Historic Blizzard Underway...CT forecast/discussion update

Just to reiterate from yesterday, this storm is NOT historic in nature because this isn't something we've never seen before, this storm is historic in nature due to the fact of what it will produce, and that's widespread snowfall totals ranging from 1' to 2' throughout New England and extending back through NYC and into portions of NJ...so we are looking at an extremely large geographical area receiving feet of snow.  In fact, some areas, may even close in on 3' of snow!!!

Currently the storm is evolving beautifully!  The circulation is beginning to deepen rather rapidly just off the coast of the Carolina's and you can see there is already an immense amount of moisture available and working into this system:

We have already seen light to even moderate bursts of snow across this state which started in the late morning and early afternoon hours.  These bursts of light to moderate snow bands will continue through the remainder of the evening before snowfall rates substantially begin to pick up after 11:00 PM or so.  Winds have also started to increase and will become an issue overnight as well.  

This will be a very long duration storm with periods of light to moderate snows continuing throughout the afternoon tomorrow along with strong wind gusts.  

What To Expect

Statewide, I am expecting an average of 18-24'' of snowfall..this is an average meaning some locations will end up with more than 24'' and some locations could perhaps end up with less than 18'' (but still more than 12'').  Why is this?  Well when you have these very intense systems you get what is called a deformation band...these bands are notorious for producing very heavy snowfall rates and dropping alot of snow in a very short amount of time.  These bands can be a complicated mess when it comes to forecasting.  The exact placement of them is something that isn't particularly modeled very well and also these bands can sometimes sit over a general area an pivot for hours...this is a situation in where areas can get upwards of 30"+ of snow.  When taking to gauge where this banding may setup we typically look at the 850/700mb levels and look at their low pressure tracks along with vertical velocities.  We will just highlight the 700mb level for time constraints:

The above image is taken from just one of the computer forecast models used for forecast guidance.  However, the consensus is we will see a favorable enough low track to get the deformation band to pivot across CT, however, the question is does that occur across western and central CT or central and eastern CT?  Something we won't 100% know until it gets going, however, at this time I'm leaning more towards the area highlighted in red on the snow map below as the highest likelihood of getting under the banding.  

Another issue the deformation bands present is they also produce something called a subsidence zone...or what we weather weenies call "screw zones".  As Newton once said, "what goes up, must come down" and that's pretty much what this is.  Remember, the very strong lift describe above...that is air rising vigorously...well in response air must sink vigorously.  So typically just to the north and west of this band will be an area of sinking air...this sinking air shuts off or lessens the rate of the precipitation falling...so while several locations are getting creamed, others are getting screwed.  All this means that is there will be some areas that probably receive less than 18-20'' of snow and there is no way to tell where that will happen.  

With this piece of info out of the way let's look at what to expect:

• Snowfall rates pick up as the evening and overnight progresses
• Snowfall rates will approach 2-3' per hour' at times and under the deformation band could approach 3-5'' per hour
• Thunder/lighting possible (also helping to enhance snow rates locally)
• Winds pick up...gusts along the coast approaching 60-70 mph.  Further inland 45-55 mph
• Blizzard conditions expected tonight and tomorrow morning
• Numerous power outages likely due to strong winds and downed trees and utility poles
• Heaviest of the snow occurring between the hours of 11:00 PM and 9:00 AM tomorrow

Below is an updated map.  Keep in mind the 24-36'' possibilities could be shifted depending on the deformation band...where it sets-up, where it pivots, and the duration the band sits over certain areas.

Historic snowstorm Looking Likely 1/26/15 - 1/27/15 Across CT

     A historic snowstorm is set to impact the state of Connecticut beginning on Monday and lasting through at least the first part of Tuesday.  First off, let's briefly discuss what will make this historic.  Snowstorms producing snowfall amounts of this magnitude certainly aren't unheard of around these parts, and while over the past several years we have seen more than a few, getting snowstorms to produce such widespread amounts greater than 12'' and even 18'' over a large geographic area don't happen very often.  Cities such as Boston, Providence, Hartford, and New York City could all be looking at over a foot-and-a-half of snow when all said and done and that's what makes this historic...how such a large area is expected to receive such massive totals.

     We are looking at a very long duration snowstorm to impact the state beginning on Monday and lasting through much of the day on Tuesday.  As the system begins to develop just to the south of CT and slowly sort of slide northeast, the system will stall out.  During this process, the system will undergo rapid cyclogenesis (strengthening), with a great deal of thanks to a strong thermal (temperature) gradient that will be in place in the lower levels of the atmosphere and at the surface.  A strong area of high pressure to the north will continue to pump in very cold air aloft over the state (and this high will also work to keep the storm movement very slow) and warm and moist air working into the storm will setup this gradient.  As the system strengthens rather rapidly, the precipitation field will begin to blossom rather quickly and precipitation rates will rapidly begin to increase.

     The first flakes are expected to break out across the state perhaps as early as noon Monday afternoon.  As the afternoon progresses, light snows will begin to overspread the state, however, snowfall rates should remain on the lighter side.  As afternoon progresses towards evening, we will begin to see more in the way of moderate bands of snowfall begin to work into the state.  As the evening progresses, we will begin to see periods of moderate to heavy snowfall become much more common across the state.  In addition to snowfall rates intensifying, the winds will begin to pick up as well.  The heaviest of the snowfall rates are expected to occur between the hours of 11:00 PM Monday night and 9:00 AM Tuesday morning.  During these hours is when the majority of the snowfall accumulations will occur.  Snowfall rates could approach 2-3'' per hour with snowfall rates up to 4-5'' per hour possible in any bands containing thunder and lightning.  Winds during this time will also become sustained between 25-35 mph gusts as high as 45-55 mph possible.  In fact, coastal CT, especially SE CT, could see gusts as high as 65 mph!  The combination of very strong winds will lead to blizzard conditions at times and will also cause numerous power outages due to downed trees and utility poles.  

   
     While snowfall intensities and rates will be decreasing during the morning hours of Tuesday, periods of light to even moderate pockets of snow will continue into the early evening hours.  Winds will also remain rather strong through much of the day on Tuesday before beginning to subside in the late afternoon hours.

     Snowfall totals may be lower across the southeastern part of the state and far northwestern CT and back in Fairfield county towards New York City.  Across southeastern CT, there may be a pocket of dry air which works in aloft, but this depends on the exact track of the 700mb low.  This pocket of dry air could cut-off snow for a period across this area leading to lower totals.  Back across northwestern CT and southwestern CT, there may be an area of subsidence (sinking air) which cuts off precipitation.  This will occur just to the northwest and west of the heaviest banding that will set-up known as a cold conveyor belt (CCB).  In fact, where this band sets-up and pivots is where the highest snowfall totals will occur and these totals could approach two-and-a-half to three feet.  However, at this time it's very difficult, if not impossible, to determine where this band will setup and we may not 100% know until the event is on-going.  This is due to the fact that they are typically very narrow in nature and our computer forecast models aren't coarse enough to accurately depict where they will setup.  However, as we get closer we can zero in on a specific area and can have a general idea of where the highest totals may occur.

Below is the first call snow map and expect another updated map tomorrow evening, and with a little more detail of where perhaps the heaviest banding will occur.

Messy Saturday in Store Across CT on Saturday

A messy Saturday is in store across the state as a coastal storm is set to bring a mixture of snowfall, sleet, some freezing rain and even rain to the state.  Given how we will be dealing with multiple varieties of precipitation, this will certainly yield to quite a complex forecast, especially with regards to how much snows and where the highest snowfall totals will be across the state.  Let's get into this!

Brief Meteorological Background:

The below image shows the 500mb level (chart on the left) and sea-level pressure (chart on the right).  These two images come from today's 12z run of the GFS and there are 3 highlighted areas;

1.  This area is what is responsible for the development of the low pressure.  Strong energy associated with the mid-level trough is leading to the development of the surface low which you can see in the chart on the right where the 3rd highlighted area is.

2.  This highlighted area is a weaker piece of energy embedded in the northern stream moving into the northern-tier of the country.  This energy will sort of combine with the southern stream energy, or "phase" and this will help the coastal storm to become a little bit stronger and also keep it a bit closer to the coast.

3.  This highlighted area is the surface low pressure.

We will now fast-forward and look 24-hours down the road and the projected 500mb pattern and sea-level pressure chart:

1.  With the first highlighted area, we can now see the trough and associated energy draped right along the east coast now.  The trough here has a negatively-tilted component to it and this is something that will also favor the storm track to be closer to the coast.

2.  The second highlighted area is our surface low pressure or our coastal storm.  The system here is shown stronger than what it was just 24-hours prior and the system will continue to strengthen throughout the day on Saturday as it passes just to our south and east.

3.  For the third highlighted area there is a blue circle here.  One thing to notice is the lack of a surface high pressure in and around this region.  Having an area of surface high pressure in this general region is a great way to keep colder air in the lower levels locked in place and this high can also prevent a storm track too close to the coast.  Unfortunately, with no high pressure here and a storm track just off the coast, this will allow for warmer air to work in both at the surface and within the lowest 10,000ft of the atmosphere and that will be the reason why for multiple precipitation types.

With all this said what can we expect?

Forecast:

• Everyone will start off as snowfall with precipitation breaking out across the state between 4:00 AM and 6:00 AM with southern portions of the state beginning closer to 4:00 AM.  While there is a big of dry air in place aloft, forecast soundings indicate the airmass should begin to saturate rather quickly.  
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• As the morning progresses snowfall will begin to fall more moderately to heavy at times.  Snowfall rates may approach as much as 1'' to 1.5'' per hour at times during the heaviest bursts of snow.  
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• As the morning also progresses and the system works its way northward, warmer air will begin to work in the state in the lowest 10,000ft of the atmosphere.  As this warmer air work in, precipitation across the state will begin to transition over from snow to sleet, freezing rain, and even rain, especially across southeastern CT.  While some freezing rain is expected this is not expected to be a major issue, however, there may be enough to provide a coating of glaze across some communities.  
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• The system is expected to be a quick mover with the precipitation begin to wind down during the later half of the afternoon and be out by 7:00 PM, however, some light precipitation may linger, especially across the northeastern portion of the state.  
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• The heaviest of the precipitation will fall between about 12:00 PM noon and 4:00 PM.  Much of the snowfall accumulations, however, should occur prior to noon as this is when we will begin to see the changeover process occur.  
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• The highest snowfall totals should be confined to the northwest hills and northeastern CT where cold air may stay in place a bit longer.  
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• Anyone traveling should take it extremely slow and expect numerous delays.  Travel is not advised unless absolutely necessary.  

Below is the projected snowfall forecast:

Major Blast of Arctic Cold Set to Arrive...But How Long Does it Last?

     The first week of 2015 looks to be quite cold across Connecticut as a major blast of Arctic air is pushing towards the state.  An upper-level trough which was responsible for the messy storm system this weekend, is continuing to push through the central-tier of the United States.  As the trough and attendant cold front slide east it will usher in much colder air into the state.  In fact, we may be looking at our coldest temperatures of the 2014-2015 winter season later in the week.

     Right now it appears the worst of the cold will occur between Wednesday and Friday.  This is when computer forecast guidance suggests that the core of the coldest low-level airmass will slide overhead.  During this time, daytime temperatures will only get into the teens with nighttime lows falling close to and even below-zero in the typically colder spots.

     With a much colder airmass becoming established across the state this week, the million dollar question is, will see see any decent snowstorms or snow threats?  The answer to this question is a big fat NO.  The weather pattern this upcoming week is nothing sort of brutal cold and dry.  While there does look to be a developing area of weak low pressure to the south of CT late Tuesday, moisture looks rather starved and the system pretty weak so nothing significant is expected.  Some snow showers can't be ruled out but this may be more likely across far SE MA.

     Looking beyond this upcoming week and next weekend, there are some signals that the pattern will begin to relax a bit and we will see temperatures much closer to average return and perhaps even some days of above-average temperatures.  8-10 day progs from the GFS and ECMWF computer forecast models all suggest that we will see more in the way of ridging developing across the eastern United States and into our area.

Until then, however, it's nothing but brutal cold and dry with no storms expected this week at this time...how boring!        

First Winter Storm of 2014-2015 Set to Impact Connecticut

The first winter storm of the 2014-2015 season is only several hours away from beginning across Connecticut and the timing couldn't be any more opportune as we approach the Thanksgiving holiday.  We are also looking at a timing in which the most substantial impacts will occur from mid-to-late morning through early evening.  While this will be a fairly quick hitting storm, lasting for maybe 12 hours or perhaps a little less, the timing will cause major travel problems.  When forecasting winter storms across the state, they can often times be challenging as we often deal with the possibility of mixed precipitation such as sleet, freezing rain, and rain.  This case will be no different.

Brief Technical Look

A strong piece of shortwave energy is moving through a fairly deep digging trough as it progresses eastward across the United States.  The shortwave will be responsible for a developing area of surface low pressure just off the southeast coast.  As the area of low pressure moves north/northeastward it will strengthen:

Discussion

Computer forecast guidance are all in very strong agreement that the surface area of low pressure will pass just to the southeast of southern New England.  This storm track favors heavier precipitation impacting the state of Connecticut.

Surface temperatures across the state will be confined to about the lower to mid 30's from southeastern CT to more lower 30's elsewhere, with the exception of the Litchfield Hills where temperatures will be into the upper 20's to near 30F.  This indicates the snowfall will be more wet in nature and heavy in nature.  However, across the Litchfield Hills the snow will have a bit more fluff factor to it.

The main questions we are presented with is what happens with the mid-level temperatures, particularly, between about 8,000ft and 12,000ft.  While the surface low pressure is going to track southeast of the state, the developing area of low pressure at 700mb (roughly 10,000 ft AGL) is expected to not only track over CT but intensify as it does so:

This feature presents some trouble as with warmer air now working in aloft, this will eventually lead to a transition from snow to sleet across portions of CT.  However, it's a little uncertain as to exactly where this snow/sleet line will setup.

If the 700mb low not only develops earlier and intensifies quicker, this will mean a further surge of warmer air across the mid-levels over more of the state, meaning more locations transition over to a mix of snow and sleet which would reduce snowfall totals a bit.  However, if the 700mb low develops a bit later and doesn't intensify as quickly, the warmer air won't move much further into CT than the eastern portion of the state,  This feature will also allow for a pretty big snowfall gradient across the state in terms of accumulations.  

One point of interest is the track of the 850mb low with respect to CT.  While the system will be a quick hitter, typically something that can preclude much higher snowfall totals, computer forecast guidance indicates the 850mb low will track just to the southeast of CT,  As this occurs, this will favor a period of extremely strong low-level lift moving through the state of Connecticut meaning there will be a period of very heavy precipitation rates across the state.  This is when the majority of the snowfall accumulations will occur.  

Forecast

• Precipitation breaks out statewide between 6:00 AM and 8:00 AM
• Precipitation may begin as all rain, especially southern CT with more of a mix across the rest of the state.  
• Precipitation will quickly changeover to all snow with the exception of southeastern CT
• The heaviest of the snowfall will occur between 11:00 AM and 6:00 PM.  This is when snowfall rates will approach 2'' per hour and perhaps may even briefly approach 3'' per hour.  
• The highest snowfall totals should be confined to areas along and west of I-84.  This is where precipitation will remain mostly snow, however, some mixing is possible towards the end.  
• The storm begins to wind down between about 6:00 and 8:00 PM.

Numerous Showers and T'storms Expected Saturday, September 5th, 2014 across Southern New England

Saturday is shaping up to be quite an active day across southern New England as an area of low pressure moving through Quebec will allow for a cold front to slide through southern New England during the afternoon/evening hours.  Out ahead of the cold front the combination of surface temperatures in the mid to upper 80's with surface dewpoints in the lower 70's will yield to quite an unstable airmass across the region.  While winds aloft will be on the modest side, shear should be sufficient, especially given how unstable the atmosphere will be for showers and t'storms to develop and a few of these t'storms may become strong to severe.

The day will start off predominately dry, however, there may be a few showers around during the early morning hours.  While there will be clouds around, strong surface heating is expected to occur.  This will allow for temperatures to quickly jump and soar well into the 80's, perhaps near 90F in a few locations.  As mentioned above, with dewpoints in the low 70's, this combination will yield to quite an unstable airmass:

Computer forecast models indicating we could see CAPE values approach or exceed 2000 J/KG, suggesting quite an unstable atmosphere:


The strongest winds aloft in the mid-levels of the atmosphere are expected to lag just behind the cold front, however, computer forecast models do suggest that these winds will slowly be increasing throughout the afternoon.  While shear values at this time aren't expected to be very impressive (perhaps only upwards of 25 knots of vertical shear) the degree of instability will be enough for t'storms to develop.  There are some differences within the computer forecast models with regards to the winds aloft.  One forecast model indicates winds will begin increasing earlier as opposed to later on.  If winds aloft do increase on the quicker side and ahead of the approaching t'storms, this could lead to more in the way of severe weather potential:

The NAM computer forecast model showing area of stronger winds aloft approaching southern New England late in the afternoon, a bit slower than a few other computer forecast models:


For tomorrow we will see showers and t'storms develop in the afternoon and become rather numerous.  While numerous showers and t'storms are expected only a few of these storms are expected to become severe and pose the threat for strong to damaging winds and perhaps large hail.  If winds aloft do increase earlier and ahead of the approaching storms than the risk of strong to damaging winds and hail would increase.  A few rotating storms can't be ruled out either but this all depends on the strength and intensity of the lower-level wind fields.  Besides the threat for strong to damaging winds and hail, a very moist atmosphere will lead to torrential downpours and with this we will see a risk for localized areas of flash flooding, especially for the typical flood prone areas.

Stormy Wednesday On Tap Across Southern New England

A strengthening storm system just west of southern New England is slowly allowing a warm front to push northward.  Currently, the warm front is draped across the mid-Atlantic region, however, that front will continue sliding north towards southern New England during the overnight and early morning hours.  As the warm front approaches moisture will begin to rapidly increase and this will be accompanied by periods of extremely heavy rainfall overnight tonight and through the morning hours tomorrow.  Pretty much everyone across southern New England will see at least one period of heavy rainfall between tonight and tomorrow afternoon.  In fact, rainfall amounts on average should be between 1-2'' with isolated rainfall amounts as high as 3-4'' when all said and done.  This will lead to pockets of flash flooding.  What's the cause for the heavy rainfall?

A very strong an anomalous low-level jet and anomalously high precipitable water values will be the drivers behind very heavy rainfall:

18z GFS model showing a 40+ knot low-level jet working into southern New England by 5:00 AM Wednesday morning:

18z GFS model showing precipitable water values approaching and even slightly exceeding 2'' by 5:00 AM across portions of southern New England:

While the focus for tomorrow will mainly be on the periods of heavy rainfall, another focus, albeit perhaps to a lesser extend will be the potential for a few strong to perhaps severe t'storms.  This potential all hinges upon how much sunshine occurs and how much instability develops once the back end of the heavier rainfall moves through.  If just enough instability can develop we will have to pay close attention to the development of t'storms.

Winds aloft will be quite strong thanks to the strengthening storm system off to the west and with the warm front in the vicinity of the region this will lead to winds turning with height, especially in the lowest 3km of the atmosphere.  If t'storms are able to develop and become strong enough to utilize these winds aloft, they will have the potential to produce strong to damaging wind gusts and perhaps the potential for a brief tornado.

While these setups don't always materialize and generate strong to severe t'storms or tornadoes, these type of setups have produced strong to severe t'storms and tornadoes in the past, including 3 weeks ago.  It's all dependent on how much instability can be generated and this typically is not known the night before...it's known more the next morning.

All in all we are looking at widespread heavy rainfall totals (although it will not be raining the entire time) with the potential for isolated areas of flash flooding.  Rainfall totals on average should be 1-2'' with isolated amounts as high as 3-4''.  While heavy rains are the main threat we can't rule out a few strong to severe t'storms capable of strong to damaging wind gusts or even a brief tornado.