AWUS01 KWNH 201200
FFGMPD
VAZ000-NCZ000-201800-

Mesoscale Precipitation Discussion 0669
NWS Weather Prediction Center College Park MD
800 AM EDT Sat Jul 20 2024

Areas affected...much of central/southern VA...far northern NC

Concerning...Heavy rainfall...Flash flooding possible

Valid 201200Z - 201800Z

Summary...Slow moving showers producing rainfall rates of 1-3"/hr
may result in localized totals of 3-5" through morning. Isolated
to scattered flash flooding is possible (with a locally
significant event or two possible).

Discussion...Low-topped, highly efficient showers are percolating
around and north of a weak stationary boundary draped near the
VA/NC border region this morning. Some of these showers have
already been capable of 2-3"/hr rainfall rates (per MRMS
estimates), embedded within stratiform rainfall and moving very
slowly (as 850-700 mb flow ranges from only 5-10 kts). While
forcing is not very significant, a weak shortwave trough is
evident via GOES-East water vapor imagery and RAP analysis just
upstream (over TN/NC border region), and this looks to contribute
to additional development through the morning. In addition, the
current activity is moving into a more favorable area of
instability (SB CAPE 500-1000 J/kg), and instability will only
increase with the increase in solar insolation. Precipitable water
levels are quite high (1.8-2.1", between the 90th percentile and
max moving average per GSO and WAL sounding climatology), and warm
rain processes (collision and coalescence) will continue to
dominate with freezing levels between 14-15k feet (and a
significant cap above 700 mb, particularly closer to the coast
line).

While hi-res guidance is not very excited about overall rainfall
coverage, there are indications of locally significant rainfall
accumulations. While some hourly HRRR runs depict isolated to
widely scattered 3-5" totals, the 06z HREF also has a meaningful
signal for localized 3" exceedance (per 40-km neighborhood
probabilities of 20-40%, and a small bullseye of 70% near/north of
Williamsburg, VA). These exceedance probabilities correspond with
5-10 year ARI exceedance, which are also very near the Flash Flood
Guidance (for 6-hr period). Given the observational trends and
available guidance, isolated to scattered flash flooding is
considered possible (with a locally significant event also
possible, particularly over far eastern VA where the experimental
RRFS guidance also indicates a strong signal for 3" exceedance).

Churchill

ATTN...WFO...AKQ...LWX...RAH...RNK...

ATTN...RFC...MARFC...OHRFC...SERFC...NWC...

LAT...LON   37817816 37787596 37317553 36587690 36297871 
            36108065 36488153 37268077 37587971 
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