Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 25 2025 18:34:42 FOUS30 KWBC 251834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 234 PM EDT Tue Mar 25 2025 Day 1 Valid 1831Z Tue Mar 25 2025 - 12Z Wed Mar 26 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST TEXAS INTO SOUTHERN OKLAHOMA AND SOUTHEAST FLORIDA... ....Florida... Introduced a focused Marginal Risk area mainly in the urban corridor=20 along the southeast Florida coast from this afternoon into early=20 evening. Radar has shown an evolution similar to the 12Z run of the HREF with convection expanding north and eastward along the=20 urbanized I-95 corridor...and at least enough of a signal in the 1- and 2-inch per hour probabilities to warrant a Marginal Risk area. Refer to Mesoscale Precipitation Discussion 0083 for further=20 details.=20 Bann ....Texas/Oklahoma... Convection is expected to develop by 06z tonight across portions of southern OK near and just north of a stationary front. Southerly flow should advect enough moisture northward to generate upwards of 1000-2000 j/kg of CAPE with a base just above the boundary layer. This elevated convection will track off to the southeast, generally parallel to the low level convergence axis...which supports some training potential. The 3km NAM is the wettest model, but we have seen elevated convective setups like this verify towards the higher end of the model spread, and the 3km NAM has performed decently at these events in the past. So while it depicts a lower probability outcome based on all available guidance, and a swath of 1-3" is most likely, can not rule out a localized swath of 3-6" of rain across portions of northeast TX into southern OK late tonight into Wednesday morning. Overall the flash flood risk at any location is low given the dry antecedent conditions and narrow nature of any heavy rainfall swath. However intense rainfall rates and some training could allow for a localized flash flood threat to evolve, particularly if the heavy rain overlaps any more sensitive urban or low lying area. We opted to introduce a focused Marginal risk area to account for this isolated potential. Chenard 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". Convection initiating across the Sierra Madre in northeast Mexico will cross into SC and South TX Wed afternoon and overnight, and while considerable shear in the low-mid levels should keep the convection moving along, the slow progression of the mid-upper trough will likely result in slow-moving and/or repetitive areas of more organized convection, as evidenced by the higher-end QPF totals per the latest (00Z Tue) guidance...as much as 5-7+ inches per the Regional GEM, ECMWF, and FV3. Dry antecedent soils and initially high FFG values (3-4" over a large portion of the outlook area) will likely result in a more limited or localized flash flood threat, and as a result, have hoisted a Marginal Risk across these areas in the Day 2 ERO. 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 off the Gulf, becoming more parallel to the mean 850-300 mb flow with time by late Thursday, will lead to an increase in upward propagation and thus potential for cell training Thursday evening and overnight. This as low-level inflow taps the more unstable 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 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 that for now would likely end up being a higher-end Slight Risk despite the dry antecedent soils. These areas (closer to the Gulf coast) would also be an area to watch for a potential upgrade to Moderate Risk during subsequent outlooks, given the closer proximity to the more unstable airmass just off the coast and thus the greater potential for multiple hours of 2.0+ in/hr rainfall rates (higher probability of >6" of total rainfall), especially late Thu-Thu night with any cell 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 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 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. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9ve3IecHqx3CSJsoyLBirTeXLXy41_c2m2iUlDZk_9dO= zBhZhy3rNtNK38SuzscDR0LdaxX9JSn_WOS5acN3cRqwXKQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9ve3IecHqx3CSJsoyLBirTeXLXy41_c2m2iUlDZk_9dO= zBhZhy3rNtNK38SuzscDR0LdaxX9JSn_WOS5acN3eQqC8O4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9ve3IecHqx3CSJsoyLBirTeXLXy41_c2m2iUlDZk_9dO= zBhZhy3rNtNK38SuzscDR0LdaxX9JSn_WOS5acN3Ku5OWLY$=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) .