Flash Flood Guidance
Issued by NWS
Issued by NWS
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244 AWUS01 KWNH 271608 FFGMPD NYZ000-NJZ000-PAZ000-272200- Mesoscale Precipitation Discussion 0333 NWS Weather Prediction Center College Park MD 1208 PM EDT Mon May 27 2024 Areas affected...Eastern Upstate NY...Eastern PA...Western NJ... Concerning...Heavy rainfall...Flash flooding possible Valid 271615Z - 272200Z SUMMARY...A few rounds of heavy rainfall pose spots of 2-3", some spots up to 1.75"/hr may induced widely scattered incidents of flash flooding through the afternoon into early evening hours. DISCUSSION...GOES-E WV denotes a potent longwave trof over the Great Lakes with a compact upstream kicker to further sharpen the wave throughout the day. This timing/orientation is also supporting the leading edge of the modified EML to sharpen a shortwave trough across the the upper Ohio Valley providing strengthening broad scale ascent/DPVA downstream. Additionally, this is drawing deeper, more unstable, higher theta-E air off the eastern Atlantic becoming increasingly confluent with moisture stream emerging out of the Cumberland Plateau through the Mid-Atlantic. A subtle southern stream shortwave has lifted northward across Northern VA/Central MD, further helping height-falls as well as backing and strengthening low level wind fields. As such, total PWAT values are inching up to 1.5-1.75" across Central PA, Delaware River Valley and leaking into the Hudson Valley over the next few hours. RAP analysis also notes increasing MUCAPE along/ahead of the weakening southern stream warm front currently reaching 750-1000 J/kg. Further downstream clearing but also steepening lapse rates with some eastern edge of the modified EML nosing in, should allow for CAPEs to reach 1000-1500 by mid-afternoon across the area of concern. Current RADAR shows some increasing congestion across the area of greatest ascent, as well as, along the western edge of the mid-level stratus in Central PA. Hi-res CAMs remain bullish on rapidly intensifying updrafts through this sector. Given available moisture, unstable air and flux, expectation is for rapid efficient rainfall production with 1.5-1.75"/hr rates likely to become frequent in the warm sector line from SE Upstate NY into E PA over the next 2 hours. Streaks of 2" in 1-2hrs are possible with southern/upstream edge of convective line having best chances for back-building short-term training before western cells approach. Flash flooding is not necessarily expected with this line with the exception of urban centers with hydrophobic ground conditions that support high runoff. However, upstream, the approaching secondary line should also be intensifying through late afternoon. While deeper layer steering should allow for faster and more west to east cell motions, and additional 1-2" stripes with the intense downdrafts may intersect with earlier round and result in widely scattered 2-3" totals in 3-4hrs resulting in possible widely scattered incidents of flash flooding to occur; particularly in complex terrain of NE PA, SE NY and NW NJ. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ALY...BGM...CTP...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 43537427 42977371 41637376 40257473 39797608 40177662 40887676 41937686 42817651