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: