Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 04 2025 20:58:50 FOUS30 KWBC 042058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 458 PM EDT Mon Aug 4 2025 Day 1 Valid 2038Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND LOWER OHIO VALLEY... ....Southeast... 16Z UPDATE: The updated outlook is fairly consistent with the previous version. The main change was to extend the Slight Risk area a bit further to the northeast into Upstate South Carolina and extreme southwest North Carolina so that it largely matches the area outlined in the Day 2 outlook. Hi-res model signal for a focused area of heavy rain has improved for the overnight period, although there is still a lack of consistency on placement. At this time, the most plausible placement of the 06-12Z rainfall max would seem to be near the ECMWF-AIFS, although the magnitude of the QPF is likely too low. This is also fairly close to the 12Z HRRR, 12Z HiresW-ARW, and new 12Z RRFS, and centered along a line from AHN- GSP. The ingredients are there for a low-topped, efficient convective rain band with low-level convergence, the right entrance region of an upper jet, and precipitable water values increasing to the 95th percentile or higher overnight. The main question would be the level of instability. Some models show zero instability, and this would favor more of a moderate rain scenario. However, even a couple hundred j/kg CAPE would likely support greater organization to the rain band and potentially very heavy localized rainfall that may extend in to the early part of the Day 2 period. The margins between a flash flood scenario and relatively benign rainfall are quite thin in this case, but certainly there is enough risk to warrant an upgrade to a Slight Risk. PREVIOUS DISCUSSION: Wet and unsettled conditions to persist across the Southeast as a mid-level trough axis lingers to the west allowing for moisture to stream northeast from the Gulf. The trough draped from the Great Lakes to the Arklatex will weaken slightly as ridging from the Atlantic expands northwestward. Heavy rain will spread across the region as PWs remain 1.75 to 2.25 inches, highest within a narrow channel from the FL Panhandle into coastal South Carolina, overlapping MUCAPE that is progged to rise above 1000 J/kg. The presence of the strong upper level jet will enhance forcing for ascent over the region while the surface front remains draped across the region, and a shortwave ejects northeast from the Gulf. This environment should support expanding showers and thunderstorms, with just a slight shift Northwest expected in the axis of heaviest rain due to the expansion of the mid-level ridging from the east. Antecedent moisture has lowered FFGs and increased soil sensitivity, especially for portions of Alabama and Georgia. The Slight Risk areas was expanded a bit further west into Alabama and northeastward across Georgia and far western South Carolina. With rainfall rates likely eclipsing 2 inches/hour at times, any of this heavy rainfall falling atop these vulnerable soils could quickly become runoff leading to instances of flash flooding. ....Northern Plains... 16Z UPDATE: The Marginal Risk area was removed for western Minnesota and adjacent portions of the far eastern Dakotas. Although some models still show some very localized heavy rainfall, this may not meet the thresholds needed for flash flooding, and the probability at any given 25-mi neighborhood is generally lower than 5 percent, even if it's not zero. A lack of significant instability and strong focusing mechanism should be limiting factors. ....Lower Ohio Valley... 20Z UPDATE: Upgraded the Day 1 ERO in this area to (1) broaden the Marginal Risk area a bit and (2) include a targeted Slight Risk across parts of southest MO, southern IL, southwest IN, and far westesrn KY. The Slight is essentially along the mid level deformation axis/comma head region, where there is a localized uptick in 0-6 km bulk shear (~25 kts) and were we're seeing more southward propagation, while the mean 850-300 mb layer flow (individual cells) are southerly around 5-10 kts. With TPW values between 1.7-1.8" within this corridor, and mixed layer CAPEs between 1000-2000 J/Kg (positive dCAPE/dT over the past few hours), expect slow-moving and training convection to produce localized hourly rainfall rates of 2-2.5+ inches. Hurley PREVIOUS DISCUSSION: An inverted surface trough is expected to lift northeast today into the lower Ohio Valley. Low- level convergence along this boundary will combine with broad thickness diffluence and a shortwave pivoting northeast to drive pronounced lift into a region of high PWs (above 1.5 inches) and a collocated plume of MUCAPE above 1000 J/kg. Very weak flow across this area suggests that as convection blossoms, cells will move slowly, generally from south to north, with repeating rounds of 1-2"/hour rainfall rates leading to pockets of 3+ inches of rainfall. The Marginal Risk area was maintained. ....Northern Rockies... 16Z UPDATE: A small change was made to the Marginal Risk area to extend westward through central Idaho, to better match the existing Flood Watch area. Although probabilities for 1 inch per hour rain rates and FFG exceedance are generally higher in Montana, there is some potential for localized heavy rainfall extending into portions of central Idaho as well. PREVIOUS DISCUSSION: During this period a shortwave trough will emerge from the Pacific Northwest and track into the Northern Rockies. Impressive ascent, primarily through height falls and increasing mid-level divergence, helping to spawn a wave of low pressure tracking into the High Plains by Tuesday morning. Showers and thunderstorms are expected to increase in coverage as a response, and with easterly low-level flow feeding in the higher PW values, rainfall rates of 1 inch/hour or greater will be possible. Lamers/Campbell Day 2 Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN THE SOUTHEAST... ....Southeast... A heavy rainfall and flash flood event may be ongoing at the start of the period, somewhere from northeast Georgia, into Upstate South Carolina, and potentially far southwest North Carolina. The overall setup seems conducive, with a low-level convergence zone and the right entrance region of a strong upper level jet streak superimposed over the same area, along with anomalously high moisture and PWs over the 95th percentile. The main question for this initial round of rain is how much instability will be present. Some models show practically zero CAPE, while others show some elevated instability present. This will ultimately make the difference between a steady rain and heavier banding that could lead to flash flooding. The inherited Slight Risk area covers this threat, and the Day 1 outlook was adjusted to match, as the event may begin between 06Z and 12Z. Further development is then expected in a broad warm sector from the central Gulf Coast to south-central Georgia, and the rest of South Carolina, with stronger instability and PWs in excess of 2 inches. Hi-res models indicate fairly high probabilities of 2 inch per hour rain rates, and a complex pattern of cell mergers and colliding outflow boundaries and sea breezes. This may lead to periods of 1-2 hours where convection is maximized in particular areas, and scattered instances of flash flooding. The Slight Risk was expanded further south through much of South Carolina to better account for the 12Z HREF probabilities in those portions of the state. ....Northern Plains and Minnesota... There continues to be a fairly strong signal for thunderstorms to initiate in the northern extent of an instability plume in the Plains, near the nose of a moisture transport maximum, in the afternoon on Tuesday. Consensus among the hi-res models is this will rapidly organize into an MCS and then likely propagate quickly to the southeast in the general direction suggested by the thickness contours. The potential for relatively fast forward motions may mitigate some of the flash flood risk. However, precipitation anomalies over the past 14 days indicate the pattern has been relatively wet of late in the Northern Plains, and precipitable water values will be anomalously high. Deep moisture combined with strong instability should support high instantaneous rain rates. Therefore, any areas where the duration of heavy rain could be lengthened (such as backbuilding along the periphery of a cold pool, cell mergers with antecedent convective development) may lead to flash flooding. The Marginal Risk was maintained and only slightly adjusted to account for the latest guidance. ....Ohio River Valley... A Marginal Risk was introduced for a potential repeat of the pattern from today (Monday), just displaced slightly to the east over S IN, C KY, SW OH. A slow moving mid-level wave would continue to slowly shift east, in the presence of relatively strong instability. Therefore, scattered afternoon thunderstorms are expected. Although moisture levels are not expected to be anomalously high, storm motions should be very slow with mean winds in the lowest 8km around 5 knots or so. This could lead to localized flash flooding by extending the duration of heavy rain with the most intense storms. Rain rates should reach the 1-2 inch per hour range. Lamers Day 3 Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025 ....THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS THE UPPER MISSISSIPPI VALLEY AND FOR THE COASTAL SOUTHEAST AND MID- ATLANTIC REGION... ....Upper Mississippi Valley... A region of anomalously high moisture, with PWs in excess of the 90th percentile, should shift east during the Day 3 period, and generally continue to be superimposed with moderate to strong instability on the periphery of a ridge. Confidence in the convective details are low at this time, but the overall environment will remain supportive of organized convection with high instantaneous rain rates. The pattern may still lead to a forward-propagating MCS, which would mitigate flash flood risk overall, but the overall environment is favorable enough to maintain a Marginal Risk across the region. ....Coastal Southeast and Mid-Atlantic... There is relatively low confidence in the placement of the heaviest rainfall on the Day 3 period, with the ECMWF-based models and ensembles showing higher QPF concentrated in Virginia and North Carolina, whereas the GFS and CMC-based models and ensembles are focused further south in SC, GA, AL, and FL. For now, we have opted not to show any particular preference, with a relatively broad Marginal Risk expanded down to the Gulf Coast in portions of North Florida. Regardless of the precise details, it does seem like a plume of anomalously high precipitable water values will remain present across the coastal Southeast and Mid-Atlantic region into the Day 3 period, and the flash flood threat areas will likely come down to mesoscale details that may only come into focus in subsequent outlook periods. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5J1eieZnupAL6SPYS3Rfxm-UYJz1dG4GGmxhK82fT5Px= MYi2qXbCB_hV0FLTRlZu1USaITLDlAuINq93WOgN3rPxkaQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5J1eieZnupAL6SPYS3Rfxm-UYJz1dG4GGmxhK82fT5Px= MYi2qXbCB_hV0FLTRlZu1USaITLDlAuINq93WOgN7zQWLyY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5J1eieZnupAL6SPYS3Rfxm-UYJz1dG4GGmxhK82fT5Px= MYi2qXbCB_hV0FLTRlZu1USaITLDlAuINq93WOgNGFyqOhw$=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) .