2/17 Overnight Moderate Event for DC/Balt Metros

A low-pressure system tracking in southern VA will bring our region snow again. It will move out fast, but will bring good dynamics with it, and intensify as it slides across, bringing heavy rates. Winter storm warnings are up for those in the I81 corridor and N of I66 for 4-6" of snow, with more in the mountains.

A strong 170-200kt UL jet will lead to divergence(therefore, rising air) on the left exit of the jet(over us). This, along with strong frontogenesis/WAA will support lift, which looks to stretch throughout the DGZ, leading to optimal snow growth and support for heavy banding. The intensifying low pressure will also deepen the lift in the DGZ even more, strongly supporting the idea of 1-3"/hr rates and higher accumulations; despite short duration. 

A sounding in N MoCo indicates a favorable environment for snow growth barring the above-freezing surface temps. N of 66 and NW of 95 temps should be 30-32 during the peak precip, and we should dynamically cool to around 32 when the precip sets in.

Additionally, the 850 MB low(image 2) is in a favorable position for those just north of its track, and the SLP is in an ideal location. We will see strong WAA(image 1) and healthy PVA(image 3). Large dynamic cooling is expected given the rates of the precip and the profile being dry enough right now(image 4). 

Thus, 1-3"/hr rates are expected with this system. The duration will be around 3-6 hours, but the impressive dynamics and rates will make up for the short period of time, in my opinion. The overall setup is just supportive of heavy snowfall rates, and ratios aren't horrible either(should be around 10:1, slightly higher in peak banding). Overnight timing also really helps here with any melting/sun angle concerns, so long as we are around or below freezing it will have no problem sticking.

Thus, this is why I went aggressive with my snowfall and forecasted widespread 4-6" N of 66, with mountains getting 6-8" because of orographic lift leading to heavier rates in banding(as well as being colder). Thundersnow is possible too due to the strong forcing and instability in the column. Heavy snow is almost a lock, but this event will almost fully wrap up by 7-8 am. Although the warm is ground initially, initial precip should help saturate the column and enforce dynamic cooling for the heavier rates. It will stick to roads due to the overnight timing and rates. Precip type should generally be all snow N of 66(maybe you might see a bit of rain mix in at the start but should transition quickly), S of there you should start as rain first but transition to snow. Overall, a solid snowstorm is upcoming for N and W of DC!