Everyone seems to be in anticipation of a quick hitting yet impactful event tonight. Asking when it will happen and how much snow will fall. Here is my forecast, which may be a little different than what most forecasters are saying right now but I will explain later.
WHEN: Snow will appear on radar as soon as 5 PM for most spots North and West of Baltimore like Westminster and Frederick then by 8 PM for most areas in Central Maryland. However, my fear is that that this will be virga, where the snow sublimates right into the air before reaching the ground due to dry air. It is tough to predict when and what areas will have snow travel through the atmosphere and reach the ground first, but it will into the region from NW to SE as the precipitation becomes heavier and the air becomes more saturated, between 10 PM to 2 AM for most regions.
HOW MUCH: There will be moderate to heavy snow for most of the region between 1 AM and 4 AM but exactly how much snow some areas get versus other areas depends on how long it takes for the snow to saturate the air. For areas North and West of Baltimore current model guidance thinks the air will saturate sooner than other areas. Therefore, I expect a vast part of Maryland to get between 1-3 inches with some areas NW get 2-4 inches due to less Virga and some areas towards Baltimore and south to get 2-4 inches due to heavier snow bands.
FOR THOSE INTERESTED IN METEOROLOGY
I will go over the storm in more detail and how the track affects precipitation and virga.
First up will be Virga because it occurred for north central Maryland in the last storm and may present itself again in this storm. Virga is when precipitation is falling and can be seen on radar however it never completes its journey to the ground. This happens because the atmosphere is too dry, therefore the rain evaporates on its way down and snow sublimates. In most cases this happens for light precipitation in dry conditions, then the air becomes saturated enough for the precipitation to fall all the way down. Sometimes this can be a good thing when temperatures are marginal for snow because this process cools the air allowing for further formation of snow. When forecasting this and looking at model guidance, a good indicator would be conquering the 85% humidity mark at the surface.
For this storm, a cold front from the prior storm dropped through the region and following a cold front is dry air. Moreover, with this storm, the fear of virga arose from a slower more onshore/westward position of the low where it is taking longer to transition to a coastal low. Therefore, the counterclockwise flow is no longer originating from the warm water, and now from drier, colder land. Another effect of that track is that precipitation will initially be flowing in from the west and fail to travel over the mountains and into the dry air until the low moves east and moisture can finally filter into the Maryland region.
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