Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 29 2025 08:16:21 FOUS30 KWBC 290814 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Tue Jul 29 2025 Day 1 Valid 12Z Tue Jul 29 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... ....Central and Northern Plains... A stationary front will likely extend across southern IA/NE and=20 into the High Plains on Tuesday afternoon/evening. Easterly flow to the north of this front will likely help trigger convective=20 development over the High Plains as shortwave energy moves out of=20 the Rockies and upper level divergence increases in the right=20 entrance region of an upper jet. This activity will likely push=20 east into the overnight hours across NE and eventually IA=20 along/north of the stationary front. Upscale development into an=20 MCS appears probable driving a heavy rainfall threat. Any eventual=20 MCS will probably propagate off to the east fairly quickly, however additional development downstream over NE/IA is possible given the convergence near the front and what should be CAPE around 4000=20 j/kg. PWs should be plenty high enough for heavy rainfall, generally 1.5 to 2 SD above late July normals.=20 There remains some latitudinal uncertainty with the axis of convection during this period. The trend amongst the 00z high res guidance is for two convective clusters...one moving across southern SD, and another from NE into IA. The Slight risk was made broader to account for this potential. Still think the greatest rainfall magnitudes are more likely to occur with the southern=20 swath across NE into IA, where instability and surface convergence will be higher. EAS probabilities from both the HREF and REFS=20 indicate the best coverage of 2"+ rainfall to focus over portions=20 of eastern NE into western IA. Both of these ensemble suites also=20 indicate a 30-50% chance of exceeding the 3hr FFG across this same=20 corridor. Thus higher end Slight risk probabilities exist from=20 eastern NE into western IA where some training of convection may=20 result in a greater flash flood risk Tuesday evening/night. ....Southwest... Scattered to numerous areas of thunderstorms have persisted Monday evening/night across central and southern NM. A well defined=20 shortwave/MCV will likely persist into the day today. While this=20 feature should act as a forcing mechanism today, abundant cloud=20 cover associated with this feature may help reduce convective=20 coverage/intensity.=20 However, confidence remains higher on a flash flood risk today=20 over the Sacramento Mountains and vicinity. Probabilities from both the 00z HREF and REFS show more persistence of high 1"/hr=20 exceedance today...with elevated probabilities at least from=20 19z-22z. There is some hope that enough cloud cover persists that=20 we are not able to destabilize enough for maintained deep=20 convection. However most indications suggest we do see a gradual=20 destabilization take place by late morning into the early afternoon over southeast NM. If this occurs then flash flooding near the=20 Sacramento Mountains appears likely...and significant impacts are=20 possible near area burn scars. Confidence is lower over the rest of NM. The 00z CAMS really struggle to build instability back after the widespread convection of Monday night. Thus the coverage and intensity of convection is muted compared to Monday. Since day shift just introduced the=20 Slight risk we will leave it for this cycle given the uncertainty. It very well may not verify outside of the aforementioned=20 Sacramento Mountain region...however with the remnant MCV and PWs=20 similar or slightly higher than Monday, if we are able to=20 destabilize then Slight risk level coverage of flash flooding could still materialize.=20 ....Southeast... Expecting greater convective coverage today from the Southeast=20 into the southern Appalachians. Not much in the way of forcing to=20 organize convection, and generally expecting pulse activity that=20 does not last all that long at any one location. This will act as a limiting factor for flash flooding...however there should be=20 enough convective coverage and chaotic cell motions (weak mean winds) that some cell mergers along outflows will be a=20 possibility. PWs in the region are forecast between 2" and 2.4",=20 with CAPE over 2000 j/kg...so the thermodynamic environment will=20 support heavy rainfall rates. Thus even with a relatively short=20 duration of rain at any one location, rainfall totals locally over=20 3" appear probable driving a localized flash flood risk. Both the HREF and REFS have 3"+ neighborhood probabilities in the 40-70%=20 range over most of the Marginal risk area, however the spatial=20 coverage of these amounts is quite small. Thus while localized=20 areas of flash flooding are likely today, not currently seeing=20 enough coverage to justify a Slight risk at this point.=20 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... ....Mid MS Valley... At 12z Wednesday it seems likely that we will have some degree of=20 organized convection ongoing in the vicinity of IA. It appears=20 probable that a better defined shortwave/MCV will be associated=20 with this convection and slowly move east across IA/MO/IL and=20 vicinity Wednesday into Wednesday night. Meanwhile we will have a=20 slowly southward shifting cold front helping focus convergence, and the right entrance region of the upper jet adding some divergence=20 aloft. With plentiful CAPE near the front and PWs locally over 2",=20 the ingredients for areas of excessive rainfall are in place. There is actually quite a bit of uncertainty with the convective=20 details Wednesday into Wednesday night. There has been a subtle=20 northward shift in the forecast MCV location, seemingly now=20 tracking across IA into northern IL. However the northern extent=20 of the excessive rainfall risk will likely be capped by a lack of=20 instability well north of the cold front. The 00z ECENS and AIFS=20 ensembles are focusing the best rainfall potential from eastern IA into northern IL. This seems reasonable, although the magnitudes=20 remain in question and are dependent on the degree of instability=20 in place. The better instability may actually reside over portions=20 of central MO into central IL, and so if convection is able to=20 organize this far south then a greater flash flood risk could=20 evolve. All this to say that confidence is low, and thus a broad=20 Slight risk remains to encompass the various possible scenarios.=20 Higher end impacts are possible in this setup, but far from=20 guaranteed given the questions that remain regarding the MCV=20 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=20 low level flow into the Front Range of CO will likely result in=20 slow cell motions and the possibility of at least isolated flash=20 flooding. Guidance is coming into better agreement that some=20 upscale growth and eastward propagation into northeast NM,=20 southeast CO and the OK Panhandle is probable as well, focusing=20 near the cold front. ....Elsewhere... A broad Marginal risk extends from the central Gulf Coast all the=20 way into portions of New England. High PWs and CAPE and convective=20 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 the=20 OH valley into New England. Generally quick cell motions and the=20 lack of a significant QPF footprint in the 00z models keep this=20 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... ....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.=20 There is quite a bit of model spread with the details of this=20 convective risk. The driver of most of this spread appears to be a shortwave/MCV that intensifies associated with the NE/IA=20 convection Tuesday night into Wednesday. This feature then tracks=20 eastward and enhances lift over the Mid-Atlantic and Northeast.=20 Some models have a stronger more well defined feature, which then=20 helps keep the front a bit farther north resulting in a farther=20 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=20 unlikely that the heaviest QPF swath will be as far north as they=20 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,=20 generally from DC to PHL to NYC and into southern New England. This also seems to fit the expected placement of higher instability/PW=20 overlap. Also will note that the 00z RRFS has a very similar axis=20 as well. Thus the Slight risk will remain positioned across this=20 corridor, although confidence on this exact placement remains=20 average at best given some of the run to run differences we are=20 seeing. Wherever the event does focus there is certainly a potential for=20 higher end impacts. PWs near the slow moving front will be over 2" over a broad area, with embedded values likely getting towards=20 2.3". We should also have plenty of instability to work with, which combined with the high PWs, should drive heavy rainfall rates.=20 Combine that with some training potential near the slow moving=20 front, and can not rule out the possibility of eventually needing a Moderate risk...especially if the more sensitive Urban corridor=20 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=20 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=20 more organized risk somewhere within this broad Marginal risk. But at the moment not enough confidence to go with any Slight risk=20 areas for this region. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!54mVDCmK-im9M9uA42FtWSysqevb2s-sSeLY1SclsLGj= O70_x5TTcXznX7-PBa7Q6NuLfeK8VasMHFtLSUB2aovvEHc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!54mVDCmK-im9M9uA42FtWSysqevb2s-sSeLY1SclsLGj= O70_x5TTcXznX7-PBa7Q6NuLfeK8VasMHFtLSUB2Q1pidks$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!54mVDCmK-im9M9uA42FtWSysqevb2s-sSeLY1SclsLGj= O70_x5TTcXznX7-PBa7Q6NuLfeK8VasMHFtLSUB2iJq6gFs$=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) .