Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 06 2025 19:10:23 FOUS30 KWBC 061908 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 308 PM EDT Wed Aug 6 2025 Day 1 Valid 1907Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE COASTAL SOUTHEAST AND SOUTHERN MID-ATLANTIC REGION... ....19z update... The day 1 slight risk area was expanded into southern-central areas of Virginia in coordination with the Wakefield field office. This was to account for 2-3"+ rainfall that has occurred so far today,as well as the potential for more convection to occur in and around this sensitive area this evening. ....16z update... Minor edits were made to the day 1 ERO. Upper-level dynamics continue to support widespread convection across the Southeast/Mid- Atlantic throughout the day. 12z HREF 40km neighborhood probabilities of 24h qpf exceeding 5 inches remains high (50-70% range) over parts of the Carolina coast from near Charleston up through Wilmington. However the EAS probabilities of exceeding 3 inches remain capped at 15%. In the Central Plains to Middle/Upper Mississippi Valley, model spread continues to be the main reason for no upgrades there. Favorable instability and moisture will support heavy precipitation, but models struggle to converge on placement of impactful qpf. Kebede ....Coastal Southeast and southern Mid-Atlantic... Relatively high coverage of convection is expected once again today, as a stalled frontal boundary remains draped across the region. While storms should generally remain relatively short-lived over any one location with shear being the main limiting factor, more significant convective organization is possible in the vicinity of the NC/SC Coastal Plain border region where deep layer shear is progged to be closer to 20-30 kts (versus less than 20 kts from surroundings). This is due to the slow eastward progress of the blocked upper trough to the west, and a vorticity maxima may also contribute to convective proliferation later today via DPVA and enhanced upper divergence via right-entrance region of ~80 kt jet streak over the Northeast. While the 00z HREF 5" exceedance probs are quite high (50%+ over portions of the Coastal Plain), uncertainty remains high with regard to the extent of resulting flash flooding, as the QPF spread between the HREF members remains quite large (with the NAM-nest and FV3 being the wetter outliers, while the ARW and ARW2 are the drier outliers). Maintained the inherited Slight Risk, and a targeted upgraded Moderate Risk could be possible later today should models come into better agreement. ....Central Plains into Middle and Upper Mississippi Valley... A mature MCS is diving southward early this morning from SD/MN into IA/NE (see MPD #876 for more information) along with anomalously high tropospheric moisture (PWs nearing 2.0", or around the 90th percentile). Maintained a Marginal Risk for lingering convection associated with the MCS this morning, along with the potential sustained or redeveloped convection in association with the remnant MCV later today. Churchill Day 2 Valid 12Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025 ....THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST/GREAT LAKES, AS WELL AS FOR THE COASTAL SOUTHEAST AND SOUTHERN MID-ATLANTIC REGION... ....2030z update... A marginal risk area was introduced for portions of southern Wisconsin and northern Illinois. 12z hires guidance suggests the development of some sort of MCS over parts of southeastern Minnesota/southern Wisconsin Thursday afternoon/evening. This is likely to occur beneath an shortwave impulse propagating through the Northern Plains/Upper Midwest within a broader upper-level ridge. 1-2 ....Northern Plains... Models continue to indicate a potent upper-level shortwave digging southward into the PacNW by Day 2, and have maintained a Marginal Risk centered on ND where convection is anticipated to initiate in the afternoon. The approach of the trough will result in another round of low-level return flow into the Northern High Plains, sending PWs to near (or above) the 90th percentile once again. Moderate to strong instability with daytime heating should yield organized convection by late afternoon/evening, and a targeted Slight Risk upgrade may be necessary with subsequent outlooks. ....Coastal Southeast and southern Mid-Atlantic... Confidence in the rainfall forecast is relatively low going into Thursday, as much depends on how convection ultimately evolves today. That said, the recent trends in QPF have been downward with most models showing anomalously high tropospheric moisture and low-level convergence shifting to the immediate coastline. Have maintained an inherited Marginal Risk for this cycle, which may be further reduced in coverage should these model trends continue. Churchill Day 3 Valid 12Z Fri Aug 08 2025 - 12Z Sat Aug 09 2025 ....THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, AS WELL AS FOR THE COASTAL SOUTHEAST AND SOUTHERN MID-ATLANTIC REGION... ....Northern Plains into Upper and Middle Mississippi Valley... Inherited Marginal Risk was expanded to include a relatively large portion of the north-central portion of the CONUS, a continuation of the threat from Day 2 from a potent upper trough/closed low. Convective details remain quite fuzzy at this range, but guidance suggests the potential for localized 3-5" totals (high spatial spread in these depictions) which may necessitate future targeted upgrades. ....Coastal Southeast and southern Mid-Atlantic... Low confidence forecast continuing into Day 3, maintaining a Marginal Risk area that is largely focused on the same area. At this juncture most models depict heavy rainfall remaining largely offshore, but given recent heavy rains and the potential for significant amounts today have opted to keep the Marginal Risk. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7pbvWue52_LkTYk98IqFgm7fj1FyUTeLdH6NwEtTgFlw= 7xQaRO7Oy1-BvIZeNjMZc3TOnhr1GrINnVF8avqebUm_P8U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7pbvWue52_LkTYk98IqFgm7fj1FyUTeLdH6NwEtTgFlw= 7xQaRO7Oy1-BvIZeNjMZc3TOnhr1GrINnVF8avqeU8iI-9M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7pbvWue52_LkTYk98IqFgm7fj1FyUTeLdH6NwEtTgFlw= 7xQaRO7Oy1-BvIZeNjMZc3TOnhr1GrINnVF8avqeyO86gF4$=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. 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