Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 11 2025 00:40:21 FOUS30 KWBC 110040 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 840 PM EDT Wed Sep 10 2025 Day 1 Valid 01Z Thu Sep 11 2025 - 12Z Thu Sep 11 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 THE NORTHWEST AND FLORIDA'S SPACE COAST... ....Florida... A stationary front draped across the central Florida Peninsula=20 will remain in place as abundant tropical moisture, characterized=20 by PWATs over 2 inches, remains focused in its vicinity. Post-=20 frontal convection is possible late tonight into early tomorrow=20 morning in and near the Space Coast in an area where onshore flow from the Gulf Stream brings instability to the coast in an=20 environment of slightly cooler air aloft. A short, training band or two cannot be ruled out, so maintained this small portion of the=20 preexisting Marginal Risk area. ....Pacific Northwest... A large upper level low and associated trough covering much of the West Coast will gradually drift east/inland across the Intermountain West today. Shortwave energy rotating around the low, interacting=20 with the instability caused by the cold air aloft and unusually=20 high amounts of atmospheric moisture (anomalies to 2.5 sigma above=20 normal) has allowed for clusters of showers and a few storms=20 capable of heavy rainfall to form. The 18z HREF suggest that any heavy rain-related issues over burn scars could persist for another several hours before fading. Roth Day 2 Valid 12Z Thu Sep 11 2025 - 12Z Fri Sep 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA... ....Florida... 21Z update... The latest guidance trended a little drier across portions of central Florida so the northern boundary of the Marginal was trimmed southward. Additionally, there was an uptick in QPF for locations west and southwest of Lake Okeechobee which prompted a minor westward expansion of the Slight Risk area. Elsewhere the highlighted level of threat remains appropriate. Campbell The stationary front plaguing Florida for well over a week will begin to move as a cold front on Thursday, as a reinforcing shot of cooler and drier air pushes south across the Southeast. As the front pushes south down the Peninsula, the front should act as a source of forcing for the storms, allowing for greater organization and greater areal rainfall across South Florida as compared with previous days when the forcing was far less defined. Add in the saturated soils across south Florida, and the area where flooding could be more frequent than isolated instances expands north up the I-95 corridor towards the advancing front. Thus, the Slight was expanded north accordingly. ....Pacific Northwest into the Northern Plains... 21Z update... The latest model runs hinted that more QPF would reach eastern Montana and far northwest North Dakota than some of the previous cycles. Therefore a portion of the Marginal boundary was adjusted to include more of/expand into the following counties- Yellows= tone, Big Horn, Treasure, Rosebud, Custer, Prairie, McCone, Garfield, Dawson, Richland, Roosevelt, Sheridan, Williams, Divide and Burke Counties. Campbell A plume of anomalously high moisture in place from the Pacific Northwest to the Northern Plains will support scattered showers and thunderstorms forming once again along a corridor from eastern Oregon through much of northern Montana. A separate cluster of storms may sneak south across the Canadian border and into far northeastern North Dakota and northwestern Minnesota as well. Due to the potential for heavy rain in all of these areas, a Marginal Risk is in place and has been expanded eastward to cover the areas most likely to see isolated instances of flash flooding from heavy rain from these features. ....Western Colorado and Far Northwestern New Mexico... Southerly flow on the western edge of a low-level jet across the Plains will transport Gulf moisture into the region on Thursday. Meanwhile the large trough out west will become positively tilted with time. This will allow an 80 kt jet southwesterly jet streak to form, which will increase the divergence aloft across this area. Expect upslope into the high peaks of southwestern Colorado to locally enhance rainfall totals, thereby increasing the flash flooding risk. Soil moisture measurements from NASA Sport show much of this area to be below average/drier than normal. Thus, expect much of the rainfall Thursday in this region to work to saturate the soils in anticipation of Day 3/Friday's bigger rainfall event. Wegman Day 3 Valid 12Z Fri Sep 12 2025 - 12Z Sat Sep 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH FLORIDA AS WELL AS FOR PORTIONS OF NORTHWESTERN NEW MEXICO INTO SOUTHWESTERN COLORADO... ....Northwestern New Mexico into Southwestern Colorado... 21Z update... In general, the areas highlighted with the Slight and Marginal Risks for excessive rainfall remain in good order. Minor reshaping of the Slight was done to reflect latest WPC QPF, noting that north side was expanded further to include northern Gunnison and southern Pitkin Counties in the Slight. Campbell A positively tilted longwave trough centered over much of the Intermountain West on Friday will become more neutrally tilted on Friday as a strong shortwave trough causes the southern half of the trough to begin to race eastward. The result will be even more enhanced upper level divergence out ahead of the trough over New Mexico and Colorado. Atmospheric moisture will be largely unchanged in this area from Thursday. With peak heating Friday afternoon, numerous showers and thunderstorms will form over nearly all of the High Plains. However, coverage and intensity will be much higher in the Slight Risk area in Colorado and New Mexico. The area is likely to be the same one hard hit from the Day 2/Thursday period, so expect soil moisture amounts to be higher and FFGs to be lower over much of the area by Friday's event. Given all of these ingredients in place and in coordination with GJT/Grand Junction, CO forecast office, a Slight Risk upgrade was introduced with this update. ....South Florida... 21Z update... QPF trended a bit southeast for this period but still focusing the highest amounts in the Miami-Dade/Broward vicinity. Western portions of Collier, Hendry, Palm Beach and Martin Counties were trimmed from the Slight. All or portions of Sarasota, Charlotte, Manatee, DeSoto, Hardee, Highland, and Polk were trimmed out of the Marginal, while portions of Osceola and Brevard were added. Campbell The cold front across central Florida at the start of the day Friday will make slower, but still incremental progress in its southward movement down the peninsula. The plume of tropical moisture across south Florida ahead of the front will also still be in place. Thus, as is similar to Thursday over the middle of the state, the axis of heaviest moisture will simply shift southward to far south Florida on Friday. The inherited Slight Risk area was expanded west over much of the Everglades and portions of southwest Florida with this update, to follow more closely to where the axis of heaviest rainfall is likely to set up. Given the focus will shift into the more vulnerable and flood prone urban areas of south Florida, its entirely possible that a higher end Slight risk will need to be issued for the Ft. Lauderdale to Miami corridor with future updates. ....Northern High Plains... 21Z update... The Marginal Risk area was reshaped across portions of northeast Wyoming, southeast and northeast Montana, northwest South Dakota and southwest North Dakota. The latest guidance trended less wet for portions of western South Dakota, northwest North Dakota and parts of northeast Montana near the North Dakota border. However the guidance focused the higher QPF across south Montana and into the surrounding counties of North Dakota and South Dakota. Campbell Similar to Day 2/Thursday, anomalous moisture will remain in place across much of Montana and portions of adjacent states again on Friday. The upper level low and trough will move overhead or just south of the region on Friday. As the cold air aloft peaks in depth on Friday, numerous showers and thunderstorms are likely to redevelop again on Friday afternoon with peak heating and maximum instability. The axis of heaviest rain from central Idaho through northwestern Montana will also be largely unchanged and unmoving from Thursday, meaning these areas will likely have a second day of showers and storms capable of heavy rain and flooding again on Friday. Thus, here too a Slight Risk upgrade may be needed with future updates. This will largely be determined by how much rain falls on Thursday in this area, which will prime the soils for Friday's round of rain. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7nOhw-lypM05lTF1HKK-3FWOInLjw4r--h2Nzetg_1ud= ncwQTtN5TIac2N3tZUijKbc3pq9DX-9DfQMehoKhRGhkBx8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7nOhw-lypM05lTF1HKK-3FWOInLjw4r--h2Nzetg_1ud= ncwQTtN5TIac2N3tZUijKbc3pq9DX-9DfQMehoKheUcC1i0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7nOhw-lypM05lTF1HKK-3FWOInLjw4r--h2Nzetg_1ud= ncwQTtN5TIac2N3tZUijKbc3pq9DX-9DfQMehoKhVXs-ov8$=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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