Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 06 2023 19:50:17 FOUS30 KWBC 061950 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Sun Aug 06 2023 Day 1 Valid 16Z 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 AS WELL AS PORTIONS OF THE MID-ATLANTIC... ....16Z Update... ....Iowa through Indiana... A Slight Risk remains in effect for much of IA, IL, and into southern IN. A vigorous mesolow centered near Omaha this morning will move slowly southeastward. Convection is beginning to develop east of the low center and along the leading warm front. This convection is likely to continue to develop through the afternoon and into the overnight, wrapping north of the low. Thus, a 50 mile or so wide corridor of 3-5 inch totals is likely from central IA through IL and into southwestern IN. These rainfall totals are likely to cause widely scattered flash flooding. Areas near the IA/IL/MO border junction which were hard hit with heavy rains recently will be particularly vulnerable if the heaviest rains cross nearby. ....DC Metro area into Eastern PA... Showers moving across WV this midday will erupt into thunderstorms and widespread convection down from the Blue Ridge later this afternoon as the front forcing the precipitation continues moving east. The storms should be fairly progressive/fast-moving, but there are some indications there may be localized training as more storms form behind the main line of storms. This is most likely to occur in the Slight Risk area. ....Portions of the Intermountain West... 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/Bann Day 2 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... ....20Z Update... No significant changes were made to either Slight risk area with this update. In the Northeast, the guidance continues to fluctuate on rainfall amounts, with no really strong signals for particularly heavy rains in any one area, just a few localized amounts that vary from run to run and model to model. Given the favorable antecedent conditions across much of the Slight Risk area, from the I-95 corridor to the wet soils from eastern PA into New England, even scattered 2-4 inch rainfall amounts, even if localized, have the potential to produce flash flooding. Thus, the Slight remains in effect. The higher risk areas extend northeast from the Poconos north through eastern NY and much of VT. PWATs locally exceeding 1.75 inches on a 30-40 kt LLJ will advect plentiful moisture into the region. While storm motions are likely to be fast, the influx of moisture will support training storms, particularly across northern and western areas, while the urban corridor's lower FFGs make for a similar flash flooding risk there, despite some agreement that the heaviest rains will be north and west of there. A Moderate risk was considered for portions of the hardest hit areas of central and southern VT and far eastern NY, but in collaboration with BTV/Burlington, VT and ALY/Albany, NY forecast offices, opted to keep it a Slight until there's better agreement in the area. Expect especially in the area where FFGs are below 2 inches in 3 hours, that more widespread flooding may develop with a bit of over-performance of actually rainfall as compared with the forecast. In the Northwest, no significant changes were made, although the rainfall signal has decreased a little bit from 24 hours ago. Think given more favorable antecedent conditions from today's rains in the area adding to previous' days, that the flash flooding threat in the Slight Risk area remains elevated. Wegman ....Previous Discussion... ....Much of the Northeast... Guidance continues to struggle with the placement of heaviest rainfall across 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. This encompasses areas which are still hydrologically sensitive following heavy rainfall and flooding over the past few weeks...particularly northern NY into northern New England. Some expansion was done to the Slight risk area in eastern Pennsylvania given an increase in QPF with little need for changes in areas where orographics influence rainfall totals compared with the previous outlook. 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. Bann 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!8RtNCxfdSnUsDRh0jjQs6HpuigP7RGKi0Mx0_UKz4Q_w= FJPu2RqENO0vixLJLNqNU87eed_hM7kmt7MK8PNMU3wi_U0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8RtNCxfdSnUsDRh0jjQs6HpuigP7RGKi0Mx0_UKz4Q_w= FJPu2RqENO0vixLJLNqNU87eed_hM7kmt7MK8PNM8SiulrE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8RtNCxfdSnUsDRh0jjQs6HpuigP7RGKi0Mx0_UKz4Q_w= FJPu2RqENO0vixLJLNqNU87eed_hM7kmt7MK8PNM-aBVGcc$=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) .