Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 27 2025 00:53:21 FOUS30 KWBC 270053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Mon May 26 2025 Day 1 Valid 01Z Tue May 27 2025 - 12Z Tue May 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL TEXAS TO THE GULF COAST AND THE SOUTHEAST U.S. STATES... 01Z Update... Despite uncertainty displayed by the latest runs of the HRRR and RRFS...the previous outlook required only minor adjustments. Split the Slight Risk area across the Gulf coast and Southeast U.S. into two areas. The Southeast U.S. area was a bit more uncertain...but there was a consistent enough signal from the HRRR to warrant an expansion of the Slight Risk area to cover parts of eastern Georgia and a small part of neighboring South Carolina given a bit stronger low level convergence than depicted by earlier runs...as much a result of high pressure to the northeast being a bit stronger and keeping more of a northerly component to the wind across the Carolinas. The higher-end Slight Risk area was still on-track across Texas based on early-evening radar and guidance continuing to show expansion later this evening as southerly low level flow accelerates a bit and the potential for repeat convection/training increases. Only other change was to pull the Marginal Risk area off of the highest terrain of the Rockies given the loss of daytime heating. Bann 16Z Update... The 12Z HREF guidance including recent runs of the HRRR and RRFS=20 reflect considerably uncertainty with the finer scale/mesoscale=20 evolutions of much of the convective threat involving the southern=20 High Plains/Texas and also areas downstream across the Gulf Coast=20 states. However, generally the consensus suggests an active period=20 of convection with high rainfall amounts possible this evening and=20 into the overnight period across areas of central TX and the Hill=20 Country where recent HRRR/RRFS runs suggest locally significant=20 rains of 5+ inches where intersecting outflow boundaries with=20 substantial instability along their leading edges will noted. The=20 Slight Risk area has been pulled farther southwest to account for=20 this. Elsewhere, the Marginal and Slight Risk areas farther north have been trimmed to account for where heavy rainfall has ended or will be coming to an end soon, and also where the airmass is much more stable. However, the Slight Risk area has been extended farther east across the Gulf Coast states where the ongoing MCS/MCV activity should continue eastward while interacting with plenty of moisture and instability along an east/west oriented frontal zone. Orrison Previous Discussion... ....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... 2030Z Update... Changes to the D2 ERO for this cycle include expanding the Marginal and Slight Risk areas up across the coastal plain of the Southeast where proximity of a frontal zone with strong instability and rather anomalous moisture pooling along it will be in place as upstream shortwave energy arrives. The 12Z guidance suggests a wave of low pressure developing along this front which may enhance the moisture convergence and overall concentration of convection near the NC/SC border region. Some hires models suggest enhanced rainfall totals (5+ inches) impacting portions of this area, but confidence is limited with respect to the details given proximity to the coast. Elsewhere, some southward expansion of the Slight Risk area down into the Cumberland Plateau was accommodated given potential for at least a narrow instability axis and corridor of strong moisture convergence for multiple bands of convection to focus here. This will further be aided by orographics and could support sufficient rainfall for some scattered areas of flash flooding. A Marginal Risk area was added to portions of the central High Plains (eastern CO and western KS) where a narrow axis of instability along with upslope flow into eastern slopes of the Rockies/Front Range will initially drive a convective threat over the terrain with potential for some small-scale MCS development/evolution downstream into the High Plains. Locally a couple inches of rain may fall with this, and thus will pose an isolated threat for some runoff issues. Orrison Previous Discussion... ....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 Day 3 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, AND PORTIONS OF THE GULF COAST... 2030Z Update... The inherited Marginal Risk areas will be maintained for the time being, but noting that there will likely at some point be a need for a Slight Risk to be embedded within potentially both Marginal Risk areas. Heavy rainfall in particular may be rather concentrated and impactful across areas of south-central TX early in the period as a new MCS traverses the region and heads gradually toward the coast and offshore. There is guidance led by the GEM regional, UKMET and even the RRFS that suggests a strong MCV associated with this will lift northeastward and potentially bring a heavy convective rainfall threat into areas of southwest to south- central LA, but there is a relative lack of agreement spatially and temporally regarding the exact details, so for now, the Marginal Risk area will be maintained. Over the central Plains, northwest mid-level flow will allow for shortwave energy to drop southeastward and interact with the pooling of increasingly moist and unstable return flow across the region. This will favor an MCS that drops southeastward over the region with impacts especially across parts of western KS and northwest OK, but latitudinal differences are noted in the latest guidance with respect to placement. Thus the Marginal Risk for now will be kept here as well. Orrison Previous Discussion... ....High Plains... Scattered areas of convection will occur on Wednesday afternoon and evening across the Central and Southern High Plains with locally heavy rainfall anticipated in any cell development. A notable theta_E ridge will align from the Rio Grande up through Southwest TX with the northern extension reaching the Eastern NM portion of the Caprock. MUCAPE between 2000-4000 J/kg will be positioned once again within the theta_E tongue during Wednesday afternoon, just in time for an interaction with a weak shortwave reflection moving into the area between 18-00z. Expect cells to fire within the terrain of West TX, eventually drifting off the topography and propagating into the adjacent Stockton Plateau and Pecos River Valley as we move into the late afternoon and early evening hours. Further north, a cells will fire in-of the Sacramento Foothills and drift towards the western Caprock in Eastern NM. A few cells will likely fire across the Caprock as well given the favorable RER jet dynamics sprung about by the approaching shortwave. This is a textbook MRGL flash flood risk with scattered convective elements capable of impacts, both severe and flooding in any given area. Further north off the Colorado Front Range and Western KS High Plains, cells will fire within a passing shortwave on the western flank of a large closed ULL centered over the Midwest with mean storm motions generally northwest to southeast out of the Front Range. Expect cells to flow in that general motion with some overlap of cells as they move through the area one after another. The setup will end with the cold pool mergers eventually making it into the TX Panhandle and rooting as they move away from the primary forcing. Locally heavy rain will be plausible in the setup with some guidance showing upwards of 3" during the setup. A low- end MRGL risk was maintained from previous forecast highlighting the threat and matches up with current ML First Guess Fields for D3. ....Southeast U.S... More thunderstorm activity will located within the confines of the stubborn quasi-stationary front situated near the Gulf Coast. Coverage and magnitude of potential convection is subjective to how each deterministic output evolves the front and any shortwave progressions nearby. Considering the bias corrected QPF mean of 2.5-3" in portions of the Southeast from the Upper TX coast to the Central Gulf Coast, wanted to cover for the low-end threat with at least MRGL risk and will assess as time goes. This is highly variable pending eventual frontal placement and convective evolutions upstream in the period prior. Areas of greatest concern will likely be more urbanized areas where runoff capabilities are highest. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-VAm_cLiIIzbKaLAr5JnNIuHAYG7GyDxUcSbLETLh9l4= Iae8pORUyI-grpIEd1CVSOt6zvuGnrnZXDmZ4JO7Xzp7YrI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-VAm_cLiIIzbKaLAr5JnNIuHAYG7GyDxUcSbLETLh9l4= Iae8pORUyI-grpIEd1CVSOt6zvuGnrnZXDmZ4JO77cjGwI8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-VAm_cLiIIzbKaLAr5JnNIuHAYG7GyDxUcSbLETLh9l4= Iae8pORUyI-grpIEd1CVSOt6zvuGnrnZXDmZ4JO7Hv9_FTg$=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) .