Subj : MESO: ASB Heavy Rain/Floo To : All From : Mike Powell Date : Mon Aug 07 2023 16:03:00 AWUS01 KWNH 071927 FFGMPD NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-WVZ000-080030- Mesoscale Precipitation Discussion 0882 NWS Weather Prediction Center College Park MD 326 PM EDT Mon Aug 07 2023 Areas affected...Western & Central MD...Central & Eastern PA...Northern DE...Northern VA...DC... Concerning...Heavy rainfall...Flash flooding possible Valid 071925Z - 080030Z SUMMARY...Very intense rainfall to accompany severe thunderstorms, cell orientation or even mergers may allow for increased duration to result in localized instances of flash flooding through the evening. DISCUSSION...GOES-E WV shows core of upstream height-falls are starting to press into the Mid-Atlantic with numerous strong updrafts already forming along the front range of the of the Blue Ridge Mtns as well as into south-central PA. Discrete supercells are currently the mode across PA into West-central MD/NW VA starting to approach more unstable axis of 2000-2500 J/kg of MLCAPE from central VA into central PA. CIRA LPW and RAP analysis suggest, a WSW-ENE wedge of lwo to mid-level enhanced moisture is crossing N WV across the Mason-Dixon line. This has resulted and likely to continue, a more expansive area of convection that continues to be favorably oriented for multiple rounds of heavy rainfall across this area, slowly sinking southward with time as southeasterly return moisture off the Atlantic lifts northward as well combining toward 1.75-2" values by mid-evening. This should increase rainfall efficiency with time with rates of 1.5-2"/hr (though sub-hourly rates would be much higher with 1-1.5" totals in 15 minutes suggested by HRRR or .35-.5" in 5 minutes by numerous WoFS members); however, forward cell motions are likely to limit totals by any one given storm to about 2". Stronger rotating updrafts may enhance this a tad given Bunker's right moving propagation vectors suggest slightly reduced forward speed combined with isallobaric acceleration of moisture flux into the supercell itself. The key to greater flash flooding potential will be the requirement for additional upstream thunderstorms. Hi-Res CAMs and WoFS solutions, combined with current observational trends suggest this is more likely across E WV, Maryland and far Southern PA, though there are a few guidance members that suggest a enhanced potential of back-building environment over north-central VA as well, particularly later into the evening. This is less likely further north in central to northeast PA, but overall, the FFG values are a bit lower relative to MD/NoVA to suggest perhaps one really strong round could exceed these lower values and induce a localized flash flood instance or two. As such, a broad possible risk of spotty/localized flash flooding will accompany the severe weather risk through the evening across the Mid-Atlantic. Gallina ATTN...WFO...AKQ...BGM...CTP...LWX...PHI...RNK... ATTN...RFC...MARFC...OHRFC...NWC... LAT...LON 41487566 40897508 39707547 38477641 37887780 37817897 37947969 38387964 38857930 39237899 39787889 40137892 40607850 41157768 41387679 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .