Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 06 2023 19:50:46 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 Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN NEW ENGLAND AND NORTHERN NEW YORK... ....20Z Update... There also were no significant changes made to the Slight Risk area across the Northeast with this update. The guidance has largely settled on a widespread area of 1-3 inches of rain, particularly from NH through much of ME, but there are differences in thought as to how much flooding this amount of rain will translate to. In coordination with BTV/Burlington, VT; GYX/Gray, ME; and CAR/Caribou, ME forecast offices, the Slight was left as is for this update, though a Moderate Risk is a very real possibility with future updates. The surface low will track from Lake Ontario to north of northern ME through the Day 3/Tuesday period. This will spur on a 50 kt southerly LLJ into western ME, which may advect 2 inch PWAT air into the Maine coast. Topography will likely locally enhance rainfall totals, and it is in these areas that the greatest threat for flash flooding is possible. Further west into VT, there will be less time in the Day 3 period for it to rain, but the rain is likely to be well underway from Monday night. Any additional uptick in forecasted rainfall in this area will likely require a Moderate Risk upgrade. No significant changes were made to either Marginal Risk area, as the flooding threat across the Plains will greatly increase into Day 4 once the forcing moves to the MS River Valley. The stalled out front across the Deep South may cause localized flash flooding, particularly in any urbanized areas. Otherwise FFGs are high enough that flooding concerns are low. Wegman ....Previous Discussion... ....Northeast... Increasing dynamics associated with low pressure that will have made its way eastward from the Upper Midwest and Great Lakes region will aid increasing coverage of showers and thunderstorms capable of producing heavy rainfall rates and excessive rainfall.=20 A plume of precipitable water values in excess of 1.75 inches should already by in place from the Mid-Atlantic region north/northeastward that can be tapped by the vertically stacked system. The threat of excessive is expect to be driven by intense rainfall rates...especially more in the south and in the east...while concern for portions of upstate New York into Vermont will be a combination of heavy rates with the deep moisture followed by the upper system over Canada that begins to change course and possibly bring renewed convection later in the period.=20 This area in particular is still recovering from recent flooding.=20 There was too much spread in the guidance to place a Moderate with any confidence. The 06/00Z NAM and UKMET depict 2 to 4 inch amounts just north of the international border while the ECMWF is consistent in 4 or 5 inches along coastal Maine. If the NAM solution verifies a bit farther east...the concern for flash flooding will be considerably higher given their hydrologic sensitivity. Surrounding the Slight is a broader apron of Marginal Risk given uncertainty in how quickly rainfall will be moving away and how much will have already fallen by then. ....Southeast U.S.... Maintained the Marginal Risk area along and immediately south of an unusually strong cold front for this time of year where precipitate water values at or above 2 inches are expected to be in place. Exactly where there will be interaction between any mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area to receive excessive rainfall.=20 ....Western Plains... Another day of late day and evening convection...with at least some threat of excessive rainfall...is possible as return flow draws moisture farther into the western Plains on the west side of a quasi-stationary front. Thinking is that the convection will initiate over the Western High Plains with the concern for excessive rainfall increasing as the convection moves east/southeastward towards deeper moisture as shown by precipitable water values of 1.5 inches. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6pr0hFee19lmBtthRG4z7fGSL1sXPF3rVCoYb4whmByU= wyUDl5grm-QJL_qYp7grofBQjp93cjNZMObFmh8zEj7Ysj8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6pr0hFee19lmBtthRG4z7fGSL1sXPF3rVCoYb4whmByU= wyUDl5grm-QJL_qYp7grofBQjp93cjNZMObFmh8zIeWCa10$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6pr0hFee19lmBtthRG4z7fGSL1sXPF3rVCoYb4whmByU= wyUDl5grm-QJL_qYp7grofBQjp93cjNZMObFmh8zxb_HWxU$=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) .