Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 27 2025 15:57:53 FOUS30 KWBC 271557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Tue May 27 2025 Day 1 Valid 16Z Tue May 27 2025 - 12Z Wed May 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY, SOUTHEAST U.S., AND TEXAS... ....16Z Update... No wholesale changes were needed to the previous ERO issuance. Was able to connect the Slight Risks through the Tennessee Valley=20 where recent CAMs have shown (low) probabilities for exceeding FFG=20 and heavy QPF in what was the gap between them. In areas like=20 eastern Louisiana/western Mississippi where initial convection has=20 cleared, trimmed the Slight Risk out of those areas. CAMs show some showers possibly popping up again this afternoon/evening in the=20 Lower Mississippi Valley, but with the atmospheric turnover leading to less instability, expect those showers to stay light enough the Marginal will cover it. Regarding the Carolinas, recent models=20 have shown convection focusing offshore of Wilmington NC and=20 vicinity rather than onshore, so the Slight Risk was removed into=20 NC and eastern SC. See the previous discussion below for further=20 information on the south-central to southeastern U.S./Ohio Valley=20 threat areas.=20 ....North-Central High Plains... A shortwave diving southeastward in the north-central Plains=20 rounding the north-central U.S. upper low will provide support=20 aloft for convection, while a tongue of higher theta-e streams into the High Plains. CAMs show instability (surface based CAPE) of=20 around 1000 J/kg but with ensemble max CAPE possibly exceeding=20 2000. A trough or weak front at the surface with the theta-e=20 gradient just to the east could enhance surface convergence to=20 force convection. In this issuance, extended the Marginal north=20 into Wyoming given the same type of environment there as farther=20 south, plus the HREF probabilities were 30 to 70 percent for=20 exceeding 2 year ARIs in portions of Wyoming. Tate ....Previous Discussion... ....Ohio Valley... A multi-round rain threat with stronger convective roots in the afternoon and evening will occur across portions of the Ohio Valley. Initial impacts will lean more on the tamer side with the approach of a mid-level vorticity maxima currently moving=20 northeast out of TN. Approach of the shortwave energy between=20 12-18z will allow for accompanying light to moderate rainfall=20 providing some initial priming of the soils in areas like Eastern=20 KY up through the western half of WV. Initial progs indicate little to no instability with the initial batch, so the threat for flash=20 flooding is minimal in the first portion of the forecast. Later in=20 the afternoon, a strengthening speed max will nose into the Central Ohio Valley with increasing RER dynamics to couple with a tongue=20 of relatively modest instability running west of the Appalachian=20 front. 00z CAMs are on-board with a period of convection firing=20 within the confines of the Cumberland Gap up through Eastern KY and Southern WV, enough of a signal to warrant flash flood prospects=20 within an area that is more prone to flash flood risks given the=20 complex topography. The area of greatest focus will likely be the=20 area across North-Central TN up through Eastern KY where the core=20 of the greatest theta_E advection will occur and steeper lapse rate presence as you position south of RLX territory. 00z HREF neighborhood probs for >1" are over 90% for all of Southwest WV down through Eastern KY with 70+% over much of Eastern TN. >2" probs do fall over WV with more of a signal positioned across Eastern KY and TN, a testament to the expected range of precip for the convective period. It's unlikely to see widespread significant flooding in this scenario, but these setups within a formidable deep moisture presence (low to mid-level RH >80% and PWATs ~ +2 deviations) typically can surprise within a good dynamical pattern. The previous SLGT risk was maintained with some trimming around the western and northern edges of the risk area. It's possible the risk gets scaled back across WV if trends allow for it, but wanted to maintain some continuity overall with the threat still lingering across Southwestern WV when assessing the accompanying dynamics. The SLGT risk is very much in play for locations further south in Eastern KY and the Cumberland Gap of TN. ....Southeast U.S... Shortwave energy currently analyzed over TX will move eastward into the Lower Mississippi Valley and continue to plug eastward through the course of Tuesday allowing for scattered to widespread convective activity during its progression. Quite a large MCS is maneuvering through Central TX with cold pool mergers helping to feed the complex and maintain its forward momentum beyond the I-35 corridor. Energy analyzed over TX is helping fuel the disturbance with a strong likelihood of the energy associated migrating eastward over the next 24 hrs with regional ascent maximized within proxy of the shortwave. Deep moisture pooling with Gulf roots will lead to widespread 1.8-2.1" PWATs across all of the Southeast with a core of elevated theta_E presence aligned west to to east from LA over into GA during the daytime hours Tuesday. Despite a relatively progressive forward motion of convection, heavier convective cores will materialize across the South with rates between 2-3"/hr possible, including some heavier intra-hour rates that could drive localized flooding concerns higher, an issue that is profound in these types of environments. 00z HREF EAS depiction for >1" is over 70% for large chunk of the Southeastern U.S. with modest probs (20-35%) for at least 2" in the same corridor of Central LA through West-Central GA. Neighborhood probabilities back up the threat with >3" signaling 50-80% over much of the area stretching from Baton Rouge to just west of Atlanta. Considering 1/3/6 hr. FFG's being slashed considerably over the past 24 hrs. due to waves of convection over MS/AL, the prospects for flash flooding are heightened a bit more than usual. The areas of greatest concern will be those that saw significant rainfall this previous period and those urban centers that exhibit greater runoff potential normally. The previous SLGT risk was generally maintained with only some minor adjustments based off the expected blended mean QPF from the hi-res ensemble suite. Across Southeast NC, surface low development off SC coast will spur some significant rainfall over the coastal plain between OBX down to northeastern fringe of SC. Recent hi-res trends have seen a pretty substantial uptick in-of that corridor with some CAMs members boosting QPF over 6" during a 6-12 hr. span Tuesday night into Wednesday morning. Strong low-level convergence and prevailing flow directly off the Atlantic will be the culprit for enhanced rainfall potential in proxy to the coast. It's a tight window to contend with leading to some uncertainty on totals, but the threat is within reason synoptically, so long as guidance has a handle on cyclogenesis. The previous SLGT risk was maintained given the recent short range trends. ....Texas... A series of strong mid-level perturbations will eject out of Coahuila and Chihuahua thanks to the affects of a migrating shortwave trough currently situated over Sonora. A significant uptick in convective development across the Serranias del Burro and Chihuahua will ultimately lead to heavy thunderstorms plowing east out of Mexico, eventually impacting the Lower Trans Pecos, Stockton Plateau, and eventually the Edwards Plateau down through the Rio Grande after 00z. Environmentally speaking, there will be a abundance of instability available during peak convective impact with a strong buoyant signature situated from the Stockton Plateau to points southeast with a large span of 2000-4000 J/kg of MUCAPE in that expected zone. PWATs will be ~1 deviation above normal, a relatively favorable moisture regime that can yield some elevated precip potential, albeit sort of capped in higher potential. The main concern with the setup will occur overnight where cold pool mergers can lead to cells maturing and collapsing over the same area for several hours leading to prolonged heavy rainfall. The initial surge of the convective impacts will be most notable in the towns along the Rio Grande up to the Stockton Plateau. Areas downstream over the hillier terrain will be most susceptible to the eventual cold pool merger/decay pattern with 2-5" of rainfall plausible in either of these impact scenarios. Blended mean QPF from the latest HREF output is signaling a 2-4" bullseye within proxy of the Rio Grande, putting places from Del Rio down towards Laredo at the brunt of the convective surge. Neighborhood probabilities for >3" of rainfall in 6-hrs (40-60%) are relatively high, but the key is the anticipated hourly rates to be generally 1-2"/hr with higher intra-hour potential given the steep lapse rates and formidable mid-level ascent over the region. This likely spur scattered flash flood instances over areas impacted with a higher threat further north into the Edwards Plateau due to recent priming from previous heavy rainfall. The SLGT risk inherited was maintained with some extension back into the Stockton Plateau given the correlation of convection development off the Davis and Glass Mountains that could lead to flooding concerns along I-10 in Pecos County Texas. The SLGT risk was also expanded north into the Concho Valley as convection over the same areas hit hard recently will offer a greater potential for flash flood prospects as FFG's have taken quite a hit the past 24 hrs. Kleebauer Day 2 Valid 12Z Wed May 28 2025 - 12Z Thu May 29 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE SOUTHERN AND CENTRAL HIGH PLAINS, TEXAS, LOWER MISSISSIPPI VALLEY, AND CENTRAL GULF COAST ... ....Central High Plains... Longwave pattern across the CONUS will become "blocky" with a large closed ULL centered over the Midwest with vorticity maxima pivoting around the general circulation. A shortwave currently analyzed over the High Plains of Canada will get caught in the western periphery of the ULL and rotate quickly to the south and southeast by the middle and latter stages of D2 leading to convective enhancement with a likely MCS definition as we move into the end of the period. There's a growing consensus on the evolution beginning upstream over the Front Range, eventually crossing through the Central High Plains of KS with upscale growth of any convective segment sliding down prevailing northwesterly steering flow. As the complex loses latitude and reaches the southern periphery of the sprawling ULL, the disturbance will slow and eventually pivot eastward allowing a training axis to occur somewhere over Southern KS. Ensemble bias corrected QPF is fairly aggressive in the signature with max QPF orientation aligned from DDC down towards North-Central OK. This has been pretty well-documented so far with relatively consistency in guidance, so the threat is certainly increasing for a regional bout of heavy rainfall. The area expected to see the most rain is actually one of the "drier" areas of KS leading to modest FFG indices in the 1/3/6-hr temporal scales. Areal average of 2-3" with max potential of up to 4" is forecast across the axis referenced above. 1-2" will surround the forecast maxima leading to some isolated flash flood concerns in those areas if the orientation of the complex shifts. Overall, a MRGL risk is in place for the threat with a potential for a targeted upgrade if the current assessment proves to be favored leading in. ....New Mexico and Texas... Persistence in a narrow theta_E ridge from West TX up through the Southern Rockies will lead to another shot at scattered thunderstorms over the terrain with locally heavy rainfall causing some flash flood concerns in complex topography. Remnant burn scars up over Northern NM have been hit the past few days with high runoff capabilities leading to flash flooding in those targeted zones. This threat will remain for D2 with some isolated heavy cores also possible as far south as the Sacramento Foothills. West Texas will see an initiation of thunderstorms across the Big Bend up to the Stockton Plateau and adjacent terrain of Southwest TX. Threat is very localized, but considering the environment in place, any cells will have the opportunity for a quick 1-2" of rainfall that would ultimately cause issues over the area. A relative min in QPF is anticipated over the Permian Basin and Caprock area of Eastern NM and High TX Plains. This is well documented within the QPF distribution allowing for a "donut hole" for a nil ERO to be positioned in the area of the QPF minimum. Across Central and East TX, thunderstorm complex will materialize the back end of D1 and roll into D2 with heavy rainfall forecast across the Piney Woods and Upper coastal areas of TX. This will be back to back days this is forecast or will have occurred leading to some threat of localized flooding pending soil saturation and recovery response in the area. Totals between 2-3" will be possible across portions of the TX coast up into the Lower Sabine, so realistically, localized flooding will remain a threat for at least another period. Any convective development across Central TX Wednesday will lead to another isolated flash flood threat after this evenings rainfall and any Tuesday additions will lead to solidly primed soils within a region that is prone to flash flood concerns due to higher runoff capabilities. For each of the areas mentioned in the above sections, a MRGL risk is forecast with the best opportunity for upgrades potentially across the Upper TX coastal area into the Piney Woods. ....Lower Mississippi Valley and Gulf Coast... Complex developing upstream over TX will eventually work its way into the Lower Mississippi Valley with more heavy rainfall concerns in-of the energy associated with the progressing complex. This area has relatively high FFG's leading in, so the threat will remain fairly isolated overall with max QPF outputs generally within the 2-4" range with the heaviest more likely to be closer to the Lower Sabine than areas further east. Still, there's reasonable consensus on a stripe of elevated QPF from the expected convective impacts moving overhead leading to a lower-end MRGL risk for the Lower Mississippi area, mainly LA. Across the Central Gulf Coast, heavier cells within a moisture rich environment could spawn over areas a little more sensitive (urban areas) between New Orleans to Tallahassee. The risk is low-end, but some guidance has been trying to put heavier QPF signatures within proxy of those areas. Steering flow will be relatively weak so a shot for cells to retain some longevity over the same areas. A MRGL risk remains in place over each area. Kleebauer Day 3 Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... A tricky setup is on tap for much of the CONUS situated east of the Rockies with a broad ULL helping dictate the pattern encompassing two-thirds of the nation. Across the Southeast, a corridor of elevated PWATs and associated instability will be present south of the primary low. A cold front will begin a southward advancement with cooler, stable air making headway through the Ohio Valley the beginning of the period. As the front sags south a broad axis of pre-frontal convergence will transpire with scattered to widespread thunderstorms likely situated across much of the South. The issue becomes a discrepancy in timing of the front, precip location, and any boundaries that could enhance convective posture. What is known is the environment favors heavy rainfall for anything that does develop and that trend will remain steady all the way through the eventual setup. Ensemble depictions are more stable in the QPF distribution compared to the deterministic where several iterations of output are available with maxima situated anywhere from Tennessee down to the Gulf Coast to as far west as TX. Ensemble means are positioned over an axis from Jackson, MS towards Western GA, a corridor that will be well-saturated by the D3 time frame leading to a greater threat for flooding in those areas. With a pretty high chance of somewhere in the South getting anywhere from 2-4+" of rainfall, the forecast SLGT risk in place outlines the region that would have the greatest threat considering the ensemble QPF and relevant FFG's expected by D3. The risk area could very well adjust, but went with the area of greatest interest, correlating well with the current ensemble output of heavier QPF. More scattered to widely scattered thunderstorms will develop across much of Texas leading to localized flash flood concerns for yet another day with the best threat occurring Thursday afternoon and evening. The threat will be most suitable within areas of higher saturation from previous periods of rainfall, urban areas, and complex topography. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4qAwRxENaybQOWJvniPzI5fPZfIl4tzU-xEuAoyoTm05= CiOntaH0bgNaKQjiAiiBWGUaDv0QgKGLr7iHyghkZRL8-JM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4qAwRxENaybQOWJvniPzI5fPZfIl4tzU-xEuAoyoTm05= CiOntaH0bgNaKQjiAiiBWGUaDv0QgKGLr7iHyghkozpGYHo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4qAwRxENaybQOWJvniPzI5fPZfIl4tzU-xEuAoyoTm05= CiOntaH0bgNaKQjiAiiBWGUaDv0QgKGLr7iHyghk8Ph0nqo$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .