Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon May 26 2025 09:07:00 FOUS30 KWBC 260800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Mon May 26 2025 Day 1 Valid 12Z Mon May 26 2025 - 12Z Tue May 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS TO THE TENNESSEE VALLEY... ....Texas through Southern Ohio Valley... The current pattern will yield another round of appreciable rainfall within the confines of a wavy quasi-stationary front positioned over the Southern Plains, east towards the Southeast U.S. coast. This evening is quite active with multiple convectively driven outflow patterns with broad attendant cold pools helping to maintain thunderstorm longevity from Northwest TX through the Red River Basin extending into MS/AL. The western half of the setup this evening will be the precursor to "round 1" of anticipated heavy convection across TX through the ArkLaTex tomorrow morning with greater coverage occurring by the afternoon as large scale ascent increases once again within the diffluent region of an approaching trough axis. The whole flash flood scenario is contingent on the behavior of various cold pools and the overall positioning of the stationary front, a complexity that can evolve into a convective paradigm that can shift in a relatively short period of time. In simplistic terms....the setup has a lot of potential, but there will be some uncertainty in specifics with regards to EXACTLY who gets how much rainfall during the period. Probabilistically, there is some merit to the threat with a broad area extending from I-35 across Central TX to points northeast through the ArkLaTex and AR/LA state line. This is coincident with the higher probabilities for significant totals, as well as the favored alignment of the quasi-stationary front for the forecast. Tomorrow morning between 12-18z will be busiest over Central and Northeast TX extending through the ArkLaTex as the cold pool merger overnight across North TX will propagate east-southeast along the tight theta_E gradient encompassing the stationary front in question. As the complex moves east, the southwestern flank of the boundary will fan out and become elongated south of I-20 with the upshear side of the complex likely to induce some backbuilding when assessing forecast soundings from the latest 00z CAMs indicating easterly corfidi vectors with very slow mean cell motions typically indicative of backbuilding potential. The mean flow will become pretty much parallel to the boundary leading to heightened training along the front, an output that is yielding some of the higher local totals within the 00z hi-res suite (3-4+"), and associated higher neighborhood probabilities for >3" within the 00z HREF prob fields (50-80%) from I-35 north of Austin all the way across the AR/MS state line. The >5" probs are not as robust comparatively, but still signal 20-35% bullseyed along I-20 east of the DFW metro to Shreveport. There is some indication that an MCV will originate from the primary complex of thunderstorms over the northeastern flank of the line that transpires overnight. If this were to occur, local QPF maxima due to enhanced low and mid-level convergence within the MCV confines would likely spur a heightened prospect of flash flooding as the MCV makes headway across the Lower Mississippi Valley. These types of setups are highly variable in expectations, but the concept is there for possibly a secondary max being depicted downstream of what will occur over TX. There's also increasing favor of this complex exiting northeast into the Southern Ohio Valley with areas of Western TN seeing a convective flare up within proxy of the disturbance. These types of patterns are very tricky but can yield higher end threats if they materialize. The greatest threat will likely be places like Memphis down into the MS Delta for flash flood potential considering the expected path of the remnant complex. The SLGT from previous forecast was relatively unchanged due to the threat. The remainder of the period will be exhibit a large, remnant outflow bisecting Central and Eastern TX with increasing large scale ascent expected later in the afternoon and evening hours Monday. Another round of convection will likely spawn in-of a narrow theta_E ridge positioning across the Edwards Plateau up through the Southeast Permian Basin and adjacent Concho Valley. These cells will form and begin migrating eastward with scattered development likely to grow upscale and impact areas of Central and Eastern TX again during the evening and overnight period of D1. There's still some discrepancy on exactly how the convective threat will unfold, but given the current depiction, some overlap of locally enhanced precip would cause some problems for those that saw impacts earlier in the period. As a result, a higher-end SLGT risk is positioned over Central TX, including the Austin metro through the ArkLaTex and along that Southern AR and Northern LA zone previously mentioned. These areas have the greatest threat for overlap and/or significant rainfall totals from enhanced rates in a favorable synoptic pattern and mesoscale environment. ....Southern/Central Rockies and High Plains... Another area to focus will lie over Northeast NM through the TX Panhandle where energy ejecting out of the Southern Rockies will likely spur another round of convection tomorrow morning with the main energy sliding east into OK/KS with more thunderstorm threats impacting areas that have been hit the last couple days. The overall impacts are not expected to be as prolific as the previous setups with the energy moving fairly progressively and away from the surface reflections that have garnered more training activity. This is round 1 for this particular area as more convection is expected to spawn off the Southern Rockies and move east late- afternoon and evening as upstream trough will kick east acting as a trigger for more convective initiation. The back to back nature of the precip after getting hit pretty solidly today will offer some mid-range flash flood prospects, but the overall progressive nature of the cell motions should curb higher end flood scenarios. The MRGL risk was maintained over those above areas with the risk extending up through the lee-side of the Rockies in NM and CO with the northern periphery through the Southeastern-most Front Range of WY. ....Southeast Coastal Plain... Another round of heavy convection is forecast across the South Carolina Low Country as weak mid-level energy moves in from the west and interacts with a progressing sea breeze boundary that will propagate inland between Charleston to Savannah. There's some discrepancy on timing, but the chances for 3-6" locally are elevated when assessing the 00z HREF neighborhood probs and the blended mean output between 2-3" in a narrow corridor between the two coastal urban centers. Sandier soils will likely limit extensive flash flood prospects, but the there is certainly a chance a quick 3-6" to inundate local areas that get under persistent thunderstorm activity. A MRGL risk remains from previous forecast given the threat. Kleebauer Day 2 Valid 12Z 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... ....Ohio Valley... Anticipating a period of scattered to widespread elevated convective initiation across portions of the Ohio Valley on Tuesday afternoon and evening with emphasis on the area encompassing Northeast KY up through the western half of WV and portions of Western MD. Complexities in the expected approach and magnitude of a mid- level vorticity maxima ejecting northeast out of the Tennessee Valley will create some uncertainty on specifics, mainly the coverage and rates of precip occurring during the peak of impact (21-06z), but ensemble mean QPF distribution has been favoring a period of heavier convective premise in those above zones. There's a distinct difference in the thermodynamic fields within the global guidance with the ECMWF more favored for a tongue of low-end MUCAPE up through the Central Ohio Valley into adjacent valleys west of the Appalachian Front. A stronger upper speed max will pivot northeast out of the Mississippi Valley with the nose of the jet approaching the Eastern Ohio Valley towards nightfall. In this scenario, the setup would likely yield some heavy precip for a short time as the area enters a favorable RER jet dynamic that would generate a round of enhanced precip before quickly shoving off to the northeast. PWAT anomalies between +1.5 to +2.5 deviations across the region would be sufficient to support localized heavy precip cores, especially in any elevated convective cores. This setup is more likely to yield a quick 1-3" in any area from Eastern KY up through the Western slopes of the Laurels before diminishing, a forecast that would put this specific area right on the cusp of the MRGL/SLGT risk threshold when assessing 3 and 6-hr FFG exceedance probs. Some of the areas of greatest concern will be those smaller urban zones along I-79 in WV up towards Cumberland, MD where poor drainage and funneling affects can have considerable impacts for localized flash flood concerns. Considering the inheritance of the SLGT from previous forecast, have decided to maintain general continuity, but will keep an eye on the setup closely to potential of expansion OR downgrades pending the expected mid and upper level evolution as we get closer in time. ....Southeast U.S... Positioning of a persistent quasi-stationary front and advancement of multiple shortwaves out of the Southern Plains will lead to waves of convection across the Southeast with the footprint likely situated from as far west as the Lower Sabine all the way into western and central GA. This setup is pretty tricky overall as cold pool mergers from previous convection will likely orient the front differently than what is being depicted in the current guidance. Despite the potential discrepancy, the environment is ripe for enhanced convective cores with hourly rates between 2-3"/hr during any stronger cells which then boils down where these occur. When assessing the recent ensemble mean QPF, there's two areas of favorability; one situated over the Lower Mississippi Valley from LA through the southern half of MS, and the second positioned across AL into Western GA. The QPF is less bimodal than you would think, but the time frame of impact for each respective area will generate slightly different results overall. The further west area will be the benefactor of stronger low-level convergence along the front with a peak maturation of the anticipated complex moving out of TX. As the complex moves east, it will encounter a better thermodynamic footing leading to convection refiring in-of the mid-level perturbations path with scattered heavy convection situated over AL into GA by the second half of the forecast. General maxima between 2-4" with locally higher is forecast in either zone with the best threat of flash flooding likely within the confines of the boundary, as well as over any urban footprints. A SLGT risk was added to account for the above threat, but look out for small shifts in the coverage area of the SLGT in the coming updates as CAMs get a better handle on the forecast upstream that will have implications further into the period. ....Texas... A strong mid-level shortwave will eject out of Coahuila thanks to the affects of a migrating shortwave trough situated over the Southwestern U.S. A significant uptick in convective development across the Serranias del Burro will ultimately lead to heavy thunderstorms plowing east out of Mexico, eventually impacting the Lower Trans Pecos, Stockton Plateau, and eventually the Edwards Plateau after 00z. The area of great convective focus will be along the Rio Grande south of the Big Bend over into the western Hill Country where eventual cold pool mergers will lead to cell conglomeration overnight Tuesday with heavy rain situated over those more sensitive terrain areas south of I-10. 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 2-4" of rainfall plausible in either of these impact scenarios. Given the robust signature within multiple global deterministic and ensemble mean QPF output, a SLGT risk was added to encompass the above areas where the threat is the highest. Kleebauer $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .