Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 17 2025 05:35:11 AWUS01 KWNH 170532 FFGMPD SDZ000-NDZ000-171100- Mesoscale Precipitation Discussion 0948 NWS Weather Prediction Center College Park MD 132 AM EDT Sun Aug 17 2025 Areas affected...Northwest South Dakota...South-central North Dakota... Concerning...Heavy rainfall...Flash flooding possible Valid 170530Z - 171100Z SUMMARY...Expanding convective clusters with potential repeating/merging elements. Rates of 1.75-2"/hr and localized 2-3" totals pose localized possible incidents of flash flooding overnight. DISCUSSION...GOES-E WV suite shows an elongated north to south shortwave feature along the eastern edge of larger scale trough in the Northern Rockies, lifting northeastward out of northwest SD into southwest ND continuing to be enhanced by broad right entrance of 300mb 50kt jet entrance along its northwest quadrant. This is supporting a strong surface to low level wind response, further confluent/convergent along and north of the Black Hills across W SD. 05z surface analysis shows stationary front along the SD/NEB border with Tds in the low to upper 70s throughout the SD fluxed on solid, weakly confluent easterly flow. The bulk of the moisture resides north of the surface front, though 850-700mb moisture flux convergence is further increased deep layer flux given directional confluence from southerly to southeasterly along the southern flank of the elongated shortwave trough north of the Black Hills. As such, Total PWat values have increased from 1.75 along the convergence axis. As such, strong clusters continue to expand and merge with pre-cursory convergence axes across northwest SD; noted in the RADAR mosaic and expanding/cooling 10.3um IR canopy tops. Ample mid-level lapse rates and higher theta-E air support CAPEs over 2000 J/kg which further increase within the upstream inflow region north of the frontal zone. As such, cells are already becoming rather efficient in rainfall production with rates of 1.75-2"/hr noted throughout the expanding clusters.=20=20 As the trough is lifting, differential shear within the steering profile (as the elongated trough lifts north) is helping to orient cell motions toward a repeating/short-term training profile, which is expected to become further convergent as the wave is reaching the apex of the synoptic ridging over south-central ND. As a result, an axis of 2-3"+ totals are likely to expand toward a larger cluster toward the central common border of the Dakotas over the next 3-5hrs. The rates alone are near/just above the 1-1.75"/hr FFG values in all but a few scattered locations within the area of concern and localized areas are probable to exceed the 2-2.5"/3hr values, suggesting flash flooding is possible through the overnight period. And while the top layers have been in prolonged drought with 0-40cm saturation ratio below 25%, the sheer rate on hard soils will further limit infiltration and result in increased runoff. Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9A1tL6ALpcWkq0Q1jTWNfI1ulZQ3PVJDOTEd9YVG12RiO_Knvvl5ztfrimIZYxeLqCwP= DZ3lb1GNFPxlCSsBYCu2rCQ$ for graphic product... ATTN...WFO...ABR...BIS...UNR... ATTN...RFC...KRF...NWC... LAT...LON 46850021 46699900 46299858 45569891 45059981=20 44620088 44070304 44470377 45150381 45670352=20 46030294 46550185=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .