Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 10 2025 00:20:13 FOUS30 KWBC 100020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 820 PM EDT Tue Sep 9 2025 Day 1 Valid 01Z Wed Sep 10 2025 - 12Z Wed Sep 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS=20 OF THE SOUTHERN HIGH PLAINS, FLORIDA, THE UPPER GREAT LAKES, & INTERIOR TERRAIN OF THE PACIFIC NORTHWEST... ....Florida... Showers and thunderstorms continue across portions of FL at the present time. A small convective cluster is expected to cross southernmost portions of the peninsula over the next few hours, moving along an outflow boundary from previous convection across South FL. Thunderstorms with heavy rain are also possible across the northeast Peninsula in the airmass just behind a front. Any excessive rainfall/flash flood concerns are expected to be isolated. ....Upper Midwest... A boundary across the Upper Peninsula of MI and northern WI will=20 continue to focus additional showers and thunderstorms ahead of an=20 upper level shortwave approaching from southeast MN. Due to recent=20 and ongoing rainfall, soils across the western U.P. and northern=20 Wisconsin are near saturation, so additional rain should largely=20 convert to runoff. The 18z HREF guidance shows a slow fading of=20 this activity over the next several hours. ....Pacific Northwest... A large trough and embedded cutoff low just inland of the Oregon=20 coast has led to the development of showers and thunderstorms over the mountains of western OR and northern CA. Portions of the=20 mountains have large burn scars wildfires in recent years.=20 Rainfall over these burn scars could cause localized instances of=20 flash flooding. The Marginal Risk inherited is largely unchanged.=20 ....Western KS south into the TX Panhandle... Widely scattered showers and thunderstorms have had some increased concentration across the TX Panhandle as of late, which has led to heavy rainfall/flash flooding near Amarillo. Other activity moving into western KS is expected to congeal into a complex that moves south-southeast overnight. As flash flood guidance values are modest in this region, and flash flooding has already occurred near Amarillo, figured it a new Marginal Risk area as a precaution for heavy rainfall potential into tonight. Roth Day 2 Valid 12Z Wed Sep 10 2025 - 12Z Thu Sep 11 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN FLORIDA PENINSULA... 2030 UTC Excessive Rainfall Discussion Update... Introduced a Slight Risk area over portions of the southern Florida peninsula in areas largely covered by a Slight Risk area on Day 1. Instability forecasts were comparable with what was observed on Monday...2000 to 3000 J/kg...with weak low surface pressure or surface trough near the area helping to focus the activity. Initial flash flood guidance values were generally 2 to 4 inches...although amounts were in the 1 to 3 inch range in the urban areas and in areas south of the metro area where locally heavy rainfall had recently suppressed the flash flood guidance. There was enough deterministic QPF guidance and ensemble forecast guidance that could challenge the guidance to warrant a Slight Risk. In the Pacific Northwest...the 18Z WPC QPF had increased enough in parts of southwest Oregon and northwest California such that the Marginal Risk area was expanded. It now covers an area similar to the territory covered by a Marginal Risk area on Day 1. Elsewhere...little change was needed and the forecast reasoning remained unchanged. Bann 0830 UTC Excessive Rainfall Discussion... ....Pacific Northwest... The upper level low over western Oregon will remain nearly stationary on Wednesday as the trough becomes positively tilted, and a strong shortwave rides north along the eastern periphery of the low. The storms will be embedded in an air mass that is up to 3 sigma above normal for this time of year, with PWATs at or around 1 inch. Since the greatest forcing will be with the northbound- moving shortwave, the heaviest rain is likely across much of eastern Oregon, in areas where FFGs are very low, due to normally dry conditions. Thus, while there's a fair amount of uncertainty as to precipitation intensity and duration, the fact that the heaviest rains will be moving over flood sensitive areas should result in isolated to perhaps even widely scattered instances of flash flooding. A Slight Risk may be needed over portions of Oregon with future updates. ....Florida Peninsula... A cold front, which had been stationary over the northern part of the state on Tuesday, will continue to slowly press southward across the northern half of the Florida Peninsula on Wednesday. Meanwhile, the stationary surface low will remain over the western Gulf. Thus, once again, the clashing of the southerly flow around the Gulf low, and the front will cause numerous showers and thunderstorms to form Wednesday afternoon as on Tuesday afternoon, albeit a bit further south down the Peninsula. Otherwise, the storms will remain disorganized and in clusters, so the flood risk will remain isolated due to this lack of organization. Wegman Day 3 Valid 12Z Thu Sep 11 2025 - 12Z Fri Sep 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN FLORIDA PENINSULA... 2030 UTC Excessive Rainfall Discussion Update... Introduced a Slight Risk area over portions of the southern Florida peninsula in areas largely covered by a Slight Risk area on Day 1 and Day 2 given little change in the overall synoptic pattern during the period. Instability is expected to remain in the 2000 to 3000 J/kg range to the south and east of a slow moving front that helps to focus and support convection capable of 1 to 2 inch per hour rainfall rates. Given the lowered flash flood guidance due to the urbanization and the prospect for scattered storms capable of producing downpours over impervious surface...the potential for excessive rainfall is elevated there compared with other places in the southern Florida peninsula. Elsewhere...little change was needed and the forecast reasoning remained unchanged. Bann Previous Excessive Rainfall Discussion... ....Pacific Northwest... The slow-moving upper level low will continue to drift eastward across the Intermountain West on Thursday. Numerous shortwaves and vort maxes will move through the flow while moisture levels across this region remain between 2 and 3 sigma above normal. Thus, expect an area of showers and storms to develop across Oregon and Idaho through the day Thursday, then spread into Montana Thursday night, allowing more areas into the potential for flash flooding. The Marginal Risk was expanded northeastward to account for this fast flow around the slow-moving upper low. Depending on where the high-resolution guidance ends up over the next couple days, enough rainfall may fall on the southwestward facing slopes to possibly need a Slight Risk upgrade with future updates. ....Colorado/New Mexico... Portions of western Colorado and New Mexico will be in the most favorable divergence region around the upper level low parked over the Pacific Northwest. The guidance that simulates the radar at the Day 3 time range, like the RRFS, suggests showers and few storms will track northeastward across western Colorado starting in the afternoon, peaking in coverage and intensity Thursday afternoon, then gradually subsiding overnight. There is some uncertainty as to how quickly the rain dies out. Regardless, much of the Marginal Risk area has soils that are at or drier than average. This should help to mitigate the flooding risk a bit, except in the slot canyons where soil moisture levels are far less relevant. ....Florida Peninsula... The front over the northern part of the state will continue to sag southward across the central part of the state on Thursday. Showers and storms will once again develop along and ahead of the front in the abundant tropical moisture that will persist over the southern part of the state. The air mass will be characterized by PWATs above 2.25 inches in some areas. Thus, any storms that form will still have access to ample moisture needed to produce heavy rain. Since the front will be the primary forcing for the storms, they will likely form along the front, then most southward down the Peninsula Thursday afternoon. This should result in enough of a progressive movement to limit the flash flooding potential somewhat, but given the multiple days of heavy rain, the flooding threat may be gradually increasing with time due to the saturated soils. It's not out of the realm of possibility that a Slight Risk upgrade may be needed with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!72R-ThCvjk4x76OTIIsj21gAVeDYoRoBj7ke9ywdWpiE= 04RxFWgNfSL15P_u6MXD2BGICfgprL3UqqoehpQcb62YVks$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!72R-ThCvjk4x76OTIIsj21gAVeDYoRoBj7ke9ywdWpiE= 04RxFWgNfSL15P_u6MXD2BGICfgprL3UqqoehpQcFUlGtsE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!72R-ThCvjk4x76OTIIsj21gAVeDYoRoBj7ke9ywdWpiE= 04RxFWgNfSL15P_u6MXD2BGICfgprL3UqqoehpQcq7q3wvg$=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) .