Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 30 2025 20:17:35 FOUS30 KWBC 302017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM 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 the southern periphery of strong cold pool and mesohigh situated over SC OK. Over the past couple hours, the convective line in N TX has become increasingly oriented NE-SW, or even ENE-WSW; in other words, more parallel to the mean flow vector. This has increased the potential for training and backbuilding, especially into the afternoon hours as the LLJ begins to weaken and low-level 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 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 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. 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 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 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 SLIGHT RISK OF EXCESSIVE RAINFALL IN SOUTHERN OKLAHOMA, FAR NORTH TEXAS, AND FAR WESTERN ARKANSAS... The most significant change for this outlook period was to introduce a Slight Risk primarily over southern Oklahoma, where previously the probabilities were below the Marginal Risk (5 percent) threshold. This was due to an emerging consensus in model guidance that thunderstorms will blossom on Thursday Night ahead of a digging shortwave in an area of relatively strong upper level divergence. Although the mid levels will be drier than in recent days, leading to lower PWs overall, strong instability will be in=20 place which should support organized convection with more intense=20 rain rates at times. A key uncertainty at the moment is the overall structure of convective clusters and lines, and whether they will=20 be more progressive or include some training. However, the=20 potential for heavy rain rates in an area that has received a lot=20 of rainfall already in recent days does raise the risk of flash flooding. Hourly rainfall on the order of 1-2 inches will be possible in this environment, and in areas with already wet ground conditions this would be more than sufficient to cause flash flooding. Elsewhere, the broad Marginal Risk that was already in place was maintained with few changes. A broad plume of moderate to strong instability will favor scattered lines and clusters of thunderstorms in advance of a surface low and trailing cold front with fairly high instantaneous rain rates. The key uncertainty would be the persistence of those rain rates and the degree to which any training could occur, but the overall environment with PWs around 1.3 to 1.4 inches raises the probability high enough to warrant the broad Marginal Risk. The only exception to the continuation of the risk area was over much of IL, IN, and MI, which should largely be behind the cold front by 18Z and so most of the QPF on model guidance would be associated with post-frontal showers that would tend to have lower rain rates overall. Lamers Day 3 Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FROM THE SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY... A flash flood event may be ongoing at the beginning of the period, with convection developing on Thursday Night in southern Oklahoma and continuing into Friday morning. This may continue beyond 12Z Friday (into this Day 3 period) in SE OK and AR in the region of strong upper-level divergence. The existing Slight Risk over TX was expanded northward to account for this scenario. Whether that round of convection wanes due to more extensive cloud cover and limited daytime heating, or progresses southeast and becomes reinvigorated during the afternoon, the threat of heavy rainfall and flash flooding should expand southeast on Friday into the Lower Mississippi Valley (S AR, N LA, W MS) and much of C and E TX. This would be via progression of the early round of storms, or redevelopment in the afternoon and evening hours.=20 Strong instability will remain over much of the warm sector with=20 CAPE values potentially in excess of 3000 j/kg in some areas, and=20 PW values are expected to increase above 1.5 inches as SSE flow=20 increases off the Gulf. The combination of strong instability and=20 high PWs will favor very heavy rain rates in organized convection, and this is supportive of the broad Slight Risk on the new=20 outlook. At the moment, model guidance is showing the most=20 consistent signal for heavy rain from Texas Hill Country in C TX=20 east- northeast into the Piney Woods region of E TX. Given the=20 overall environment, it is possible an upgrade to a Moderate Risk=20 may be needed at some point. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!6xRxQekbUzFBT0Qg_LxjCsF7ZP3yAmkoM8lbioY-aLmehrj-mYvjVklzNW85PXg= 3fMYSpxyelbZpIe-AyurHgrfoRgw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!6xRxQekbUzFBT0Qg_LxjCsF7ZP3yAmkoM8lbioY-aLmehrj-mYvjVklzNW85PXg= 3fMYSpxyelbZpIe-AyurHrhh3wFU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!6xRxQekbUzFBT0Qg_LxjCsF7ZP3yAmkoM8lbioY-aLmehrj-mYvjVklzNW85PXg= 3fMYSpxyelbZpIe-AyurHnQq4WZ4$=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) .