Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 26 2025 00:51:03 FOUS30 KWBC 260050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 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 THE NORTHWEST GULF COAST & LOWER MISSISSIPPI VALLEY... An east-west band of convection is slowly settling southward across portions of South-Central and Southeast TX. It lies on the western side of a region of 850 hPa confluence, southeast of a deep layer cyclone, which has precipitable water values of 1.5-2", effective=20 bulk shear of 35-60 kts, and MU CAPE of 1000-2000 J/kg. Activity=20 should fire up in the Lower Mississippi Valley overnight. Even=20 though antecedent conditions are quite dry, the concern is that=20 overnight/early morning convection has a ~30% chance of exceeding=20 5"+ within the 18z HREF and 12z REFS guidance, with the best=20 overlap from the Teche/sugar growing region of LA northeast into=20 southwest MS. This should be sufficient for at least scattered=20 areas of flash flooding in that area, particularly in urban centers where impacts could be on the higher end of a Slight Risk. The=20 concern is for training band potential, mesocyclone formation holding up otherwise progressive convection, and cell mergers=20 within this environment leading to hourly amounts to 2.5" and local totals up to 6". Roth Day 2 Valid 12Z Sun Oct 26 2025 - 12Z Mon Oct 27 2025 ....THERE IS A SLIGHT RISK OVER PORTIONS OF THE CENTRAL GULF 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!6YjaMJJCCbMIp8ffBCb19Es5Opq6UXkr4IgYZ7Wgh_lH= I7dcV5lw8VTTWAXoDL23WANVsmU7nlRSCuLTTZpCwR9t1No$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6YjaMJJCCbMIp8ffBCb19Es5Opq6UXkr4IgYZ7Wgh_lH= I7dcV5lw8VTTWAXoDL23WANVsmU7nlRSCuLTTZpC9vgO3SY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6YjaMJJCCbMIp8ffBCb19Es5Opq6UXkr4IgYZ7Wgh_lH= I7dcV5lw8VTTWAXoDL23WANVsmU7nlRSCuLTTZpCdqeeL8s$=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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