Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 09 2023 15:57:15 FOUS30 KWBC 091557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat Sep 09 2023 Day 1 Valid 16Z 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... ....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 ....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 Valid 12Z Mon Sep 11 2023 - 12Z Tue Sep 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST, MIDWEST, AND SOUTHERN PLAINS... ....Southern Plains... The digging shortwave trough over the central Plains from Sunday will continue to push south and east during the Day 3/Monday period. Heavy rain will diminish during the day with the weakening of the low level jet (LLJ). However, by Monday night, rainfall rates with developing storms will once again increase as the LLJ reestablishes over north TX. The heaviest rainfall rates will therefore be over portions of north Texas, generally west of the Dallas-Fort Worth metroplex. Like areas further north, antecedent soil conditions have been very dry in this area. However, there is a little bit more soil moisture here than in western KS, and most of the storms will be a little bit weaker as most of the best forcing stays further north. In turn, think the flash flooding threat is low enough to keep the area in a high-end Marginal, but will definitely continue to be monitored for a possible Slight Risk upgrade with future updates. Further west, rainfall will continue to develop to the immediate lee of the Front Range from central CO south across much of northeast NM. Isolated flash flooding will also be possible in this region on Monday and into Monday night. ....Midwest... The same cold front that will help trigger convection across the Southern Plains will extend northeastward into the Midwest. As mentioned above, the forcing will be better across the Midwest as the upper level trough will be better aligned with this area. However, the LLJ will be oriented parallel to the front (southwesterly), but the front will be tracking orthogonal to the LLJ towards the southeast. This in turn will favor one or two quick hits of more fast-moving storms across the Midwest, reducing the threat for training/repeating. A Marginal Risk area was added for those strong storms that track mainly over urban areas such as Chicago and Detroit, but any urban areas from northeast IL through southern MI may experience local flash flooding from any training/repeating storms, which will be possible albeit less likely than further south. Antecedent dry conditions should preclude any more than localized/isolated flash flooding in the Midwest. ....Northeast... The shortwave trough will continue tracking northeast through Monday, while the entire Northeast remains in the favorable downstream area of the advancing positively-tilted longwave trough over the Midwest. Coverage of storms on Monday will be greatest across New England, but could reform as far south as Wilmington, DE. Thus, the Marginal Risk area was expanded south and west to include the NYC metro, as this may be the next consecutive day where storms impact the NYC area. The heaviest rain is likely from CT northward into ME. It's probable a Slight Risk area may be needed for portions of New England, but that will be heavily dependent on how much the antecedent soil conditions change as a result of storms expected to track over this area both Saturday and Sunday. Of course any increase in forecasted rainfall would also support a Slight Risk upgrade. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_B0CDRGKva5korPeoJ-A-stWF6JXbHUpRLvhZnBYzaxG= BY18OPzZx6zAebIda5RS0W5E5sYbleJzebc9yfMMAg_pQCw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_B0CDRGKva5korPeoJ-A-stWF6JXbHUpRLvhZnBYzaxG= BY18OPzZx6zAebIda5RS0W5E5sYbleJzebc9yfMMkVYOtLA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_B0CDRGKva5korPeoJ-A-stWF6JXbHUpRLvhZnBYzaxG= BY18OPzZx6zAebIda5RS0W5E5sYbleJzebc9yfMMfdmWszU$=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) .