Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 30 2025 08:47:29 FOUS30 KWBC 300845 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 445 AM EDT Wed Jul 30 2025 Day 1 Valid 12Z Wed Jul 30 2025 - 12Z Thu Jul 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND FOR PORTIONS OF THE FRONT RANGE AND ADJACENT HIGH PLAINS... ....Mid MS Valley... An eastward moving squall line will be ongoing at 12z this morning across portions of eastern IA into northern MO,and given the=20 downstream instability axis do expect this to generally survive=20 across much of central and northern IL into early this=20 afternoon. The progressive nature of this convection should limit=20 the extent and magnitude of flash flooding, however hourly rainfall of 1-2" will remain likely. This will be enough to drive at least=20 an isolated flash flood risk as convection moves eastward. This=20 MCS/MCV will help drive a cold front south across MO and KS, and do expect convection to expand over these areas by afternoon. A bit=20 unclear whether it takes the form of a progressive squall line=20 (limiting the flash flood risk), or if more discrete cell formation allows for some training/cell merging.=20 The overall environment...a slowly southward shifting cold front=20 helping focus convergence, the right entrance region of the upper=20 jet adding some divergence aloft, plentiful CAPE near the front and PWs locally over 2"...certainly supports areas of excessive=20 rainfall. At a minimum, isolated instances of flash flooding can=20 be expected across portions of KS/MO/IA/IL/IN. The magnitude and coverage of this threat will come down to convective mode and evolution today/tonight. As mentioned above, convective mode generally favoring more progressive squall lines may end up=20 keeping the risk isolated. However if we are able to get any=20 recovery after this initial round of convection the MCV and=20 favorable synoptic and moisture ingredients will remain in place.=20 Thus, higher end impacts are still possible in this setup, but far=20 from guaranteed given the questions that remain regarding=20 convective mode/evolution and instability persistence.=20 ....Southwest and High Plains... Isolated to scattered convection will likely develop near areas of terrain from NM into the Rockies. Easterly low level flow into the Front Range of CO will likely result in slow cell motions and the=20 possibility of at least isolated flash flooding. Increased convergence near the front could support a bit more convective coverage and eventual organization from portions of eastern NM into the OK and TX Panhandles as well. Only change to the inherited Slight=20 risk was to expand it northward into WY where both the HREF and=20 REFS show an uptick in 1" per hour probabilities by this evening as convective coverage increases and a few cell mergers become=20 likely. ....Elsewhere... A broad Marginal risk extends from the central Gulf Coast all the way into portions of southern NY. High PWs and CAPE and convective coverage will drive the risk from the Gulf Coast into the TN=20 valley. A lack of organization may keep FFG exceedance coverage=20 down, but high rainfall rates will likely locally approach or=20 exceed FFG. A cold front will drive the risk form portions of WV into southern NY. Convective coverage and organization should be lacking...however with the front slowing cells should be slow moving and could drop a quick 1-2" of rain. Likely enough to drive at least a localized urban flash flood risk. Chenard Day 2 Valid 12Z Thu Jul 31 2025 - 12Z Fri Aug 01 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-ATLANTIC INTO SOUTHERN NEW ENGLAND... ....Mid-Atlantic into New England... Heavy rainfall and at least some flash flooding appears likely Thursday across portions of the Mid-atlantic and possibly into southern New England. Convection is likely to focus along a slow moving cold front, with broad mid level troughing and height falls providing large scale ascent. The upper level jet is displaced pretty far north, however still likely to get some enhanced upper level divergence into the Mid-Atlantic from this feature. Still quite a bit of uncertainty with the details of this=20 convective risk. Some of this uncertainty is tied to the evolution of the shortwave/MCV associated with the ongoing convection over IA/IL this morning. The strength of that feature and how it interacts with the front and developing longwave troughing will likely impact the frontal timing and both the axis and magnitude=20 of the flash flood risk. The front should become more progressive=20 by Thursday evening, but how things evolve before then are=20 unclear. There is a concern that we could see cell development=20 Thursday afternoon just ahead of the front as southerly flow=20 increases which could set the stage for a fair amount of cell=20 merger activity driving up rainfall totals. Also it looks like an=20 area of low pressure tries to spin up along the front Thursday=20 night with an inverted trough axis extending northward. The ECMWF=20 and 3km NAM keep this troughing inland, which could be a pretty=20 ideal setup for low topped warm rain convection through the=20 overnight. However, most other models push this troughing offshore=20 and clear things out by the overnight.=20 The environment (slow moving front, increasing large scale=20 forcing, PWs over 2" and plentiful instability) supports excessive rainfall...its just going to come down to location and duration, both of which remain uncertain. The current consensus among the=20 models still supports a corridor from DC to PHL to NYC (including=20 much of eastern PA and NJ) as the areas most likely to be=20 impacted. The 00z experimental REFS ensemble neighborhood=20 probabilities of exceeding 5" are 40-70% across this corridor. The=20 REFS can run high with its QPF output, and may be under=20 dispersive...but nonetheless still a signal worth watching. We gave some consideration to a MDT risk upgrade this cycle, but after=20 collaboration with local WFOs we opted to maintain a Slight risk=20 given some of the lingering questions on how the details of the=20 event will evolve. As the event comes more into range of the HREF=20 today and tonight we will hopefully gain more confidence on these=20 details, and a MDT risk upgrade is still a possibility with later=20 updates. For now this is still considered a higher end Slight risk=20 from DC to eastern PA, NJ and NYC, and locally significant impacts=20 could evolve. ....Gulf Coast into OH Valley... Another day of slow moving pulse thunderstorms is expected over a large swath of the region to the south of the cold front. Similar=20 to Wednesday, high PWs and CAPE and convective coverage will drive=20 a localized flash flood risk. A lack of organization may keep FFG=20 exceedance coverage down, but high rainfall rates will likely=20 locally approach or exceed FFG. ....Rockies into the Plains... Scattered convection is likely from eastern AZ into NM northward all the way into MT. Currently not seeing a strong indication of organization that would lead to Slight risk level flash flood coverage. However localized flash flooding will again be possible over this broad region. There are some height falls beginning to approach from the west, so that could perhaps end up driving a more organized risk somewhere within this broad Marginal risk. But at the moment not seeing enough of a signal in the global models=20 or CAMs to go with any Slight risk areas. Chenard Day 3 Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS... ....Southeast and Mid-Atlantic... A Slight risk was maintained across much of the carolinas. Likely to have a well defined cold front dropping south over NC that will help trigger a more organized corridor of convection Friday=20 afternoon. Meanwhile both terrain driven and coastal convergence=20 driven convection are also likely by afternoon. This combination=20 will result in good convective coverage, which will ultimately=20 likely lead to a decent degree of cell merger activity. PWs are=20 forecast as high as 2.3"-2.5", with plentiful instability. Very=20 heavy rainfall rates are expected in this thermodynamic=20 environment, which combined with the likelihood of cell mergers,=20 should drive an isolated to scattered flash flood risk. There is still some degree of uncertainty with the details by=20 Friday. The 00z ECMWF is an outlier really hanging the front and=20 better upper level support well to the north. Thus it keeps a heavy rainfall risk over NJ into NYC and less of a concentrated risk over the Carolinas. This is not supported by the GFS/UKMET/CMC...and=20 the 00z ECMWF AIFS ensemble also looks more in line with the=20 GFS/GEFS. Thus we will continue to lean the EROs to be more in line with the non ECMWF solution. However we will need to continue to=20 monitor trends going forward. ....Lower MS Valley and Vicinity... A cold front should make it all the way into portions of TX/LA and MS by Friday afternoon. This should act as a focus for convective development, and with a large pool of PWs over 2", heavy rainfall=20 rates are likely. Large scale forcing and moisture transport is=20 weaker here, so not expecting widespread heavy rainfall...but=20 localized heavy rainfall rates should be able to drive an isolated=20 flash flood risk. ....Rockies and Plains... Convection is likely from NM into MT Friday. The best environment=20 for heavy rainfall will probably be across the northern end of=20 this risk area, centered across MT and vicinity. More in the way=20 of mid level troughing will be over this corridor, to go along=20 with the highest PW anomalies in the CONUS...near early August=20 peak values. Combine this with CAPE forecast around 2000 j/kg and=20 the environment appears conducive to areas of flash flooding.=20 However none of the 00z global models are all that aggressive with QPF output, and the one CAM that goes this far (the 00z RRFS)=20 suggests a more progressive convective mode. Thus we will stick=20 with a Marginal risk and continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!82ksXFNoYSnoxRc80G53rd19UBNHUucfkPSnukOSd0oT= 8wzbgEfpTdWvJw04p2-ABVllCnj3X-8-xf-BOTbAQpWKkqI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!82ksXFNoYSnoxRc80G53rd19UBNHUucfkPSnukOSd0oT= 8wzbgEfpTdWvJw04p2-ABVllCnj3X-8-xf-BOTbAKdM4KrI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!82ksXFNoYSnoxRc80G53rd19UBNHUucfkPSnukOSd0oT= 8wzbgEfpTdWvJw04p2-ABVllCnj3X-8-xf-BOTbAubpPyVc$=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) .