Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 28 2025 07:40:56 FOUS30 KWBC 280739 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 AM EDT Mon Apr 28 2025 Day 1 Valid 12Z Mon Apr 28 2025 - 12Z Tue Apr 29 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS AND INTO THE UPPER MIDWEST... ....Northern Plains & Upper Midwest... The period in question will remain active on the rainfall front, but=20 a few key pieces will be missing, relegating flash flood prospects=20 closer to the MRGL risk category as inherited from the previous=20 update. Upper level progression from the Inter-Mountain West through=20 the Northern Plains and Upper Midwest will exhibit a decaying mid to=20 upper level structure of a closed ULL out west with a more sheared,=20 progressive pieces ejecting into the High Plains and propagating=20 northeastward through MN and northern WI. The evolution is such that=20 a reasonable upper forcing pattern will materialize with scattered=20 to widespread convective initiation located downstream of a broad=20 diffluent axis situated from the Front Range through the Upper=20 Midwest. Instability on the order of 1500-2500 J/kg of MUCAPE will=20 be present ahead of the mean trough with ample mid-level=20 perturbations ejecting into the northern tier of the CONUS during=20 the morning and afternoon periods. To round off the setup, a budding=20 140-150kt jet streak will be oriented over south-central Canada=20 providing a solid RER dynamic suitable for maintaining a more=20 organized precipitation element over the northern tier of the CONUS.=20 Despite the above support for convective prospects, the pattern will=20 be in the process of breaking down as the upper low out west becomes=20 a more open wave with the primary bulk of mid-level energy quickly=20 ejecting northeast with a textbook strong PVA to NVA signature as=20 the energy vacates over the northern Lakes. This setup has shifted=20 to a very front loaded convective scheme over the Northern Plains=20 into the Upper Midwest with a majority of the convective pattern=20 quickly ushering east-northeastward through the course of the=20 morning and afternoon periods. By the evening, remnant convective=20 elements will become more scattered in nature, eventually moving out=20 of the primary area of interest after nightfall leading to a=20 diminished threat for the end of the forecast cycle. 00z HREF EAS probs remain pretty elevated (50-80%) for at least 1"=20 across the eastern half of ND through Northern MN, but a sharp=20 decline in probs for >2" leading the case for more isolated to=20 widely scattered flash flood prospects in the given pattern. There=20 are some respectable probabilities in the neighborhood scheme for=20 >2" within the Arrowhead of MN back over the the Dakotas (50-80%),=20 but again a sharp decline once into the >3" fields limiting the=20 upper bounds of the potential. A few areas may see between 3-5" at=20 the peak, but those will likely be few and far between for an area=20 that is actually in a low-end drought overall. Considering the soil=20 type and antecedent dry conditions in the region, the inherited MRGL=20 was maintained with little change in the risk area over the Northern=20 Plains into the Upper Midwest.=20 ....Central and Southern Plains The Central and Southern Plains will reside within a broad warm- sector environment positioned between a vacated warm front pressing=20 north through the Mississippi Valley and a slow-moving cold front=20 through the High Plains with a secondary cold forecast to plunge=20 southeast through the Northern and Central High Plains by the end of=20 the forecast cycle. A fairly meridional upper pattern will end up=20 being a key factor in the eventual ejection of stronger shortwaves=20 out of Chihuahua, MX, barreling northeastward through the Southern=20 Plains with scattered to widespread thunderstorm development along=20 and east of a dry line situated over West TX. The combination of=20 relevant buoyancy/instability positioned over the Southern and=20 Central U.S. coupled with favored regional ascent and=20 thermodynamics will create a period of late-afternoon and evening=20 convective development capable of localized flash flood prospects.=20 Recent heavy rain episodes have contributed to an elevated soil=20 moisture presence from the Red River Basin up through much of OK=20 into southeast KS, and MO. Rainfall totals up to 2-3" in a short=20 time frame will be capable of causing flash flooding within the=20 bounds of those areas that exhibit those moist top-layer soils=20 since the time frame for recovery will have been short. 00z HREF=20 neighborhood probs for >1" are respectable (50-70%) within portions of the Central and Southern Plains with the focus generally over=20 north-central OK, Concho Valley, and Western Rolling Plains of West TX. Probabilities for >2" are very low leaving the threat for=20 flash flooding fairly conditional and likely more reserved for more sensitive areas within urbanized zones and areas more susceptible=20 due to low-water crossings.=20 Kleebauer Day 2 Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS INTO THE MISSOURI VALLEY... The combination of favorable buoyancy and upper level dynamics=20 within the confines of a quasi-stationary front across the Southern=20 Plains to Mid-Mississippi Valley will create an environment=20 conducive for locally heavy rainfall over an area still recovering=20 from previous heavy rain episodes. Models are in agreement on the=20 placement of a west-southwest to east-northeast oriented stationary=20 front extending from the Western Rolling Plains of TX up through the=20 Red River Basin, bisecting much of central and northeastern OK, up=20 through the Ozarks of MO. Multiple mid-level shortwaves will eject=20 northeast out of MX creating a multi-wave convective initiation=20 cycle that will eventually move over the same areas repeatedly=20 during the course of the D2 time frame. Instability markers are very=20 favorable within the axis denoted above with sufficient buoyancy=20 on the order of 1500-3000 J/kg of MLCAPE situated across West TX to points northeast through the Red River into OK. Assessment of NASA SPoRT soil moisture anomalies pinpoint a fairly large area of=20 well- saturated top-layer soils running upwards of 90-98th=20 percentile across southwestern OK up into the Norman/OKC metro=20 corridor thanks to a significant flash flood episode that occurred=20 less than 36 hrs prior. Further northeast, soils are still=20 recovering from multiple rounds of convection the past several days with elevated top-layer soils also running high within the 75-90th percentile across the Tulsa CWA up through southwestern MO.=20 Considering guidance privy to these areas also being the=20 beneficiary of multiple convective rounds over the course of=20 Tuesday into Wednesday, there is a growing necessity for a higher=20 risk level forecast in these zones. Ensemble bias corrected=20 guidance has steadily increased the areal QPF averages from run to=20 run within these confines of northeast OK up through western MO, a=20 testament to the growing consensus in the heavy QPF footprint.=20 00z CAMs are only within the initial 12 hrs. of the period in=20 question, but the signs even prior to the bulk of the heavier=20 convective signature (Heaviest after 00z/29) are very much favorable=20 for potentially significant flash flood prospects. 00z HREF 6-hr FFG=20 exceedance probabilities for the 18-00z time frame are already=20 between 15-30% on the initial stages of heavier precip across the=20 Red River Basin of North TX into Southwestern OK up through Central=20 and Northeast OK. This is very likely due to the convective episode=20 stemming from the first mid-level shortwave ejecting northeast=20 generating a period of convection during the afternoon hrs. on=20 Tuesday. By Tuesday evening, large scale ascent will increase=20 substantially over the Southern Plains a potent mid-level shortwave=20 pivots over NM with a robust diffluent signature present downstream=20 into West TX and the adjacent Red River Basin. At the surface, a=20 cold front will motion south through the High Plains with an=20 expected convergence pattern developing as it moves into the already=20 present quasi-stationary front in place across the region. The=20 combination of the evolving surface and upper pattern will yield a=20 robust convective footprint across the Southern High Plains of TX up=20 through the Red River Basin, pressing northeast given the steering=20 pattern aloft. The episodic nature of the heavy precip will garner=20 limited recovery for areas already impacted by one round of=20 thunderstorms earlier in the period leaving a better opportunity for=20 flash flooding potential for the nocturnal period. There's some=20 discrepancy on exact placement of the more prolific convective=20 cores, but the one constant is the area expected to see the threat=20 is generally pinned down at this lead.=20 After coordination with local WFOs over North TX through OK,=20 Southeast KS, and Southwest MO...the previous MDT risk was expanded=20 to include much of Central and Northeast OK up through Southeast KS=20 into Southwest MO. A large SLGT encompasses areas surrounding the=20 MDT risk with areas like the eastern Permian Basin, Concho Valley,=20 and the Southern half of MO (Excluding the MDT risk area) within the=20 SLGT corridor. A broad MRGL extends from the rest of the Permian=20 Basin all the way through the Central and Northern Ohio Valley into=20 Northwest PA and far Southwest NY state as scattered convective=20 clusters will have the opportunity in these zones to produce locally=20 heavy rainfall capable of isolated flash flood prospects. Kleebauer Day 3 Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE ARKLATEX, EASTERN OKLAHOMA, AND WESTERN OZARKS... The evolving upper air pattern along with the slow progression of a=20 cold front will yield another period of significant rainfall within=20 the confines of the Southern Plains to points east into the ArkLaTex=20 and Lower Mississippi Valley. Strong mid-level vorticity maxima will=20 translate eastward around the base of a potent mean trough=20 traversing the center of the CONUS. At the surface, a cold front=20 will be propagating eastward into the above corridors with ample=20 convection situated along and ahead of the front as it maneuvers its=20 way east. By the afternoon period Wednesday, sufficient diurnal heat=20 flux from typical daytime heating will provide ample instability=20 along and east of I-35 with a very buoyant forecast sounding being=20 depicted from multiple deterministic outputs, including the global=20 scale models, a testament to a pretty agitated environment. Above=20 normal PWATs on the order of +1 to 2 deviations coupled with a=20 textbook boundary layer convergence pattern will yield a swath of=20 heavy thunderstorms extending from Central TX to points northeast=20 into the vicinity of the ArkLaTex up through northwestern AR.=20 Latest deterministic output is between 2-4" on an areal average=20 basis with local maxima between 5-8" pending model depiction.=20 Ensemble means are generally between 2-3" for the period with the=20 bias corrected mean closer to 3-4" across the ArkLaTex up through=20 the western Ozarks, a signal that usually precludes a "ramp up" of=20 the precip depiction once we get closer to the range of the CAMs=20 guidance. Area soil moisture anomalies are once again fairly wet=20 across portions of eastern OK up into northern AR, leading to a=20 lower FFG parameter to meet for flash flood potential. Rainfall=20 rates will be between 1.5- 3"/hr during cell peaks, enough of a=20 signal to induce flash flood prospects even into northeastern TX=20 where soils are drier compared to areas further north. The D2-3=20 total QPF is now between 3-6" over the above areas, a defined=20 corridor for waves of heavy rainfall anticipated.=20 In coordination with local WFOs across OK, AR, TX, and MO...a MDT=20 risk was introduced to cover for the expected heavy rain threat over=20 areas anticipated to see significant convective impacts for=20 Wednesday into Thursday morning. Future updates will be provided as=20 we move closer to the event as later CAMs integration for the=20 forecast will provide better detail on forecast QPF maxima=20 placement, as well as magnitude.=20=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!6ykpAxWmwiEw-A-hCaI63OzbNCQK1eED7_o7AoFYC0pCbJStzKQBKTSlMbAR7Un= 7GpCUt4kU2LoCKFDdTlAXUn3ehag$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!6ykpAxWmwiEw-A-hCaI63OzbNCQK1eED7_o7AoFYC0pCbJStzKQBKTSlMbAR7Un= 7GpCUt4kU2LoCKFDdTlAXXUSLH_4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!6ykpAxWmwiEw-A-hCaI63OzbNCQK1eED7_o7AoFYC0pCbJStzKQBKTSlMbAR7Un= 7GpCUt4kU2LoCKFDdTlAXHGjToTQ$=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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