Flash Flood Guidance
Issued by NWS
Issued by NWS
402 AWUS01 KWNH 220319 FFGMPD PAZ000-DEZ000-MDZ000-220718- Mesoscale Precipitation Discussion 1041 NWS Weather Prediction Center College Park MD 1119 PM EDT Sat Sep 21 2024 Areas affected...central/eastern Pennsylvania, north/northeastern Maryland, northern Delaware Concerning...Heavy rainfall...Flash flooding possible Valid 220318Z - 220718Z Summary...A cluster of deep convection over central Pennsylvania was producing spots of rainfall rates approaching 2 inches/hr. Isolated flash flood potential will exist with this activity as storms migrate toward the southeastern Pennsylvania/northeastern Maryland border region through 06-07Z. Discussion...A vigorous shortwave trough over Pennsylvania has helped to force a relatively fast-moving convective complex that currently extends from near Williamsport to State College. Ahead of this complex, cells have developed within a warm/moist axis characterized by ~1000 J/kg MLCAPE and 1-1.5 inch PW values - highest near Chesapeake Bay. The cells ahead of the linear complex were moving considerably slower (around 5-10 mph), allowing for several cell mergers and prolonged rainfall, with rates approaching 1.5-2 inches/hr at times generally from Williamsport to Harrisburg. Both models/observations and objective analyses indicate potential for the southward-moving convective complex to continue for a few hours, potentially reaching the MD/PA border region through 0530Z and the I-95 corridor thereafter. The combination of cells and linear convective structures suggests potential for at least an isolated flash flood risk to persist with this activity during that timeframe - especially near cell mergers and slower-moving storms. FFG thresholds in much of the region are in the 2.5 inch/hr range, suggesting that the best risk of excessive runoff should remain 1) isolated and 2) tied to low/sensitive spots and urbanized land surfaces. Cook ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...CTP...LWX...PHI... ATTN...RFC...MARFC...NWC... LAT...LON 41157682 40537581 39897560 39197581 38917653 39237727 40497798