Update! Pre-Thanksgiving Snowstorm Snow Forecast and Impacts

Update: 2:30 pm

Rather than update the timeline set in place. I will do my best to interpret and evaluate the models output that has been presented in terms of how Tuesday Night into Wednesday looks for this region and the rest of the Mid-Atlantic/Northeast.

A large 500 mb trough is stretched out across the entire country and with it brings in high pressure with a large amount of upper level divergence. This trough is expected to move through the Midwest and Ohio Valley Tuesday night into Wednesday. A separate cold front will move across our region late today into tonight which will cool down temperatures. Once the cold air starts to interact with the warmer waters of the Atlantic, a low pressure starts to form. Where this low forms and how it interacts with the cold air along its track up the coast is key with forecasting the precipitation forecast of the storm. There are several different situations that could occur that leave us with several different situations. I will do my best to explain each one and the probability of each of them occurring Tuesday Night through Wednesday night.

The low pressure system rapidly deepens off the Atlantic coast and moves in a nearly parallel pattern to the East Coast. Temperatures on Tuesday Night into Wednesday will remain in the middle to upper 30’s throughout most of the metropolitan region with higher ranges towards the western suburbs and warmer closer to the water. Precipitation will begin as rain during the late night/early morning hours on Wednesday. Soon though during the mid-morning hours into midday there will be a changeover to snow. Most likely this change over will occur at around the 11:00 am mark with the accumulation beginning around 1 pm. While dew points will be at freezing or below, surface temperatures will remain above freezing during the early afternoon hours. This will slowly change though as a small amount of cold air advection will bring in enough cold air to cool off the entire region leading to the changeover to snow. When and how this occurs throughout the day is truly dynamic so the difference between accumulation or not is really a matter of a minuscule shift in the rain snow line. Within this model, we could very well see all rain and just snow showers showing up midday without accumulation on the surface.

Situation #1: 

Accumulation amounts: For the western suburbs, especially right near the Blue Ridge, accumulations will be in the 3-6 inch range with higher amounts possible depending on when the changeover to all snow occurs. For the immediate metropolitan region though, a dusting to 1 inch of snow is expected with around a quarter to half an inch of rain expected also. This will be very wet snow also meaning it will melt fairly quickly due to the dew point being right near the freezing mark.

Impact (Low, Medium, High)-For our region I will put this situation as a Low impact as the amount of snow expected is nothing to warrant any concern. Using caution while traveling is definitely important in this situation, especially those who are traveling to the North and West where totals are forecasted to be higher and heavier due to colder temperatures. Traveling along the 95 corridor, expect minor delays mostly due to high volume, but weather shouldn't be an impact at this time.

Probability: Very likely. I will say this is about 80-90% chance of occurring.

Timing: While precipitation will start Tuesday night into Wednesday morning, the models are in agreement that the heaviest amount will occur between 9 am and 4 pm Wednesday. Most of the snow will move out by Wednesday night with lingering snow showers mostly to our west possible early Thursday.

Situation #2:

The models are not exactly in agreement over the snowfall amounts. There is a true possibility that this could be a Low to Medium impact storm even though it is relatively early in the season. Within the situation we are looking at right now, where temperatures are right near the freezing mark and a large swath of moisture from a low in the Atlantic will bring in precipitation (rain or snow) for the entire region, the probability of a “busted” forecast is very high. The GFS model right now has the low being much closer to the Atlantic coastline than the NAM and the Euro model has us getting more snow than both of those models.

It is really tough to forecast so far out from a winter storm even as there are so many variables that need to be set into place to properly say with any kind of certainty that snow will occur.

In this scenario of higher precipitation, the rain/snow line moves over the DC area early and temperatures fall near the freezing mark earlier than expected, thus leaving the amounts of snow higher. Roadways could begin having slush on them by the 3-4 pm time-slot and if snow stays in the region, rush hour will be also affected.

Accumulation: In this situation, the 95 corridor will be more affected by snow covered roads and the 3-6 inches of snowfall will shift to the East while higher amounts (6-10 inches) will remain to the North and West and lower amounts to all rain will be likely in the East of our region (trace-2 inches).

Impact: Low to Medium-Roads will be slippery in this situation, not high enough impact that you should plan on changing travel plans, but allow more time to reach your destination and anticipate backup and accidents along major roadways.

Probability: Not very likely 10-20% chance.