Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 25 2025 08:22:32 FOUS30 KWBC 250822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 AM EDT Tue Mar 25 2025 Day 1 Valid 12Z Tue Mar 25 2025 - 12Z Wed Mar 26 2025 The probability of rainfall exceeding flash flooding guidance is less than 5 percent. Hurley Day 2 Valid 12Z Wed Mar 26 2025 - 12Z Thu Mar 27 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS... Diffuse mid-upper level trough moving slowly eastward across the Southwest-Four Corners Region will lead to an uptick in upper diffluence across northern Mexico and SC-South TX Wed and Wed night, with a WNW-ESE lobe of mid level vorticity (shear axis) lifting northeast across the outlook area along the elevated warm between 18Z Wed and 12Z Thu. Strengthening low-level inflow (30-40 kts at 850 mb) will boost the low-level moisture transport, with TPW values climbing between 1.75-2.00".=20 Convection initiating across the Sierra Madre in northeast Mexico=20 will cross into SC and South TX Wed afternoon and overnight, and=20 while considerable shear in the low-mid levels should keep the=20 convection moving along, the slow progression of the mid-upper=20 trough will likely result in slow-moving and/or repetitive areas=20 of more organized convection, as evidenced by the higher-end QPF=20 totals per the latest (00Z Tue) guidance...as much as 5-7+ inches=20 per the Regional GEM, ECMWF, and FV3. Dry antecedent soils and=20 initially high FFG values (3-4" over a large portion of the outlook area) will likely result in a more limited or localized flash=20 flood threat, and as a result, have hoisted a Marginal Risk across=20 these areas in the Day 2 ERO.=20 Hurley Day 3 Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF COAST... ....Southern Texas... Aforementioned southern stream trough pivots slowly eastward across the Southern Plains Thu-Thu night. Rather healthy, deep-layer exit region forcing continues, with low-mid level moisture flux anomalies increasing to near 3 standard deviations above normal (TPW anomalies per the 00Z 3/25 GEFS) while 1.75-2.00" TPW values overspread the outlook areas. Continued healthy low-level inflow=20 off the Gulf, becoming more parallel to the mean 850-300 mb flow=20 with time by late Thursday, will lead to an increase in upward=20 propagation and thus potential for cell training Thursday evening=20 and overnight. This as low-level inflow taps the more unstable=20 airmass (1000-1500 J/Kg mixed-layer CAPEs) across the western Gulf. The guidance is beginning to converge with the heavy rainfall signal with the 00Z 3/25 models, as the GFS, ECMWF, GEM (esp. GEM Regional), and UKMet now all show a corridor of 4-8+ inches of=20 rain across portions of SC-South TX into the Gulf Coast during the 24hr period from 12Z Thu-12Z Fri. While there is still some areal and timing differences, the growing model consistency (especially among these global runs) has bolstered the confidence for a more targeted Slight Risk area, including areas closer to the Coast=20 that for now would likely end up being a higher-end Slight Risk=20 despite the dry antecedent soils.=20 These areas (closer to the Gulf coast) would also be an area to=20 watch for a potential upgrade to Moderate Risk during subsequent=20 outlooks, given the closer proximity to the more unstable airmass=20 just off the coast and thus the greater potential for multiple=20 hours of 2.0+ in/hr rainfall rates (higher probability of >6" of=20 total rainfall), especially late Thu-Thu night with any cell=20 training. ....Pacific Northwest... Another modest Atmospheric River (narrow ribbon of 500+ kg/m/s IVTs) will result in pockets of 3-5+ inch 24 hour rainfall totals=20 along the West Coast from Northern CA northward through OR-WA. Among the 00Z 3/25 models, the Regional GEM is by far the wettest, with isolated totals of 6-8+ inches. Despite the lack of=20 instability, the wet antecedent soils along with the potential of 0.3-0.5 in/hr rainfall rates could lead to localized short-term runoff issues, particularly along small streams and over burn scars.=20 Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6fq8bDC2j2ogS7-BjZUAwo57YQeQ5ssRcbX5jHRkrVd9= nDJeq9BajFUTD-fAIRKthcCTIGgs0ycI2BOC5LIZAU3tXlg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6fq8bDC2j2ogS7-BjZUAwo57YQeQ5ssRcbX5jHRkrVd9= nDJeq9BajFUTD-fAIRKthcCTIGgs0ycI2BOC5LIZ8YVI80o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6fq8bDC2j2ogS7-BjZUAwo57YQeQ5ssRcbX5jHRkrVd9= nDJeq9BajFUTD-fAIRKthcCTIGgs0ycI2BOC5LIZ2yIFtOI$=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. 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