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St. Paddy's Day Storm Obs


nj2va

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You're way off base here...this is 3/3 all over again. :P

 

Haha... while this event does possess some passing similarities to what happened a few weeks ago, I think the key difference here is that the extensive southern stream shortwave is quite a bit stronger (and isn't really weakening as quickly as 3/3's event). Thus, we won't see the ageostrophic forcing weaken as quickly as it did with that event. I think northern VA is going to do very nicely with this event. Unfortunately for those further south, it looks like a significant MCS is robbing mid-level moisture transport into the Carolinas and so beyond the frontogenetical forcing across northern and central VA, the rates further south could be lower, and that combined with marginal thermodynamics might really hamper significant accumulations south of I-64

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The tiny flakes should be expected at the very beginning of the event. In these CAD cases the low-level column is very dry at first, so the snow that does reach the ground at first has almost completely sublimated (helping to both cool the column as it transfers moisture from the snowflake into the drier atmosphere). Watch the flake size start to increase steadily as the forcing increases, the column moistens up and the temperature drops to freezing or below. Cloud top heights are between 235-245K (-38 to -28 C) so we know that there is moisture well up into the dendritic snow growth zone (-12 to -18C) so there is nothing to worry about as long as forcing for ascent continues up to this level. 

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What HM and Phil were describing...you can see how this is frontogenesis. Check out the converging wind vectors tightening up the thermal gradient. South winds over central VA, west winds near the PA border. Tighten that gradient over time and the atmosphere is not in a thermal balance that is strives to be at. The result if a circulation that develops on the "warm" side of the gradient...and hence the lift.

 

This will start as a thump, the question is how organized will it be into the overnight hours. Sometimes the actual swath of snow dries up, but you are left with one narrow ribbon of snow left while everyone else is just getting very light snows. It's possible that occurs after 06z or so, but certainly no guarantee.

 

 

post-33-0-25156900-1395005442_thumb.gif

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