Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 09 2023 20:19:19 FOUS30 KWBC 092019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Sat Sep 09 2023 Day 1 Valid 2004Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE CENTRAL AND SOUTHERN APPALACHIANS AND ADJACENT PIEDMONT... ....20z Special Update... An expansion of the Marginal Risk was necessary across portions of the Northeast, Mid-Atlantic, and Southeast based on observational trends. Please see the latest MPDs (1045, 1046, and 1047) for more locally relevant information on the expansions. Also trimmed back the Slight and Marginal Risk areas across portions of western PA, based on the observational trends. Churchill ....16Z Update... Main adjustment this morning was expansion of the SLGT risk on the eastern and western edges to align with the latest 12z HREF blended mean QPF and neighborhood probabilities for 2"/hr or greater rainfall being placed a bit further to the west across areas of eastern TN/KY. MCV across western PA has continued to move to the north over the course of the morning with moderate to locally heavy rainfall along and west of the Laurel Highlands. This will prime portions of the area for later this afternoon as large scale ascent will generate widespread convective coverage within a broad area of deep moisture as indicated by the 1-2 standard deviation PWAT anomalies currently in place from the central Mid-Atlantic through the Northeast U.S. FFG's have come down a bit compared to the last few days given the total rainfall from the last few rounds of convection. FFW's were more prominent than the day before and today's setup is very similar with regards to potential impacts. The zone with most concern lies within the NC Piedmont, west through the Appalachian front from Northeast GA up through PA. The complexity of terrain and guidance consistently targeting the higher elevations lead to maintaining the SLGT and providing a necessary expansion after analyzing the latest forecast bufr soundings and hi-res trends. Kleebauer ....Previous Forecast... ....Northeast and Mid-Atlantic... A deep trough/cutoff low over the Ohio and Tennessee Valleys will remain nearly stationary today. To the east of the low, enhanced upper level divergence will be present to the immediate lee of the trough axis. Meanwhile, tropical Atlantic moisture will continue advecting northward up the east coast, providing a steady supply of moisture into the Mid-Atlantic. The result of this will be Day 3 of the same weather pattern across the area, where periodic showers and thunderstorms will develop...mostly during peak heating, but like right now continuing into the overnight hours as additional shortwaves round the base of the trough. The shortwaves will repeatedly add forcing to the moisture-rich atmosphere, resulting in repeated redevelopment of storms along the Appalachians and Mid-Atlantic. The storms will track northward with the upper level flow. Since this pattern has been stuck for the past couple days, many areas of the Appalachians and Mid-Atlantic have been hit with repeated storms over the past couple nights. This in turn has primed the soils in those areas to be fairly saturated, and unable to handle heavy rainfall. The showers and thunderstorms that have developed have been capable of 2 inch per hour rainfall rates, which locally overwhelms most soils. Meanwhile some areas have seen repeating/backbuilding storms, resulting in several inches of rain over a small area, resulting in local flash flooding. Since the pattern remains the same, this will continue today and into tonight as well. The greatest rainfall totals expected today will be across the western Carolinas, which like previous days has been a consistent signal in the guidance. This is because the upper level cutoff low/trough will be immediately west of the area, leading the western Carolinas to be in the area of greatest local upper level divergence. This area will also have the benefit of local upslope flow, as well as the first areas to see renewed moisture moving in from the south. The western Carolinas haven't been as hard hit as areas further north in previous days, but with the greatest likelihood of repeated storms, the area is considered in a higher-end Slight, especially portions of western NC from Charlotte north and west. Local rainfall amounts of 2 to 3 inches are likely from repeated thunderstorms. Flash flooding is most likely in urban and poor-drainage areas, such as in Charlotte, should storms track over the city. Further north into the Mid-Atlantic, the Slight Risk area was expanded to include much of the central Appalachians, the DC/Baltimore area, and up to the Catskills of southern NY. While rainfall totals broadly will be lower here than in the western Carolinas, antecedent conditions including multiple rounds of storms over the past few days supports the development of flash flooding even with lesser rainfall totals. Nonetheless, the risk of slow-moving and repeated storms over this area is high, so the flash flooding threat should be higher than surrounding areas, hence the upgrade to a Slight Risk. ....Northeast Wyoming and the Black Hills... No significant changes were needed to the Marginal Risk area. A digging positively-tilted shortwave will approach the area tonight, so much of the associated rainfall will occur after sunset. Amounts around 1 inch are expected, though local totals with any stronger/repeating storms could approach 2 inches. This will result in isolated flash flooding, so the Marginal Risk area was maintained. Wegman Day 2 Valid 12Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE CENTRAL PLAINS AS WELL AS THE MID-ATLANTIC AND NORTHEAST... ....2030Z Update... Expanded the MRGL on the northern and eastern flank of the risk area, as well as a general expansion of the SLGT given the latest trend in guidance from both a global ensemble and hi-res ensemble perspective. As stated in the previous discussion, the Slight Risk expansion was warranted given the antecedent conditions anticipated ahead of tomorrow as large scale ascent over the Northeast U.S will shift the convective coverage further to the north where the Carolinas will be mostly spared the worst, but New England enters the fray for heavier precipitation potential. FFG's are still pretty low across portions of central and southern New England which enhances the potential for flash flooding, especially within the terrain and urban corridors stretching from western CT up through western MA and southern VT/NH. 12z neighborhood HREF probabilities for 1-2"/hr are up around 20-30% for the aforementioned area which coincides well with the 1-1.5 standard deviation(s) above normal for areal PWATs. CIPS analogs are also pinpointing heavy rainfall with neighborhood 2+" rainfall totals approaching 60-70% for a good portion of Northern NJ up through southern and central New England. Did expand the SLGT through the Mid-Atlantic given the latest rainfall today with more on the way. FFG's over eastern WV were already low to start the day and should trend lower given the current rain falling overhead. The MRGL was expanded on the eastern side of the previous forecast as models have been struggling with convective coverage west of the Chesapeake up through NJ. There was little update necessary for the central plains SLGT and MRGL forecasts, although did take the MRGL risk down through the Caprock of west Texas with hi-res deterministic keying on convective development over the Panhandle moving south-southeast along a tight theta-e gradient in place where the nocturnal jet on Sunday night is positioned. Despite the lack of rainfall in the region, soils will be very hard and an abundance of low-water crossings could create some localized flooding concerns if storms move over the area. The previous forecast otherwise remains on track. Kleebauer ....Previous Forecast... ....Mid-Atlantic into New England... The nearly stationary upper level longwave trough over the Midwest will very slowly drift eastward with time Sunday and Sunday night. Meanwhile, the upper low from Saturday over the Tennessee Valley will open up into a shortwave trough Sunday, which will pick up speed and track northeast across the Mid-Atlantic by Sunday night. The result will be a northeastward shift in the axis of heaviest rainfall to include the northern Mid-Atlantic and much of New England. After 3 consecutive days and nights of repeating storms over the Mid-Atlantic, especially from the Baltimore area north through eastern PA, northern NJ, and the Catskills of NY, a Slight Risk area was introduced with this update, largely accounting for the favorable antecedent conditions for flash flooding expected. Further north and east into New England, rainfall amounts the past few days haven't been quite as high, as storms have been largely weakening by the time they reached New England. This will no longer be the case by Day 2/Sunday, as the added forcing will allow for much larger coverage of storms. Trends in instability have been decreasing, which could help preclude the heavy rainfall rates needed for widespread flooding, so the Marginal risk area inherited for northern NY and all of New England was maintained. Meanwhile the Marginal Risk area was expanded well south and west down to the southern Appalachians with this update. This too is largely driven by antecedent conditions with expected widespread heavy rainfall over this area today/Saturday. While the rainfall is not expected to be as heavy or widespread on Sunday as compared with today in this area, the rainfall picked up again on Sunday will be the latest in a multiple-day rainfall event, so isolated flash flooding is quite possible. ....Central Plains... Rainfall starting out over WY with a digging and intensifying shortwave trough tracking south down the Plains will also push south and intensify as well, especially going into Sunday night. A developing low level jet (LLJ) Sunday night will advect an airmass characterized by PWATs over 1.75 inches at times northward out of Texas towards the intensifying front and upper level forcing. This will result in strong showers and thunderstorms developing, with a focus over western KS. Soils/antecedent conditions in this area have been very dry, which while normally being a strong factor in the column working against flash flooding, could in this case work to locally enhance flash flooding, as dry soils are not often readily receptive to heavy rain, which results in increased runoff until the soils have enough time to absorb the rainfall. Certainly repeated rounds of lighter rain should gradually reduce the total runoff as the topmost soils more readily absorb the rainfall that occurs. This is most likely with any rain that occurs after midnight, when there's less instability supporting heavy rainfall rates. The surrounding Marginal was expanded north and west to include much of the northeastern half of WY with this update. Rain will likely be ongoing at the start of the period from Saturday night, so any resultant flash flooding from Saturday will continue in this area into Sunday with similar rainfall amounts expected again on Sunday. 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!4Mp8R7GmskRO77G-WgDaPID4wbo-CKXetVqe8H-LNiBC= 5w_PwrQza73XSRSa-ukTyL8Mg31FsE1N45wfJ3SQGQddIVw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Mp8R7GmskRO77G-WgDaPID4wbo-CKXetVqe8H-LNiBC= 5w_PwrQza73XSRSa-ukTyL8Mg31FsE1N45wfJ3SQAJtMJko$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Mp8R7GmskRO77G-WgDaPID4wbo-CKXetVqe8H-LNiBC= 5w_PwrQza73XSRSa-ukTyL8Mg31FsE1N45wfJ3SQwY04TWM$=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) .