Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 01 2025 20:24:36 FOUS30 KWBC 012024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Mon Sep 1 2025 Day 1 Valid 16Z Mon Sep 01 2025 - 12Z Tue Sep 02 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, TEXAS, CENTRAL PLAINS, TENNESSEE VALLEY, AND EAST COAST OF FLORIDA... Monsoonal moisture persists from southern Arizona through central New Mexico with diurnally terrain driven activity expected with an isolated flash flood risk. The Marginal Risk is maintained with some more of the Mogollon Rim in AZ added. Slowly south-moving frontal zone and abundant Gulf moisture persists over south-central Texas. 2 to 2.4" PW along this zone is 2 sigma above normal and will result in further heavy thunderstorms with at least some organization to redevelop today and last into tonight. This is generally south of the areas impacted with heavy rain yesterday except around the Houston metro. The Marginal Risk is trimmed a bit from the north given the front location. MCV over far southeast Nebraska will continue shifting east through midday with heavy rain to sweep through the KC metro. Further development to the west over eastern Neb/KS is expected later this afternoon. Trimmed the Marginal Risk on the north side where rain has become more stratiform (including over Omaha). Slow-moving convection is developing over the northern TN Valley which is on the southern end of an upper low centered over the OH Valley. This activity should continue to develop and shift south/southwest today over middle TN into northern AL. Marginal Risk is maintained. Easterly onshore flow into Florida from a low located along the same frontal zone that extends to Texas will allow for focused heavy rain south of Jacksonville again, though the risk extends to Miami today, so the Marginal Risk is maintained for the eastern coastal areas. Jackson Day 2 Valid 12Z Tue Sep 02 2025 - 12Z Wed Sep 03 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHWEST AND PORTIONS OF THE UPPER MIDWEST, THE MID-SOUTH AND=20 TENNESSEE VALLEY, AND FLORIDA PENINSULA... ....Mid-South through Tennessee Valley...=20 Remnant boundaries from ongoing convection today combined with=20 additional lift from a digging trough from the central U.S. should=20 allow convective activity Tuesday to be in similar areas (and west) from the coverage today. However, moisture is limited with the=20 1.25" PW today increasing to around 1.5" Tuesday (neither of which=20 are anomalously high). This uptick will allow for some greater=20 rates than today and motion should remain slow overall, but overall the rainfall magnitude continues a decreasing trend. The Slight=20 Risk was downgraded to Marginal per coordination with WFOs OHX HUN=20 and MEG.=20 ....Upper Midwest...=20 A cold front leading a developing upper low descending from Arctic Canada will push across North Dakota by midday and Minnesota=20 overnight. Moisture that has streamed up the Plains over the past=20 couple days will greet this front with a PW of 1.25" which is 1=20 sigma above normal. Diurnally enhanced convection is expected fire=20 up along and ahead of the frontal boundary, with potential for=20 heavy rainfall. The Marginal Risk area remains in effect for all=20 but northwest Minnesota, northern Wisconsin, the western UP of=20 Michigan into northern Iowa. ....Florida... The frontal boundary across the Florida Peninsula will continue to sag south with mainly diurnally driven convection south of the front. The farther south front means north side of the Marginal=20 Risk can be trimmed and the 12Z CAM consensus allows the Tampa Bay=20 metro to be removed.=20 ....Southwest... An anomalous surge in monsoonal moisture shifts up the Sierra Nevada tonight through Tuesday due to the position of the upper ridge axis to the east and a weak upper low along the central CA coast to the west. PW anomalies of 2 to 3 sigma are forecast from the lower CO River up trough the Sierra Nevada and even into southern Oregon by 00Z Wed. Scattered diurnally driven convection=20 expected across this region with a maintenance of the Marginal Risk up through the northern Sierra Nevada. Guidance is generally struggling to produce notable QPF along the Sierra Nevada, though the ECMWF did increase its precip there with the 12Z cycle. CAM guidance is more keen on southern CA deserts and terrain activity over central AZ and NM despite less moisture anomaly in NM. Jackson Day 3 Valid 12Z Wed Sep 03 2025 - 12Z Thu Sep 04 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PARTS OF THE SOUTHWEST INTO THE GREAT BASIN, THE UPPER MIDWEST, MISSOURI, AND THE SOUTHERN FLORIDA PENINSULA... ....Southwest... Monsoonal moisture continues to stream north from the lower CO Valley through the Sierra Nevada and much of Nevada on Wednesday, particularly according to the ECMWF. Diurnally-driven activity can be expected again over the Sierra Nevada which may be locally excessive, as well as much of Nevada through western Arizona and southwest Utah. The Marginal Risk is maintained there and trimmed away from southeast Arizona and New Mexico where moisture trends below normal east of the upper ridge axis. ....Central U.S... Elevated moisture ahead of the strong cold front sweeping down the Plains will trigger heavy rainfall again. Enough focus is present to warrant maintenance of the Marginal Risk around the central Great Lakes though timing is in question. The experimental RRFS indicates some potential repeating lines over southern WI, so the Marginal was expanded a bit to include Milwaukee. While much of the activity should be progressive, there is a repeating threat on the southern end of activity along the southern KS/MO border. A Marginal Risk was introduced here. ....Florida... Deep moisture along the stalled frontal boundary will continue to=20 generate convection capable of producing heavy rain across the southern Florida Peninsula. Given the southern drift of the front is expected to continue, the northern portions of the Marginal=20 Risk were trimmed out as it was for Day 2. This includes removing the Tampa Bay metro from the Marginal.=20 Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8JiT7xF-5Dbzb-lApZmHzOrtPi2Wjb-TaEUzeXe8sLJb= l_qeS4asbfLLsq-Z0Uk9kJ__POoqL7ilwzL6DWTQ8zDBqQA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8JiT7xF-5Dbzb-lApZmHzOrtPi2Wjb-TaEUzeXe8sLJb= l_qeS4asbfLLsq-Z0Uk9kJ__POoqL7ilwzL6DWTQ4VJ8904$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8JiT7xF-5Dbzb-lApZmHzOrtPi2Wjb-TaEUzeXe8sLJb= l_qeS4asbfLLsq-Z0Uk9kJ__POoqL7ilwzL6DWTQ8vo78Q8$=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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