Wednesday, March 7th, 2018 Snowfall Forecast Update

The forecast continues to be on track for a rather significant snowstorm across a large chunk of Connecticut tomorrow. Potential for mixing and a change to rain as you near southeastern Connecticut will keep snowfall amounts on the lower side here. There is even the possibility for mixing as far as north and west of Hartford. In addition to concerns about mixing of sleet and rain, there is also a big concern for the possibility of a rather extensive dry slot to push into central and eastern Connecticut late afternoon which could further hold back snowfall totals across these parts and there is the potential for frontolysis late afternoon as well. This process will be discussed later on.

Late yesterday afternoon and overnight, forecast guidance continued to come in extremely impressive for substantial snowfall totals for a large portion of Connecticut. This had me very tempted to increase snowfall totals with my afternoon update. However, while this morning's data continued being impressive, a few flags showed up which prevented me from raising totals. While I did slightly expand my 10-14'' zone slightly eastward, (I also did include the possibility for isolated amounts of 18'') I expanded the mixing line further to the north and west which resulted in lesser snows across these parts.

Today's data indicated a pretty sizeable westward storm track which not only would allow for warmer air both at the surface and aloft to work into a larger portion of the state but it also introduces the potential for a rather extensive dry slot to move into the state as well. Analyzing 700mb (~10,000' above the ground) relative humidity (RH) values on both the NAM and GFS forecast models we see RH values of less than 30% overspreading much of central and eastern Connecticut on the NAM with these values just to the south of Connecticut on the GFS. This is due in part to the 700mb low tracking overhead. As low pressures become very strong drier air is typically ingested into the storm in the mid and lower levels of the atmosphere and this happens typically to the south and east of the 700mb low track:

 
Both the NAM and GFS also indicate rather impressive 800mb-600mb vertically averaged frontogenesis across the entire state during the afternoon hours. This would be indicative of moderate to heavy snowfall across the majority of the state away from any areas mixing. Bufkit profiles also show excellent snowgrowth during the afternoon hours along with sufficient moisture within the snowgrowth zone (not shown) and very strong upward vertical velocities (indicated by the red contours) with omega values exceeding 50 units. All of this confirms that we will see moderate to heavy snows during the later morning hours through the majority of the afternoon. As a note the NAM is a bit more impressive than the GFS:

Both models also indicate the process of frontolysis happening late in the afternoon and early evening. This process is the opposite of frontogenesis. If frontogenesis is the strengthening of the horizontal temperature gradient with height (which strengthens upward vertical motion) frontolysis is the weakening of this horizontal temperature gradient with height. This processes reduces the degree of upward vertical motion. This is something which could result in a rapid lessening of snowfall intensity by late afternoon which is something that could result in the lower end of the snowfall forecast verifying as opposed to the higher end of the range:

A point-and-click forecast sounding for interior southwestern Connecticut shows a moist absolute unstable layer (MAUL) as well. This is a strong signal for not only extremely heavy snowfall but also suggests the possibility for thundersnow as well which would further increase snowfall intensities:

What to expect:

• Light snow begins to break out between 11:00 PM and 2:00 AM and remain quite light through mid-morning with some breaks during this time as well. 
• After 10:00 AM-11:00 AM the snow rapidly begins to increase in intensity with the heaviest snows occurring between this time and about 5:00-6:00 PM. During this time snowfall rates could approach 2-3'' per hour at times, especially in areas under the heaviest banding and the potential for thundersnow would exist as well. 
• After 6:00 PM the snow intensity may begin to taper down, especially if drier air moves in. 
• Light snows would then persist into the late evening/early overnight hours. We have to watch timeframe closely because if the dry slot does not work in we could be looking at much more snow, especially the central part of the state. We also need to watch and see if the heaviest banding sits and pivots over western Connecticut which would mean higher totals here. 
• While winds won't be as strong as Friday, gusts of 25-45 mph will be likely (strongest at coast) and this could yield near blizzard-like conditions at times along with isolated power outages. 

I may have another update tomorrow on snowfall totals once we know how the banding will transpire and whether or not the band will sit in place and then pivot or just move through. Here is my updated snowfall map:

Plowable Snows Likely Wednesday Across Connecticut

The weather pattern has certainly become much more active as we've entered the month of March and that trend will continue this upcoming week as there is continued growing confidence for plowable snowfall across the entire state of Connecticut this Wednesday. Computer forecast models have been in strong agreement for the past week on the possibility of a rather significant storm to impact the state o n Wednesday and that agreement continues. There is, however, some disagreement with regards to the exact track of the storm and this will ultimately affect exactly how much snow we see, where the heaviest banding/highest snowfall totals occur (and whether this happens to be somewhere in Connecticut), and where we see an area of subsidence which would mean reduced snowfall totals somewhere. Over the course of the next 24-36 hours these questions should be resolved.

We are watching two key pieces of energy which are expected to phase and once they phase the result will be the development and cyclogenesis (rapid strengthening) of a low pressure just off the coast of the Carolina's where surface convergence and upper-level divergence will be favored:

As the storm works up the coast there is some uncertainties with regards to exactly how close to Connecticut the storm tracks. Some of the forecast models track the storm farther off to the east. This result would mean the heaviest snows likely being confined to central and eastern Connecticut and also a reduced possibility for mixing along the southeast coast. Other forecast models are farther west with the track and this result would mean heavy snows across western and central Connecticut...perhaps even the heaviest occurring in eastern New York and northwestern Connecticut and a higher likelihood for mixing across southeastern Connecticut. These details will be ironed out over the next 24-36 hours.

Most forecast models develop an extremely potent storm with the potential for extreme upward vertical motion with this storm. This indicates that there will likely be a zone of extremely heavy and intense snowfall, the question just is where does that occur? This is the biggest question/challenge at hand with regards to snowfall forecast. We will examine below:

The NAM also indicates that the best frontogenesis (strengthening of horizontal temperature gradient with height) would remain west of Connecticut with frontolysis (weakening of horizontal temperature gradient with height) moving into Connecticut late afternoon and early evening. This indicates that the heaviest snowfall and most extreme rates would be just to the west of Connecticut:

However, prior to frontolysis and perhaps as the system works east and the best frontogenesis pivots through western Connecticut we could see some extremely heavy banding and very heavy snowfall rates per the NAM and this indicated very well by some bufkit soundings. This bufkit profile over Waterbury Connecticut yields two windows for extremely heavy snowfall; one during the midday and the second during the early portion of the evening as the system is beginning to depart. This profile shows a very favorable snowgrowth zone with impressive omega (red contours indicate upward vertical motion) penetrating the snowgrowth zone. One possible question here though is would moisture be lacking in this zone and this questions arises with less than 90% relative humidity values within the dendritic snowgwoth zone:

Using the same image but with relative humidity overlayed this shows relative humidity values less than 90% within the dendritic snowgrowth zone. Not a major problem but this could be something which inhibits excellent snowgrowth a bit:

The GFS also shows substantial lift within the snowgrowth zone and even hints at the possibility of some subsidence within the lower levels looking at a bufkit sounding from Waterbury, CT:

The GFS also advertises rather strong vertically averaged 800-600mb frontogenesis over the entire state during the afternoon, however, like the NAM the frontogenesis weakens and we see frontolysis develop. This indicates that as we approach the early evening snowfall intensities could weaken before finally picking back up briefly as the system departs:

So as of now what can we expect?

The details mentioned within this post are all critical to how much snow we see, where the heaviest banding/highest snowfall totals set up, and whether or not we see any areas of reduced snowfall totals thanks to subsidence or weakened upward vertical motion. As these details become ironed out the snowfall map can be adjusted to account for these details.

• Light snows begin to break out between 11:00 PM Tuesday night and 2:00 AM Wednesday morning. 
• Snowfall intensity begins to increase in intensity by late Wednesday morning with a period of moderate to very heavy snowfall lasting through mid afternoon. During this time snowfall rates could approach 2-3'' per hour in the areas which are under the heaviest banding. 
• Between mid-afternoon and late afternoon there is the possibility of reduced snowfall intensities. This is due to the strongest frontogenesis and upward vertical motion perhaps pushing west and north of the state. This will have to be watched because a prolonged duration of this phenomena could result in lesser snowfall totals for many locations. If we don't see reduced snowfall intensities during this time this would result in the likelihood for even higher snowfall totals across many locations. 
• As we near the height of the storm, strong gusty winds will begin to develop thanks to a strengthening pressure gradient across the region. Winds could gust as high as 25-45 mph with the higher gusts occurring near the coast. 
• As the system begins to depart the back edge of the storm will begin pushing through and there is the potential for this band to be extremely heavy which could add to some of the snowfall totals. This is something which will also influence the potential for higher end snowfall totals. 
• There is the likelihood for snow to mix with sleet and rain across southeastern Connecticut and this will result in lesser totals here. There is also the possibility for some mixing to occur across the majority of the state during periods of lesser intensity if the storm tracks further west and we get an influx of warmer air in the boundary layer. 
• Snow begins to wind down during the mid to late evening hours.

Below is my fist call snowfall map. Keep in mind that this will likely be adjusted tomorrow as the details mentioned above become more clear:

Significant Coastal Storm Possible End of the Week

Computer forecast models are in very strong agreement for a substantial storm system to impact Connecticut at the end of this week. We are looking at the likelihood for heavy rains, strong to damaging wind gusts (ESPECIALLY along the coast), the potential for significant coastal flooding, and there is even the possibility of snow as well. What is leading to the potential for such a potent event? Substantial changes are occurring within the atmosphere and these changes will be tied to a big sudden stratospheric warming event (SSWE) which just occurred. First off, what is a sudden stratospheric warming event? A sudden stratospheric warming event is defined by a substantial increase in temperatures in the lower stratosphere which can be up to 50°C in just a couple of days! These events can play substantial roles in the evolution of the weather patterns down in the troposphere. 

Using the period from February 18th to February 24th, we see above-average temperature anomalies at 10mb (~100,000 ft above the ground!) along with above-average height anomalies (indicating ridging):

This event is rather significant in that it will lead to something we have not seen since late November...the negative phase of the North Atlantic Oscillation (NAO). For us here in New England, the negative phase of the NAO during the winter and early spring months can be a dominant player in our weather. When the NAO goes from the positive to negative phase it can coincide with shots of very cold weather and it can also be associated with stormy weather, particularly an increased chance for snow. In the end, however, there is much more to than whether the NAO is just positive or negative. The structure of the NAO is extremely important as well but we will not get into that here. For the past few weeks, long-range forecast models have been very aggressive and consistent with the negative phase of the NAO developing. In fact, forecasts are for the NAO to dip to -2 standard deviations below-average:

The negative phase of the NAO is correlated with above-average height anomalies (ridging) over Greenland. We can see this by looking at the forecast 500mb height anomalies for Thursday morning. The red's and purple indicate above-average heights and ridging. The purple shadings indicate much above-average height anomalies which is why the NAO is forecasted to become so negative:

Since about mid-February we have also been in a negative phase of the Pacific-North American teleconnection pattern (PNA). This, along with a pretty positive NAO were big players in the record warmth we saw last week. The PNA is expected to remain fairly negative through the week:

A negative phase of the PNA is typically associated with a trough across the western United States with ridging into the eastern United States. This would usually correlate to below-average temperatures in the west and above-average temperatures in the east. Given the extreme negative development of the NAO, however, this will really help to suppress any ridging come Friday and establish a trough across the northeastern United States on Friday which will be enhanced by ridging across the central portion of the country:

A combination of a negative NAO and negative NAO holds a fairly high correlation to a rather active storm track and has even been associated with some rather potent storms and that looks to be the case once again come the end of the week. So now that we sort of took a quick look into how the evolution of the atmosphere over the past week will lead up to the potential of this event let's looks at how things will evolve this week. 

Currently there is shortwave energy entering the Pacific Northwest. This shortwave energy will continue digging into the western United States and phase with the sub-tropical jet stream. As it does so, the energy will continue progressing northeast through the central Plains before phasing with northern stream energy on Thursday:

The result will be rapid cyclogenesis (strengthening) of a surface low across the Ohio Valley during the day on Thursday:

As this system continues to rapidly strengthen we will see low pressures develop at 925mb (~2500 ft off the ground), 850mb (~5000 ft off the ground), 700mb (~10000 ft off the ground), and 500mb (~18000 ft off the ground). The system is also expected to become vertically stacked rather quickly. A system is vertically stacked when these series of low pressures throughout the troposphere are vertically aligned. This is when a storm system is at it's strongest. In the below image, come Thursday evening the placement of all these low pressure centers is roughly over NE Ohio which indicates the vertically stacked nature of the system:

During this process, a secondary area of low pressure will begin to develop just south of New England as the main energy begins to shift towards the southeast from the Ohio Valley. Typically when we see storms track to our south this time of year we have to keep an eye on the possibility for snows. At this time it appears there will be just too much of an easterly component to the winds and not much of a cold air mass in place ahead of the system for a major snow event. The easterly component to the winds will advect in warmer air in the lowest few thousand feet of the troposphere from the Atlantic Ocean. This isn't to say, however, we won't see snow with this system. But this aspect of the forecast is a bit too complex now. As the system strengthens and the winds aloft begin to strengthen a process called dynamic cooling will occur. Dynamic cooling occurs when very rapid upward vertical motion occurs along with heavy precipitation falling. This will work to cool the atmosphere about several thousand feet up. At this juncture, it does not appear snow will be a big threat but snow could be possible within the hills and we could see rain perhaps end as snow but this potential will continue to be tracked as the week progresses. 

What we will focus on right now is the closed off circulation at 500mb as this will add some complex challenges to the forecast. When systems close off at 500mb soon after systems will occlude and this process begins to shut off the influx of warmer and moist air into the system which is an ingredient necessary for both strengthening and precipitation production. Forecast models indicate that the system will close off at 500mb well west of Connecticut. This could mean less in the way of heavier precipitation here but this is something to watch as the week progresses. 

Regardless, there is the potential for a quite a bit of rainfall as forecast models suggest a very anomalous low-level jet on the order of -3 to -4 standard deviations indicating a rather strong easterly component to the winds. This would supply a great deal of moisture into the state:

At this juncture this is what we are tracking for the state:

• Potential for heavy rainfall (but again how much rain we see may be tied to the evolution of the 500mb low and whether it occludes too early and shuts off inflow of warm/moist air into the system.
• Potential for strong to damaging wind gusts, especially along the immediate shoreline. Very strong winds aloft and a very strong pressure gradient will yield the possibility for strong to damaging wind gusts statewide, however, the biggest threat will be along the shoreline. 
• Coastal flooding could be a fairly big concern. A prolonged period of easterly/northeasterly winds coincident with astronomical high tides and strong winds could make for a moderate-impact coastal flooding event, especially along the southwestern shoreline. 

As the week progresses details will begin to become much more clear but for now the idea of a rather significant storm is certainly in the cards. 

Updated Snowfall Forecast 02/17/2018 into 02/18/2018

No big changes to yesterday's forecast, however, forecast models have become a bit more aggressive with total precipitation accumulation across the state thus have decided to increase snowfall totals for northwestern CT. There are some indications that the immediate shoreline will see warmer air towards the surface work in thanks to a developing southerly flow transporting warmer air off the ocean so this will work to keep snowfall totals down here. However, with this influx of warmer air, this will make for snow to be on the wet side across these areas and this could result in some pockets of power outages. While the atmosphere is rather dry right now, forecast models indicate we will saturate rather quickly and because of this the timing of the onset of the snow may be on the earlier side.

Forecast models continue to indicate a several hour period of very intense upward vertical motion moving over the state and there are strong indications that the maximum degree of frontogenesis will coincide with maximum upward vertical velocities. This coupled with excellent snowgrowth and strong upward vertical motion and moisture within the snowgrowth zone will result in a several hour window of moderate to heavy snowfall. This window should be from 11:00 PM this evening to 4:00 AM tomorrow morning. During this time snowfall rates will approach or perhaps even briefly exceed 1.5'' per hour. It will be during this period where the majority of the snowfall accumulations will occur. The latest run of the European forecast model drops upwards of 0.5'' of liquid equivalent over much of CT during a 6-hour window which suggests very heavy precipitation falling and this precipitation will be in the form of snow. Computer forecast soundings also show modest instability within the 700mb-500mb level which further indicates the likelihood for moderate to heavy snows.

What to expect:

• Snow begins to breakout between 7:00 PM and 9:00 PM. 
• Snow quickly begins to increase in intensity with the heaviest snows falling between 11:00 PM and 4:00 AM. During this time snowfall rates may approach or exceed 1.5'' per hour. 
• While snow will be on the fluffier and lighter side across inland Connecticut, closer to the shore were warmer air will work in, this will allow for snow to be wetter and heavier. This could result in some isolated pockets of power outages.
• There is the possibility for a few towns to receive upwards of 9-10'' of snow, however, due to the very fast movement of this storm those amounts are not expected to be widespread.
• Snow begins to wind down after 4:00 AM and will be out the state by 7:00 AM, however, some leftover flurries may remain across eastern CT. 
• Below is the updated snowfall map:

                      

Quick Hitting Storm to Bring Accumulating Snows 02/17/18-02/18/18

A quick hitting but potent storm system will bring plowable snow to the state of Connecticut overnight Saturday into Sunday morning. The snow is expected to begin mid-to-late evening, however, the snow should be cleared out by 7:00 AM Sunday morning.

A potent piece of shortwave energy propagating through the northern branch of the jet stream will interact with shortwave energy moving through the southern branch of the jet stream. Associated with the southern stream piece of shortwave energy will be an area of surface low pressure. As the shortwave associated with the northern stream continues to amplify (strengthening) and interact with the southern stream shortwave, the result will be for further strengthening of the low pressure system as it begins to pass south of Long Island. One question we still face is how far south of Long Island the system tracks as this will ultimately dictate if the heaviest of the snowfall pushes into Connecticut or not:

While this will be a rather fast moving system, there are numerous indications we will see a period of moderate to heavy snows work into the state in which snowfall rates could approach 1.5'' per hour. Forecast models indicate that a period of intense upward vertical motion will move over the state during the overnight hours on Saturday. It is during this period when we would see the heaviest snowfall intensities and snowfall rates:

Forecast soundings indicate we will have excellent snowgrowth potential with good upward vertical motion within the snowgrowth zone and plenty of moisture. This is how you get the production of dendrites which are the snowflakes which accumulate quickly. This bufkit profile sounding from the latest run of the NAM forecast model for Windsor Locks, CT shows excellent snowgrowth and decent omega within the snowgrowth zone:

While perhaps not a true MAUL (Moist Absolute Unstable Layer) some forecast soundings indicate some fairly unstable layers within the troposphere. This could act to further enhance upward vertical motion and snowfall rates:

Based on the above (along with several other factors) the thinking is for a several hour period of moderate to heavy snows to overspread the state during the overnight hours. With a marginal southerly component to the surface winds, this could lead to some mixing issues across the immediate shoreline which may reduce snowfall totals here. Across the northwestern part of the state totals may be reduced due to being just a bit too far northwest of the heaviest snows. Below is what to expect:

• Snow begins between 7:00 PM and 9:00 PM Saturday evening. 
• Snow quickly begins to increase in intensities with the heaviest snowfall rates occurring between 11:00 PM Saturday and 4:00 AM Sunday. 
• During the peak, snowfall rates could approach 1.5'' per hour. 
• Snowfall begins to wind down by 5:00 AM Sunday and will be cleared out by 7:00 AM Sunday. Some leftover flurries may remain across eastern CT.

                  

Wintry Mess on Wednesday, February 7th, 2018

Our active weather pattern looks to continue and we are tracking quite the mess for Wednesday as an approaching storm system looks to give us accumulating snows with much of the state even transitioning over to sleet, freezing rain, and rain. The northwest hills stand a better likelihood of remaining mainly all snow.  This is a very challenging setup with many uncertainties and those uncertainties will all be looked at in detail below:

Wednesday morning an area of low pressure will approach from the southwest. Initially, temperatures statewide will be cold enough to support all snow, however, with a southerly flow developing and becoming established in the lowest 5000' of the atmosphere this will eventually change snow over to sleet, freezing rain, and even rain for extreme southern CT. One of the biggest questions right now is how long does the cold air remain in place and another big question is do we lock in colder air at the surface which could enhance the potential for a prolonged period of freezing rain and this could be a major concern for some. 

At 500mb (about 18,000' off the ground) a shortwave trough within the northern stream will dig into the upper mid-west on Wednesday morning. This main shortwave energy will lag quite a bit behind the surface low pressure which indicates a "tilted" system. Strong shortwave energy within the southern stream will help with the generation of a surface low pressure across the Gulf coast states/Tennessee Valley:

 With a big disconnect between the main upper energy and surface low pressure, this will work to establish a southwesterly flow in the lower levels of the atmosphere which will promote the advection of warmer air in the lowest 5000' of the atmosphere. Initially the temperatures through the troposphere will be cold enough to support precipitation to begin as snow state wide, however, with warmer air working in aloft, this will promote snow to begin transitioning towards sleet, freezing rain, and even rain for extreme coastal CT:

One of the bigger questions and challenges at this time is what happens with the track of the surface low pressure as this will play a big impact on surface temperatures which will be key in how precipitation transitions. While there is a bit of uncertainty with regards to where the surface low will track, the general idea seems to be for the surface low to either track over CT or track just to the southeast of CT. If the track of the surface low is over CT this will allow for a southerly flow to become established across a good portion of the state which will allow for warmer air off Long Island to to work in at the surface. This would yield a transition from snow to sleet/freezing rain to plain rain for many. If the surface low track is more southeast of CT this would enhance the likelihood for colder air at the surface to remain in place and this would increase the odds for more in the way of sleet and freezing rain for many. One feature we need to watch extremely closely is an area of surface high pressure just to the north of Maine. This feature could work to keep colder locked in place across a good part of the state, especially if the surface low tracks just to the southeast of CT. This would be a strong signal for a prolonged period of freezing rain and this has to be watched very closely:

Another challenging aspect of this system is with regards to how much snow falls before we see snow begin to transition over to sleet, freezing rain, and rain. The potential exists for upwards of several inches of heavy, wet snow to fall across portions of the state (with the highest totals across the northwest hills where they will remain mainly all snow) before the transition process occurs. 12z GFS bufkit data for some locations across the state indicate the potential for a several hour thumping of heavy wet snow before any changeover occurs. The below soundings are from Waterbury, CT and Windham, CT. They show excellent snowgrowth with high values of omega and relative humidity within the snowgrowth zone. This is a strong signal for moderate to heavy snows:

Given the marginal temperatures in the lower portion of the atmosphere, this will make for snow to become very wet and heavy and will also yield snowfall ratios likely on the order of around 8:1 (maybe even slightly less). The snow will not be fluffy by any means and will stick to trees and utility wires. This will pose a major problem for any areas which see a prolonged period of freezing rain. 

What are we looking at right now?

• Snow begins to break out across the state between 7:00 AM and 9:00 AM Wednesday morning. 
• Snowfall rates quickly increase and could approach and exceed 1''/hour at times. Expect snow to be heavy and wet making it difficult to move. 
• A several hour period of moderate to heavy snows can be expected, especially across the northern portion of the state and northwest hills. The northwest hills will likely remain all snow. 
• By early afternoon we begin to see snow transition to sleet/freezing rain across much of the state. The immediate shoreline will see this changeover happen by late morning thus much less snow will be expected here. The shoreline is expected to transition over to all rain, however, a further south and east track of the surface low would mean more in the way of frozen precipitation. 
• There is the potential for a moderate-impact freezing rain event with the possibility for some areas to receive upwards of 0.25'' of freezing rain accretion. If this occurs on top of heavy, wet snow this could yield power outages and some tree damage. This potential needs to be watched EXTREMELY closely. 
• Everything begins to wind down early evening. 
• Travel on Wednesday is not advised and numerous school closings should be expected. Road conditions will deteriorate extremely quickly and travel will become especially dangerous with the onset of freezing rain. 
• Below is what I am expecting right now: 

                            

 

 

Accumulating Snows Possible Thursday into Friday

We aren't looking at a major snow event Thursday night into Friday but portions of the state could see some light snowfall accumulations as an area of low pressure develops along a cold front just to the south and east of CT. As has been the case this winter this is a bit of a tricky forecast, however, there are some signals that we could see a band of moderate snow develop and traverse over the portions of the state. While this window may be rather brief there is the possibility for a brief period of moderate to perhaps heavy snows. Given the timing of this potential, this could have an impact on the Friday morning commute.

As we move towards the end of the week and amplifying (strengthening) long-wave trough will approach our region along with an attendant cold front. Within the amplifying trough will be strengthening shortwave energy. This strengthening shortwave energy will help to enhance mid-level forcing and lift across the state along with aiding in the development of a weak area of surface low pressure along the cold front:

While computer forecast models aren't particularly impressive with their output of liquid precipitation totals, there are some signals this could be a bit underdone. We will explore some of these signals below.

One signal is with regards to a blob of warmer than average sea-surface temperatures just east of New Jersey and south of Long Island and the very cold low level airmass which will work into the region as the trough begins to push through:

What is the significance of this? When you have such a cold airmass pushing over waters which are much warmer this creates atmospheric instability due to the large temperature gradient which exists between the water temperature and temperature of the airmass. This works to enhance upward vertical motion. This is one driver in the development of lake effect snows. This may also explain why some of the "smaller" forecasted events thus far this season have overperformed, especially closer to the coast where the effects of this enhanced gradient is stronger. The warmer water temperatures can also add increased moisture and water vapor for a system to work with, especially if the system tracks right over these warmer waters.

The second signals is an enhanced area of frontogenesis and vertical velocities which some computer forecast models indicate will develop across portions of CT, particularly east of the CT River. Frontogenesis, simply defined is the increase in horizontal temperature gradient over time. The stronger the frontogenesis, the stronger the horizontal temperature gradient which indicates a steep temperature change as you ascend through the vertical in the atmosphere. This is another feature which can enhance the degree of upward vertical motion. Vertical velocity is a measure of upward vertical motion with the greater the value the stronger the upward vertical motion:

A bufkit sounding from this morning's run of the GFS computer forecast model over Windham, CT suggests the potential for a window of moderate snow Friday morning. Bufkit soundings show the snowgrowth zone between 12,000 and 18,000 ft (preferred zone for the generation of excellent snowgrowth) with sufficient moisture (not shown here as it would complicate the picture) within the snowgrowth zone along with sufficient upward vertical motion:

What should we expect?

As we move through this evening a warm front will be approaching and as it approaches we will run the risk for a few snow or rain showers. During the day on Thursday temperatures will warm a bit and we will continue to run the risk for a few showers. Late Thursday afternoon and evening the cold front will approach and then eventually push through and usher in colder air. At this time, this is when computer forecast models suggest the development of a very weak area of surface low pressure along the front which was described above. At this time we will begin to see the blossoming of some precipitation which should be in the form of some snow as the surface temperature begins to drop towards the wet-bulb. The timeframe of snow should be between 3:00 AM and 9:00 AM Friday morning where periods of light to even moderate (at times) snow will occur. Given the timing, there could be some impact to Friday morning travel. Based on the above this is what I'm thinking right now: