Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 10 2025 20:16:23 FOUS30 KWBC 102016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Sun Aug 10 2025 Day 1 Valid 16Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF KANSAS, MISSOURI, AND NORTHERN OKLAHOMA... 16Z Update... The meteorological synopsis provided by the overnight shift remains in sound shape with updates this cycle based off changes in the 12Z guidance suite. Most guidance has shown the heaviest rainfall axis is shifting farther south and west with HREF/REFS neighborhood probabilities for >3" of rainfall around 40% in southwest KS and far northern OK. The expectation remains for a blossoming area of thunderstorms activity over the central Plains as the LLJ strengthens tonight and favorably orients itself along a frontal boundary draped over the region. 12Z HREF and 00Z REFS guidance continue to highlight southeast KS as featuring the highest odds for rainfall totals >5" (30-45% 40km neighborhood probabilities on new 12Z HREF) through tonight. Farther west, the Slight Risk was expanded as far west as the Palmer Divide with new 12Z HREF PMM guidance showing some isolated max amounts approaching 2" in some cases. Only minor tweaks to the risk areas were made in the Southeast and Southwest based on a blend of new incoming 12Z guidance, which largely shows the risk areas remain on track. Mullinax ---Previous Discussion--- ....Central Plains through the Upper Midwest... The synoptic pattern today continues to be indicative of a Maddox Frontal rainfall event, although confidence in the exact evolution has decreased and significant changes are needed to the inherited ERO. Some of these changes are due to exceptional model uncertainty at this time range, but also accounting for what occurred Saturday when the event did not really pan out as expected. Synoptically, the setup still remains extremely favorable for an axis of very heavy rainfall. An anomalous mid-level trough (500mb and 200mb heights locally below the 10th percentile according to NAEFS) will remain aligned from ND/MN southwest through the Four Corners. Although the leading impulse embedded in the core of this trough will advect east across southern Canada, spokes of vorticity and associated shortwaves rotating through the flow to the south will force the trough axis to remain to the west. Downstream of this trough axis, a persistent upper jet streak will arc poleward into Ontario, placing at least modest but persistent upper level diffluence within its RRQ. Beneath this evolving setup, a surface front will oscillate at least subtly NW to SE as a stationary front, providing additional focus for ascent beneath the weak height falls and periods of PVA. Additionally, any stronger impulse interacting with the surface front could result in waves of low pressure rippling along to the northeast. East of this front, low-level return flow will continue to advect from an open Gulf, drawing elevated thermodynamics and theta-e northward. PWs are progged to climb to above 2 inches as the 850mb LLJ surges to 20-30 kts, overlapping intense MUCAPE of more than 3000 J/kg to provide an extremely favorable environment for heavy rain, which is additionally supported by specific humidity values between 925-850mb above the 97th percentile (NAEFS). At the same time, the theta-e ridge will blossom and align into the front, with an intense gradient suggesting heavy rainfall on its NW axis as is typical with the Maddox Frontal conceptual model. In this setup, with persistent ascent acting upon impressive thermodynamics, and mean 0-6km winds aligned to the front, it is likely streaks of heavy rainfall will occur, and most deterministic models showing 1-4 inches of rainfall through 12Z Monday, with high-res CAMs indicating locally 5-7 inches, so heavy rainfall is almost a certainty. However, uncertainty is exceptional (especially for a 24-hr forecast) with the placement of this heavy rain, and the spread in placement ranges from southwest WI to southern KS, and almost anywhere in between, leading to a challenging ERO forecast. While the front will be the primary focus for convection, along with a wave of low pressure expected to develop along it, and the LLJ is likely to be parallel to it, there is increasing concern that convective clusters (both overnight Saturday night into Sunday, and additionally on Sunday) will leave outflow boundaries aligned more west-to-east outrunning the front. This could drastically change the position of the heaviest rain axes, and would support heavy rainfall south of the global members in a typical bias. If these clusters do not develop, convection and heavy rainfall would likely remain more aligned SW to NE, but have noted a trend in the Corfidi vectors to be more pointed S/SE after SW early, which would additionally support more W-E heavy rainfall. With all the uncertainty in placement, the ERO was modified heavily and tailored primarily to HREF/REFS neighborhood probabilities, but with additional changes influenced by the EAS probabilities, the CSU first guess fields, and coordination with the impacted WFOs. ....Southeast... Another active day of convection is likely from the coastal Carolinas through Florida. Broad ridging extending from a Bermuda-type but elongated high pressure over the Atlantic will be modified by an inverted trough aligned from the eastern Gulf to the NC coast. Onshore and confluent flow into this inverted trough will maintain impressive moist advection into the area, manifesting as PWs above 2.25 inches, well above the 90th percentile according to the SPC sounding climatology. This will overlap with MUCAPE of 500-1000 J/kg, modest overall instability indicative of a tropical airmass, to produce impressive thermodynamics to support heavy rainfall. Additionally, this robust thermodynamic advection onshore will result from 850mb winds as high as 15 kts, which, while modest, will exceed the mean 0-6km winds which are progged to be just around 5 kts. This suggests widespread convergent ascent across the Southeast, with the accompanying WAA surging warm cloud depths to around 15,000 ft, supporting efficient warm rain processes within a deeply saturated column noted in regional forecast soundings. The CAMs are in generally good agreement in widespread convection today. Scattered slow-moving thunderstorms with rainfall rates of 1- 3"/hr are expected across much of FL and into GA/AL. However, the focus for the heaviest rainfall is likely along the coastal plain of GA through southeast NC. Here, propagation vectors are aligned against the weak mean wind, and with subtle mid-level impulses lifting northwest within the weakly forced environment, waves of repeating rounds of convection with heavy rainfall are likely. In this region, the HREF indicates a 20-30% chance of 2"/hr rain rates, with locally more than 3"/hr possible at times, at least briefly. With backbuilding offshore leading to regenerating and training rounds of heavy rain, total rainfall could exceed 5" (30-50% chance from the REFS/HREF), with the greatest potential along the SC coast. The inherited SLGT risk was cosmetically adjusted, but scattered instances of flash flooding appear likely today, even across these areas that are generally more difficult to flood. ....Southwest... A MRGL risk of excessive rainfall continues for portions of the Southwest today as monsoonal moisture returns to the area leading to a more active convective pattern. A mid-level trough extending down from the Northern High Plains will deepen today, producing lowered heights across the Four Corners and Desert Southwest, through which weak spokes of vorticity will rotate. These lowered heights and locally enhanced ascent will impinge into an increasingly favorable thermodynamic environment to support convection with heavy rainfall as low-to-mid level flow backs gradually to the SW, allowing for a better draw northward of PWs exceeding 1", which may spread as far north as the Mogollon Rim and combine with MUCAPE of 500-1000 J/kg across the southern half of AZ and NM. This will support scattered to widespread showers and thunderstorms which will move slowly eastward through the day. With rainfall rates as much as 0.75"/hr to 1"/hr at times as shown by the UA WRF and HREF neighborhood probabilities , this could result in at least isolated instances of flash flooding, especially across more vulnerable soils or terrain. The MRGL risk was curtailed a bit on the eastern side across NM where CAPE is progged to be much weaker, but in general changes to the inherited risk were minor. ....Central High Plains... The inherited SLGT risk was maintained across eastern CO, primarily for renewed thunderstorm activity on Sunday evening falling atop soils that will be primed from ongoing (Saturday night) convection. The elongated stationary front across the Central Plains early today will drop steadily southward, leaving NE upslope flow in its wake. This will be most pronounced into eastern and southeastern CO, where PWs will likely surge to above 1 inch. As convection develops across the higher terrain of CO during the aftn, it should steadily progress east and then grow upscale as it moves off the terrain in response to bulk shear of 40 kts. While these clusters should move steadily eastward on mean 0-6km winds of 15-20 kts, rainfall rates of 1-2"/hr are likely which could be sufficient to overwhelm soils that will be saturated from antecedent rainfall. Additionally, some local backbuilding is possible as reflected by CAMs across southeast CO, lengthening the potential duration of heavy rainfall in a few areas. Weiss Day 2 Valid 12Z Mon Aug 11 2025 - 12Z Tue Aug 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS THROUGH THE UPPER MIDWEST, THE COASTS OF THE=20 CAROLINAS, & THE FLORIDA PANHANDLE... ....Southern Plains & MO-KS-AR-OK... The South Central U.S. will be sandwiched between two upper level disturbances; one upper trough approaching from the Southern Rockies, and mid-upper level trough axis over southern Texas. Atop the atmosphere, modest diffluent 250mb flow will support divergence aloft while a nearby surface frontal boundary acts as a trigger=20 for thunderstorms. Any residual outflow boundaries from Sunday night's convection or from Monday afternoon's convection will also likely play a role in firing off thunderstorms to the south and east of the surface front. PWATs will generally be >1.8" along the Red River and points north and east, while areas as far west as=20 the TX Panhandle and cap rock are between 1.4-1.6". In terms of=20 instability, lesser values (MLCAPE 500-1,000 J/kg) will be=20 available in the TX Panhandle, with greater over the MO-KS-AR-OK=20 region through Monday night. The 12Z HREF does show spotty areas of moderate-chance probabilities (40-60%) within the Slight Risk area for >3" of rainfall, as well as low-chance probabilities (10-30%)=20 for >5" in central OK. Given these factors, the Slight Risk was=20 focused more here compared to the previous forecast cycle. ....Middle MS Valley... The Slight Risk was scaled back here given the reduction in available instability, latest QPF reduction, and lesser footprint of >3" rainfall probabilities in the 12Z HREF. With soils growing more saturated and the lingering presence of near 2" PWATs, there is still a localized flash flood threat, hence the presence of a rather large Marginal Risk area that stretches from the I-70 corridor from KS/MO on north through the Great Lakes.=20 ....Florida Panhandle & Big Bend... In coordination with TAE, introduced a Slight Risk for areas along and south of I-10 in the FL Panhandle and Big Bend. As a mid-to- upper level inverted trough axis approaches, it will be accompanied by a wealth of anomalous PWATs that will approach 2.5" in some areas. Modeled soundings show warm cloud layers just shy of 16,000ft deep in some cases with little-to-no capping inversions Monday afternoon. On the eastern flank of the inverted trough, 925-850mb winds will be uniformly out of the SE. ECMWF soundings over the Big Bend show Corfidi Upshear vectors that are <10 knots=20 Monday afternoon, suggesting the potential for back-building and=20 training thunderstorms. Given the abundance of tropical moisture=20 and MLCAPE up to 1,000 J/kg, it is quite possible to have=20 thunderstorms generate 2" of rainfall in as little as 30 minutes.=20 The scale of these storms are making QPF forecasts across CAMs=20 guidance widely varying on location, but the atmospheric parameters support an increasingly concerning setup for flash flooding for=20 Monday. Mullinax ---Previous Discussion--- ....Southeast... Yet another active day across the Southeast is expected Monday as an inverted low-to-mid level trough drifts northeast along the coast of the Carolinas. This trough will move within a favorable thermodynamic environment characterized by still highly anomalous PWs of 2 to 2.25 inches, although the slow retreat of the mid-level ridge to the east should help ease some of this moisture the latter half of D2. Still, the overlap of these elevated PWs with MUCAPE of 1000-2000 J/kg will support convection with heavy rainfall. In general, convection across the Southeast on Monday should be scattered in coverage, but some organization, or at least better clustering, is likely in the vicinity of any shortwaves and the inverted trough, primarily from the Grand Strand of SC to the Crystal Coast of NC. Here, repeating rounds of convection with efficient rain rates exceeding 2"/hr are likely, driven by impressive low-level confluence. Elsewhere across the Southeast, heavy rainfall rates are also likely, but with less organization, but still slow storm motions. The rainfall probabilities for more than 3" peak along the SC/NC coasts and the SLGT risk was adjusted cosmetically for the new guidance. Otherwise, a broad MRGL risk continues for slow moving thunderstorms across areas that have had recent heavy rainfall, and an expansion was made into the Upstate of SC where some of the high res CAMs suggest enough upslope to enhance rainfall locally in that area. Weiss Day 3 Valid 12Z Tue Aug 12 2025 - 12Z Wed Aug 13 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST/GREAT LAKES, THE SOUTHWEST, AND GULF COAST/SOUTHEAST... 2030Z Update... There were some minor adjustments made this forecast cycle to the Marginal Risk areas based on latest 12Z guidance and WPC QPF. The Southeast will be closely monitored given the same feature supporting a Slight Risk on Day 2 in the FL Panhandle and Big Bend areas will move farther inland on Tuesday. However, there remains a fair amount of spread regarding how much rainfall occurs and which areas are favored. Given the lingering uncertainty, opted to hold on to a Marginal Risk area with no upgrade this forecast cycle.=20 Mullinax ---Previous Discussion--- ....Upper Midwest... The anomalous mid-level trough that will produce the heavy rainfall D1 and D2 across the Upper Midwest/Central Plains will weaken Tuesday as shortwaves embedded within the trough become more rapidly absorbed into westerlies across the northern tier of the CONUS. A lingering shortwave trough, likely convectively enhanced, pivoting across KS/MO will drive sufficient ascent for additional rainfall along the weakening front, but with lesser moisture and instability than was available earlier in the week. Still, at least short term training of intense rainfall rates is likely as mean layer winds align to the weakening front, which could result in isolated instances of flash flooding. ....Southeast... Elongated mid-level ridge extending westward from the Atlantic to along much of the Gulf Coast will subtly retreat Tuesday as a shortwave emerging from the Gulf lifts northward. The modest height falls and PVA accompanying this feature will drive ascent into an otherwise weakly forced environment, producing a favorable setup for scattered to numerous thunderstorms from the central Gulf Coast northeast along the periphery of the ridge into Georgia. Thunderstorms that develop will contain heavy rainfall rates of 1-2"/hr in response to favorable thermodynamics characterized by PWs of 2-2.25 inches overlapping MUCAPE as high as 2000 J/kg. The general SW return flow will push storms along steadily to the N/NE, but with repeating rounds possible and some organization beneath the shortwave, a few instances of flash flooding could result as rainfall reaches 2-3" in some areas. ....Southwest... Mid-level ridging will expand but shift subtly westward on Tuesday while weak impulses rotate around its periphery. This will help to intensify instability across the region, and progs indicate a larger plume of 1000 J/kg MUCAPE spreading across the southern portions of AZ and NM, especially during peak aftn heating. This will combine with PWs that will be above 1" in many areas south of the Mogollon Rim, leading to a favorable environment for convection. Where any of these impulses rotate aloft, enhanced ascent will yield scattered to widespread thunderstorms with rain rates approaching 1"/hr at times, and these storms will move slowly/chaotically beneath the ridge. This could result in isolated instances of flash flooding, most likely where any storms track across more vulnerable terrain features or burn scars. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6SEaZuyrmJqTVnBbPGy8ZTlf5gn3tbkz1_pA61F_TF0m= FuMgIZCf3yPyN49zvkwspwDLxcuveXEqGb5vVUMc6QEe8hg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6SEaZuyrmJqTVnBbPGy8ZTlf5gn3tbkz1_pA61F_TF0m= FuMgIZCf3yPyN49zvkwspwDLxcuveXEqGb5vVUMcjTTMIzg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6SEaZuyrmJqTVnBbPGy8ZTlf5gn3tbkz1_pA61F_TF0m= FuMgIZCf3yPyN49zvkwspwDLxcuveXEqGb5vVUMceVwvHOE$=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) .