Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 19 2024 17:53:05 AWUS01 KWNH 191752 FFGMPD NDZ000-MTZ000-200000- Mesoscale Precipitation Discussion 0906 NWS Weather Prediction Center College Park MD 152 PM EDT Mon Aug 19 2024 Areas affected...Eastern Montana Concerning...Heavy rainfall...Flash flooding possible Valid 191800Z - 200000Z Summary...Clusters of showers and thunderstorms will continue to develop through the afternoon across eastern Montana. Rainfall rates at times may reach 2"/hr which through training could result in 1-3" of rain. Flash flooding is possible. Discussion...The regional radar mosaic early this afternoon shows showers and thunderstorms expanding along a stationary front and wave of low pressure analyzed by WPC encroaching into eastern Montana. A shortwave/vorticity maxima is lifting out of NW WY into central MT at the same time, with the resultant local backing of low/mid level winds driving enhanced thermodynamic advection northwestward from the Great Plains. Recent PWs have surged to as high as 1.4 inches as measured by GPS, above the daily max for GGW, with a stretching of higher PWs noted in the 700-500mb and 500-300mb LPW fields representative of a deformation axis aloft. MLCAPE as high as 1500-2000 J/kg is advecting from SD into this region to overlap with the elevated PWs to provide an extremely favorable environment for heavy rainfall, with recent radar-estimated rain rates within deep convection reaching 2-2.5"/hr from KGGW WSR-88D. As the afternoon progresses, the shortwave lifting northward should continue to have a positive impact on the environment as low/mid level moisture and instability continue to surge NW on the locally backed flow. This will impinge into a sharp moisture gradient aligned N-S seen via moisture transport vectors, which will overlap with the deformation axis aloft. With the stationary front continuing to serve as a focus for convective initiation, this suggests storms will develop and lift continuously northward with only slow translation to the east. Mean 0-6km winds of 15-20 kts will support progressive cells, but parallel flow and bulk shear of 25-35 kts will organize storms into clusters which will then repeatedly lift north across the same areas. Rain rates are progged by the HREF neighborhood probabilities to have a 20-25% chance of exceeding 2"/hr, and the HRRR 15-min rainfall accumulations indicate brief rates up to 3"/hr are possible. This could cause 0.5-1.5" of rain in less than 30 minutes, and where training occurs, locally as much as 3" of rain is possible. FFG across this region is low, at generally 1.5"/3hrs, and the HREF exceedance probabilities for this threshold reach as high as 40% by this evening. While impacts related to flash flooding may remain isolated and confined to urban areas or more sensitive soils, anywhere training of these intense rain rates can occur could experience rapid runoff to produce instances of flash flooding. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4LGAbhUYNwm2Rp-YVWRu3x9SwVSYjr8Mv158UezmzOzUpvjhkGWjc0IctnlNctLjCWev= Al5B047NGESG6OoGfpE9FO4$ for graphic product... ATTN...WFO...BIS...BYZ...GGW... ATTN...RFC...MBRFC...NWC... LAT...LON 49050451 48800411 48010395 46880395 45820412=20 45320447 45110489 45050557 45360609 45970649=20 46930685 48490652 49030569=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .