Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 05 2023 19:15:59 FOUS30 KWBC 051915 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 PM EDT Sat Aug 05 2023 Day 1 Valid 16Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST/NORTHERN PLAINS... ....16Z Update... ....Northern Plains... The Slight across the Plains looks largely on track this morning. Widely scattered deep convection is spaced out all around the low, with more concentrated light to moderate rainfall across northwestern SD. As the day goes on and the low crawls eastward, expect more scattered convection capable of 1-2 inch per hour rainfall rates to develop in eastern SD/MN/northwest IA. Anywhere these storms train, flash flooding will be possible. ....Northeast IL/Northwest IN... A compact mesolow is tracking east into the Slight Risk area. This low too is moving very slowly, but convection has been downtrending until recently. Likely the storms on the south side of the low are tapping into increasing diurnal instability. This is likely to continue as the area of rainfall moves into IN this afternoon. Agreement rapidly diminishes as the rain reaches the eastern side of IN as to how much rain will fall in this area. Given recently dry antecedent conditions, the Slight was maintained for northwestern IN and in coordination with IWX/Northern IN forecast office, left northeastern IN in a Marginal. ....Northern OK/southern KS/southwest MO/northwest AR... An MCS with various lines of fast moving storms over northern OK is expected to weaken over the next few hours, though that process has been painfully slow. The storms have been scattered enough and fast-moving enough that the flash flood threat has been low. Some of the guidance suggests a renewed round of storms in this same area tonight, which if it overperforms may require a last-minute Slight upgrade. ....Gulf Coast... No significant changes were made with this update. Wegman ....Previous Discussion... ....Northern Plains... A Slight Risk remains in place across parts of the Upper Midwest/Northern Plains as a vigorous vertically-stacked low makes its way eastward across much of the Northern Plains as a compact but vigorous vertically stacked low makes its way across the Dakotas and into Iowa and southern Minnesota by late this evening. An axis of 1.75 inch precipitable moisture plume will continue to wrap into the system...helping provide a sounding profile favorable for heavy rainfall and rainfall rates. Given operational model deterministic amounts...3 to 5 inch amounts somewhere in the Slight Risk area seems plausible and the HREF continues to be bullish. So some places may meet the criteria for Moderate risk area...but soils in the eastern Dakotas have been dry, which may allow for more efficient absorption of the heavy rain and instances of which are expected to be widely scattered. Saw little reason to alter the Slight Risk area that was extended as far south as Kansas City. ....Northern IL and northwest IN... A separate compact low that that has been moving across Missouri into far western Illinois during the overnight hours will continue to make its way eastward today...bringing the potential for several inches of rain along its path. Being small and compact...its slow forward motion is expected to continue with the risk of heavy to excessive rainfall as it taps deep moisture over portions of Illinois and Indiana.=20 ....Coastal S. Alabama and the western FL Panhandle... Maintained the Marginal Risk area introduced earlier...but downward trend noted in deterministic model QPF and the ensembles pointed to removal of the Slight Risk area. Convection is still expected to form along the sea breeze and an old frontal boundary...so this hardly telegraphs a dry forecast. But flooding or run off will be mainly a concern in urban areas and low lying areas with poor drainage.=20 Bann Day 2 Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST AND MIDDLE MISSISSIPPI VALLEYS... ....1930Z Update... ....Upper Midwest and Middle Mississippi Valleys... The guidance continues to shift southwestward with the axis of heaviest rainfall expected across the Upper Midwest and Middle Mississippi Valley. A crawling upper level low will continue to drift eastward from the Plains to the Great Lakes on Sunday. As it drifts, it will continue to tap into anomalously high moisture with PWATs above 1.75 inches. Diurnal heating will increase instability along this corridor, increasing the likelihood for slow-moving strong storms with potential for training and backbuilding. The biggest question remains where the heaviest rain will form. At the moment it is expected across north central IA, however continued south and westward shifts may result in the corridor of heaviest rain shifting to along the Mississippi into hard-hit eastern Missouri. To cover this possibility, the Slight was expanded south to include much of eastern Missouri, though for the moment the forecasted rainfall is not likely to produce that much flash flooding. Much of southeast MO, southern IL, western KY and northwest TN were added to account for the very low FFGs in the area due to all the recent rains, but heavier rains than forecasted currently would need to move over the area to result in more than isolated flash flooding. ....Pacific Northwest... Highly anomalous moisture also persists across portions of the interior Pacific Northwest, with PWATs to 1.25 inches running more than 3 sigma above normal for this time of year. The concern is with daytime heating Sunday afternoon that the storms that form are likely to train westward across portions of WA that are not equipped to handle much heavy rain all at one time. Since the forcing is weak on Sunday, the area remains in a Marginal Risk. Wegman 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!8Yq0vEK8YOqszpl0rRNhw-vTc5VFhdo087aRAgAE-j2J= spDTJtepnevWHrwud22g660-DKCDgKqsgL97tuOaN_IW5AI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Yq0vEK8YOqszpl0rRNhw-vTc5VFhdo087aRAgAE-j2J= spDTJtepnevWHrwud22g660-DKCDgKqsgL97tuOa9a4xgNg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Yq0vEK8YOqszpl0rRNhw-vTc5VFhdo087aRAgAE-j2J= spDTJtepnevWHrwud22g660-DKCDgKqsgL97tuOaheAsHFA$=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) .