Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 25 2023 18:44:00 AWUS01 KWNH 251843 FFGMPD WIZ000-MNZ000-IAZ000-252300- Mesoscale Precipitation Discussion 1102 NWS Weather Prediction Center College Park MD 243 PM EDT Mon Sep 25 2023 Areas affected...Central and Eastern Minnesota, Western Wisconsin Concerning...Heavy rainfall...Flash flooding possible Valid 251842Z - 252300Z Summary...Training showers and thunderstorms with rainfall rates of 1-2"/hr will likely expand in coverage through this evening. This could result in 1-3" of rainfall with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this afternoon shows expanding bands of showers and thunderstorms from central MN through eastern WI and into IA/IL. These bands are rotating SSE to NNW around a vertically stacked area of low pressure west of Minneapolis, with the surface low occluded to secondary low pressure in IA according to the recent WPC surface analysis. This entire system will continue to drift slowly southeast this afternoon, with accompanying height falls and downstream divergence driving increased ascent. This ascent will occur within extremely favorable thermodynamics characterized by PWs of 1.2-1.4 inches, above the 90th percentile for the date according to the SPC sounding climatology, and SBCAPE climbing to as high as 1500 J/kg by this evening. The improving ascent overlapping the impressive thermodynamics should result in an expansion and intensification of convection. The high-res guidance, in general, appears to be a little too slow developing convection when compared to current activity, but the overall trends appear relatively accurate. As the stacked low dives southeast, a deformation axis is forecast to sharpen from NW to SE, while at the same time the theta-e ridge axis pivots cyclonically to the NW. This should result in even more impressive instability due to the theta-e rich air surging overhead, with additional ascent being produced within the axis of deformation. This suggests that showers and thunderstorms will increase to the south and east, and then lift N-NW on 0-6km mean winds of around 10 kts, weaker directly beneath the upper low. At the same time, Corfidi vectors are progged to collapse to just 5 kts, suggesting slow moving cells with backbuilding to the SE into the greater instability likely. Rain rates within this development will likely eclipse 1"/hr as reflected by HREF probabilities reaching 70%, with brief 2-3"/hr rates possible noted by 15-min HRRR rainfall reaching 0.5-0.75". Training of these rates is almost certain, which will likely produce 1-3" of rain. Locally higher amounts are possible, especially beneath the deformation axis, where focusing and organization could result in longer duration training of the most intense rain rates. In this area, generally around the Coulee region of MN and WI, HREF and RRFS TL probabilities for more than 3" peak above 25%. FFG across this region is moderate, around 1.5-2.5"/3hrs due to generally dry antecedent conditions. However, some moistening has occurred in the past week thanks to 7-day rainfall that has been 200-300% of normal leading to increased 0-10cm soil moisture according to NASA SPoRT. This has lowered the FFG from previous weeks, and the HREF suggests at least a 20% chance of exceedance. While the next several hours will likely feature only an isolated flash flood risk, generally within the most intense training or atop more urban areas, additional heavy rainfall is possible into tonight, which could necessitate further discussions with a greater overall flash flood risk. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4hreh1Lyvv3Vow7VfDnokQM66NibXyNM_HdTzaD0wl2__NxgQEmm0aUKP6C1r3eAuX1A= 7nYzLV5pII3QH0lTfV0hZFE$ for graphic product... ATTN...WFO...ARX...DLH...GRB...MPX... ATTN...RFC...NCRFC...NWC... LAT...LON 46999323 46629209 45809120 44489012 43809019=20 43389066 43389125 43399136 43519230 43779302=20 44749391 45649393=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) .