Flash Flood Guidance
Issued by NWS
Issued by NWS
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849 AWUS01 KWNH 181746 FFGMPD NYZ000-NJZ000-PAZ000-MDZ000-WVZ000-OHZ000-182345- Mesoscale Precipitation Discussion 0459 NWS Weather Prediction Center College Park MD 145 PM EDT Tue Jun 18 2024 Areas affected...Upper OH Valley...Central Appalachians...Northern Mid-Atlantic Concerning...Heavy rainfall...Flash flooding possible Valid 181745Z - 182345Z SUMMARY...Scattered areas of showers and thunderstorms are expected to develop this afternoon and continue into the early evening hours. High rainfall rates and relatively slow cell-motions may foster some isolated instances of flash flooding. DISCUSSION...A moist and unstable airmass is in place across much of the Upper OH Valley through the northern Mid-Atlantic region characterized by midday MLCAPE values that are on the order of 1500 to 2500 J/kg and PWs of 1.5 to 1.75 inches. The boundary layer in particular is quite moist with surface dewpoints in the low 70s across large areas of eastern OH through western and northern PA, and also across southern NY. Additional strong diurnal heating going through mid-afternoon will only contribute to a further destabilization of the boundary layer and should set the stage for scattered areas of showers and thunderstorms to develop. A combination of localized differential heating boundaries and orographics involving the higher terrain of the Appalachians will be key drivers of this. However, a rather strong subtropical ridge is currently centered over the Mid-Atlantic states and will be strengthening a bit further this afternoon. This suggests poor mid-level lapse rates, and with a lack of shear, the convective mode going through the afternoon hours should generally be pulse in nature. However, as smaller-scale convection develops, there may be sufficient outflow boundary activity to promote additional convective growth/activity. Areas also farther west across northeast OH, northwest PA and southwest NY in close proximity to Lake Erie will have the consideration of a lake-breeze boundary which will help to provide some low-level forcing/convergence for convective initiation this afternoon. Can`t rule out some locally focused areas of convection across these areas with slow cell-motions in the vicinity of this boundary. The 12Z HREF guidance suggests rainfall rates should easily reach 1 to 2 inches/hour with the stronger storms, with the potential for a lot of this to fall in as little as 30 minutes given the high CAPE environment. Some localized storm totals of 2 to 3+ inches will be possible, and this may result in some isolated instances of flash flooding. This will include potentially some urban impacts as well for locations such as Pittsburgh and Cleveland. Orrison ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ALY...BGM...BUF...CLE...CTP...LWX...OKX...PBZ... PHI...RLX... ATTN...RFC...MARFC...NERFC...OHRFC...NWC... LAT...LON 42757731 42427483 41877418 41377449 40947537 40877657 40537795 39767864 38488014 38828100 39938126 40918246 41638226 42088089 42687898