Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed May 28 2025 08:10:00 FOUS30 KWBC 280740 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 AM EDT Wed May 28 2025 Day 1 Valid 12Z Wed May 28 2025 - 12Z Thu May 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND COASTAL TEXAS... ....Central and Coastal Texas... Energy ejecting out of Coahuila tonight will migrate southeast towards Deep South TX before turning east-northeast around the western periphery of a ridge positioned over the Caribbean. Deep moisture advection regime will be ongoing through the period with PWAT's between 1.9-2.2" likely across all of South TX with some localized maxima exceeding 2.2" likely when assessing the latest CAMs output. This puts the Texas coastal plain with a +2 deviation anomaly moisture wise, indicative a fairly unstable environment prior to the approach of the expected shortwave. MUCAPE between 3500-4500 J/kg is forecast over the Lower RGV up through the TX coast with the highest forecast in proxy to the immediate coast and southern-most portion of the Rio Grande. Hi-res deterministic QPF shows a bullish depiction of local 4-6" maxima cropping up near and over some of the urban centers within the above zone, including places like McAllen, Corpus Christi, and nearby the Houston Metro. These areas are more prone to flash flooding due to the urban footprint providing higher runoff capabilities, but also the previous evening's MCS that blew through the area truly cut the FFG indices ~50% compared to where they were even 48 hrs. prior. The 1/3/6 hr. indices are well within reach considering the blended mean QPF output over these areas are now between 2-3" with local maxima littered across the region. Neighborhood probs from the 00z HREF painted a broad 50-80% area for >3" with the entire coastal plain highlighted from South Padre up through the Upper Texas coast. Considering consensus from latest hi-res deterministic/ensemble combo and pertinent prob fields, and in coordination with the coastal TX WFO's (BRO/CRP/HGX), a SLGT risk was added along a vast majority of the TX coastal plain with emphasis on higher potential inside the urban zones. Further northwest, another shortwave will exit off the TX Caprock with a steady migration eastward through the northern Concho Valley, eventually into Big Country and Central TX late-afternoon Wednesday. A well-defined theta_E ridge will bisect much of Central TX, arcing northwest through Northwestern TX leading to a tongue of elevated instability characterized by a persistent corridor of 2000-3000 J/kg SBCAPE when assessing the 00z CAMs. This is defined very well within the ensemble mean SBCAPE output with a 90th percentile output closer to 4000 J/kg positioned between San Angelo/DFW/Austin when taking a look at the spatial SBCAPE max from the 00z HREF. This area has been impacted for multiple days with locally heavy rainfall bringing regional QPE between 2" to as much as 8" in the last 72 hrs. This has prompted FFG responses to drop considerably with even the 3hr FFG marker a paltry 2-3", a far cry from the ~5" marker just a few days ago. Streamflows across the region impacted are relatively high as well meaning the threat is a bit more pronounced when you take everything into account. Precip means are between 1-1.5" with some local maxima of up to 4" showing up in the CAMs, especially in the area between DFW and Austin, near and along I-35. Hourly rates will be driver of the threat, but there's plenty of favor for 2-3"/hr rates considering the environment. In coordination with some of the local WFO's (FWD/SJT/EWX), a SLGT risk was added to portions of the Concho Valley, Southern Big Country, and Central TX with emphasis on that area near and along I-35 between DFW/Austin. ....West Texas and New Mexico... 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 recently with high runoff capabilities leading to flash flooding in those targeted zones. This threat will remain for D1 with some isolated heavy cores also possible as far south as the Sacramento Foothills. West Texas will see an initiation of thunderstorms across the western half of the TX Big Bend up through the western 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, eastern Big Bend, and the 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. Despite a relatively sporadic coverage, the potential for heavy rates between 1-2"/hr will be the key for any flash flood potential, outside burn scar locales. The previous MRGL risk was generally maintained with only some minor adjustments near the nil ERO area. ....High Plains... A shortwave currently analyzed over the High Plains of Canada will get caught in the western periphery of a broad ULL positioned over the Midwest leading to pieces of energy fragmenting off the main vorticity lobe and pivoting south and southeast through High Plains of MT/WY later this morning. As the energy moves over the WY/CO Front Range, convective initiation will occur with cold pool maturation and consolidation likely leading towards a defined MCS as we move into the second half of the period. Consensus was maintained across all major deterministic on the upscale growth of any convective segment sliding down prevailing northwesterly steering flow aligned across Northeast CO down through Western KS as we move into nightfall. As the complex loses latitude and reaches the southern periphery of the sprawling ULL, the disturbance will slow and eventually pivot eastward allowing a short-term training axis to occur over South-Central KS. There has been a minor displacement of the previous QPF maxima with the 00z hi-res suite insinuating the maxima be positioned between Dodge City to Wichita, KS with the magnitude generally between 3-4" where the heaviest precip occurs. The shift was fairly minor in the spatial aspects of the forecasts, but hinting at places a little further east would put areas of South-Central KS closer to needing a risk upgrade (SLGT) if the trend continues. There is a sharp delineation between elevated FFG's and lower FFG's within that part of the CONUS, much of it stemming from a barrage of repeated convective impacts this past weekend. HREF prob fields are pretty insistent on at least a widespread 1" of rainfall given the 60-90% probabilities of >1" in the EAS depiction. The 2" EAS probs, however are much lighter (10-25%) over the same areas meaning guidance is still not settled on the QPF "bullseye" in the area. At this juncture, the threat for a targeted SLGT remains, but the corridor of where the heaviest rain will occur will likely be more suited for a MRGL risk with greatest flash flood potential likely within those Wichita/Dodge City urban zones. The previous MRGL risk was maintained given the assessment. ....Lower Mississippi Valley and Gulf Coast... Very little has changed with regards to anticipated complex developing upstream over TX working its way into the Lower Mississippi Valley with more heavy rainfall concerns in-of the energy associated with the progressing complex. This area will have 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 smattering 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 Mobile. 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. ....Coastal Mid Atlantic... Surface cyclone will induce a persistent easterly regime off the Atlantic with a tongue of elevated theta_E's positioned across Cape Hatteras up through the Hampton Roads area. CAMs are indicating periods of convection to fire within the confines of these areas with the large scale forcing increasing as we step into the late- morning and afternoon periods today. General QPF maximum (2-3") in that time frame over Eastern NC up into the Tidewater will have the opportunity to induce some localized flash flood prospects with the best risk over the Tidewater where urbanization factors can tip the scales. The previous MRGl risk inherited was unchanged with the current QPF footprint and thermodynamic regime still in play after assessment of the 00z deterministic suite. Kleebauer Day 2 Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....Southeast U.S... The combination of shortwaves ejecting eastward out of the Southern Plains and the approach of a cold front from the north will lead to ample ascent between sfc-500mb along a stretch of elevated moisture between +1 to +2 deviations in PWAT anomalies. Consensus has increased in a broad, scattered convective regime from the Lower Mississippi Valley into the Southeastern U.S. leading a potpourri of QPF maxima within areas of convective impact. Signals at the end of the 00z HREF were quite bullish in the 12-hr window from 12-00z Thu/Fri with neighborhood probabilities for >3" between 35-60% across several small areas over the Central Gulf Coast, including some "bullseye" patterns within a few urban centers between Baton Rouge to Tallahassee. Thunderstorm concerns will continue until the front passes through any specific area south of I-40 in the period, so the prospects for local maxima capable of flash flooding are elevated when you add the lower FFG indices currently in place between MS/AL. This will come after seemingly 3 straight days of heavy rainfall at any point in time for the region, so the FFG recovery is pretty unlikely at this juncture. The previous SLGT risk inherited was generally maintained, but did expand west given the timing of the approaching shortwave out of TX and the accompanying higher QPF signatures over LA between 12-18z Thursday. ....Ohio Valley... A broad ULL positioned over the Midwest will pivot southeast with a positioning across the Mid-Mississippi and Western Ohio Valley through D2. Large scale forcing under presence of the ULL will generate periods of convection across the above areas with a focus on MO/IL/KY/IN. QPF maxima based on the ensemble bias corrected output was between 1.5-2.5 over Central Ohio Valley with highest signature over the Ohio River basin. The current outlook is still within the MRGL risk threshold due to the lower instability presence at the surface as much of the ascent is rooted in the mid to upper levels given the ULL/trough influence. The previous MRGL risk was relatively unchanged with only minor tweaks around the periphery to account for the latest heavy QPF distribution. ....Texas... More convection is expected over the Lone Star State on Thursday afternoon through the end of the period. West TX will be the initiation point with convection firing in-of the theta_E ridge positioned over the Big Bend up through the Pecos River Valley. A cold front will be pressing in from the north with a sharp cutoff of any instability presence as MUCAPE will flat-line with fropa as drier air advects behind the front. The best threat for convection will likely occur over Southwest TX with some question as to whether the cold front will subdue convective concerns for areas north of I-20 by the second half of the forecast. Guidance is split on how it wants to handle the shallow cold push with the NAM Nest the most aggressive in its latitudinal push and globals lagging enough to warrant storm threats as far north as the Caprock. For now, left a broad MRGL over the region, but there will likely be some fluctuations in the northern periphery of the MRGL risk area in place. Will be monitoring over the next succession of updates. Kleebauer $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .