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March 20th White Rain/Slush Event Discussion and Obs


yoda

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Dropped to 36 at DCA and 33 at IAD. Didn't help much, but LAMP was high.

0.48" precip. Pretty healthy.

Regular MOS wasn't bad.. Had 36 and 38 at the time forget which was which. Figured LAMP was high just my role plus as Matt noted the writing was on the wall for the city at midnight.
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The High was not in an ideal position, but the air was pretty dry. Probably mostly evap cooling. Dynamic cooling is a bit misunderstood, its not simply cold air aloft  coming down to the surface with the heavy precip. One of the processes involved there is the snow melting as it falls into "warmer air", thus removing heat. In this case I think it was more evap cooling with the low dew points. Dynamic cooling gets talked up a lot, but evap cooling is more efficient.

thanks for the explanation!  Pure evaporational cooling should cool temps down to the wet bulb temperature (assuming no change in airmass), but in this case the wet bulb temperatures fell a few degrees after the precip started, which seems to indicate some cold air seepage from somewhere.  With a southeast wind, I'm curious as to where the colder air (in terms of wet bulb) came from

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thanks for the explanation!  Pure evaporational cooling should cool temps down to the wet bulb temperature (assuming no change in airmass), but in this case the wet bulb temperatures fell a few degrees after the precip started, which seems to indicate some cold air seepage from somewhere.  With a southeast wind, I'm curious as to where the colder air (in terms of wet bulb) came from

Dunno. Based on my temp, DP, and pressure, the wet bulb should have been 32 or a tad higher, and that is exactly what the temp was once the snow began falling in earnest. But perhaps there could also have been some column cooling due to dynamic processes, as the column was cold.

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Dunno. Based on my temp, DP, and pressure, the wet bulb should have been 32 or a tad higher, and that is exactly what the temp was once the snow began falling in earnest. But perhaps there could also have been some column cooling due to dynamic processes, as the column was cold.

Probably a small combo of melting cooling, dynamic cooling, and precip dragging down colder air from aloft.

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World of difference from AFB to Crofton this morning. I drove through the rain/snow line. Did some work in the rain/sleet. Then drove back through the rain/snow line back into snow. Hardly anything down there. Soon as I got within a couple miles of Crofton. Boom, all white. Then it turned to rain shortly after. Eye balled about 2".

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This event was definitely better the further N you were. WinterWX did much better than me with this one. Where are you for the season?

 

I'm at 30.75".  You've got 10" on me.

 

I think last year you got me by 12".  Seems like Winchester is the rubicon, if you will, of the valley winter climate.  Stark contrasts just north of the city from just south of it.

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