northern MA will mix in some snow.  We are looking at 1-2'' of rainfall region wide with some areas picking up 3'' of rain.  Flash flooding will be an issue in the typical flood prone locations.  As this system departs another surge of Arctic air will arrive, perhaps even with some snow squalls later in the day on Sunday.

Record Warmth With a Sunday Washout (1/10/2016)

An area of strong low pressure will be passing well to the west of southern New England on Sunday and this track will allow for a warm front to push through much of southern New England.  With the passage of the warm front will come a highly unseasonably warm/moist air mass as temperatures and dewpoints will climb well into the 50's.  In fact, a few areas, could even get close to 60F.  This warmth will indeed set some records in some spots.

With the warmth, however, comes rain...and quite a bit of rain.  Some areas will pick up as much as 1-2'' of rain with a few locations perhaps picking up as much as 3-4'' of rainfall.  Given how the ground is partially frozen (especially the top layer) this could create some issues with puddles and localized flooding.  Below is a graphic from the January 9th 12z GFS computer forecast model showing Precipitable Water Values "PWATS" for early Sunday afternoon.  We see PWAT values around 1.1'' to 1.3'' which is pretty impressive for this time of year and suggestive of the heavy rain potential we will be seeing:

 

We will also see the potential for thunderstorms as well embedded in the heavy rainfall as some weak instability pushes in thanks in part to the warm/moist low-level airmass advecting in.  Its within any thunderstorms that some areas could pick up the 3-4'' of rainfall.  The thunderstorms could arrive in what is called a forced line of low topped convection.  Unlike warm-season thunderstorms which are formed mainly due to strong instability, these thunderstorms are formed from very strong wind dynamics aloft which forces air to rise upwards, hence the term "forced convection".

Below we see the forecast from the January 9th 12z GFS computer forecast model for 500mb winds (winds about 18,000ft off the ground) during the early afternoon hours on Sunday.  The mid-level jet streak modeled is about 80 knots or so and arrives from the SW.  Notice the area over CT/MA where the color shading goes from yellow to red.  At the edge of this boundary we call this the nose of the jet.  This provides a good focal area for enhanced lift (stronger winds arriving) and it is along this boundary we could see a forced line of convection develop.

With winds just above the surface fairly strong it is also possible some of these stronger winds are transported down to the surface, thus it could be fairly gusty at times.  If any convection does form or push into the region, this will also enhance the potential for some stronger winds mixing down to the surface.  All in all we could see winds gust as high as 40-50 mph across the region with the small potential for a gust or two reaching 60 mph.  Isolated to widely scattered power outages will be a possibility, however, they should not be a major issue.

After the passage of this storm system we will see the beginning of a drastic pattern change towards much colder weather and yes perhaps even some snow chances...but we'll talk about this in a few days.    

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.