Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 28 2025 15:47:56 FOUS30 KWBC 281547 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Thu Aug 28 2025 Day 1 Valid 16Z 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 MISSISSIPPI VALLEY... ....Southern Plains into the Lower MS Valley... 16Z Update: Organized convection across the adjacent Southern Plains/Lower Mississippi area continues to propagate to the south and southeast with the main shortwave involved analyzed over northeast OK as of 15z UA analysis. Observational trends compared to hi-res QPF footprint indicate a general north bias in the CAMs leading to QPF maxima off-centered from where this is occurring. The main concern will be the models playing catch up to the actual outcome which could have some implications on placement of=20 relative maxima as we step forward in time. The good news is the=20 current model reflections in terms of magnitude are more favorable, so the adjustments made to the inherited risk areas were fairly minor and more often just shifted ever so slightly to the southwest to account for recent trends and model bias. One area of interest will be the western flank of the convective complex as it migrates through Central OK into the Red River basin near the TX border. This is an area that could be sneaky when it comes to potential flash flood concerns as the mean flow aloft runs parallel to the quasi-stationary front bisecting the region. Low-level inflow from surface low pressure analyzed over north TX could be enough to focus a narrow corridor of organized convection right within the convergent zone situated between the in-flow area and along the front. There's some hints of this occurring within a few of the CAMs leading to a minor extension westward near and along the Red River to the northeast of Dallas. Not anticipated convective impacts as far south as the metro, but the area near Dallas will be monitored closely to see if convective trends favor any isolated cells getting close enough to the eastern edge of the metro to warrant attention. The MRGL risk is in place to account for the low-end threat.=20 As far as the expectation for the most favorable areas moving=20 forward...the pattern still denotes the expected maximum to be=20 located across the ArkLaTex with a secondary maxima potentially=20 aligned west to east across northern LA into the southern half of=20 MS with a small minima forecast over southeast AR and northeast LA. Assessing sat/radar composite over the Lower Mississippi signals=20 some gravity wave formations across southern MS with a small MCV=20 feature embedded within the convection situated near Jackson. These features could be sneaky when it comes to locally enhanced=20 convective cores over the course of the morning and afternoon with=20 some training potential aligned northwest to southeast from the=20 Mississippi Delta down into southeast MS. The SLGT risk from=20 previous forecast was adjusted a bit further southeast to account=20 for the trend.=20 As for the ArkLaTex, the threat remains relatively stable with=20 regards to the QPF output via CAMs signaling the greatest=20 convective posture to end up over the aforementioned area. The core of highest PWATs is centered over east TX over into the Lower=20 Mississippi Valley with KJAN and KFWD soundings both nestled=20 between 2-2.1" with higher PWATs focused within the progressing=20 complex. As the shortwave migrates south, we see the currently=20 analyzed warm front stall and even start to drift back south as a=20 quasi- stationary front as alluded above. This will be the focal=20 point for heavy precip threats later this evening as the LLJ kicks=20 in and we see increased convergence over the area. Models are=20 generally in agreement with secondary convective development=20 initiating over the area leading to additional heavy rainfall with=20 rates between 1-2"/hr likely in stronger cores with upwards of=20 3"/hr when you account for intra-hour maximum. High regional FFG's=20 will be one factor working against more widespread flash flood=20 potential, however the threat is certainly in the SLGT risk=20 threshold with a lean towards the higher end of the risk spectrum.=20 Greatest chance for flash flooding will likely occur from small=20 stream responses and more urbanized settings where run off=20 capabilities are maximized. The overnight period will be one to=20 monitor for targeted upgrades if the trends warrant it. For now,=20 the rates and antecedent conditions point towards a continuity of=20 the previous SLGT risk forecast. Kleebauer ....Rockies and Western U.S.... 16Z Update: The current forecast remains on track for locally heavy rainfall across the CO Front Range near Pueblo as models remain consistent in their interpretation of scattered convective development later this afternoon generating a corridor of heavy rainfall with totals between 2-4" possible where cell mergers materialized. Widely scattered thunderstorms over the west will also provide the opportunity for isolated flash flood concerns with the main focus over WY/MT late afternoon and evening.=20 Kleebauer ...Previous Discussion.. 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 to show a focus for heavier rainfall amounts over this corridor, we=20 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 best convective focus will likely be across portions of southern/central MT and northeast WY, but only modest instability should keep rainfall rates down enough to keep any flash flood 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 time...with the 00z GFS remaining an outlier. The GFS continues to be more suppressed and weaker with this feature than the other global model guidance. The ECMWF, CMC, UKMET, ICON and NAM are all farther northeast with the heavy rainfall axis stretching into 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 GFS. Thus, it seems more likely than not that the GFS is too suppressed and it represents a less likely outcome at the moment. The ECMWF has been the most aggressive with a farther northeast 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 ingredients will be conducive to heavy rainfall, with PWs over 2" and a corridor of persistent lower level moisture transport/convergence supporting some backbuilding and/or training potential. Other than the weaker GFS, models show an area of low pressure only slowly moving eastward along a low level frontal axis...with upwards of 2000 j/kg of CAPE near and south of this boundary. Thus it does seem like an excessive rainfall threat could evolve Friday into Friday night with either a persistent swath of convection or multiple convective rounds. The Slight risk area was drawn from portions of central/northern LA into central/southern MS and southwest AL...as this corridor is generally the consensus axis of heaviest rainfall potential from the non-GFS solutions. The high FFG keep this as just a lower end Slight risk for now, and I would expect to see some adjusting of the risk area as we get closer and models better converge on a solution. ....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 afternoon. It also looks like some mid/upper level moisture streaming well north and east of Juliette in the Eastern Pacific will move into these areas Friday, likely helping increase rainfall efficiency. High res models indicate convection initiating over eastern NM near higher terrain Friday afternoon, before propagating off to the east. Cell motions look rather quick off to the east, which may end up limiting the extent/magnitude of the flash flood risk. However it looks like enough of a surge in instability and PWs to suggest high rainfall rates, and we could be looking at enough convective coverage to support some upscale convective cluster development. So there seems to be enough there to justify going with a Slight risk over these areas. Elsewhere, areas of convection across portions of NE into SD and MT will pose a localized flash flood risk. Convective coverage and intensity here also looks higher than Thursday, and can not rule out eventually needing another targeted slight risk area, but for now even with the greater convective coverage, any FFG exceedance 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 the Plains resulting in increased troughing and embedded shortwave energy. This should drive rather widespread convection from TX/NM all the way into the Dakotas. A Slight risk was maintained from eastern NM into the TX Panhandle and west central TX where the ingredients appear better for an excessive rainfall threat. This area will see an uptick in instability compared to Friday, with PWs also trending slightly higher and well within the range of supporting heavy rainfall rates. The right entrance region of an 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 terrain driven convection over NM growing upscale as it moves eastward into western TX resulting in an isolated to scattered flash flood threat. ....Southeast... More uncertainty exists over the Southeast by Saturday. As was described in the day 2 discussion, still think the 00z GFS is an outlier and lower probability solution. Most other models indicate a lingering frontal boundary with a low and enhanced moisture transport. Thus it seems more likely than not that at least localized areas of heavy rainfall will remain possible from the Gulf Coast and possibly into portions of GA and SC. The northern extent of this risk is more uncertain, and quite possible the 00z ECMWF (and other recent ECMWF runs) is too far north and east. But somewhere from the central Gulf Coast into north FL and southern GA still looks in play for heavy rainfall Saturday into Saturday night. The overall environmental ingredients would support Slight risk level rainfall...however the combination of high FFG and above average uncertainty suggests sticking with a broad Marginal 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!7kGoOMXh_GcPtpfBaUIW6PkPjT_clDgJPolOtFYWrqSw= ipXVPMCgqJhS6r-Rhqv7xwRBVwCLXEFnB-Tuyy2HnWcXVjc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7kGoOMXh_GcPtpfBaUIW6PkPjT_clDgJPolOtFYWrqSw= ipXVPMCgqJhS6r-Rhqv7xwRBVwCLXEFnB-Tuyy2HkdoDGhM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7kGoOMXh_GcPtpfBaUIW6PkPjT_clDgJPolOtFYWrqSw= ipXVPMCgqJhS6r-Rhqv7xwRBVwCLXEFnB-Tuyy2HUluaGYQ$=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. 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