Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 25 2025 15:51:54 FOUS30 KWBC 251551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Sat Oct 25 2025 Day 1 Valid 16Z Sat Oct 25 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.=20 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=20 and into Mississippi today. A preceding line of storms from this=20 morning should collapse and dissipate by this afternoon.=20 A complex of storms will likely develop over eastern Texas later=20 this afternoon and propagate eastward into Arkansas, Louisiana and=20 Mississippi this evening. The heaviest rainfall is expected to=20 occur between 00z and 09z within a moisture rich environment=20 (1.75-2.25in. PWAT--90th percentile). There will also be=20 sufficient instability (500-1000J/Kg ML CAPE) over much of the=20 slight risk area. HREF neighborhood probabilities of exceeding 5in. and 3in. in 24 hours have increased over parts of southeastern=20 Arkansas/northwestern Mississippi, which supports the expansion of=20 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 MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-AND LOWER-MISSISSIPPI VALLEY, TENNESSEE VALLEY, SOUTHERN APPALACHIANS AND SOUTHEAST... Maintained the Marginal risk area already hoisted for portions of the southern United States on Sunday. Thunderstorms capable of locally intense downpours remain possible across the immediate Gulf coast early as southerly flow continues to tap Gulf moisture and advect it northward...potentially as far north as the Tennessee Valley. A front making its ways eastward should help focus convective activity early in the period...but also help shunt it eastward with time as surface high pressure builds in from the west. Isolated cells have the potential to produce rainfall rates in excess of an inch per hour prior to frontal passage. Farther north...concern is more for quarter to half inch rain rates within a weakly destabilized environment (200-400J/Kg MUCAPE). PWATs of 1-2" may support any isolated instances of excessive rainfall/flash flooding within the marginal risk area. Models are trending toward a higher qpf solution along the Central Gulf Coast, but their continued to be enough uncertainty in the 24/00Z guidance to preclude any upgrades at this time. Bann Day 3 Valid 12Z Mon Oct 27 2025 - 12Z Tue Oct 28 2025 ....|THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS A PORTION OF THE SOUTHEAST UNITED STATES... 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!_i-5ArYHtT8QmWB5sLHUSW61t_soI4Ns2mTi7IzqcVmS= Di4daAxIA5Q8eKjunwiUXbQ9Ja-zbsr_Rs3bc09-AvtDokE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_i-5ArYHtT8QmWB5sLHUSW61t_soI4Ns2mTi7IzqcVmS= Di4daAxIA5Q8eKjunwiUXbQ9Ja-zbsr_Rs3bc09-RENPgVY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_i-5ArYHtT8QmWB5sLHUSW61t_soI4Ns2mTi7IzqcVmS= Di4daAxIA5Q8eKjunwiUXbQ9Ja-zbsr_Rs3bc09-1hSwx44$=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) .