Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 04 2023 18:33:44 FOUS30 KWBC 041833 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 233 PM EDT Mon Sep 04 2023 Day 1 Valid 1759Z Mon Sep 04 2023 - 12Z Tue Sep 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES INTO THE PLAINS... ....18Z Update... Have hoisted a Marginal Risk area across much of the Lower MS Valley/Mid South into the Ozarks. The main flash flood risk will be across urban and other drainage areas more susceptible to intense short-term rainfall rates. Elongated, stagnant, and relatively shallow Warm Conveyor plume into this region east of the upper low is noted per the mid-level water vapor imagery. Weak deep layer forcing (broad upper difluence) and subtle low-level confluence/frontogenesis east of the upper low, along with an uptick in 0-6km bulk shear (20-30 kts), should support more organized/numerous convective clusters within the Marginal Risk area, while the robust thermodynamic profile (surface based CAPEs 2000-3500+ J/Kg and PWs around 2.00") will support hourly/sub-hourly rainfall rates between 2-3" underneath the strongest, slow-moving clusters. This will pose a localized threat for flash flooding, again mainly across urban and other drainage areas more susceptible to these intense rainfall rates. For further information, please refer to the latest mesoscale precipitation discussion or MPD #1035. Hurley ....16Z Update... No major changes were made to the inherited Slight Risk area. The guidance has trended just a hair further south, aligning the most significant convection along the WY/MT border, then turning northeast through northwestern SD and southwestern ND. This corridor is where the heaviest rainfall totals are expected with localized totals exceeding 2 inches likely overnight tonight. The big question will be how much flash flooding results from that given the antecedent dry conditions in the area. With MUCAPE values between 500-1,000 J/kg this afternoon, but atmospheric moisture about 2-3 sigma above normal. Localized flash flooding in flood-sensitive areas is probable. Guidance suggests an initial round of convection moving through late this afternoon through this evening, a few hour break, then additional mostly stratiform rainfall following it from north-central WY through south-central ND. The stratiform rain will result from wrap-around rainfall on the back side of a developing low, which will form along the largely stationary front, which will focus the heaviest rainfall in the Slight Risk area. Further west around the Yellowstone areas, the Slight Risk is maintained for this afternoon to account for expected additional thunderstorm development into an area that picked up 1-3 inches of rain overnight last night, so FFGs are a bit depressed. A major limiting factor will be the thick cloud cover noted on visible satellite, which appears likely to linger for the next several hours. This may limit instability, which in turn will limit the coverage of any thunderstorms. Wegman ....Original Discussion... Weakening mid-level shortwave over southeastern ID/central UT this morning will continue eastward across WY and into western SD by early Tuesday. Northern stream jet across southern Canada will dip southward into northeastern MT/northwestern ND this afternoon/evening, providing broad-scale lift to the region as a surface cold front sinks southeastward. Column moisture amounts are high in the region -- precipitable water values from 0.75" along the Divide to 1.50" on the Plains which are +2 to +3 sigma -- and the system has had a history of heavy rainfall even in its weakening trajectory. Despite the lesser dynamical support than earlier in its life cycle, daytime heating combined with the approaching shortwave and ample instability to initiate convection this afternoon over eastern WY and push eastward into SD where heavier rainfall rates may exceed local FFG values (1.50-2"/hr). 00Z HREF neighborhood probabilities of exceeding 1 and 3-hr FFG values are roughly 10-30%, and the Slight Risk outline was maintained in this region. This also stretched back to southwestern MT, where the complex terrain may make areas more susceptible, and into southwestern ND along and just north of the surface boundary where heavier rain rates are still possible. Broader Marginal Risk area covers expected/ongoing rain/convection over ID/UT this morning and ultimately eastward across ND as the system evolves later this evening into the overnight hours. Fracasso Day 2 Valid 12Z Tue Sep 05 2023 - 12Z Wed Sep 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST... Shortwave over SD early Tuesday will continue eastward into MN by early Wednesday with the attendant surface low and cold front. A weakening shortwave over the mid-Mississippi Valley lifting northeastward will bring an axis of moisture into the Great Lakes ahead of the front that has its own surge of moisture over the eastern Dakotas into MN (precipitable water values ~1.25-1.75" or around +2 sigma in the warm sector). Lower-level frontogenesis will drive higher rainfall rates just to the north of the surface low, though this does coincide with slightly depressed FFG values. Afternoon heating should spur convection over northeastern MN into WI where 00Z HREF neighborhood probabilities of >1"/hr rainfall (through 00Z Wed) touch 50%. Higher rainfall totals are expected into the Arrowhead which may cause localized flooding issues for more sensitive/vulnerable/urban locations. The entire region has been dry for at least the past 1-2 weeks (or more), so most of the rain will be beneficial and the Marginal Risk outline was maintained. Farther west, the area extends back to western SD where rainfall from D1 into early D2 may still pose a very localized flood risk early in the period. Fracasso Day 3 Valid 12Z Wed Sep 06 2023 - 12Z Thu Sep 07 2023 The probability of rainfall exceeding flash flood guidance is less than 5%. Fracasso Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6DNDACTMawGXRivbGvXecxfzbGErXVSU6xwvnphfQ803= 7geS62STDXBPeJ5N_DiLKDxzlqdQBkGKNSWA7Il2vVAG87o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6DNDACTMawGXRivbGvXecxfzbGErXVSU6xwvnphfQ803= 7geS62STDXBPeJ5N_DiLKDxzlqdQBkGKNSWA7Il20pnoLNM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6DNDACTMawGXRivbGvXecxfzbGErXVSU6xwvnphfQ803= 7geS62STDXBPeJ5N_DiLKDxzlqdQBkGKNSWA7Il21lmHNb4$=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) .