Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 11 2025 15:46:09 FOUS30 KWBC 111545 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1145 AM EDT Wed Jun 11 2025 Day 1 Valid 16Z Wed Jun 11 2025 - 12Z Thu Jun 12 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL TEXAS... ....Texas... 16Z Update: The forecast premise remains the same with 12z CAMs finally all catching on to a better degree of what has occurred, what is occurring, and general expectations going forward. Expected to see a redevelopment of convection across East-Central TX this afternoon within a pocket of higher theta_E remaining over the I-45 corridor between DFW/HOU, aligned well within the higher CAPE, lower CIN depiction on the recent RAP Mesoanalysis. This will congeal and move northeast allowing for some of the areas hit hard this morning to get hit again in wake of the departing complex moving through the ArkLaTex. Multiple FFW's have occurred to the southeast of the DFW metro, an area where local FFG's have taken a hit over the past 24-36 hrs. This setup will allow for a continued impact in those zones, so the MDT was expanded a bit further to the northeast to account for the trends in the latest CAMs and impacted areas.=20 ULL progression over West TX will maintain a posture of relatively modest buoyancy returning over the Concho Valley down through the eastern Edwards Plateau and adjacent Hill Country by late-afternoon and evening. Stronger mid-level perturbations pivoting around the ULL center will eventually rotate back around and provide a significant increase in regional ascent within the above zone leading to cell initiation back over areas hit pretty hard the past 24 hrs. Incredibly low FFG's (<1" over 1/3/6 hr intervals) are centered over Hill Country with much of the CAMs depicting some action moving back over the area creating an environment capable of inducing flash flood prospects over the region as CAMs print out 1-2"/hr rates in some of the stronger convection in the area. By the evening, the increasing large scale forcing will navigate into Central TX with the I-35 corridor between DFW to San Antonio and 50 miles either side becoming the primary target for heavy rainfall.=20 Return flow within the 850-700mb level is expected to allow for greater low-level convergence across the area with convection orienting more southwest to northeast with some training prospects along the tight theta_E gradient anticipated between eastern Hill Country and the I-45 corridor. HREF blended mean QPF is very high (3-5") within the I-35 corridor between Austin to Waco leading to 40-60% neighborhood probs for >5" over the area. Will need to monitor for areas south of Austin as a secondary maxima down towards New Braunfels and San Antonio has been showing up on guidance with some of the higher probs for >5" also expanding into that zone. With those locations inheriting higher FFG's, prospects for flash flooding a bit less than those to the north, but a higher-end SLGT is now forecast for that area with some potential of an expansion of the MDT if trends continue.=20 The SLGT was removed over CRP and parts of Deep South TX in conjunction with radar trends. The southern tip of TX, including the Brownsville/McAllen area was removed from any ERO as the area will maintain a relative min in QPF for the period.=20 Kleebauer ...Previous Discussion.. In coordination with FWD/Dallas, TX and HGX/Houston, TX forecast offices, a Moderate Risk upgrade was introduced with this update for portions of central Texas. As is typical with summertime convection, the signal is a mess. The current MCS in central Texas is largely agreed upon in the CAMs guidance to persist well into the afternoon and advance across the rest of east Texas and into Louisiana, as a broken but slow moving line of storms. Most of the guidance suggests new convection will form in the afternoon with peak heating and instability across the Texas Moderate and Slight Risk areas. Those storms will generally track north and east, which will follow behind the first round of storms into east Texas. Most importantly, yet another round forms during the overnight, likely in the Moderate Risk area. This too will organize into an MCS, but the southern end of those storms may hang back and backbuild, resulting in prolonged heavy rain in a narrow swath somewhere in central Texas. While this is the broad scenario expected, the details of each round of storms will play a large role as to the coverage and severity of flash flooding today and tonight. The majority of the guidance suggests this will be most prevalent in the Moderate Risk area of central Texas, but this is far from unanimous, with a second grouping of guidance suggesting these storms form further south towards Houston. Thus, this is an unusually low confidence Moderate Risk for the Day 1 period. Compounding concerns is that much of northern and central Texas has seen multiple rounds of heavy rain in recent days, including at present, which has sufficiently saturated soils across the area. Thus, multiple inches of rain associated with these storms which will be drawing on air with PWATs to 2 inches, will quickly overwhelm streams and creeks, especially in urban areas, resulting in flash flooding. These very favorable antecedent conditions have also contributed to confidence increasing for a Moderate Risk upgrade. HREF guidance should neighborhood probabilities for at least 3 inches of rain today and tonight in the Moderate Risk area of over 70%, and a 20-50% chance of over 5 inches of rain. Given the abundance of moisture available, both slow moving storms, multiple rounds of heavy rain in some areas, and the potential for training and back building will all work to increase the flash flooding potential across much of eastern Texas, also supporting the Moderate Risk upgrade. As the convective evolution becomes more clear through the day and into tonight, it's likely there may need to be adjustments to the Moderate Risk area, which as mentioned above, appears most likely to extend south to the Houston Metro. However, given the multiple rounds of rain that have impacted the DFW Metroplex in recent days, and the heavy rain that has hit San Antonio over the past few hours, any storms favoring any of those metros could require the Moderate Risk to expand to those areas of the Texas Triangle. Wegman ....Gulf Coast through the Carolinas... 16Z Update: Minor expansion north of the MRGL across parts of the Lower Mississippi Valley as 12z CAMs came in a bit north with the heavier precip today. Still looking at locally robust rates of 2-3"/hr in the heaviest convection, but very much more pulse variety as you align between Central LA across the South and up into the VA Tidewater. Modest neighborhood probabilities for >3" (35-50%) exist over parts of the Central Gulf Coast and Coastal NC, so the threat for at least low-end MRGL persists. No changes we made in those regions. Kleebauer ...Previous Discussion.. A stalled front along the Gulf Coast will act as a focus for afternoon and evening convection due to an abundance of Gulf moisture available for the storms to draw from. The broad Marginal Risk remains in place for all of the rest of the Gulf Coast and through the Carolinas. The signal has expanded a bit north so the Marginal was expanded north as well to the Hampton Roads area. Wegman ....Upper Midwest... 16Z Update: The MRGL was expanded a bit eastward to include western Milwaukee metro, but otherwise the threat remains on the low-end of MRGL. A relatively high probability on the EAS of at=20 least 1" across parts of Northern IA and Southwest WI exist (60-80%), but a significant drop off in the probs (<15%) for at=20 least 2" means the threat is capped between 1-3" for the general=20 max. With antecedent moisture pretty low over the area, this is a setup coincident with a MRGL.=20 Kleebauer=20 ...Previous Discussion.. The Marginal Risk in the area around the MN/IA border has been left largely unchanged, with most of the concerning convection likely occurring late tonight at the nose of the LLJ. Wegman ....Idaho/Montana... 16Z Update: PWATs running between 1.5-2.5 deviations above normal across the Northern Rockies and interior PAC NW will lead to locally heavy rainfall potential over Eastern OR through much of ID/MT/WY this afternoon and early-evening before dissipating with loss of diurnal heating. CAMs indicate the risk of 1-2" of rainfall in heavier cores which would certainly cause problems in any urban zones and remnant burn scares. The MRGL was expanded through WY and adjusted a touch south. The threat remains sufficient for a broad risk area given the environment in place.=20 Kleebauer ...Previous Discussion.. The Marginal Risk was left largely unchanged here as well with periods of rain occurring over mostly hydrophobic soils resulting in localized flash flooding, particularly around any burn scars. Wegman Day 2 Valid 12Z Thu Jun 12 2025 - 12Z Fri Jun 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARKLATEX REGION AND MUCH OF EASTERN TEXAS AS WELL AS FOR SOUTHERN MINNESOTA INTO WESTERN WISCONSIN... ....Arklatex Region & East Texas... Strong southerly flow of deep Gulf moisture will continue across east Texas and the Arklatex region on Thursday. There will very likely be an ongoing MCS across eastern Texas at the start of the period Thursday, which will progress eastward into Arkansas and Louisiana through the day. There may be a lull in most of the shower and thunderstorm activity for the afternoon hours in many areas, but renewed activity will develop through the evening as the LLJ restrengthens, and a slow moving upper level shortwave helps drive a dryline east over Oklahoma and Texas, which will cause numerous storms to form and organize into an MCS near the Arklatex region through the overnight, as PWATs remain up near 2 inches, promoting very heavy rainfall with the strongest storms. Once again, this region has been hard hit with heavy rain in recent days, and therefore this latest round of heavy rain will not take much to result in flash flooding. Depending on how much rain falls in this region on Day 1/Wednesday, it is possible a Moderate Risk upgrade may be needed for this region as well as confidence increases and the CAMs include the Thursday night period. There is enough uncertainty as to how far west the storms are (which will be highly dependent on the placement of the dryline), and therefore, the Metroplex, Houston, and Austin remain included in a higher end Slight Risk, which extends into the southeast corner of Oklahoma and adjacent western Arkansas counties. ....Southern Minnesota into Western Wisconsin... At the nose of the upper level jet, a mesolow is likely to form over the central Plains Thursday afternoon, which will then drift northeast into Iowa on Thursday night. Abundant Gulf moisture will ride northward ahead of the low, running into a strong stationary front which will likely be set up over central Minnesota. This front will provide the lift for training and backbuilding thunderstorms across much of southern Minnesota Thursday afternoon and evening. The storms appear likely to impact all of the Twin Cities and surrounding suburbs, which when added to the wetter soils from any rainfall from Day 1/Wednesday, may result in widely scattered instances of flash flooding. The Slight Risk area was nudged northward with this update, following the latest changes in the ensemble guidance, but rainfall amounts are largely unchanged. Wegman Day 3 Valid 12Z Fri Jun 13 2025 - 12Z Sat Jun 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARKLATEX REGION INTO PORTIONS OF THE MID-MISSISSIPPI VALLEY... A slow-moving upper level trough will drift east across Oklahoma and Arkansas through the period. It will draw on abundant Gulf moisture with PWATs at or above 2 inches to force numerous slow moving thunderstorms to develop from northeast Texas northeast to the Mississippi-Ohio River confluence. The southerly flow will also upslope into the Ozarks of central Arkansas, likely resulting in localized increases in rainfall rates across much of Arkansas on Friday and especially into Friday night. Given the expanded eastward movement of the storms, the inherited Slight Risk was expanded significantly north and east into the mid-Mississippi Valley with this update. A higher-end Slight is in effect for the Arklatex region through much of central Arkansas, where the heaviest rainfall is expected. Periods of heavy rain across the Arklatex and western Arkansas are expected from the Day 2/Thursday period, which will greatly moisten the antecedent soil conditions in advance of the heavy rainfall expected on Friday and Friday night. Here too, a Moderate Risk upgrade may be needed with future updates should the preponderance of guidance remain consistent at targeting the Arklatex and central Arkansas with the heaviest rainfall. New Marginal Risk areas were added across much of the Great Lakes, where training storms may cause isolated flash flooding, the northeast corner of Colorado due to some upslope with the flow turning southeasterly into an expected stationary front over that area, and the third consecutive day of moderate rainfall across much of central Montana, aided by increasing divergence ahead of a potent shortwave moving across the Pacific Northwest by Friday night. In each of these areas, only isolated flash flooding is expected as the signal for heavy rain in these areas remains diffuse. Isolated to scattered disorganized convection is likely to impact much of the eastern seaboard Friday afternoon with peak heating. Given the increasing moisture moving up the coast, any storms that remain in a given area for too long or backbuild along their cold pools could cause isolated instances of flash flooding. Given a lack of discernible forcing, for now have left the area out of a Marginal citing a less than 5% chance of flash flooding, but the threat is non-zero. The Marginal may need to be expanded eastward across much of the eastern seaboard in the coming days should current trends continue. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8LycBScaiZ1SzQ01Z_wb8e-GEBM-hxHO7UhGAP8OkRa7= CsPOEJeXkRh1f892s-jlkmCrw_KT3VF7IAROTr_jlmaUnD0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8LycBScaiZ1SzQ01Z_wb8e-GEBM-hxHO7UhGAP8OkRa7= CsPOEJeXkRh1f892s-jlkmCrw_KT3VF7IAROTr_jIOHiTMw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8LycBScaiZ1SzQ01Z_wb8e-GEBM-hxHO7UhGAP8OkRa7= CsPOEJeXkRh1f892s-jlkmCrw_KT3VF7IAROTr_jbmvpypo$=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) .