Subj : Heavy Rain/Flooding VA/NC To : All From : Dumas Walker Date : Sat Jul 20 2024 10:01 am 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 --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175) .