Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 24 2025 15:58:37 FOUS30 KWBC 241558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Fri Oct 24 2025 Day 1 Valid 12Z Fri Oct 24 2025 - 12Z Sat Oct 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....16z update... The marginal and slight risk areas were expanded slightly over=20 portions of Texas and Oklahoma for this afternoon and evening's=20 heavy rainfall threat. The overall setup described in the previous=20 discussion remains mostly in place with a line of convection moving through a favorable environment for producing heavy to excessive=20 rainfall. There's an increasing risk (at least 25% chance) of=20 excessive rainfall over portions of the Dallas Fort- Worth metro=20 area due, in part, to antecedent conditions from recent rainfall=20 (1-3" over the past 12 hours). HREF 40km neighborhood probabilities of exceeding 3" are between 20-50%, which supports the drawn 25%=20 area. ERO first guess fields between 25-40% across much of the=20 slight risk area also support increased confidence in the potential for flash flooding. The marginal risk area over the Northwest Coast remains mostly unchanged. The strongest IVT remains likely to be directed at portions of the Pacific Northwest and gradually weaken as it works its way south along a cold front into northern California. Kebede ....Previous Discussion... The large scale pattern still favors an active day for convection through early Saturday morning over portions of the southern=20 Plains as strong height falls associated with an upstream Southwest U.S. closed low/trough ejects out into the High Plains along with=20 a cold front. Low level moisture and stability will be in place=20 across much of central and northern TX as the nose of a 30 to 40+=20 kts southerly low-level jet rides up into the Red River Valley=20 region. The expectation is that organized convection should develop initially across areas of west TX and advance east by later in the afternoon/evening which will be capable of producing high rainfall rates upwards of 2 inches/hour with the stronger cells. However,=20 the organized nature of the convection suggests some forward=20 propagation potential that could mitigate the overall storm total=20 amounts. In fact, most of the guidance supports a fairly well-=20 organized MCS evolution into southeast TX and near the northwest=20 Gulf Coast region by early Saturday morning. Maintained the Slight=20 Risk area with only some minor adjustments to the western and=20 northern periphery of the area to account for where the axis of=20 better instability should be along with corridors of convective=20 initiation. This is also in better alignment with the consensus of=20 the 24/00Z HREF and 23/18Z REFS guidance. Scattered areas of flash=20 flooding, especially for the more urbanized locations, will be a=20 concern. ....Pacific Northwest into Northwest California... Only small localized adjustments were made to the Marginal Risk area as a modest Atmospheric River (AR) pushes across the coastal and Cascade ranges of Washington and Oregon. The plume of deepest moisture and associated rainfall will slide southward with respect to the coastline into northwest California by early Saturday morning. Strong height falls and precipitable water values over 1 inch total PWats into the coastal range set up the potential for rainfall rates of 0.75 inches to 1 inch through the Columbia Valley into central WA with higher rainfall rates remaining off- shore. Winds of 45-50kts through 850-700mb will result in peak IVT values of 700-750 kg/m/s. Still agree that the duration of heaviest rainfall is going to be very limited given the strength of the height falls and forward propagation of the cold front. QPF signals remain at the 2-4" range within the favored southwest facing orography with an occasional rate up to .5" though fleeting; all in line with a broad Marginal Risk of excessive rainfall. Bann Day 2 Valid 12Z Sat Oct 25 2025 - 12Z Sun Oct 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS INTO SOUTHERN ARKANSAS AND WESTERN LOUISIANA... Multiple rounds of heavy showers and thunderstorms are expected to impact the broader northwest Gulf Coast region this period, with one initial axis of convection likely ongoing Saturday morning across areas of southeast to east-central TX and advancing across western Louisiana and southern Arkansas. This will be the carryover of organized convection from the day 1 period which should be interacting at least initially 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. The arrival of an upper- level jet streak in the subtropical stream combined with the interaction with the southern stream closed low/trough axis crossing the southern Plains will favor a renewed convective outbreak across areas of eastern Texas through portions of western and southern Louisiana Saturday afternoon through early Sunday. 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. For this update, the Slight Risk area was expanded given the footprint of 2+ inch rainfall amounts within a broadened region of instability shown by the models. This is also consistent with the latest deterministic QPF from WPC. ....Pacific Northwest and Northwest California... Multiple rounds of moisture transport into the coastal ranges will continue for the period as atmospheric river activity continues. 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. Therefore, the Marginal Risk area is maintained with no changes for this update cycle. Bann Day 3 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 AND ALONG THE PACIFIC NORTHWEST COAST... Maintained a Marginal Risk area across portions of the southern and middle Mississippi Valley. The risk across the southern should diminish once a cold front passes through the area early in the period and the deeper moisture/instability get shunted eastward. Farther north where the moisture is not as deep...concern is more from mid-level frontogenesis and upper level divergence leading to locally enhanced rainfall rates where antecedent conditions have been somewhat wetter than other locations. There is plenty uncertainty as to whether there will be a second round of storms capable of producing locally heavy rainfall...so overall confidence is below average which precludes anything higher than a Marginal risk. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5yfa5ytvo6XArNuSbV3O5VQLN2JZ18qjJ8XOX3pukaMe= ehNEczkp5AB2HnnzFjH_QOPp31zq9XJv9CkAIZra13S-UHY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5yfa5ytvo6XArNuSbV3O5VQLN2JZ18qjJ8XOX3pukaMe= ehNEczkp5AB2HnnzFjH_QOPp31zq9XJv9CkAIZraZrW7sRY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5yfa5ytvo6XArNuSbV3O5VQLN2JZ18qjJ8XOX3pukaMe= ehNEczkp5AB2HnnzFjH_QOPp31zq9XJv9CkAIZrao7BM_dM$=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) .