Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 30 2025 00:34:15 FOUS30 KWBC 300033 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 833 PM EDT Tue Jul 29 2025 Day 1 Valid 01Z Wed Jul 30 2025 - 12Z Wed Jul 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND NORTHERN PLAINS AND FOR PORTIONS OF THE SOUTHWEST... Overall, the risk areas strongly resemble continuity. Some carving out of the risk areas has been made to account for places where the threat appears to be diminished where convection has moved along. ....Rockies eastward into the Midwest... An instability gradient exists from the SD/NE border into central IA which is supporting forward propagating convection across SD, which per the 18z HREF is a few hours ahead of schedule as of 00z. Post-frontal upslope flow along with sufficient instability led to afternoon storm development in a couple sections of the Rockies.=20 Guidance continues to suggest upscale development into a couple=20 MCS's. ML CAPE values of 6000+ J/kg exists from NE into IA. PWATs=20 are 1.5-2" in the Midwest and 0.75" in the High Plains, or 1.5 to=20 2 standard deviations above the late July mean. This combination=20 supports intense thunderstorms capable of heavy rainfall with=20 hourly rain amounts up to 2.5-3" per hour is possible.=20 Instantaneous rain rates of 5-7" an hour wouldn't be surprise near=20 the area of strongest instability. Local totals in the 3-6" range=20 are possible overall through Wednesday 12x. This continues to=20 support higher- end Slight Risk probabilities for areas in and near IA. ....Southwest... Thunderstorms are inbound into southern NM from south of the border. This activity supports the advertised flash flood risk for the area this evening into tonight. Hourly rain amounts of 1-2" are possible. The biggest concern is near area burn scars. ....Southeast... Activity persists across the Appalachians and Southeast, which should persist for a few more hours. Other activity is possible in the morning hours near southernmost NC. Roth/Putnam/Chenard Day 2 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... ....2030Z Update... Confidence remains low in the exact convective evolution across the Missouri to the Middle Mississippi Valley and adjacent Great Lakes region on Wednesday. Now available hi-res guidance follows prior thinking that an ongoing MCS from the day 1 period will continue eastward across the Slight Risk area, most likely from central/eastern Iowa into northern Illinois. Sources of additional convective development Wednesday afternoon are apparent south and west of the initial MCS across central Illinois and westward through the Missouri Valley along any outflow boundary(s), the slow moving cold front, and supported by the passage of an upper- level short-wave. Depending on how/where available higher instability and PWATs are present in vicinity of these features away from convective overturning, these features would support a continued threat of thunderstorms producing heavy downpours (1-2"+/hr rates) and heavier rainfall totals (2-3"+). This is especially true where any storms repeat over any prior rainfall and/or train parallel to any outflow boundaries or the slow moving cold front. A corridor of higher probabilities may be required given the signal for heavy downpours and the potential for training and/or multiple rounds of convection. However, given the noted uncertainties with the location of ongoing and renewed convection that would determine a more significant threat, a broad Slight Risk remains sufficient. For the central/southern High Plains and adjacent Plains, the prior forecast remains on track based on now available hi-res guidance. The one adjustment made for this issuance was to extend the Slight Risk further south/southwest across eastern New Mexico to the Sacramento Mountain vicinity given a notable signal for locally heavier rainfall (1-3") and ongoing Burn Scar sensitivities. Putnam ....Previous Discussion... ....Mid MS Valley... At 12z Wednesday it seems likely that we will have some degree of organized convection ongoing in the vicinity of IA. It appears probable that a better defined shortwave/MCV will be associated with this convection and slowly move east across IA/MO/IL and vicinity Wednesday into Wednesday night. Meanwhile we will have a slowly southward shifting cold front helping focus convergence, and the right entrance region of the upper jet adding some divergence aloft. With plentiful CAPE near the front and PWs locally over 2", the ingredients for areas of excessive rainfall are in place. There is actually quite a bit of uncertainty with the convective details Wednesday into Wednesday night. There has been a subtle northward shift in the forecast MCV location, seemingly now tracking across IA into northern IL. However the northern extent of the excessive rainfall risk will likely be capped by a lack of instability well north of the cold front. The 00z ECENS and AIFS ensembles are focusing the best rainfall potential from eastern IA into northern IL. This seems reasonable, although the magnitudes remain in question and are dependent on the degree of instability in place. The better instability may actually reside over portions of central MO into central IL, and so if convection is able to organize this far south then a greater flash flood risk could evolve. All this to say that confidence is low, and thus a broad Slight risk remains to encompass the various possible scenarios. Higher end impacts are possible in this setup, but far from guaranteed given the questions that remain regarding the MCV track/intensity and instability in place. ....Southwest and High Plains... Isolated to scattered convection will likely develop near areas of terrain from NM into the Rockies. Day shift introduced a Slight risk for this activity, and that still seems reasonable. Easterly low level flow into the Front Range of CO will likely result in slow cell motions and the possibility of at least isolated flash flooding. Guidance is coming into better agreement that some upscale growth and eastward propagation into northeast NM, southeast CO and the OK Panhandle is probable as well, focusing near the cold front. ....Elsewhere... A broad Marginal risk extends from the central Gulf Coast all the way into portions of New England. High PWs and CAPE and convective coverage will drive the risk from the Gulf Coast into the TN valley. A lack of organization may keep FFG exceedance coverage down, but high rainfall rates will likely locally approach or exceed FFG. A cold front will drive the risk form portions of the OH valley into New England. Generally quick cell motions and the lack of a significant QPF footprint in the 00z models keep this risk at a Marginal level. Chenard Day 3 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... ....2030Z Update... Updated deterministic and ensemble guidance has come into somewhat better agreement on the noted focused corridor of QPF aligning with the slow moving cold front stretching from the Mid-Atlantic northeastward into southern New England, particularly with the northward extent of the threat. The 12Z GFS/GEFS mean as well as the latest ECMWF AIFS guidance still support a bit more southerly corridor, roughly from DC to Chesapeake Bay and vicinity northeastward along I-95 through PHL and to NYC, though higher QPF values extend from the NYC vicinity into southern New England. The 00Z ECens mean and especially 12Z ECMWF favor a corridor shifted a bit more north/west of I-95, with particular uncertainty with southern extent into the greater DC area, though still support higher QPF extending into southern New England. Opted to broaden the Slight Risk a bit further northward across New York/New England with a particular concern for higher- end Slight Risk impacts extending further north through the NYC region and into southern New England given better agreement with northward extent along the I-95 corridor between the GFS/GEFS and ECWMF- AIFS. The southern extent of the Slight Risk, as well as concern for higher- end Slight impacts into the greater DC area, remains despite the lack of support form the ECMWF/ECens means given the noted uncertainty discussed with respect to Synoptic scale details of the system evolution. Putnam ....Previous Discussion... ....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. There is quite a bit of model spread with the details of this convective risk. The driver of most of this spread appears to be a shortwave/MCV that intensifies associated with the NE/IA convection Tuesday night into Wednesday. This feature then tracks eastward and enhances lift over the Mid-Atlantic and Northeast. Some models have a stronger more well defined feature, which then helps keep the front a bit farther north resulting in a farther north QPF axis. At the moment it does seem like the 00z NAM and 12z ECMWF are a bit too amplified with this feature...thus find it unlikely that the heaviest QPF swath will be as far north as they depict. The 00z ECMWF came south a bit, but probably still too far north. Both the 00z ECMWF AIFS and 18z NOAA EAGLE machine learning ensembles show highest probabilities closer to the I95 corridor, generally from DC to PHL to NYC and into southern New England. This also seems to fit the expected placement of higher instability/PW overlap. Also will note that the 00z RRFS has a very similar axis as well. Thus the Slight risk will remain positioned across this corridor, although confidence on this exact placement remains average at best given some of the run to run differences we are seeing. Wherever the event does focus there is certainly a potential for higher end impacts. PWs near the slow moving front will be over 2" over a broad area, with embedded values likely getting towards 2.3". We should also have plenty of instability to work with, which combined with the high PWs, should drive heavy rainfall rates. Combine that with some training potential near the slow moving front, and can not rule out the possibility of eventually needing a Moderate risk...especially if the more sensitive Urban corridor remains a focus going forward. ....Gulf Coast into OH Valley... Another day of slow moving pulse thunderstorms is expected over a large swath of the region. Similar to Wednesday, high PWs and CAPE and convective coverage will drive a localized flash flood risk. A lack of organization may keep FFG exceedance coverage down, but high rainfall rates will likely 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 enough confidence to go with any Slight risk areas for this region. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4zCa5i8V42woXKatNYTPPUDttFjIszZCsdwTucKZeV2V= 9Jajow2Na0gihMofC9-ALZ6VQqJyzcrSZuFN4z-yZ0FhTlg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4zCa5i8V42woXKatNYTPPUDttFjIszZCsdwTucKZeV2V= 9Jajow2Na0gihMofC9-ALZ6VQqJyzcrSZuFN4z-yC1Pnp9o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4zCa5i8V42woXKatNYTPPUDttFjIszZCsdwTucKZeV2V= 9Jajow2Na0gihMofC9-ALZ6VQqJyzcrSZuFN4z-yIUqeBPw$=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) .