Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 09 2023 08:31:11 FOUS30 KWBC 090831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Sat Sep 09 2023 Day 1 Valid 12Z Sat Sep 09 2023 - 12Z Sun Sep 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL AND SOUTHERN APPALACHIANS AND ADJACENT PIEDMONT... 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 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS AS WELL AS THE MID-ATLANTIC... ....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, but this area will be closely monitored for an expansion of the Slight Risk north and east into this area. A large contributor to the need for the expansion will be how much rain the area gets today/Saturday, as overperformance of storms (which has been the trend further south), would mean the soils in the area will be more primed for flash flooding when better coverage of showers and storms reaches this area on Sunday. 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. Further increases in forecast rainfall for western NC and southwest VA may require another Slight Risk upgrade in this area as well with future updates. ....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 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5uif53cw73M752ZE9WqPskR4rBuwU69cg86-Yn4VD8we= ERAZfaX_AuApcT6QgDDohTW2nqf2xJzuMkE2U2qnMc6_u90$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5uif53cw73M752ZE9WqPskR4rBuwU69cg86-Yn4VD8we= ERAZfaX_AuApcT6QgDDohTW2nqf2xJzuMkE2U2qnddNeSnE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5uif53cw73M752ZE9WqPskR4rBuwU69cg86-Yn4VD8we= ERAZfaX_AuApcT6QgDDohTW2nqf2xJzuMkE2U2qnZdxOWK4$=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) .