Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 25 2023 20:27:33 FOUS30 KWBC 252027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Mon Sep 25 2023 Day 1 Valid 16Z Mon Sep 25 2023 - 12Z Tue Sep 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY... ....16Z Update... No significant changes were made to any of the risk areas on the Day 1 ERO. The area of highest concern remains over the upper Mississippi Valley, as a slow-moving cutoff low will support training convection up along the Mississippi River. Elsewhere, concerns are lower for the lower Mississippi Valley into Texas, where dry conditions should mean any flash flooding threat is relegated to urban and flood-sensitive areas. For the OR/CA border area, the antecedent soil conditions have been abnormally dry, so this should mostly be a beneficial rainfall event. That said, for burn-scar areas in particular, localized flash flooding is possible. A couple areas not highlighted: For southern New England, steady light to moderate rain with rates generally below 1 inch per hour will continue into the afternoon. While the area received several inches of rain going into today, the light rainfall rates going forward should not result in any flash flooding concerns. For the Florida Peninsula, expect scattered showers and thunderstorms to redevelop today. Much of the peninsula has been dry lately, which should help absorption rates. Should training cells happen to move over an urbanized or other flood-sensitive area, highly localized flash flooding may be possible given very high PWAT values in the area. The flood threat appears to be below Marginal thresholds. Wegman ....Previous Discussion... ....Upper Mississippi Valley... A meandering closed low over the Upper Midwest provides a suitable environment for divergent flow atop the atmospheric column today. PWs as high as 1.25" and MLCAPE between 500-1,000 J/kg will provide sufficient moisture and instability for developing thunderstorms. Combined with a conveyor belt of southeasterly 850mb moisture flux ahead of an occluded front, thunderstorms Monday afternoon and evening will be capable of producing rainfall rates >1.5"/hr. This round of storms is also coming on the heels of a 2-day stretch where as much as 2-3" or rainfall is forecast through Sunday night, so soils will be a little more saturated compared to earlier in the week. 00Z HREF probabilities have continued to increase with 3-hr QPF > 3-hr FFGs rising to as much as 40-50% between 21Z Monday - 03Z Tuesday along the MN/WI border. Probabilities for 24-hr QPF > 3" was between 60-80% with >5" probabilities for the same 24-hr winds up to 15-20%. These all continue to support the Slight Risk that is in place. Tweaked the inherited Marginal and Slight Risk areas to account for the latest QPF guidance. Urbanized communities, poor drainage areas, and nearby creeks and streams are the most vulnerable to flash flooding. ....Pacific Northwest... A powerful upper low approaching the Pacific Northwest will direct an atmospheric river at the West Coast today, delivering a soaking rain to residents from northern California to western Oregon and Washington. The upper low off the coast is very impressive for late September with heights within the 500-850mb layer a remarkable 4-6 standard deviations below normal according to NAEFS. This powerful upper trough will direct a robust 750 kg/ms/s IVT (peaking around 9 standard deviations above normal) at northern CA and western OR where PWs up to 1.5" translate to being as high as 3-4 standard deviations above normal. This is effectively the first bonafide atmospheric river of the season for the Pacific Northwest. Fortunately, parts of far northern California and southern Oregon could use the rain as parts of the region are in D0-D2 drought. The primary concerns are residual burn scars where soils will be more susceptible to possible debris flows. The Marginal Risk remains in place as rates do not appear to pose a considerable flood threat, but detrimental impacts could be felt in burn scars. ....Southern Plains & Lower Mississippi Valley... A frontal boundary stretching from West Texas to the Ozarks is forecast to weaken and push south throughout the day while at the same time, southerly 850mb moisture flux is expected to wain. These features will still be present at a time when a weak 500mb disturbance approaches from the north, providing some modest vertical ascent atop the atmosphere. PWs are currently forecast to reach as high as 1.75-2.0" in (1-2 standard deviations above normal according to NAEFS) and MLCAPE between 1,000-2,000 J/kg from central TX to the Lower Mississippi Valley (higher values over south-central TX). The available PWs and instability should support >2"/hr rainfall rates within the more intense cells, and soils should be a little more saturated following Sunday's rainfall. The Marginal Risk remains in place with any consideration of a Slight Risk likely to be discussed in future forecast cycles when more CAM guidance becomes available to key in on the more at-risk areas. ....Northeast... While it still looks to be a wet day along the southern New England coast, there is even less instability to work with compared to what was observed on Sunday. Rainfall rates should remain within manageable levels today given the depleted instability aloft and diminishing 850mb moisture transport from northern NJ to southern New England, but cannot rule out some nuisance ponding in poor drainage areas. Mullinax Day 2 Valid 12Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF FLORIDA AND THE MIDWEST... ....2030Z Update... No significant changes were made to either Marginal Risk area with this update, as the forecast remains on track for isolated instances of flash flooding in both areas. See the below discussion for more details. Wegman ....Previous Discussion... ....Florida... There is a large reservoir of 2-2.25" PWs across the Sunshine State with a stationary front draped over the region. At upper levels, a deep trough over the Gulf of Mexico is providing some sufficient divergence aloft while a large dome of high pressure in the Caribbean will direct a upper level disturbance to approach Florida from the South. NAEFS shows these PWs are at or slightly above the 90th climatological percentile and MLCAPE values reaching up to 2,000 J/kg. These parameters should be able to support hourly rainfall rates up to 3"/hr in the most intense storms. The other factor worth noting is vertical wind shear, winds within the 1000-850mb level will be southeasterly but veer and strengthen with height. This allows for surface-6km shear values to climb as high as 20 knots. This could help keep some storms around a little longer than the usual summer-time pulse storms. Given these factors, chose to introduce a Marginal Risk for Florida. This area will likely be tweaked depending upon QPF trends in guidance over the next 24-36 hours. ....Midwest... The closed low responsible for the Excessive Rainfall threat in the Upper Mississippi Valley on Monday works its way southeast towards the Great Lakes on Tuesday. A low pressure system will funnel 850mb moisture flux on the eastern flank of its circulation with PWS rising to around 1.25". MLCAPE is expected to range between 500-1,000 J/kg and mean 1000-500mb RH values are likely to average >80%. The slow progression of the closed upper low could support training convection given slower storm motions, especially as southeasterly low level winds on the eastern flank of the low are oriented quasi-parallel north of the warm front. The 00Z HREF did shows probabilities for 3-hr QPF > 3-hr FFGs as high as 40-45% in parts of northern IL Tuesday afternoon. Some soils are a little more sensitive than others, particularly north-central IL where AHPS 7-day rainfall totals have been as high as 400-600% of normal. Given these reasons, have chosen to introduce a Marginal Risk for parts of the western Great Lakes region this forecast cycle. Mullinax Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4InpsOVSHIzERCEd-tYeGskoV_mlW2rdKQ5biNts-uKm= wo0a_ZLSL9RnITFSdXb1Chp46kg9nPJWzCdN9fyrKMkvDzY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4InpsOVSHIzERCEd-tYeGskoV_mlW2rdKQ5biNts-uKm= wo0a_ZLSL9RnITFSdXb1Chp46kg9nPJWzCdN9fyrQSjFLm8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4InpsOVSHIzERCEd-tYeGskoV_mlW2rdKQ5biNts-uKm= wo0a_ZLSL9RnITFSdXb1Chp46kg9nPJWzCdN9fyr2Ip9dDY$=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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