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
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(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: