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January 12-13th Cold Smoke Obs and Nowcast


Bob Chill
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10 minutes ago, biodhokie said:

Nothing like having to work in a windowless room on a Saturday. I’m fully expecting flakes to be flying by 4 pm and it’s a winter wonderland outside.

That said, how does the red line (outdoor portions) fare during snow events?

Metro should be fine; unless conditions are really lousy, usually it takes 8 inches of accumulation (height of the 3rd rail) to get outdoor portions shut down.

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I am in DC for a wedding tonight and have a jetblue flight back to Hartford from Reagan airport at 5 pm tomorrow. Thoughts on that flight happening? We were planning on taking a cab or uber to the airport. 

Looking forward to the storm though. Its been pretty much snowless back home in CT. 

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Just for fun, here is a good example of mesoscale banding indicated by some diagnostics of the 12z NAM.

sepv_nam_36.thumb.png.70061e5830375f5fbd067e5d4af9073e.png

The 750 mb heights (black), frontogenesis, and saturation equivalent potential vorticity (SEPV) are plotted above. SEPV is a measure of the stability of air parcels to slantwise and/or vertical displacements in a saturated environment. Regions of negative SEPV have been associated with mesoscale precipitation bands in winter storms. You can see that there is a band of negative SEPV from east of the Delmarva to northern MD, to the north of the closed 750-mb height contour.

There is also enhanced 750-mb frontogenesis in this region, associated with a thermally direct circulation (i.e., warm air rising to the south, cold air sinking north). This circulation provides lift, and in the aforementioned environment of conditional stability, leads to enhanced vertical motion within the general synoptic ascent.

An important thing to note with these diagonistics is that these circulations and regions of conditional instability occur within layers of the atmosphere, and are often sloped. For example, here is the same plot but for 850 mb:

sepv_nam_36.thumb.png.1dbdef54e0696ac3d92b43f6c9c1b08e.png

You can see here that the main region of conditional instability and enhanced frontogenesis is displaced to the south over central VA and SE MD. These two levels are different horizontal slices through the same circulation; in this case the circulation is sloped to the north with height. Where this circulation intersects the dendritic growth zone (DGZ) is often where the most intense banded precipitation occurs, due to the region having the most efficient ice crystal growth and aggregation.

For more information, this is one good resource: http://cstar.cestm.albany.edu/CAP_Projects/Project6/Mesoscale_Structure/seminar (M. Greenstein).ppt

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