White Christmas Forecast Update

No need to really get too scientific with this post as that was done yesterday but this post will reflect some minor changes to my forecast from yesterday along with discussing the potential for some thundersnows which could occur with a rather intense band of snowfall which may push through portions of the state during the early morning hours tomorrow (between 6:00 AM and 9:00 AM). Several mesoscale computer forecast models develop a very intense band of snowfall on the backside which could drop a couple inches of snow (along with thunder, lightning, and strong wind gusts) in just a short amount of time. Due to the fast moving nature of this potential band this will reduce the likelihood for significant snowfall accumulations:

There are still some uncertainties with regards to how far into the state the warm air gets and this leads to some uncertainties with where the rain/snow line will setup. This will be very critical because locations near this line can be looking at as little as an inch or so of snow or as much as 4-5'' of snow. Unfortunately this is not something that will not be pinned down until the event is ongoing.

What could be a rather underrated aspect of this event is the potential for strong to damaging wind gusts as the system departs. Computer forecast models indicate the potential for wind gusts between 50-60 mph for a time tomorrow afternoon:

Given the potential for wet snow which would stick to limbs and power lines, this combined with strong wind gusts could yield a quite a bit of downed limbs and power outages beginning tomorrow morning and lasting through the afternoon. Areas which see any water refreeze on tree limbs and branches tonight could be especially vulnerable.

What should we expect with this storm?

• Snow flurries begin to work into the state between 10 AM and 1:00 AM. Any snow should be brief along the southeast coast as warmer air will be in place and continuing to work in. 
• Light to moderate snows work into the state after 1:00 AM. At this time much of southeastern CT should have changed over to rain. 
• Also after 1:00 AM, we will begin to see warmer air in the lowest 5000' of the atmosphere work into the state, this will transition snow to a mixture of sleet and even some freezing rain in spots from the southeast to the northwest. The rain/snow line is projected to come very close to the I-84 corridor. The further to the north and west of I-84, the more likely you are to remain all snow. 
• After 5:00-6:00 AM we will begin to see some areas further south and east along I-84 transition back over to snow as colder air works back in. At this time, mesoscale computer forecast models suggest a VERY intense band of extremely heavy snowfall will push through a good portion of the state. Within this band snowfall rates could approach or even exceed 3'' per hour!  This will make for white out conditions. Also, given the degree of upward vertical motion within this band and even some very weak instability, the possibility will exist for thunder, lightning, and strong wind gusts as well within this band. If these convective elements happen it could only enhance the snowfall rates. 
• Given the natural of the thermal profile, the snowfall ratios will not be great and the snow will be more wet in nature (the exception here could be the northwest hills). This will allow for snow to stick to power lines, utility poles, and tree limbs. This combined with strong to damaging wind gusts beginning late morning will yield numerous pockets of tree damage, downed power lines, and power outages. 
• The snow will begin to taper off after 9:00 AM to 10:00 AM and should be out of the state by noon with perhaps a few leftover flurries around. 

         

White Christmas On the Way

For those of you who dream for a white Christmas that dream will be coming true this year. An approaching amplifying (strengthening) shortwave trough will spawn the development of a surface low pressure which will go through fairly rapid cyclogenesis (strengthening/deepening). As this process occurs we will see a rapid increase in precipitation. For much of the state (pretty much along and north/west of I-84) this will be all snow, however, south and east of I-84 there will be some mixing issues and then a changeover to rsain and this will lead to reduced snowfall totals for these areas.  We will have to watch how the models handle some features such as the development and track of the 850mb and 925mb low pressure centers (this will be discussed in more detail below) because there is the possibility that if these develop further west, this could allow for a surge of warmer air to push in around from around 2000-5000' above the ground. This would reduce snowfall totals even along and northwest of I-84 and introduce the likelihood for mixed precipitation as well. 

An approaching shortwave trough will undergo strong amplification as a potent 500mb (~18,000' above the ground) jet stream rounds the base of the shortwave trough:

Within the jet streak will be some potent shortwave energy and this energy will only help to enhance and fuel the development of the surface low along with the rapid strengthening of the surface low:

As the system begins to rapidly strengthen and develop we will see low pressure centers develop at 925mb (~2500' above the ground), 850mb (~5000' above the ground), and 700mb (~10,000' above the ground). This will lead to a couple different things; 1) It will enhance upward vertical motion (particularly on the northwest side of these lows) which will increase the likelihood for the moderate to heavy precipitation. 2) Influence temperatures, especially between about 2000' and 5000'. While computer forecast models do agree that these lows will develop west of here but the main question is how far west? Typically a west track of these lows indicates warm air advection into these levels and computer forecast models do suggest that a strong enough surge of warm air could advect in which could change precipitation type from snow to perhaps sleet or even some freezing rain. However, this would also enhance an extremely band of heavy snowfall just west of this temperature gradient. Typically the strongest upward vertical motion is also on the northwest side of these low pressure centers, however, computer forecast models actually still yield a fairly impressive amount of upward vertical motion with the strongest occurring across the northwestern part of the state with a secondary area across northeastern CT. We should note, however, that in between these two areas of enhanced lift (of course this is assuming it verifies this way) there could be an area of enhanced downward vertical motion (what goes up must go down!). You have air rapidly rising upward in the area of enhanced rising motion so that air needs to eventually go somewhere and that somewhere is back downward. This is why in those big snowstorms where you see a narrow zone of really high snowfall totals (this is the area where the enhanced upward vertical motion occurred and remained the longest) but right near it you'll see much less snowfall totals compared to everyone else. This is the area where the subsidence occurred and. This is VERY difficult to forecast. Anyways, while temperatures will be marginal between about 2000' and 6000' (which is usually not a good thing) in this situation it will be (that's if you like alot of snow :) ). 

In place we will have very cold temperatures between about 700mb and 500mb (~10,000' to ~18,000') with temperatures between -10C (700mb) to as low as -20C (500mb). This is one key when talking about good snowgrowth production:

With great snowgrowth we will also see sufficient upward vertical motion along with sufficient relative humidity within the snowgrowth zone. This will yield to the generation of those really big and fluffy snowflakes which accumulate quickly (dendrites). For this we will look at a bufkit profile over Windsor Locks, CT:

Back to the marginal 850mb temperatures. In this situation since we will have excellent snowgrowth, due to the presence of sufficient relative humidity within the dendritic snowgrowth zone, and very strong upward vertical motion within this zone this would normally lead to rather high snowfall ratios. In this case, the marginal 850mb temperatures will work to reduce the snowfall ratios a bit but what they will also do is lead to the development of rather strong frontogenesis (strong vertical temperature gradient within the horizontal). This will yield to produce a very intense band of very heavy snow just west of this temperature gradient. The key is just figuring out where this sets up. This will only be further enhanced by the presence of very strong upward vertical velocity:

The surge of warmth, however, will yield a mix and changeover to even rain for some areas south and east of I-84

What should we expect?

• Flurries/light snows begin to break out between 10:00 PM and 1:00 AM. 
• Snow begins to increase in intensity after 2:00-3:00 AM and periods of moderate to heavy snows will occur through mid-to-late morning before quickly beginning to taper off from the southwest to the northeast. 
• Warmer air moves in south and east if I-84 and these locations will see snow transition to a mixture of sleet and then all rain. North and west of I-84 will remain snow but some mixing of sleet can't totally be ruled out.
• Under the heaviest banding snowfall rates could approach 2-3'' per hour. This would likely occur anywhere from about 5:00 AM to 9:00 AM. This is where the highest snowfall totals will occur. There could be two areas of heavy banding with the first area across northwestern CT and the second area across parts of northeastern CT.  If we do indeed see two bands, this could yield some subsidence in between these bands which could result in some lower totals for some. 
• Snow begins to taper off late morning and push out by early afternoon.

Freezing Rain Tonight/Tomorrow Morning.

We have seen periods and pockets of mixed precipitation across the state throughout the afternoon and we are beginning to see freezing rain, and some accretions across portions of the state as well. This theme will continue into the overnight hours. We will continue to see these pockets of mixed precipitation and freezing rain move through the state through the rest of the evening and through the overnight hours, however, our attention turns to tomorrow morning when steadier precipitation will push into the state. There could be some concerns for prolonged duration of freezing rain within the CT River valley across interior CT. This is due to the possibility of colder air holding in place much longer than locations outside the Valley. (The meteorology behind this was discussed in the previous blog post).

A radar grab of the Northeastern United States from 6:42 PM EST highlights the two areas of precipitation currently. We see the pockets of mixed precipitation across CT and we see an extensive area of precipitation back off to the southwest. This area of precipitation is expected to begin pushing into the state as early as 4:00-5:00 AM tomorrow morning. (Remember we will continue to see periods and pockets of mixed precipitation through the overnight but it's the heavier and steadier stuff which begins to arrive by 4:00-5:00 AM:

Current surface observations from across the region as of 7:00 PM EST show much more mild air just to the south of CT with much colder temperatures just to the north of CT:

Also discussed in the previous blog post was an area of high pressure which would become positioned just to the north of ME. We currently have the high pressure positioned just to the north of ME and with this we have a generally northerly wind direction across the northern part of the state with a more easterly component to the winds across the southern portion of the state.  The positioning of this high (along with the strength of the high pressure) is allowing for much colder air up north to drain right down through the CT River Valley. As long as this high pressure remains in the position it is in this cold air will continue to drain down the Valley. Once the high pressure begins to move off to the north and east, the surface winds in the Valley will shift to a more easterly or southerly direction and this will allow for surface temperatures to warm above freezing:

For southern CT the high pressure isn't that much of an influence and with a more easterly component of the winds this will allow for surface temperatures to rise above freezing much more quickly and across some places they already are just above freezing. This will limit concern for any prolonged icing across southern CT.

Some of the computer forecast models do warm surface temperatures above freezing in the CT Valley as early as 10:00 AM tomorrow morning, however, there are some computer forecast models which keep surface temperatures below freezing much longer and this would pose an increased risk for prolonged icing which could lead to some downed trees, utility poles, and power lines.  This is going to be an extremely close call and we'll have to monitor this very closely. Some of what to watch include;

1) Onset of the heaviest precipitation: If that batch of precipitation off to the southwest moves in faster this gives the Valley prolonged exposure to steadier precipitation with surface temperatures below freezing. The result would be ice accretions which could approach or exceed 0.20'' which is when things could get dicey when talking about the possibility for downed limbs and power lines. If the timing is much slower than this likelihood is greatly reduced.

2) How long does the cold air remain locked in place? This right here in the million dollar question. Computer forecast models often struggle to handle these cold air damming situations very well and that is due to the resolution size of the models. They just don't have the power to analyze these very small scale phenomena. A difference of just two hours or even two degrees could mean the difference in only minor icing to accretions high enough to warrant the possibility for downed trees/limbs, power lines, and utility poles.

3) The area of high pressure to the north: How quickly does this high pressure depart off to the northeast? If it remains in place longer, this allows for a longer duration of the northerly component of the winds to remain in place across the Valley...the result: colder temperatures and prolonged icing. If it pushes out more quickly the result: Winds shifting to a more easterly or even southerly component and that will favor the advection of warmer temperatures from the east (Atlantic Ocean) and from the south.

This is going to come down to a nowcasting forecast and we'll have to watch it very closely. The map below highlights what areas have to be concerned the most from the scenario which would result in icing problems:

This map highlights the areas which could see icing issues if the worst case scenario verifies which is colder air remaining locked in longer. The rest of the state (with the exception of southern CT) will continue to see pockets of mixed precipitation and freezing rain through the evening:

Looking ahead the chances for a white Christmas are vastly increasing as plowable snows are looking more and more likely across the state but we'll look more into this tomorrow.

Ice Event Possible Across Interior CT Friday night into Saturday?

Just a day ago it looked as if we would see a major warming trend Friday night through Saturday as a storm system tracked well off to the west allowing for a surge of milder air to work into the state. However, substantial changes have unfolded within the computer forecast models and these changes suggest that we will keep the cold air locked in at the surface while the surge of warmth happens a few thousand feet above our heads. This type of scenario would favor the potential for freezing rain, especially across interior CT away from the shoreline. In my forecast video from Tuesday I went with a high of 57 Saturday along with a warming trend Friday night into Saturday...well this looks very wrong. So why the sudden change? This will be explained below.

One of the biggest key factors here will be an area of high pressure which slides through southern Canada and becomes positioned just to the north of Maine for several hours before slowly drifting off to the north and east. There is strong agreement on this scenario from the NAM, GFS, and European computer forecast models. The below image (using the GFS forecast model) is for 7:00 PM EST Friday evening showing the area of high pressure to the north of Maine. In the northern hemisphere, the flow around high pressure is clockwise (anti-cyclonic). Given the position of the high pressure, this would indicate surface winds coming from a more northeasterly direction:

This is extremely critical as the airmass to our north and east will be quite cold with surface temperatures into the teens and 20's. When we see high pressures lock in this position we have to watch for a situation known as cold air damming. When you have such a cold airmass to the north and east with the high locked in these cold air damming scenarios can be a major headache for forecasting purposes. What happens is the northeasterly winds transport this much colder (remember cold air is more dense than dry air and thus sinks and is much closer to the ground). air to the south and west. Here in New England we have the Connecticut River Valley which runs in a north-south fashion from VT/NH down through central MA and down through central CT. The CT River Valley (which has higher hills to the west and east) sits relatively low compared to the surroundings. The colder air being more dense sinks and sits in the valley and is continued to be funneled through the valley keeping the colder air locked in:

                      

While we have the surface high pressure locking in colder air at the surface much more mild air will be advecting in a few to several thousand feet off the ground Friday evening into Saturday. This will mean that temperatures a few thousand feet off the ground will warm to much higher than the freezing mark. As precipitation falls from the clouds, it will initially fall as snow as temperatures well above 10,000' certainly will be cold enough to support snow, however, once it descends into the milder air it will melt and transition into rain. Once this rain falls to within the lowest 1000' of the atmosphere it will once again encounter temperatures which will be below freezing. This will introduce the hazard for freezing rain away from the coastline:

A bufkit forecast sounding for Windsor Locks, CT from the GFS computer forecast model shows a nearly classic look for freezing rain Friday evening:

What should we expect with this?  During the day on Friday a warm front will slowly be approaching from the southwest as a storm system begins to strengthen off to our west. All indications right now are that this warm front will push into extreme southern CT but due to influences of the high pressure to the north will stall across extreme southern CT. This will lead to temperatures warming above freezing for southern CT, however, temperatures across interior CT will remain below freezing. Throughout the morning and afternoon hours periods of light to perhaps moderate mixed precipitation (snow and sleet) will fall across interior CT with rain across southern CT. While no major accumulations are expected, this will lead to slick roadways so extra time should be given when traveling.

By Friday evening, as the storm system off to the west not only strengthens but pushes closer, we will begin to see the warmth work in several thousand feet off the ground and we will see steadier precipitation push in. While southern CT will see rain this is when the potential will increase for freezing rain across the northern part of the state. The timeframe for the start of this will be between 5:00 PM and 7:00 PM. Periods of freezing rain will continue into and through the overnight hours and even into Saturday morning. While indications right now are the high pressure will depart off to the northeast, which would allow for warmer air to work in at the surface and transition the freezing rain over to a plain old cold rain, there are uncertainties with how quickly this happens so this will be heavily monitored over the next few days. As it stands right now ice accretion does appear to be a high possibility and this could lead to tree damage and power outages. Travel is NOT advised during freezing rain. Contrary to the belief, NO vehicle is safe to drive in during freezing rain and that includes Four wheel drive vehicles.

Further information will be known as well as a precise forecast as we continue to move closer to the event and the details continue to be ironed out.

Overnight Clipper System to Bring Light Snows to CT

I would like to begin this blog by pointing out that my forecast for last weekend's snowfall event was HORRIBLE...absolutely horrific. I completely dropped the bomb on it. As infuriating as it is to not only get a forecast wrong but to get the first snowfall event of the season wrong, these experiences are always used to further learn. When it comes to verifying snowfall forecasts, I like to look beyond the forecast numbers.  For example, let's say I forecast 2-4'' for CT and say that this 2-4'' is likely going to occur from banding that develops towards the end of the system.  Well CT for the most part gets 2-4'' but that 2-4'' didn't come from any banding at all.  Technically by the numbers I was correct, however, meteorologiclly I was not  In the end, an understanding of the science is just as important as a "forecast". Anyways, with this little rant done we are once again looking at some more snow. The snow will fall during the overnight hours and will mainly be light to at times moderate. However, due to the speed of the storm and lack of richer moisture, this system will not be as significant as last weekend.

As the evening progresses a fast piece of energy moving through the jet stream aloft (known as an Alberta clipper b/c it originates from the Provence of Alberta) will approach southern New England. This piece of energy is associated with moisture which it has acquired as it passed over the Great Lakes:

As the piece of energy slides south of southern New England it will acquire some additional moisture from both the Long Island Sound and the Atlantic Ocean. This will set the stage for a period of light to perhaps moderate snowfall during the overnight hours lasting through mid-morning or so tomorrow.  As the shortwave approaches we will see a window of rather decent mid-level and low-level forcing which will allow for moisture to become lifted well into the dendritic snowgrowth zone. This will work to produce those very large and fluffy snowflakes which will accumulate rather quickly:

What should we expect?

• Light snow begins to break out between 2:00 AM and 4:00 AM.
• Bursts of moderate snow will be possible between 5:00 AM and 8:00 AM.
• Snowfall rates could approach or briefly exceed 1/2'' per hour.
• Given the extend of the cold airmass, sufficient lift into the dendritic snowgrowth zone, and excellent snowgrowth expected, snowfall will be very fluffy with ratios as high as 15:1 to 20:1. 
• Snow will taper off between 8:00 AM and 10:00 AM.
• Expect slow travel, especially for the early morning commute so extra time and caution should be taken when traveling.

Saturday, December 9th, 2017 Snowfall Forecast Update

No substantial changes to my thinking yesterday, however, based on the latest information I've decided to include much of the state in the 2-4'' snowfall range. Flurries will begin to breakout as early as 7-9 AM across the state (SE sections of the state first) then slowly begin to increase in intensity throughout the morning and into the afternoon hours. During this time, snowfall will range in intensity between light and moderate with snowfall rates perhaps approaching 1/2'' per hour.  Snow will begin to wind down during the late afternoon and early evening hours.

Some factors which preclude my thinking for higher snowfall totals include; not a tremendous amount of 700mb vertical velocities encompassing the state. Computer forecast models only show bursts of higher 700mb vertical velocities moving over the state throughout the day and this is when we will see snowfall be more moderate in nature, however, the duration of the moderate snowfall will be on the brief side:

Latest bufkit soundings across the state suggest only modest omega values within the dendritic snowgrowth zone region. This only suggests that snowfall rates will vary between light and moderate. Also, this will work to reduce the snowgrowth potential and lead to lesser snowfall ratios:

Based on the above this is my latest thinking:

Expect a slow commute during the snow tomorrow so extra time should be given during travel.

Light Snow Event Saturday, December 9th, 2017

Over the past several days computer forecast guidance has hinted at the potential for some snow showers or even some light snows across parts of the state in the Friday to Saturday timeframe. While there has been a great deal of inconsistencies with the operational forecast models as to how far west offshore moisture would track, computer forecast ensembles have painted a different story hinting that the likelihood for a far enough westward displacement of this moisture to impact at least portions of CT. Throughout the day today, computer forecast guidance has continued to track a bit westward with some of this moisture, increasing the likelihood for a period of light snows across much of the state during the day on Saturday. While a significant event is expected, some areas could potentially pick up as much as 2-3'' of snow.

Computer forecast models show two key areas of energy which may interact on Saturday. The first is some energy which is going to rotate around the polar vortex.  The second is an elongated area of energy which stretches up and down the eastern seaboard. Another area of focus is an impressive swath of moisture which extends from near Bermuda, through the southern tier of the United States into Mexico!

There also exists a stalled front off the east coast (this is the cold front which passed through here late Wednesday ushering in much colder air):

Latest guidance suggests this frontal boundary will retrograde back west over the next few days and this sets the stage for the likelihood of light snows during the day on Saturday. What happens is multiple areas of weak low pressures will develop within the elongated area of energy across the east coast and ride up along the stalled frontal boundary. What will help bring this moisture west is an impressive and amplifying (strengthening) 500mb jet streak. As this amplifies, this will help to tug the moisture off-shore west into CT and this will give us the light snows on Saturday:

Bufkit model data does show excellent snowgrowth.  You want the snowgrowth zone to be between 12,000 ft and 18,000' with the -15°C line intersecting it. In order to maximize and utilize the snowgrowth you want substantial moisture (relative humidity values > 90-95%) within the snowgrowth zone and excellent vertical upward lift. Using a bufkit sounding from Windham, CT does show sufficient enough lift within the snowgrowth zone for a period of light snow (in fact this would be pretty close to perhaps a brief period of moderate snow):

With this said, this is what I am expecting:

• Snow (snow showers to light snow) begins to break out across the state between 7:00 AM and 9:00 AM) Saturday morning.
• Light to even moderate bursts of snow (especially eastern CT) will continue through early to mid-afternoon. 
• Snow tappers off late afternoon.
• Expect slow travel during the event.

At this time, this is what I am currently thinking:

Substantial Pattern Change Expected as We Move into December

For those who love cold and the potential for lots of snow some good news exists as we move into and through the first week of December. For a while now, long-range teleconnection patterns have all indicated a major shift within the northern hemispheric pattern would occur which would favor major cold into the eastern United States as well as the possibility for increased storm chances.

Last night's 8-10 day mean 500mb height anomalies from the Euro (image on the left) and GFS (on the right) both indicate an extremely favorable pattern for some significant cold to pour into the eastern United States.  Both models indicate extreme blocking developing across the north-central Atlantic extending into Greenland (indicating a rather robust negative phase of the North Atlantic Oscillation (NAO) along with substantial ridging developing across the far western United States, extending northward into far western Canada and into Alaska (indicating a positive phase of the Pacific North-American teleconnection pattern (PNA) and negative phase of the Eastern Pacific Oscillation (EPO):

The configuration of the northern hemispheric pattern above is extremely favorable for very cold air to pour right into the eastern half of the United States.  There is a great deal of model support as well, especially within the GFS ensembles (unfortunately I don't have permission to post Euro ensemble data) for the AO, NAO, and EPO to tank and for the PNA to sharply rise:

While we recently are in a rather low state of angular atmospheric momentum (AAM) the forecasts are for the low state to relax with AAM hedging more towards the "circle of death".  Typically, when the atmosphere is in a really low AAM state, especially when La Nina forcing is present, this will favor troughs to dig into the western United States with ridging developing and strengthening across the southeastern United States. The extreme high latitude blocking, however, would work to suppress any ridge development across the southeastern United States:

There is also stratospheric support for blocking to develop as computer forecast models all indicate the likelihood for warming to occur within the stratosphere above the far north latitudes which will work to weaken the polar vortex and favor southward displacement. 

All in all the signals have been in place for quite some time on the potential for a rather significant pattern change to occur and one that will favor some major cold to spill into the eastern United States.  At the same time, this projected pattern will also become quite active.  With plenty of cold air available we'll have to certainly watch for snow events! One thing to also keep in mind is, if the blocking becomes as extreme or perhaps even more extreme than modeled, this could lead to a suppressed storm pattern meaning the storm track is well south of New England or well off the coast.  Regardless, for those who like interesting winter and root for snow and cold you may be in luck!

Significant Storm Update

Significant coastal storm has been dropping torrential downpours across the state all day and as the system continues to undergo rapid and further strengthening we are now beginning to introduce the strong wind aspect of the storm.  Numerous weather stations across the state, particularly across the coastline have reported wind gusts as high as 40-50 mph.  Flash flooding, downed trees, and power outages will continue to be the biggest threats as we move into the overnight hours.

As of this writing, our system is already very strong with a 984mb closed low just east of NJ.  Based on the trend of greatest pressure falls, the low should continue moving NNW and push into northern NJ and through SE NY.  Computer forecast models strengthen the system down to the mid-970 mb range which is very, very impressive:

With the last few updates there was big focus on the development and strength of the low-level jet streak and it appears that computer forecast models will be spot on with the extremely anomalous speed of the low-level jet with 925mb (~2500' off the ground) winds expected to exceed 90 knots and the 850mb (~5000' off the ground) low-level jet exceeding 100 knots.  This is extremely, extremely impressive and shows how much wind potential exists just off the ground.  The question just is how much of this wind energy is tapped into and how much of these winds are brought down to the surface.  Currently we see the low-level jet streak off to the SE of the state with 925mb winds as high as 65-75 knots and 850mb winds already as high as 80-90 knots.  Not only will these values continue to increase within the jet streak as the storm continues to strengthen but the core of the low-level jet max may even push into extreme SE parts of CT during the overnight hours.  If this does happen and any of this energy is tapped into, the potential will certainly increase for wind gusts along the coast to exceed 65-70 mph.  This likelihood will certainly exist just off to our east across RI and far SE MA:

The major concern over the next several hours outside of torrential downpours and flash flooding is with regards to the wind potential.  As of this writing, we're continuing to see winds increase, particularly along the coast.  Over 35,000 folks are without power as of this writing and this number should continue to rapidly rise.  The latest run of the Hi-Resolution Rapid Refresh (HRRR) computer forecast model shows very strong damaging wind gust potential, particularly across the CT coast.  the bufkit sounding for Groton, CT at 1:00 AM suggests sustained winds of 47 knots (54 mph) with gusts potentially upwards of 61 knots (70 mph).  Winds of this magnitude would yield scattered to widespread wind damage.  

One of the keys to developing these damaging gusts will be how the low-level temperature profile establishes over these next few hours.  Computer forecast model soundings show a weak unstable layer developing from the surface up through about 875mb which is noted by steepening lapse rates (strong temperature decrease with height) in this layer.  This is quite evident across coastal CT, however, the signal is much more weak inland.  This is one factor which could prevent widespread damaging wind gusts across inland CT.  

Breaking this down:

Torrential Downpours

Torrential downpours continue to batter the state of CT, however, a dry slot is beginning to work into the southwest part of the state now.  Torrential downpours will continue across central and eastern CT for the next few hours before the intensity begins to let up quite a bit.  Rainfall will transition from steady and heavy to more on and off.  Flash flooding continues to be a risk.

Strong to Damaging Wind Gusts

Sustained winds and wind gusts will continue to increase in magnitude over the next several hours as the low-level jet continues to strengthen in response to the rapid deepening of the low pressure center.  The worst of the winds will be confined to the CT coast where wind gusts at the height of the storm could exceed 65-70 mph.  Winds will become sustained as well between 40-45 mph.  Scattered to perhaps widespread tree damage is possible here along with widespread power outages which we are already beginning to see.  Further inland, strong to damaging wind gusts are possible as well, however, these will be more isolated in nature.  This will still yield isolated pockets of tree damage and power outages inland.  This potential will last through 2-3 AM before winds begin to subside.  However, late tomorrow morning the winds begin to rapidly increase again as the system departs.  With strong mixing expected and steep low-level lapse rates, we will be looking at statewide wind gusts upwards of 40-50 mph.  With a very wet ground this as well will lead to pockets of tree damage and further power outages.  

Sunday, October 29th, 2017 Dynamic Storm

Over the past several years there seems to be a trend with major storms and the end of October and in 2017 that trend continues as there is extremely high confidence in a significant storm system which will develop over the next few days.  We are looking at the potential for heavy rainfall which will result in flash flooding, strong to damaging wind gusts of 35-45 mph across the state with gusts upwards of 60+ mph across the southeastern part of the state, the possibility of convection (there may not be lightning associated with it so can't necessarily say thunderstorms) which could result in the locally higher rainfall amounts as well as increasing the likelihood for strong to damaging wind gusts, some coastal flooding, and on Monday when the system departs we will see strong to damaging wind gusts statewide with gusts upwards of 45-55 mph.  The rainfall extent, however, is a tad questionable at this time as there could be a dry slot which moves into the state.  What may transpire is a brief window of extremely heavy rainfall but then it tapers off.  

Over the past several days, computer forecast models have been in rather strong agreement on a rather dynamic storm system developing.  A slow moving upper-level trough will continue to undergo strong amplification (strengthening) over the next day and dig down into the Gulf coast states.  As this trough continues to amplify we will see extremely strong shortwave energy move overhead with strong positive vorticity advection.  Associated with this upper-level trough is a rather strong cold front as well:

As this trough continues to amplify and push east, it is going to capture what is now Tropical Depression Eighteen located near Cuba.  This tropical depression is expected to continue tracking to the north and pass just east of Florida and work up along the east coast.  At this time it will interact with the upper-level trough and cold front.  Once this interaction occurs, this will accelerate the system's strengthening and also add a great deal of moisture into the system.  It is this scenario right here which will lead to the threat for extremely heavy rainfall and strong to damaging winds.  

One of the more impressive aspects of this system will be the strength of the low-level wind fields at both 925mb (about 2500' off the ground) and 850mb (about 5000' off the ground).  The NAM/GFS/Euro all continue to indicate winds at these levels exceeding 90 knots which is beyond impressive.  With the winds at these levels coming from the SE this would transport a great deal of low-level moisture into the state.  In addition, some of these winds could also get transported down to the surface as well which also leads to the potential for strong to damaging wind gusts:

  

Something to watch as well is the track of the 700mb low as computer forecast models indicate this track will be to the west of CT.  A track like this would suggest drier air working in at some point during the system.  We see this happen sometimes during snow events and this essentially shuts off the heavier snows.  This could have an impact on the upper bounds of potential rainfall totals.  Regardless, we will certainly see a period of extremely heavy rainfall across the state of CT which will result in a widespread 1-3'' of rainfall.  The potential does exist for some isolated totals upwards of 6'', however, this is all dependent on any dry slot:

Now that we sort of looked a bit into the science behind this (there is so much more to discuss but I don't have the time and don't want to make this too long) we'll break down of each hazard.

Strong to Damaging wind Gust Potential

This potential will come in two different phases; one coming with the system Sunday and Sunday night and the other coming Monday morning through the afternoon on the backside of the system.  While the potential exists for widespread strong to damaging wind gusts across the state during the storm I think the greater potential for this will exist across southeastern CT where winds could gust as high as 35-45 mph with the potential for a few gusts up to 55 mph.  Elsewhere, gusts will be in the 25-35 mph range.  There are a couple of factors which I think will prevent the higher amounts being widespread Sunday night will be potential for either a low-level temperature inversion (or a few nearly isothermal...no temperature change with height) and the core of the low-level jet sliding east of the state.  Isolated pockets of wind damage will occur, especially across southeastern CT and this will lead to isolated power outages as well.  

For Monday, however, I think much of the state sees wind gusts upwards of 45-55 mph as the system departs.  Forecast soundings all show rather strong deep mixing taking place with fairly steep low-level lapse rates thanks to a quite a bit of sunshine.  This will lead to isolated to scattered pockets of wind damage and power outages.

This bufkit sounding from the latest GFS computer forecast model for Windsor Locks, CT 2:00 PM Monday afternoon highlights the strong wind potential suggesting winds of 30+ knots with gusts upwards of 40+ knots.  We see a well mixed atmosphere and steep low-level lapse rates present:

Heavy Rain/Flash Flooding Potential

While overall it has been quite dry the past few months, we are fresh off a rather impressive rain event just a few days ago which dropped 3-5'' of rain across the central and eastern part of the state.  With at least another 1-3'' of rainfall expected tomorrow this will result in some flash flooding.  Depending on the dry slot scenario, some areas could see upwards of 5-6'' of rainfall which would greatly increase the flash flooding potential for those locations.  

Thunderstorms/Convection

Due to rather poor mid-level lapse rates (weak decrease of temperature as you ascend through the atmosphere) the amount of instability will be quite weak.  This (along with rather warm 500mb temperatures) will make for a small likelihood of thunderstorms, however, the extent of the forcing and lift will be so significant that they can't be ruled out.  The significance of this is if there are any thunderstorms (or any embedded convective elements within the rain) this would vastly increase the threat for damaging wind gusts and higher rainfall totals on a local level.  

Tuesday's Severe Weather Potential

Tuesday continues to look rather active across southern New England as the potential exists for torrential downpours and strong to damaging wind gusts (thanks to a strengthening pressure gradient and with convection).  While the ground is relatively dry, the potential will exist for flash flooding as well with as much as 1-3'' of rainfall expected and perhaps some localized higher amounts as well. 

As discussed with yesterday's post, an amplifying trough digging into the Ohio Valley will swing a strong cold front towards southern New England late in the day.  A strengthening southerly flow will help transport an anomalously warm and moist low-level airmass into the region which will work to destabilize the atmosphere.  Associated with the trough will be very strong wind shear as well. 

The combination of a warm and moist low-level airmass, strong wind shear, weak instability, and strong forcing from the cold front and upper-level support will all contribute to periods of torrential downpours tomorrow along with the potential for thunderstorms which could produce strong to damaging wind gusts.  There are two windows for this potential tomorrow; 1) anytime during the afternoon hours and 2) late evening into the early overnight hours as the cold front arrives. 

The extent of the strong to severe thunderstorm potential tomorrow will all depend on exactly how unstable the airmass becomes and how deep (or how tall convection becomes).  The more unstable the airmass, the greater the strength of the updrafts associated with the thunderstorms.  Given how strong the wind shear aloft is, a stronger updraft enhances the likelihood for updrafts to be robust enough to penetrate upwards through the wind shear without being disrupted or toppled over.  If the updrafts are toppled over the storm(s) aren't able to grow tall enough to utilize the full potential of the atmosphere. 

Computer forecast models are in rather strong agreement that upwards of several hundred joules of mixed-layer CAPE (MLcape) will develop during the afternoon hours and persist into the early overnight hours.  While several hundred joules of MLcape is not overly impressive when discussing the possibility for strong to severe thunderstorms, given the degree of wind shear in place these values may be just enough to enhance the likelihood for strong to severe thunderstorms. 

One of the more intriguing aspects of tomorrow's setup is that of the forecast hodographs.  A hodograph gives you a 'graphical' representation of wind speed with height in the atmosphere as well as wind direction with height in the atmosphere.  The below image shows the forecast hodograph for Waterbury, CT at 8:00 PM Tuesday night from both the NAM and GFS computer forecast models.  We see a rather long and somewhat curved look to the hodograph.  This indicates that not only is wind shear very strong aloft but there is a bit of directional wind shear (change of wind direction with height).  This is a strong indicator that the potential exists for a thunderstorms updraft to rotate if this potential is utilized.  Also on the hodograph is some forecasted CAPE values for different levels through the atmosphere.  Both models indicate over 200 J of 0-6km CAPE.  There are, however, some discrepancies with regards to the degree of 0-1km CAPE:

 
As tomorrow progresses we will have to watch and see exactly how unstable the atmosphere becomes.  If the atmosphere becomes as unstable as advertised the threat will exist for a few strong to severe thunderstorms, either in the form of discrete cells or within a low-topped squall line which develops ahead of the cold front.  If the degree of instability is less this potential would be vastly reduced.  However, if more instability develops (which can happen if mid-level lapse rates become a bit steeper, which some computer forecast models have hinted as a possibility) this potential would be greatly enhanced and a more significant/widespread event would be possible.  There are also some questions as to how much, if any, thunder/lightning will be associated with this activity.  It is still possible to get strong to damaging winds with the absence of thunder and lightning (which obviously would mean these wouldn't be thunderstorms) as even heavier downpours can transport stronger winds from aloft.  If the activity is associated with a great deal of thunder/lightning this would indicate rather deep (tall) thunderstorms which further enhances the severe threat as this indicates a higher likelihood for this activity to utilize the strong wind shear aloft.  What are we looking at?

• During the afternoon we will see periods of showers and perhaps a few thunderstorms.  The rain could come down heavy at times.  Strong wind gusts will be possible too with any showers and thunderstorms.
• Winds will be quite gusty during the day as well thanks to a strengthening pressure gradient as the system approaches.
• As we near evening the potential for heavier rain, thunderstorms, and strong to damaging winds will increase as the cold front approaches.  The timing of this will be from 7:00 PM to 11:00 PM.  For central and eastern southern New England the timing for this is pushed until after 12:00 AM and lasting through 6:00-7:00 AM Wednesday.  
• The main threats will be; torrential downpours leading to the possibility for flash flooding, especially in the typical flood-prone areas, strong to damaging wind gusts with any thunderstorms or heavier showers, and an isolated tornado, especially in any discrete cells.    
• Winds could potentially gust upwards of 45-55 mph with isolated gusts over 60 mph in the strongest of activity.  

Strong to Damaging Winds Possible Tuesday and Tuesday Night

A rather vigorous and amplifying (strengthening) upper-level trough will continue advancing eastward over the next few days.  As this trough approaches a strong cold front will approach the region.  Out ahead of this cold front the airmass will be anomalously warm and moist with daytime temperatures on Tuesday expected to push the 70°F mark with surface dewpoints expected to climb into the mid-60's.  The combination of such a warm and moist low-level airmass will help to contribute some rather robust (for the time of year) CAPE (Convective Available Potential Energy) values.  This combined with very strong wind shear in place will yield the potential for perhaps multiple rounds of low-topped convection yielding the potential for torrential downpours and strong to damaging wind gusts.  It is a bit unclear as to whether or not we will see much in the way of thunder or lightning given rather warm mid-level temperatures and how low-topped the convection may be, however, given the strength of the winds aloft even torrential downpours may transfer down strong to damaging wind gusts.  This potential will exist from early Tuesday afternoon lasing through the overnight hours into early Wednesday morning:

With such a warm and moist low-level airmass in place, the NAM and GFS computer forecast models both develop upwards of several hundred J/KG of surface-based CAPE during the afternoon on Tuesday and even keep this in place well until the overnight hours:

Computer forecast models also indicate that even upwards of several hundred J/KG of mixed-layer cape (MLcape) will develop as well on Tuesday with these values sticking around into the overnight hours.  This is rather impressive for this time of year. 

In addition to perhaps modest instability values, wind shear will be quite strong.  Computer forecast models are projecting a 90-105 knot 500mb mid-level jet streak to push towards the region late Tuesday placing southern New England in the right front quadrant of this jet.  This is a region of enhanced upper-level divergence which enhances upward vertical motion.  Along with instability, this feature could help fuel a low-topped line of convection just ahead of the front:

Bufkit soundings for across the region are rather ominous for Tuesday and Tuesday night and indicate the potential for strong to damaging wind gusts.  For lack of more appreciable time on my part we'll just look at the 12z NAM bufkit sounding for Waterbury, CT for 2:00 PM EDT Tuesday afternoon and 2:00 AM EDT Wednesday morning.  While the degree of CAPE is not something you would normally find impressive, given the degree of wind shear in place combined with strong lift from the cold front and the system, these CAPE values are more than enough to warrant the possibility for strong to damaging wind gusts:

2:00 PM EDT Tuesday

2:00 AM EDT Wednesday

While perhaps the degree of winds aloft is a bit weaker, the amount of CAPE is actually greater and actually is forecast to increase a bit more past 2:00 AM EDT:

What can we gather right now?

Tuesday and Tuesday night has the potential to be rather active across southern New England in terms of periods of torrential downpours and the potential for strong to damaging wind gusts and this could come in multiple rounds with the first round of potential during the afternoon hours and the second round overnight as the cold front approaches.  If instability becomes even stronger, yielding the potential for more robust updrafts, the potential for thunder and lightning would increase which would increase the risk for strong to damaging wind gusts.  Right now the potential exists for winds to gust as high as 40-50 mph with gusts potentially to 60 mph.  These higher gusts would occur with the heavier downpours which would act to tap into those stronger winds aloft.  Isolated to scattered pockets of damage would be possible.  As we get closer the details will become much more clearer.