Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 06 2023 08:29:39 FOUS30 KWBC 060829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sun Aug 06 2023 Day 1 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/NORTHERN PLAINS... ....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 ever so slowly across the region. An axis of 1.75-2" precipitable moisture plume will continue to wrap into the system...providing a sounding profile favorable for heavy rainfall and rainfall rates. Latest guidance shows an axis of 1 to 3 inches in a northwest to southeast axis from Iowa into Illinois...with some of the models with higher resolutions showing embedded higher maximum amounts. HREF 40 km neighborhood probabilities for 1 inch per hour amounts are at or above 30 percent along this axis by afternoon and maximize over 40 percent in parts of southeast Iowa by late afternoon/early evening and then begin to taper off by late evening. There is a better signal from the HREF that 3 hour QPF may exceed 3 hour flash flood guidance than 1 hour QPF exceeding 1 hour QPF...mainly across parts of Iowa that received heavy to excessive rainfall earlier on Saturday.=20 ....Northeast IL/Northwest IN Eastward to Parts of New York and the Mid Atlantic... A compact mesolow has occluded while tracking east into northern Indiana by early this morning with generally light to moderate rainfall amounts. With the models still pointing to the possibility of an uptick in rainfall rates later today...re-aligned the Marginal risk area a bit in lower Michigan. Still not expecting much in the way of backbuilding, cell merger activity, or training. As the system continues to push eastward...precipitable water values of 1.8 to 2,2 inches get drawn into the system by this evening with growing coverage of showers and thunderstorms. Some of the convection should be able to produce some downpours given that environment. Opted to introduce a Slight Risk area over parts of the Mid-Atlantic/I-95 corridor where convection from the late afternoon into the late evening or or early Monday morning will be force-fed the moisture by an increasing low level jet...at least enhancing the risk of excessive rainfall totals. A greater concern this far east occurs after the end of the Day 1 period. ....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. Bann Day 2 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 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 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Zpa-2BukfC16p8ytixHycPESZOyddhf1WJXrC2F-c1L= YQIzr1Sq0M7Vfabo1EQnu7byYMwgkA1-V1dOkrfOyv_FAco$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Zpa-2BukfC16p8ytixHycPESZOyddhf1WJXrC2F-c1L= YQIzr1Sq0M7Vfabo1EQnu7byYMwgkA1-V1dOkrfOW5NP9Gk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Zpa-2BukfC16p8ytixHycPESZOyddhf1WJXrC2F-c1L= YQIzr1Sq0M7Vfabo1EQnu7byYMwgkA1-V1dOkrfOgtoOEek$=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) .