Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 25 2023 08:03:22 FOUS30 KWBC 250803 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 AM EDT Mon Sep 25 2023 Day 1 Valid 12Z Mon Sep 25 2023 - 12Z Tue Sep 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MISSISSIPPI VALLEY... ....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 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5XbOdQ3tFMpLKjJVwh-wULS0ijDa6L9kAoHYcw7EZ2n9= mXlNpMxgzwZsQXI0AeuGR0ogh5GiouzI1Kpt8aFv_jJ3RM8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5XbOdQ3tFMpLKjJVwh-wULS0ijDa6L9kAoHYcw7EZ2n9= mXlNpMxgzwZsQXI0AeuGR0ogh5GiouzI1Kpt8aFv48EHBik$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5XbOdQ3tFMpLKjJVwh-wULS0ijDa6L9kAoHYcw7EZ2n9= mXlNpMxgzwZsQXI0AeuGR0ogh5GiouzI1Kpt8aFvZJ0uKjc$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .