Flash Flood Guidance
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Issued by NWS
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082 AWUS01 KWNH 300230 FFGMPD NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-WVZ000-300830- Mesoscale Precipitation Discussion 0518 NWS Weather Prediction Center College Park MD 1029 PM EDT Sat Jun 29 2024 Areas affected...northern MD and DE into surrounding portions of PA/WV/VA/DC Concerning...Heavy rainfall...Flash flooding possible Valid 300230Z - 300830Z Summary...Hourly totals of 1-2" may locally train/repeat across vulnerable urbanized terrain. Isolated instances of flash flooding are possible. Discussion...A relatively small line of thunderstorms has gained organization over portions of western MD into adjacent portions of WV/VA, taking on a training orientation in the past hour or so with hourly totals up to ~2" (per MRMS estimates). The mesoscale environment is highly favorable for continued heavy rainfall, with ample instability (SB CAPE 1500-2000 J/kg), highly anomalous tropospheric moisture (PWATs 2.0-2.3", near or above the max moving average per IAD sounding climatology), and impressive effective bulk shear (30-40 kts) for convective organization. Along with moderate low-level (925-850mb) southerly moisture transport and frontogenesis (moist pronounced at 1000-925 mb) indicated by RAP mesoanalysis, upwind propagation vectors of only 10-15 kts towards the southeast should allow for some backbuilding and training/repeating along the southwestern flank of the organizing cluster of storms, increasing concerns for short term accumulations of 2-4" (and possible subsequent flash flooding, particularly in more vulnerable urban areas). While the 12z/18z hi-res CAMs haven`t done a great job handling the convective evolution (and particularly so with more recent runs of the HRR), there is still a meaningful signal (10-20%) for 3" exceedance (per 40-km neighborhood probabilities of exceedance from the 18z HREF) though 09z, favoring portions of the populated I-70/95 corridor (encompasing the Baltimore and Wilmington metro areas, and possibly getting as far south as Washington D.C. and surroundings). While Flash Flood Guidance (FFG) is as high as 4-5" for much of the area, more vulnerable urbanized terrain has FFGs of 3" or less. As a result, localized/isolated instances of flash flooding are possible (and could be quite impactful for low-lying, vulnerable localities). Churchill ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...CTP...LWX...PHI... ATTN...RFC...MARFC...NWC... LAT...LON 39877579 39617552 39277581 38937636 38807712 39027822 39337799 39577747 39767674