Tuesday, January 27, 2009


1-27 850 12z shortwaves

UPDATE: TUE JAN 27 - 4:45 PM. My projection is that all schools under a Winter Storm Warning WEST of the Chesapeake Bay will be closed Wednesday (including most of PA, the WV panhandle, and Northern Virginia.) County schools under a Winter Weather Advisory are more likely to have a 2 hour delay (such as Anne Arundel, Prince Georges, etc).

THE "SHORTWAVE" DISCUSSION: TUE JAN 27 - 2:15 PM. I've analyzed the upper level flow at 850 mb and 500 mb and identified two more shortwave ridge-trough combinations that upon arrival will set off additional wintry precipitation between now and 6:30 AM Wednesday. A quick guide to the chart above: Blue are the leading edge ridges, red are the trailing edge troughs. Precip associated with the short-wave occurs in between the two, so if you time the arrival of the ridge nased on analyzing the wind barbs, and how long it will take to traverse your area, you know the duration of snow/sleet/freezing rain.

There are many more SW's behind those, but am just focusing on next 12 hours for simplicity. I agree with commenter Nick's assessment: Shortwave # 2 shown above will cross the Baltimore Metro region before 5:00 PM, last 4-5 hours, contain mostly snow, and taper off around 10:00 PM. Shortwave # 3 will arrive around 2:00 AM (or sooner), contain snow/sleet/freezing rain and last until roughly 6:30 AM. The second one is going to introduce more warm air because the "twist factor" at that level is causing the flow to bend more north, ushering in warm, moist air from the ocean as the wave passes.

Bottom line of all this: Even if the precip cuts off at 6:30 as I expect, it's too late for a 2-hour delay, as the time required to resalt parking lots/clear roads following the 3rd shortwave will not allow enough bounceback time to get buses rolling by 7:30 for high school pickup that in some areas begin a little after 8:00 AM. So the only logical thing to do is close again.

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