Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 28 2025 00:59:39 FOUS30 KWBC 280059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Tue May 27 2025 Day 1 Valid 01Z Wed May 28 2025 - 12Z Wed May 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST U.S. AND TEXAS... ....Southeast and Mid-Atlantic... A Slight risk was maintained from portions of the central Gulf Coast into central TN. Ongoing convection will continue across the corridor through the evening, and with lowered FFGs from previous rainfall, isolated to scattered flash flooding will remain possible. Recent HRRR and RRFS runs indicate some upscale growth=20 on the southern edge of this convection over the next few=20 hours...potentially allowing for some brief training across=20 portions of far southern MS, AL and the far western FL Panhandle.=20 Much higher FFGs over this area should limit the extent of flash=20 flooding...however localized rainfall over 3" in a short period=20 could result in a few flood related issues within any more=20 sensitive locations. Portions of coastal NC will also need to be monitored into the overnight hours. Convection along a coastal trough is resulting in slow storm movements and locally heavy rainfall. Indications that the MCV associated with ongoing GA convection may help enhance=20 this coastal convection overnight. The main question remains=20 whether the bulk of this potential training convection ends up=20 onshore or just offshore. Blending the 22z and 23z HRRR gives an=20 impressive 5-10" swath of rainfall in between Wilmington and=20 Newport. However recent radar trends indicate the convection slowly propagating off to the east, getting close to moving offshore.=20 Thus quite possible the HRRR is too far inland with its axis of=20 more extreme rainfall amounts. So While the HRRR can not be ruled out the more likely scenario seemingly keeps a bulk of any extreme rainfall just offshore...either way will need to closely monitor=20 radar trends. ....Texas... A Slight risk remains for portions of southwest to central TX.=20 Most signs point towards the ongoing convection organizing and=20 growing upscale into an MCS, or at least a cluster or two,=20 overnight. In the organization phase (near the western edge of the Slight risk) cell mergers should result in isolated to scattered=20 flash flood issues. If/when the convection organizes it should=20 generally be more progressive off to the east...however FFG lowers=20 to the east and hourly rainfall could still exceed 2" even in a=20 quicker moving system...so a Slight risk still seems appropriate=20 for these areas overnight. Chenard 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 HIGH PLAINS, TEXAS, LOWER MISSISSIPPI VALLEY, CENTRAL GULF COAST, AND THE COASTAL MID-ATLANTIC... ....20Z Update... One change made to the previous ERO issuance was to add a Marginal Risk to coastal North Carolina into the Mid-Atlantic. This is the back end of the rainfall occurring on Day 1/Tuesday night that may linger after 12Z Wednesday. Many CAMs in particular show some lingering rain before pulling offshore later Wednesday, and some localized flooding issues are possible. With the existing Marginal Risk, expanded the Marginal Risk north into Wyoming. Similar to the Day 1/Tuesday period, some moisture and instability could lead to slow moving convection, and HREF probabilities for 2 year ARI exceedances are localized at more than 50 percent, with low end potential for 10 year ARIs to be exceeded. Also adjusted the Marginal to include more of the central Plains per recent guidance. Highest QPF still looks to be in south-central Kansas or so, from possible Wednesday morning convection and the potentially heavier Wednesday night convection. But continued to hold off on any Slight Risk upgrade given the general antecedent dryness and the movement of the convection with possibly only a brief period of training. Farther south, certainly some moisture (probably above the 90th percentile for PWs) and instability in southeast Texas for some flooding risk. 12Z HREF probabilities of exceeding FFG showed some relatively high probabilities for that region, but this was likely influenced by the briefly lowered FFG from this (Tuesday) morning storms. Given that FFG rebounds in these coastal and near coastal areas relatively quickly, expect these probabilities may go down in future HREF cycles. Still, will continue to monitor if this may need a Slight Risk in the Day 1 period. Farther east, global and CAM guidance has decreased with QPF toward the Florida Panhandle, with just some scattered showers and storms. Though localized storms may be slow moving, the scattered nature and higher FFG in that region seems like the risk for flash flooding is less than 5 percent. Tate ....Previous Discussion... ....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... ....20Z Update... As mentioned in the previous discussion, the pattern Thursday is quite tricky, with a possible MCV tracking east across the Southeast the start of the period while a shortwave rounding the upper low will both provide upper level support for thunderstorms but with uncertainty in the placement of both. Generally the signal this cycle is for the MCV to push through faster and show the heaviest QPF farther southeast on Thursday than in the previous cycle, and perhaps lighter with max QPF amounts, but still with considerable model spread. Adjusted the Slight Risk southeast given the model trends. Hated to leave the Slight out of the higher FFG in the central Gulf Coast because of models (like the UKMET) and ensembles (like the EC ensemble) show the heaviest QPF more along the coast. Thus show the Slight Risk covering from the central Gulf Coast and inland into the Southeast, but future adjustments will likely be needed. The Marginal Risk is similar to the previous issuance though with some minor expansion northward into the Lower Ohio Valley for a possibly separate QPF axis there closer to the vort max. Tate ....Previous Discussion... 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!7pTKqL-07pKnxhcyDPTAeA8m847RlQNH7h-5FHtZ7QmQ= LLyuQWBIoV4r-PZIEuMkmM-sTuptLOE3bTapHfEovBov5jQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7pTKqL-07pKnxhcyDPTAeA8m847RlQNH7h-5FHtZ7QmQ= LLyuQWBIoV4r-PZIEuMkmM-sTuptLOE3bTapHfEoU_XKFQU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7pTKqL-07pKnxhcyDPTAeA8m847RlQNH7h-5FHtZ7QmQ= LLyuQWBIoV4r-PZIEuMkmM-sTuptLOE3bTapHfEoMu2z-Qs$=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) .