Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 21 2025 21:28:54 FOUS30 KWBC 212128 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 528 PM EDT Mon Apr 21 2025 Day 1 Valid 2126Z Tue Apr 22 2025 - 12Z Tue Apr 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS AND PORTIONS OF THE DEEP SOUTH... 2130Z Update: MRGL risk was expanded further south to encompass the ongoing flash flood threat over the New Orleans metro with heightened concerns up through Baton Rouge. Rates between 2-4"/hr in the heaviest cells will be capable of producing totals between 3-5" in short time, enough to cause flood concerns through the rest of the evening.=20 Kleebauer ...Previous Discussion.. Texas Coast... On-going locally heavy rainfall over the TX coast will continue for the next few hours in response to persistent low-level confluence near a slowly-moving/stationary surface front. See MPD #154 for more information in the near term. Several inches of rain has already fallen, and even with high FFG values to start, sensitive areas will see some flooding where rain is heaviest and continues to fall (also in urban areas). PW values are high (>1.6 from CRP and near 1.5 at LCH per 12Z RAOBs which is near the 95th percentile for this time of year), winds are light from the sfc to 700mb, and rain rates >2-3"/hr have occurred and are forecast for this afternoon (per 12Z HREF) before subsiding this evening/overnight. Will continue with the Marginal Risk here given the high tolerance for rainfall and generally sandy soils. MS/AL... To the north, similar moisture exists and the cold front has been a little more progressive but will slow and perhaps stall this afternoon as it nudges a bit more to the southeast over northern portions of MS/AL. Isolated showers/storms this morning will become more scattered during the afternoon/evening - supported both by surface heating and low- level warm advection. Focus for higher rainfall and rain rates will be near/north of the LA/MS border to northern AL where FFG values are around 2-3.5"/hr. 12Z HREF neighborhood probabilities for >2"hr rates are around 30%, but some CAMs were showing a few areas of >3-5" rainfall this afternoon. Slowing front may allow for some training and a locally higher threat for flash flooding (especially in urban areas like Birmingham) so we have maintained the Marginal Risk outline. Fracasso Day 2 Valid 12Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TEXAS PANHANDLE INTO WESTERN OKLAHOMA AND THE CENTRAL GULF COAST... ....Texas and Oklahoma... Positioning of a west to east oriented warm front along with a dryline over west TX will maintain a surface setup capable of being focal points for convective development Tuesday afternoon and evening. Aloft, a quick moving shortwave ejecting out of the Southern Rockies will navigate over the Caprock aiding in focused mid-level ascent within the confines of the boundary and relative instability maximum situated near the front and points south and east. Scattered convection will likely billow up off the eastern NM Caprock down into the Stockton Plateau leading to strong mesoscale cores maintaining a prevalent convective footprint over the Southern Plains. Cell propagation will be east to southeast motions given the 850-250mb shear field, entering into fairly unstable environments downstream that will be capable of holding the convective pattern well into the evening. PWAT anomalies >1 deviation above normal will be present meaning rainfall rates between 1-2"/hr will be plausible during cell life cycles. 12z HREF probs for >2" are running between 15-40% which typically isn't a significant output for threats of flash flooding, however the elevated >1" signals (50-80%) for a threat that will likely be 1-3 hrs over any one location in the zone of impact will likely be problematic for an area that has a large coverage of low water crossings and poor drainage areas. These types of setups tend to be tricky, but the setup is historically one that favors isolated flash flood concerns, especially within any urbanized areas across the Caprock into the western Red River Basin. The previous MRGL risk was generally maintained and expanded to include some areas where probs have improved for heavy rain threats. ....Gulf Coast... Slow-moving frontal progression will shift focus of convection across the Southeast into the Lower Mississippi area with emphasis across all of Southern LA. The proxy of high theta_E's aligned within a shearing frontal zone will enhance prospects for locally heavy rainfall with cores generally anchored to any remnants of the front as it decays within the southern Parishes of the state. 12z CAMs were insistent on some type of heavy convective footprint across an area from Beaumont, TX over into Southeastern LA, including the New Orleans Metro. Areas outside the urban corridor are less likely to see flooding in these scenarios, but plenty of urbanization exists in that aforementioned zone. HREF probs for >5" are upwards of 30-50% with a general bullseye just west of the New Orleans metro corridor. This area is one of the more prone locations for flash flooding due to road density and impervious surfaces more conducive for runoff. The threat is modest at best due to the uncertain nature of where the pulse convective regime will initiate, however the individual totals being depicted via CAMs (5-8") and some global deterministic (3-5") are very much capable of causing some flash flood concerns across the area. Decided to introduce a MRGL risk to cover for the threat with emphasis again on the urban areas being most at risk over Southern LA. Kleebauer Day 3 Valid 12Z Wed Apr 23 2025 - 12Z Thu Apr 24 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... A myriad of shortwaves will eject out of the Rockies through the course of Wednesday with several pockets of convective development sprawled across the Central and Southern High Plains by Wednesday evening. General assessment of the pattern yields lower confidence in QPF maxima, but higher confidence in the potential for scattered flash flood prospects where convection does percolate and fester within the overall pattern. Ensemble depictions are pretty aggressive considering the lack of cohesion in the proxy of precip placement settling between a widespread 1-2" totals, locally higher positioned from the Front Range of the CO/WY/NE intersection down through KS/OK with a southern inflection across the Red River Basin bordering TX/OK. For ensemble means at lead, this is fairly robust considering the circumstances of limited agreement in any one area to see heavy precip. Some of these areas will have seen significant rainfall prior to any additional impacts leading to some localized lower FFG indices once the event occurs. This led to First Guess Field forecasts for D3 depicting a massive MRGL risk area encompassing much of the center of the CONUS. Considering the variability in precip specifics, but a pattern supportive of locally heavy rainfall, decided to add a MRGL risk across much of the Central and Southern High Plains with and extension northwest into parts of the Front Range where some guidance indicates a threat of a complex developing in-of the tri- state intersection off the High Plains in the lee of the Laramie Range. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_zeaHM1mKS9hpL6rw_mOtTQfHKSeDeoEqdcjuWNZYevX= YG4Q0Jm337K-mli4MZhLdcZb15__MK_Oz4YxZwkElGZDY08$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_zeaHM1mKS9hpL6rw_mOtTQfHKSeDeoEqdcjuWNZYevX= YG4Q0Jm337K-mli4MZhLdcZb15__MK_Oz4YxZwkEp0fDRaE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_zeaHM1mKS9hpL6rw_mOtTQfHKSeDeoEqdcjuWNZYevX= YG4Q0Jm337K-mli4MZhLdcZb15__MK_Oz4YxZwkEqJSdoLQ$=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) .