Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 05 2023 20:02:28 FOUS30 KWBC 052002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Sat Aug 05 2023 Day 1 Valid 1954Z 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... ....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 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 which could fall in a couple hours. 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. Scattered convection capable of 2.5" rainfall in an hour is developing across the region. Anywhere these storms train, backbuild, or merge, flash flooding will be possible. ....Northeast IL/Northwest IN... A compact mesolow is tracking east across 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... Some of the guidance suggests a renewed round of storms in this same area tonight, which is maintaining the risk area in this region. ....Gulf Coast... The Marginal Risk area has been expanded northwest based on radar reflectivity trends. Convection is forming along the intersection of the sea breeze, outflow boundaries, and an old polar front.=20 Precipitable water values are 2.25" and ML CAPE maximized currently in the 4500 J/kg range, which will be difficult to exhaust. This could lead to hourly rain totals to 3" and local amounts of 5"+. Flooding or run off will be mainly a concern in urban areas and low lying areas with poor drainage. Roth/Wegman/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 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE NORTHEAST AS WELL AS NORTHEASTERN WASHINGTON AND NORTHERN IDAHO... ....1930Z Update... ....Much of the Northeast... Guidance continues to struggle with where the heaviest rain will fall in this region on Monday. A developing surface low starts the day near Chicago and will track to near Lake Ontario by Tuesday morning. Along this track, the areas likely to be hardest hit with the rain are from the I-95 corridor from Baltimore to NYC northward into northern NY and New England. Isolated rainfall totals to 4 inches are possible. Much of this area is still recovering from heavy rains over the past few weeks, particularly northern NY into northern New England, but topography may influence rainfall totals further south across eastern PA as well. In coordination with OKX/Upton, NY; PHI/Mt. Holly, NJ, BOX/Norton, MA; and GYX/Gray, ME forecast offices, the Slight Risk was introduced for the Monday afternoon and evening time frame. Expect more widely scattered convection from the southern Appalachians north through the Midwest. ....Northeastern WA through Northern ID... A strong, but slow-moving and shearing shortwave in the upper levels will track across central WA on Monday. With highly anomalous atmospheric moisture already in place, it's likely this upper level forcing will enhance local rainfall totals across this region. This area has been hit with rainfall yesterday, today, and is expected tomorrow, so the likely wettest day of the group being the last day of the rain will mean FFGs will be at their lowest in the area when the heaviest rain falls. Thus, a Slight Risk was introduced in coordination with OTX/Spokane, WA and MSO/Missoula, MT forecast offices. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5H1xQOpYJOp1Zs73QuWx3H1SgXNzu3_K818nyk-q6yAi= Z6Kp790AaztKTN6hl0ul6VMsKkB1dge_v0V99Xy58hs2_FI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5H1xQOpYJOp1Zs73QuWx3H1SgXNzu3_K818nyk-q6yAi= Z6Kp790AaztKTN6hl0ul6VMsKkB1dge_v0V99Xy54XL1TLY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5H1xQOpYJOp1Zs73QuWx3H1SgXNzu3_K818nyk-q6yAi= Z6Kp790AaztKTN6hl0ul6VMsKkB1dge_v0V99Xy5Z7ONPFs$=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) .