Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 25 2025 21:05:35 FOUS30 KWBC 252105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 505 PM EDT Sat Oct 25 2025 Day 1 Valid 01Z Sun Oct 26 2025 - 12Z Sun Oct 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN TEXAS INTO CENTRAL ARKANSAS AND WESTERN MISSISSIPPI... ....16z Update... The marginal risk area over the Northwest coast remains mostly unchanged. In the Southern Plains/Lower Mississippi Valley, ripples of energy will rotate around the base of a mid-level low helping to generate thunderstorm activity across portions of eastern Texas, Louisiana and into Mississippi today. A preceding line of storms from this morning should collapse and dissipate by this afternoon. A complex of storms will likely develop over eastern Texas later this afternoon and propagate eastward into Arkansas, Louisiana and Mississippi this evening. The heaviest rainfall is expected to occur between 00z and 09z within a moisture rich environment (1.75-2.25in. PWAT--90th percentile). There will also be sufficient instability (500-1000J/Kg ML CAPE) over much of the slight risk area. HREF neighborhood probabilities of exceeding 5in. and 3in. in 24 hours have increased over parts of southeastern Arkansas/northwestern Mississippi, which supports the expansion of the slight risk into those areas. Kebede ....Previous Discussion... Multiple rounds of heavy showers and thunderstorms are expected to impact the broader northwest Gulf Coast region this period, with one initial axis of on-going convection moving across parts of eastern/southeastern Texas and advancing across western Louisiana and southern Arkansas. Convection presumably is interacting with a 40 kt southerly low-level jet off the western Gulf. Precipitable water values at or above 1.75 inches and MUCAPE values locally of 2000 J/kg will support high rainfall rates potentially exceeding 2 inches/hour with the stronger storms. Additional organized convection is expected later today/tonight with the arrival of an upper- level jet streak in the subtropical stream which interacts with the southern stream closed low/trough axis crossing the southern Plains. Even though antecedent conditions are quite dry, the concern for multiple rounds of convection which combined may yield some 3 to 5+ inch totals, will probably be enough to support a threat for scattered areas of flash flooding. The more sensitive urbanized locations will be at greatest risk for impacts. ....Pacific Northwest and Northwest California... Multiple rounds of moisture transport into the coastal ranges will continue as an atmospheric river activity continues...with greatest rates/accumulation from later this morning into the afternoon. Locally a few additional inches of rain will be possible for the windward slopes of the terrain, but given the early-season and relatively modest nature of this atmospheric river activity, any flooding concerns are expected to be isolated at best. Bann Day 2 Valid 12Z Sun Oct 26 2025 - 12Z Mon Oct 27 2025 ....THERE IS A SLIGHT RISK OVER PORTIONS OF THE CENTRAL GULF=20 COAST... A slight risk was introduced with this afternoon's update across portions of coastal Mississippi/Alabama and far western panhandle of Florida. Storms that will impact parts of the Lower Mississippi Valley Saturday night will continue into the Southeast and along the Central Gulf Coast on Sunday. More energy rotating beneath a mid-level low centered over the Middle Mississippi Valley will help propel the convective complex through the Southeast. Storms will move into a moisture and instability rich environment across the immediate Mississippi/Alabama/Florida coast Sunday afternoon/evening where the slight risk is now in place. Along the immediate coast, precipitable water values will be between 1.5-2", which will be in the 90th percentile of PWATs for this time of year. MUCAPE between 1000-1500J/Kg should provide enough instability to support 1-3in./hr rain rates in some places. Urban areas will be especially susceptible to runoff from excessive rainfall. HREF 40km and EAS neighborhood probabilities of exceeding 5" and 3" respectively will be well over 15% over far west portions of Florida's panhandle. 40km probabilities of exceeding 5in. are well over 30% across coastal Mississippi and Alabama. CAMs have also increased their qpf footprint within the current slight risk area in response to a growing consensus for slower cell motions and backbuilding storms over the immediate coast Sunday night. Kebede Day 3 Valid 12Z Mon Oct 27 2025 - 12Z Tue Oct 28 2025 ....|THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND SOUTHERN APPALACHIANS... ....2030z Update... A small expansion was made to the marginal risk area in the Southeast in the northwest direction into the Southern Appalachians. There's an increasing qpf trend in the Euro suite over the terrain of North Carolina on Monday. Kebede ....Previous Discussion... Low pressure moving across the southern United States on day 2 will begin to take a more northeastward trajectory. Given less favorable low level flow and a more stable atmosphere...the risk of excessive rainfall will be diminished. Even so...the 25/00Z suite of numerical guidance does show spotty convection with isolated bulls-eye amounts of 2 inches or greater mainly in the Carolinas. The NAM was far and away the most aggressive with its amounts and was not given much consideration given the placement of the heaviest rainfall in a region where stable air was banked up against the east aspect of the terrain in response to the ridging of surface high pressure to the north. While initially easy to downplay the spotty nature of the model QPF...the machine learning first guess algorithm did point the area as a marginal risk of excessive roughly co-located with the WPC deterministic QPF. As a result...we opted to shrink the Marginal issued on Day 4 and place it closer to the southern portion of the Appalachians. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5JI1ZE_udSFMywt3tV3j0o3FOanZMfHVIzOjJpkSuYa6= tWcuMRwDWo5tqRFibQ6PTHH3m7QrVdzihuHmQPX3T-4o4dA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5JI1ZE_udSFMywt3tV3j0o3FOanZMfHVIzOjJpkSuYa6= tWcuMRwDWo5tqRFibQ6PTHH3m7QrVdzihuHmQPX3tZu68KY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5JI1ZE_udSFMywt3tV3j0o3FOanZMfHVIzOjJpkSuYa6= tWcuMRwDWo5tqRFibQ6PTHH3m7QrVdzihuHmQPX3QtuJZhM$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .