Flash Flood Guidance
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Issued by NWS
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026 AWUS01 KWNH 291941 FFGMPD NYZ000-NJZ000-PAZ000-300000- Mesoscale Precipitation Discussion 0341 NWS Weather Prediction Center College Park MD 340 PM EDT Wed May 29 2024 Areas affected...Northern to Northeast PA...Southern Upstate NY...Northern NJ... Concerning...Heavy rainfall...Flash flooding possible Valid 291940Z - 300000Z SUMMARY...Steepening ascent pattern with colliding features may result in short-duration uptick of updrafts with quick downdrafts capable of highly focused 1-1.75" totals in less than an hour. Given lower FFG in complex terrain, an incident or two of flash flooding is possible. DISCUSSION...GOES-E WV shows a compact inner core to the shortwave across SW PA with an elongated shear axis up and downstream across NE OH into NE PA/N NJ. As a result, broad SSW return, warm advection exists across Northern PA. At the surface an associated cold front and low-level CAA is dropping south across central Upstate NY. As such, the combination of the two is resulting in strengthening FGEN ascent pattern with steepening isentropes for greater vertical ascent even with modest lapse rates. CAPE is around 750-1000 J/kg to support narrow but intense updrafts. Given deep layer moisture is modest ~1" , it is maximized along the deformation zone and will support 1-1.5"/hr though given the broad area of ascent and slow cell motions/chaotic interaction some mergers/collisions are possible bringing totals toward the 1.5-1.75" range in about 1 hour time. FFG values are naturally lower across the region and these rates would possibly exceed them in these localized intense downdrafts. Currently the intersection of the deeper layer colliding boundaries is best maximized further west into north-central PA, but will continue zipper toward the east into the Poconos and perhaps southern Catskills and northern NJ terrain toward later this evening. Currently this area is more cloud free to allow for some increased insolation perhaps resulting in some slightly increased instability for broader updrafts; and therefore broader areal coverage of those 1-1.5" totals. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...BGM...CTP...OKX...PHI... ATTN...RFC...MARFC...NERFC...OHRFC...NWC... LAT...LON 42187642 42157545 41707468 41247455 40647477 40557550 40737694 40967785 41087820 41217850 41427885 41717877 41937768