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: