Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 05 2025 08:33:53 FOUS30 KWBC 050832 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Tue Aug 5 2025 Day 1 Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND INTO THE MID-ATLANTIC... ....Southeast and Mid-Atlantic... A heavy rainfall and flash flood event may be getting started at=20 the beginning of the period, as convection is expected to increase in coverage and intensity from northeast Georgia into Upstate=20 South Carolina (potentially extending into far southwest North=20 Carolina) this morning. A conducive overall setup for locally heavy downpours exists, with a low-level front/convergence zone and the=20 right entrance region of a strong upper-level jet streak=20 superimposed over the same area (along with anomalously high=20 moisture and PWs over the 90th percentile). The main limiting=20 factor for organized heavy rainfall is certainly instability,=20 though a modestly strengthening low-level jet (15-25 kts) will=20 provide some isentropic upglide to kick things off this morning=20 (with hourly totals likely limited to 1-2"). Most hi-res models=20 depict a break in the precipitation during the day, but another=20 round may ensue later tonight over some of the same areas, as the=20 frontal zone and parent trough to the west move very little.=20 Further diurnally-driven development is expected within the broader=20 warm sector from the central Gulf Coast to south-central Georgia,=20 and the rest of South Carolina, with stronger instability building today with daytime heating and PWs in excess of 2 inches. Hi-res=20 models continue to indicate fairly high probabilities of 2"/hr=20 rain rates, and a complex pattern of cell mergers and colliding=20 outflow boundaries and sea breezes. This may lead to periods of 1-2 hours where convection is maximized in particular areas, and=20 scattered instances of flash flooding. The inherited Slight Risk was expanded a bit to cover coastal portions of SC/GA with the highest probabilities for localized 5" exceedance.=20 ....Northern Plains and Minnesota... There continues to be a fairly strong signal for storms to initiate in the northern extent of an instability plume in the Plains, near the nose of a moisture transport maximum, in the afternoon today.=20 Consensus among the hi-res models is this will rapidly organize=20 into an MCS and then likely propagate quickly to the southeast in=20 the general direction suggested by the thickness contours. The=20 potential for relatively fast forward motions may mitigate some of=20 the flash flood risk. However, precipitation anomalies over the=20 past 14 days indicate the pattern has been relatively wet of late=20 in the Northern Plains, and precipitable water values will be=20 anomalously high. Deep moisture combined with strong instability=20 should support high instantaneous rain rates. Therefore, any areas=20 where the duration of heavy rain could be lengthened (such as=20 backbuilding along the periphery of a cold pool, cell mergers with=20 antecedent convective development) may lead to flash flooding. The=20 Marginal Risk was maintained and adjusted southward based on the=20 latest guidance (factoring in upwind propagation favoring southern propagation late).=20 ....Ohio River Valley... An inherited Marginal Risk was maintained and adjusted slightly=20 for a potential repeat of the pattern from yesterday, just=20 displaced slightly to the east over S IN, C KY, SW OH. A slow=20 moving mid-level wave is expected to continue to slowly shift=20 east, in the presence of relatively strong instability. Therefore,=20 scattered afternoon thunderstorms are expected. Although moisture=20 levels are not expected to be anomalously high, storm motions=20 should be very slow with mean winds in the lowest 8-km around 5 kts=20=20 or so. This could lead to localized flash flooding by extending the duration of heavy rain with the most intense storms. Rain rates=20 should reach the 1-2"/hr range. Churchill/Lamers Day 2 Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025 ....THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND FOR THE COASTAL=20 SOUTHEAST AND MID-ATLANTIC REGION... ....Upper Mississippi Valley... A region of anomalously high moisture, with PWs near the 90th=20 percentile, should shift east during the Day 2 period, and=20 generally continue to be superimposed with moderate to strong=20 instability on the periphery of a ridge. Confidence in the=20 convective details remain low at this time, but the overall=20 environment will remain supportive of organized convection with=20 high instantaneous rain rates. The pattern may still lead to a=20 forward-propagating MCS, which would mitigate flash flood risk=20 overall, but the overall environment is favorable enough to=20 continue to maintain a Marginal Risk across the region. ....Coastal Southeast and Mid-Atlantic... Confidence remains relatively low in the placement of the heaviest rainfall on the Day 2 period, as the ECMWF/ECENS suite continues=20 to concentrate QPF farther north (mostly across North Carolina)=20 whereas the bulk of the other global models and ensembles are=20 focused south and east (mainly in SC and GA). Regardless of the=20 precise details, a plume of anomalously high precipitable water=20 values is likely to remain present across the coastal Southeast=20 and Mid-Atlantic region into the Day 2 period, and the flash flood threat areas will likely come down to mesoscale details that will come much more into focus in subsequent outlook periods.=20 Churchill/Lamers Day 3 Valid 12Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025 ....THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND NORTHERN PLAINS, AS WELL AS FOR THE COASTAL SOUTHEAST AND MID-ATLANTIC REGION... ....Upper Mississippi Valley and Northern Plains... An inherited Marginal Risk area was maintained and adjusted across North Dakota and surroundings, as models are in good agreement in indicating a potent upper-level shortwave digging southward into=20 the CONUS by Day 3. The approach of the trough is likely to result=20 in another wave of low-level return flow into the Northern High=20 Plains, sending PWs to the 90th percentile or higher once again.=20 Moderate to strong instability is expected to build once again with daytime heating on the periphery of a ridge, and organized=20 convection is likely to ensue in the late afternoon and evening.=20 ....Coastal Southeast and Mid-Atlantic... Confidence remains low in the placement of the heaviest rainfall going into the Day 3 period, with substantial differences still between the global model suites. As anomalously high tropospheric moisture is likely to remain near the coast, maintained an inherited Marginal Risk area across portions of the area most likely to realize heavy precipitation with daytime heating. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_TxLAx17v3GqvNmsBISQVKPvNlaTT444wNwholcQBIIP= rM0ou07vGd5hFFDdAbY8_L6iAVe7JDKugLNf_wkAW7v5j5c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_TxLAx17v3GqvNmsBISQVKPvNlaTT444wNwholcQBIIP= rM0ou07vGd5hFFDdAbY8_L6iAVe7JDKugLNf_wkAQ-dKV7E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_TxLAx17v3GqvNmsBISQVKPvNlaTT444wNwholcQBIIP= rM0ou07vGd5hFFDdAbY8_L6iAVe7JDKugLNf_wkAEOeHMjc$=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) .