Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 28 2025 08:25:49 FOUS30 KWBC 280825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 AM EDT Thu Aug 28 2025 Day 1 Valid 12Z Thu Aug 28 2025 - 12Z Fri Aug 29 2025 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL ROCKIES AND THE SOUTHERN PLAINS INTO THE LOWER=20 MISSISSIPPI VALLEY... ....Southern Plains into the Lower MS Valley... Convection will be ongoing at 12z this morning across portions of southeast KS, northeast OK and into central and northern AR. Some northwest to southeast training of convection is expected, and scattered instances of flash flooding will probably be ongoing and continue into the morning hours. Instability is a limiting factor, and interesting to note that the 05z HRRR has significantly less CAPE at 12z than the 00z HRRR and other 00z HREF members. This downward trend in instability during the morning hours should=20 limit the coverage and magnitude of the flash flood risk from=20 eastern OK into AR...however convection should still train enough=20 to produce isolated to scattered flash flooding, especially over=20 any more susceptible areas, with amounts of 2-4" still probable. By later this morning into the afternoon hours indications are=20 that convection should begin to forward propagate off to the south, picking up speed as it pushes into and through southern AR.=20 Instability should be on the increase by this time resulting in=20 higher rainfall rate potential, but the expected quicker cell=20 motions should still cap the flash flood risk in the isolated to=20 scattered range. Another uptick in convection is likely Thursday night into Friday=20 morning as low level convergence increases. More uncertainty with=20 the details by this time, but an area of expanding backbuilding=20 convection could develop over the ArkLaTex during this timeframe,=20 potentially driving and increased flash flood risk. Model QPF forecasts are getting to the level of MDT risk consideration. However do note that HREF probabilities of exceeding 3hr FFG do not get much above 25%, and probabilities of this=20 magnitude are not widespread. Typically we see higher HREF FFG=20 exceedance probabilities in MDT risk level events. The high FFG=20 over much of the area is likely the primary driver of these lower=20 probabilities, however the aforementioned lower instability in=20 place this morning is also likely to keep rainfall rates a bit=20 lower and thus less likely to exceed FFG on a more=20 numerous/widespread basis. Nonetheless will need to continue to=20 closely monitor model/observational trends today. At the moment,=20 the activity tonight into Friday morning over the ArkLaTex seems to have the better chance of reaching MDT risk levels, especially if=20 rainfall this afternoon is able to lower FFG ahead of what should=20 be an uptick in backbuilding/training tonight into Friday morning.=20 Not enough confidence in the details to go MDT, but still think=20 higher end Slight risk probabilities are justified across portions=20 of eastern OK into western AR, far northeast TX and far northwest=20 LA. ....Rockies and Western U.S.... A small Slight risk was maintained across portions of southeast=20 CO. With easterly upslope flow into the Front Range high res models indicate convective initiation this afternoon, with cells moving=20 eastward into the CO Plains, before diving off to the south. Some=20 initial slow cell motions and the possibility of a cell merger or=20 two will allow for areas of locally heavy rainfall. Most models=20 really only indicate one or two stronger cells persisting long=20 enough to drive a flash flood risk...so there is a question whether the coverage of flash flooding will reach Slight risk levels. But=20 given the risk is already out, and the fact that models continue=20 to show a focus for heavier rainfall amounts over this corridor, we opted to maintain the Slight risk. Elsewhere a broad Marginal risk covers much of the Intermountain West. Not expecting as much coverage of convection or as heavy of rainfall totals as we saw on Wednesday, but localized areas of heavy rainfall remain possible within the Marginal risk area. The=20 best convective focus will likely be across portions of=20 southern/central MT and northeast WY, but only modest instability=20 should keep rainfall rates down enough to keep any flash flood=20 risk isolated in nature. Chenard Day 2 Valid 12Z Fri Aug 29 2025 - 12Z Sat Aug 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN NEW MEXICO INTO THE TEXAS PANHANDLE, AND OVER PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST... ....Lower MS Valley into the Southeast... The mesoscale and synoptic features responsible for the day 1 convection over the Southern Plains and Lower MS Valley will shift southeast on Friday. More model spread is noted by this=20 time...with the 00z GFS remaining an outlier. The GFS continues to=20 be more suppressed and weaker with this feature than the other=20 global model guidance. The ECMWF, CMC, UKMET, ICON and NAM are all=20 farther northeast with the heavy rainfall axis stretching into=20 portions of MS and AL than the GFS is. The AIFS and both the ECMWF and GFS initialized Graphcast are also farther northeast than the=20 GFS. Thus, it seems more likely than not that the GFS is too=20 suppressed and it represents a less likely outcome at the moment.=20 The ECMWF has been the most aggressive with a farther northeast=20 axis, but do note that is has been gradually trending southwest the last few runs...and so the ECMWF may be a bit too far northeast. Given the above model diag, it seems prudent to go ahead with a Slight risk across portions of the region. The environmental=20 ingredients will be conducive to heavy rainfall, with PWs over 2"=20 and a corridor of persistent lower level moisture=20 transport/convergence supporting some backbuilding and/or training potential. Other than the weaker GFS, models show an area of low=20 pressure only slowly moving eastward along a low level frontal=20 axis...with upwards of 2000 j/kg of CAPE near and south of this=20 boundary. Thus it does seem like an excessive rainfall threat=20 could evolve Friday into Friday night with either a persistent=20 swath of convection or multiple convective rounds. The Slight risk area was drawn from portions of central/northern LA into=20 central/southern MS and southwest AL...as this corridor is=20 generally the consensus axis of heaviest rainfall potential from=20 the non-GFS solutions. The high FFG keep this as just a lower end=20 Slight risk for now, and I would expect to see some adjusting of=20 the risk area as we get closer and models better converge on a=20 solution.=20 ....Rockies into the High Plains... A Slight risk was also added for eastern NM into the TX Panhandle. We should see an uptick in both CAPE and PW compared to Thursday, and a shortwave feature and upper jet on the northern periphery of the upper level ridging will help initiate convection Friday=20 afternoon. It also looks like some mid/upper level moisture=20 streaming well north and east of Juliette in the Eastern Pacific=20 will move into these areas Friday, likely helping increase rainfall efficiency. High res models indicate convection initiating over=20 eastern NM near higher terrain Friday afternoon, before propagating off to the east. Cell motions look rather quick off to the east,=20 which may end up limiting the extent/magnitude of the flash flood=20 risk. However it looks like enough of a surge in instability and=20 PWs to suggest high rainfall rates, and we could be looking at=20 enough convective coverage to support some upscale convective=20 cluster development. So there seems to be enough there to justify=20 going with a Slight risk over these areas. Elsewhere, areas of convection across portions of NE into SD and=20 MT will pose a localized flash flood risk. Convective coverage and intensity here also looks higher than Thursday, and can not rule=20 out eventually needing another targeted slight risk area, but for=20 now even with the greater convective coverage, any FFG exceedance=20 looks isolated in nature. Chenard Day 3 Valid 12Z Sat Aug 30 2025 - 12Z Sun Aug 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW MEXICO INTO WEST TEXAS... ....Rockies into the Plains... Mid/upper level riding will gradually breakdown by Saturday over=20 the Plains resulting in increased troughing and embedded shortwave energy. This should drive rather widespread convection from TX/NM=20 all the way into the Dakotas. A Slight risk was maintained from=20 eastern NM into the TX Panhandle and west central TX where the=20 ingredients appear better for an excessive rainfall threat. This=20 area will see an uptick in instability compared to Friday, with=20 PWs also trending slightly higher and well within the range of=20 supporting heavy rainfall rates. The right entrance region of an=20 upper jet and an increase in low level moisture transport Saturday evening support an organized convective threat. Convective details this far out are uncertain, but the overall pattern supports=20 terrain driven convection over NM growing upscale as it moves=20 eastward into western TX resulting in an isolated to scattered flash flood threat. ....Southeast... More uncertainty exists over the Southeast by Saturday. As was=20 described in the day 2 discussion, still think the 00z GFS is an=20 outlier and lower probability solution. Most other models indicate a lingering frontal boundary with a low and enhanced moisture=20 transport. Thus it seems more likely than not that at least=20 localized areas of heavy rainfall will remain possible from the=20 Gulf Coast and possibly into portions of GA and SC. The northern=20 extent of this risk is more uncertain, and quite possible the 00z=20 ECMWF (and other recent ECMWF runs) is too far north and east. But somewhere from the central Gulf Coast into north FL and southern=20 GA still looks in play for heavy rainfall Saturday into Saturday=20 night. The overall environmental ingredients would support Slight=20 risk level rainfall...however the combination of high FFG and=20 above average uncertainty suggests sticking with a broad Marginal=20 risk is the best course of action for now. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9TNIyOe45rJDawtaENAFh4eqMSESLyRKs5r4-zTsAqvT= Gg5GhXvssHviyFqRaiJvV31gLl9erD3EsCWDO1BSLjEo9fA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9TNIyOe45rJDawtaENAFh4eqMSESLyRKs5r4-zTsAqvT= Gg5GhXvssHviyFqRaiJvV31gLl9erD3EsCWDO1BSTbsuWWY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9TNIyOe45rJDawtaENAFh4eqMSESLyRKs5r4-zTsAqvT= Gg5GhXvssHviyFqRaiJvV31gLl9erD3EsCWDO1BSvE9lOJw$=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) .