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WPC Met Watch

Mesoscale Precipitation Discussion: #0856 (2020)
(Issued at 534 PM EST Wed Nov 11 2020 )

MPD Selection

Graphic for MPD #0856

Mesoscale Precipitation Discussion 0856
NWS Weather Prediction Center College Park MD
534 PM EST Wed Nov 11 2020

Areas affected...far eastern TN, western NC into western/central
VA and southern MD

Concerning...Heavy rainfall...Flash flooding likely

Valid 112228Z - 120425Z

Summary...Flash flooding is expected from far eastern TN and
western NC as well as western and central VA into southern MD
through 04Z from an additional 2-4 inches of rain, on top of a
broad 1-3 inches which has fallen over the past 12-24 hours.

Discussion...22Z surface observations showed that a cold front
extended from western PA into northwestern GA, or just west of the
spine of the Appalachians. Moisture was highly anomalous for
November to the east of the front, with precipitable water values
ranging from 1.5 in western VA/NC to over 2 inches in
central/southeastern VA. In addition, MUCAPE was estimated to be
in the 500 to 1500 J/kg range across a broad section of the warm
sector, with CAPE values having increased 200-400 J/kg across much
of VA and MD over the past 3 hours via low level moisture
transport according to the 21Z SPC mesoanalysis. Southwesterly 850
mb flow was observed in the 20-35 kt range from western NC into
western/central VA via recent VAD wind plots.

As the cold front continues eastward early tonight, broad low
level directional confluence/speed convergence, extending from
western NC into western/central VA and southern MD, will support
an increase in the coverage of showers and thunderstorms. This
will occur beneath strengthening upper level divergence and
diffluence, located on the southern end of an upper level jet
streak, with a peak magnitude of 150-170 kt in southern Quebec.
850-300 mb flow, a proxy for cell motions, is parallel to the axis
of low level convergence, which will support repeating and brief
training of cells at times. With the degree of moisture and
instability in place, peak rainfall rates of 1-2 in/hr will be
possible with a few locations picking up an additional 2-4 inches
of rain by 04Z. Recent heavy rain over the past 12-24 hours has
lowered flash flood guidance values across a large portion of the
MPD threat area to 1 inch or less in 1 and 3 hours. Given the low
flash flood guidance, flash flooding is considered likely.




LAT...LON   38997703 38627659 38257660 37087900 36667996
            36128119 35788239 35828262 36028268 36508197
            37048115 37947971 38517804

Last Updated: 534 PM EST Wed Nov 11 2020

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Page last modified: Wednesday, 13-May-2015 19:29:02 GMT