Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 30 2025 15:51:21 FOUS30 KWBC 301551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Wed Apr 30 2025 Day 1 Valid 16Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN ARKANSAS... The threat of significant flash flooding continues today, with the focus increasingly shifting to portions of north Texas based on observational trends and the latest 12Z hi-res model guidance. A bowing convective line continues to advance eastward through S OK, with a trailing line of convection extending into N TX on=20 the southern periphery of strong cold pool and mesohigh situated=20 over SC OK. Over the past couple hours, the convective line in N=20 TX has become increasingly oriented NE-SW, or even ENE-WSW; in=20 other words, more parallel to the mean flow vector. This has=20 increased the potential for training and backbuilding, especially=20 into the afternoon hours as the LLJ begins to weaken and low-level=20 winds start to veer to the southwest. For a more detailed mesoscale analysis, please refer to MPD #202 issued at 14Z for this region. The Moderate Risk area was maintained in SE OK and W AR, even though confidence in flash flooding has decreased somewhat due to the fairly steady forward progress of the bowing line, and how thin the convective line has become. However, it was maintained out of deference to continuity and the potential to reinvigorate the=20 convection in the next few hours with some clearing occurring ahead of the line, which could increase rain rates again. Greater concern now exists in N TX. The forward progression of the line and associated cold pool in S OK and overall convective evolution has led to far more west-east orientation to the convective line in N TX than models had originally been advertising. This certainly increases flash flood potential and over the next few hours this places the Dallas-Ft. Worth metro area squarely in the greatest risk. Hourly rainfall could exceed 2 inches per hour, with unobstructed inflow from a very moist and unstable air mass (MLCAPE 1500-2000 j/kg with minimal CINH; PWs=20 1.6 to 1.8 inches per RAP analysis.) In an urban environment, this could lead to significant and life-threatening flash flooding, particularly if the rain rates are sustained for a couple hours, which is a distinct possibility in this case.=20 As the afternoon and evening progress, the hi-res models do indicate the potential for the southwestern periphery of the convective line to continue to slow or stall. While they don't necessarily agree on the specifics, many HREF members, as well as the RRFS, indicate backbuilding and training convection could become more likely in the corridor from Waco to Longview. This seems to be well supported by observational trends, and in fact many hi-res members are: (1) already not extending the convective=20 line far enough southwest based on current radar trends, and (2) have the convective line oriented far too much in a N-S fashion. Therefore, they may be underestimating rainfall potential a bit, and also underselling the potential for a significant rainfall and flash flood event further to the southwest. Probabilities and associated risk categories were increased a bit down the I-35 corridor between DFW and AUS, and this will be monitored for further updates based on observational trends. Rain rates in much of C/N TX could reach or exceed 2 inches per hour given the=20 available instability and moisture. The other biggest change for this outlook was to extend the Slight Risk a bit further northeast through MO and into W IL based on more elevated HREF probabilities of exceeding 1hr and 3hr flash flood guidance and a fairly consistent signal in the guidance for a corridor of enhanced rainfall associated with an MCV and corresponding surface low. Lamers Day 2 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF COAST... Surface cyclogenesis across the Southern Plains will lead to a steady low progression to the northeast up through the Mississippi Valley into the Great Lakes towards the end of the forecast cycle. A cold front will trail the vacating low allowing for some frontal convergence to occur across the above areas with the greatest convective coverage likely to be within the warm sector of the Ohio and Tennessee Valley into the southern Great Lakes. PWAT anomalies between +2 and +3 deviations will be present for much of the CONUS situated east of the Mississippi lending to a fairly rich environment conducive for moist convective cores from the Gulf coast up to the Great Lakes region. The reasoning for the heavier convective threat north is the added sheared profiles within the bounds of the warm sector creating better mesocyclone maturity and stronger storms. All this to say....heavy rain prospects for longer periods more likely within the zone encompassing the Tennessee and Ohio Valleys. Current forecast totals region-wide over the Southeast up into the Ohio Valley are pretty consistent with an areal average between 0.5 to 1" with some locally enhanced totals reaching upwards of 2-2.5" in the hardest hit locations. Given rainfall rates peaking between 1-1.5"/hr when assessing the probabilistic output of both the NBM and 00z HREF, there's a cap on the higher end potential of any flash flood scenario leaving this overall threat in the broad MRGL risk coverage, as inherited. There is another area of interest down across the Rio Grande extending from the Big Bend down to about Laredo for tomorrow evening as a shortwave perturbation ejects east off of Coahuila with some convection spawning over the Serranias del Burro. Some of the CAMs are pretty significant in the overall convective development, but much of the CAMs reach the end of their runs prior to the activity making its way towards the Rio Grande Valley. Other deterministic and ensemble bias corrected means tell the story in full with some respectable QPF outputs within the above bounds leading to potentially some localized flooding within some of the urbanized zones between Del Rio down to Laredo. Considering the environment to be favorable and this scenario having a long history of potential, opted to keep the MRGL risk from previous forecast issuance to outline the threat for a quick 1-3" of rainfall with locally higher amounts plausible pending cold pool convergence evolutions. Kleebauer Day 3 Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF CENTRAL TEXAS... A cold front progression stemming from a trailing cold front and pressing surface high over the Northern Plains will lead to significant low-level convergence regime over Central TX by Friday afternoon. Return flow around the western flank of a broad surface ridge positioned over the Western Atlantic will reach back towards the southern half of TX leading to a convergent pattern in conjunction with slow-moving cold front moving north to south. Models have been steadily increasing precip coverage within the confines of the boundary once it makes headway into the Hill Country of the Lone Star State by late-Friday morning, carrying through the rest of the period as the front encounters higher theta_E airmass within the western periphery of that surface ridge. PWATs between +1 and +2 deviations will be present once the front makes its presence across the region, a sufficient environment for deep moist convective development along and ahead of the boundary as it slowly migrates south. Ensemble means have upped precip output in recent runs with a solid 1-2" areal average now encompassing a large portion of the state from the Stockton Plateau, east through Hill Country, the I-35 corridor, all the way back towards the Piney Woods area of East TX. Favorable environment with likely training echoes in proxy to the cold front will generate locally enhanced QPF maxima, especially within the bounds of the terrain where an added weak upslope component could yield greatest results (3+" totals). The convergent pattern can be seen as far east as the Southeastern CONUS, but the greatest surface frontogenesis signal is situated back over the Southern Plains, south of I-20. A SLGT risk was added across the above area with a broad MRGL encompassing the higher risk as scattered convection will still be present up towards I-20 extending east through the Lower Mississippi Valley up through Northern AL and Eastern TN. Will be monitoring the setup closely as the setup will likely be most prolific within one of the flashier areas of the country. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!8JYv3_3vErz0562OCSAEyf0IRviJoNKVNSAOQ7-SPDNf-KLySIzJD7gIqnh1aUz= ihIjriYSQxTMaCJF4DosR2jry334$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!8JYv3_3vErz0562OCSAEyf0IRviJoNKVNSAOQ7-SPDNf-KLySIzJD7gIqnh1aUz= ihIjriYSQxTMaCJF4DosRbc3geoU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!8JYv3_3vErz0562OCSAEyf0IRviJoNKVNSAOQ7-SPDNf-KLySIzJD7gIqnh1aUz= ihIjriYSQxTMaCJF4DosRPURawxg$=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) .