Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 02 2025 15:44:44 FOUS30 KWBC 021544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Thu Oct 2 2025 Day 1 Valid 16Z Thu Oct 02 2025 - 12Z Fri Oct 03 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SIERRA, GREAT BASIN AND EASTERN FLORIDA... 16Z Update: The previous forecast has not changed much in the overall synoptic and mesoscale expectations over the two risk areas outlined.=20 Across FL, locally heavy rainfall within the confines of a surface trough axis aligned parallel, just inland from the coastal plain will be the threat through the period with urban flash flooding the most likely case of any potential. Highest 5" probs via 12z HREF neighborhood output are still in that 40-90% range, however the highest probs are centered between Melbourne to West Palm Beach with a centroid near Port St Lucie. This is hi-res indications of where they anticipate the strongest low-level convergence regime with relevant moisture flux situated just off the Atlantic coast at similar latitude to the elevated probs. Considering some error in convective placement, it's still reasonable to consider areas from Miami up to Daytona within the proxy for locally enhanced rainfall rates with hourly rates ~3"/hr at peak intensity when you factor PWATs hovering between 1.75-2"=20 as of the 10/12z RAOB soundings from Cape Canaveral. This signal=20 in the means and individual deterministic is more than sufficient=20 for a maintenance of the MRGL risk in place with only a minor=20 northward expansion in the area to cover for the current radar trends.=20 Across the west, the setup remains consistent among all the CAMs and global deterministic with a relative max situated over the northern Sierra along and near I-80 where modest instability and sufficient large scale ascent should initiate a period of moderate to heavy rain for multiple hours that could enhance a localized flood prospect. As we move further inland into the Great Basin, the setup shifts to more scattered/widespread convective schemes that are better suited for those highly localized flash flood concerns within topography that historically favors flashier output due to the soil composition and prevalence of dry washes and arroyos littered over the high desert. This threat expands into the Snake River basin of ID and adjacent zones later this evening. Totals between 1-2" (Locally higher in the northern Sierra) will be most=20 common in the MRGL risk in place from the Central Valley of=20 California up into Sawtooth and Salmon River Ranges of Idaho.=20 Kleebauer ...Previous Discussions.. ....Sierra/Great Basin... An upper low will dig into the West Coast while sending streams of vorticity into the Sierra Nevada, Great Basin and Northern=20 Rockies. Diffluence will spread across the aforementioned areas=20 throughout the day on Friday. At the surface, a low pressure system will propagate across the interior West, while anomalous moisture=20 (3-4 stndv PWATs) streams in behind the preceding cold front.=20 Scattered to isolated thunderstorms may develop and produce=20 isolated instances of excessive rainfall and flash flooding. Upslope enhancement across the Sierra could produce heavier rainfall totals than what are currently forecast. EAS probabilities of 24 hour qpf exceeding 1 inch are over 40% in the Sierra and between 15-40% in the Great Basin. Therefore, burn scars over central and southern Idaho are at risk of flash flooding today. ....Eastern Florida... Moist easterly flow into a surface front oriented north-south will produce scattered convection across Florida today. The surface boundary draped along the eastern coast will focus PWATs of around 2" in that area where MUCAPE between 500-1000 J/Kg will also likely produce high rain rates. Neighborhood probabilities of exceeding 5" in 24 hours are between 40-90% from near Miami metro up through Melbourne and Orlando. Kebede Day 2 Valid 12Z Fri Oct 03 2025 - 12Z Sat Oct 04 2025 ....THERE'S A MARGINAL RISK FOR PORTIONS OF THE GREAT BASIN AND EASTERN FLORIDA... ....Great Basin... The upper trough propagates through the Great Basin and begins lifting north on Friday. The excessive rainfall threat bleeds into Friday from Thursday as thunderstorms weaken and spread from the Great Basin into the Northern Rockies by Friday afternoon. Instability and moisture will be relatively modest (100-200 J/Kg MUCAPE) (0.5-0.9"). Ensemble exceedance probabilities of 1" are between 10-20% over northern Nevada with 2 day totals potentially eclipsing 2" in some places. ....Eastern Florida... Moisture advection into Florida increases on Friday as a mid-level circulation embedded within a broader upper ridge continues to direct moisture and instability into the state's east coast. Another inverted surface trough may develop along the state's east coast and focus convection along the convergence zone. Ensemble exceedance probabilities of 2" are between 5-15% along the immediate coast, while first guess fields support the marginal with 5% flash flood probabilities. Kebede Day 3 Valid 12Z Sat Oct 04 2025 - 12Z Sun Oct 05 2025 ....THERE'S A MARGINAL RISK FOR PORTIONS OF SOUTHEAST LOUISIANA AND EASTERN FLORIDA INTO FAR SOUTHEASTERN GEORGIA... ....Central Gulf Coast... Mid-level vorticity swirling over the Central Gulf Coast will generate instability and pull an increasing amount of Gulf moisture into the region on Saturday. Convection should fire along a coastal convergence zone that develops and diurnal heating should enhance afternoon/evening storms along that boundary. PWATs will likely be over 2" with MUCAPE between 500-750 J/Kg. Ensemble exceedance probabilities of 2" are between 15-30% for much of southeast Louisiana. ....Eastern Florida/Southeastern Georgia... Tropical moisture associated with Imelda will continue flowing into Florida due to and upper ridge over the East Coast and embedded troughing over Florida and the Gulf. A modest instability plume should flow up the Florida Peninsula into far southeastern Georgia where PWAT's of over 2" and an inverted surface trough could combine to produce thunderstorms capable of generating 0.75-1"/hr rain rates. Ensemble exceedance probabilities of 2" are between 5-15% along Florida's east coast into southeastern Georgia. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-TXaeIF6zJASOo8o2ZPRpSFXhdz9pbtfYb8Itozk9Wpn= YeHxPDMCzMp2yhoxlB8WXE2lf0pHGWzQUj--paKEvEq-Ky4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-TXaeIF6zJASOo8o2ZPRpSFXhdz9pbtfYb8Itozk9Wpn= YeHxPDMCzMp2yhoxlB8WXE2lf0pHGWzQUj--paKEeqNXeZE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-TXaeIF6zJASOo8o2ZPRpSFXhdz9pbtfYb8Itozk9Wpn= YeHxPDMCzMp2yhoxlB8WXE2lf0pHGWzQUj--paKEnyhorn4$=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) .