Accumulating Snows Likely Overnight into Tomorrow Across Connecticut

We are tracking yet another accumulating snowfall event later tonight into tomorrow and even at this stage there are still some uncertainties with regards to the exact track of the storm. This is just one factor which will determine just exactly how much snow we see across Connecticut and what type of snowfall totals we can expect. Part of the problem with these uncertainties too is in the end this will come down to mesoscale factors (which will be discussed below) and those are extremely difficult to pinpoint sometimes more than 6 hours out...they can even be difficult to do so just a few hours out.

One of, if not, the biggest key with regards to this system is how two pieces of energy will interact with each other. There are some differences within the forecast models on how these pieces of energies not only evolve but how they interact:

Energy #1 continues to dig towards the southeast through the Great Lakes region while energy #2 continues to dig southeast through the Tennessee Valley towards the east coast. Once energy #2 is off the coast of the Carolina's, it will then make a turn towards the northeast and this is when we begin to see the two pieces of energy begin to interact (or phase). This will lead to rapid cyclogenesis (strengthening) of a low pressure system just off the coast. This is where we are faced with some questions with the pieces of energy. There are differences within the forecast models regarding the strength of each of the pieces of energies and also with the handling of energy #2. The NAM forecast model evolution allows for the storm to get tugged in closer to the southern New England coast and also has a much larger precipitation shield.  The result is the potential for higher snowfall totals across much of the state. The GFS and European forecast models, however, are a bit farther southeast and have a much more compact (smaller) precipitation shield resulting in less snowfall potential for much of Connecticut.

This is just one piece of the puzzle, however, and there are other clues in place which I think helps to shape which direction to forecast in terms of snowfall across Connecticut. Unfortunately these clues don't tell us exactly where certain features will setup, just that these certain features will be a big player. These will be addressed below:

Rapid Storm Deepening/Storm Becoming Too Strong

Due to the interaction between the two pieces of energy mentioned above, we are going to see very rapid cyclogeneisis/strengthening of the surface low pressure occur. In fact, the system may become too strong too quickly. This can make snowfall forecasts very difficult because while you'll have some areas hit really hard, you'll have many areas struggling to accumulate. Think of tropical systems...what is one noticeable feature they have? If you answered banded precipitation as you work out and away from the storm's center you're correct! The thinking about this is such sharp pressure gradients help to shape and result in pockets of rapidly rising air...it is under these pockets where you would see the most intense and heaviest snowfall rates. However, because you have these areas of rapidly rising air, you also must have rapidly sinking areas of air. This results in extremely light snowfall rates and intensity. Where these areas set up is extremely difficult, if not impossible to pinpoint this far out. How does this make a snowfall forecast challenging? Let's see Hartford County as an example. Let's say the snowfall forecast for Hartford county was 8-12'' but we had some of these extremely heavy bands setup over parts of Hartford County and some areas were in between these bands. You would have some cities within the county likely fall within the 8-12'' range (and maybe even locally higher amounts) and you would have some cities receive much less than 8''. This just goes to illustrate how these bands can really complicate a forecast.

Frontogenesis/Subsidence 

This can somewhat be tied into the above paragraph, however, not all storms have banded like structure of the precipitation but most of these bigger storms do have areas where strong frontogenesis leads to extremely heavy bands of snowfall...we saw this exact case with this past storm) and outside of this frontogenesis you have subsidence. Frontogenesis is the strengthening of horizontal temperature gradient with height. This leads to very strong upward rising motion and heavy snowfall rates and intensity. Subsidence is the sinking motion of air and that results in much reduced snowfall rates and intensity. The NAM forecast model actually hints at the possibility of two bands of heavy snowfall to set up with subsidence right in between these two bands. The first band would be over western and portions of central Connecticut with the second band right along the Connecticut and Rhode Island border:

The GFS isn't too dissimilar from the NAM in that it also hints at the potential for two bands of extremely heavy snowfall. The exception is the GFS is a bit farther east with the second band and has it kind of setup over portions of far southeastern Massachusetts as opposed to back over the Connecticut/Rhode Island border:

The European forecast model has a nice zone of strong frontogenesis traverse right over the entire state of Connecticut. However, this could also be due to the resolution of the graphics of the source used to view the data and the resolution may not be good enough to sort of pick up what the NAM and GFS advertised. This is something which really makes constructing a snowfall map quite challenging and is a major reason as to why there will likely once again be "in-storm" updates.

Amount of Liquid Precipitation

The other question we face is just how much liquid equivalent is expected to fall across the state with this storm. As a general rule of thumb, 1'' of liquid equates to about 8-10'' of snow. However, this can vastly depend on snowfall ratios (how fluffy the snowfall is). The above, equates to 8:1 to 10:1 snowfall ratios (1 inch of liquid = 8-10'; of snow). If conditions warrant higher snowfall ratios, then 1'' of liquid can equate to as much as 15''+ of snow! In the case of this storm, it does appear ratios have a better chance of exceeding 10:1 and possibly as high as 13-15:1, however, this will largely depend on how great our snowgrowth is (we will explore this later). Forecast models generally are showing anywhere from around 0.50'' of liquid (western part of the state) to as much as almost 1.40'' across the eastern part of the state (NAM is much more pronounced with this. The GFS is generally 0.60'' to 0.70'' across the state while the Euro appears to be about 0.40'' west to near 1'' east:

After analyzing some bufkit soundings across the state we do have great snowgrowth (at least through late morning tomorrow) with sufficient moisture/ice within the snoowgrowth zone and upward vertical motion which indicates that there will be zones of great snowgrowth and fluffy snow flakes which should accumulate rather quickly. However, bufkit soundings also suggest what was mentioned a few paragraphs up and that is subsidence. Within the white box we see blue contours. These blue contours represent sinking motion (subsidence) while the red contours (highlighted in the red box) indicate rising motion. The blue contours within the snowgrowth zone are a strong signal for the potential for subsidence and reduced snowfall rates:

What can we expect?

• Flurries/light snows to begin breaking out between 12:00 AM and 3:00 AM tomorrow morning.
• Light to moderate snows will overspread portions of the state between 5:00 AM and 8:00 AM. 
• Between 8:00 AM and 1:00 PM will be the height of the storm. During this time snowfall intensities will range anywhere from light to perhaps heavy. Who sees heavy snows and light snows will all depend on how the precipitation is structured across the state and whether we are looking mainly at banded precipitation and are dealing with areas of subsidence. In the heaviest bands of snow, snowfall rates could approach 1.5'' per hour. 
• After 1:00 PM to 2:00 PM bufkit soundings show a decaying snowgrowth zone and the storm should be moving off to the north and east so snowfall will begin to wind down. 
• Below is what I am thinking, however, keep in mind this will LIKELY have to be refined once the processes described within this post better become better known and understood.