Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 23 2023 01:25:37 FOUS30 KWBC 230125 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 924 PM EDT Thu Jun 22 2023 Day 1 Valid 01Z Fri Jun 23 2023 - 12Z Fri Jun 23 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND HIGH PLAINS... ....Southeast... Upper low lifts from the TN to Ohio Valley tonight with plentiful Gulf moisture lifting up the Eastern Seaboard and raising the wedge front currently over the southern Mid-Atlantic at least through the central Mid-Atlantic. A Slight Risk covers this warm sector from central VA through the Southeast and into north Florida then back through the central Gulf Coast. Impulses rounding the upper low over the north-central Gulf of Mexico are again aiding organized heavy rain in the northern FL Peninsula east of the Big Bend/Nature Coast over similar/just south of areas hard-hit over the past several days with flash flooding likely there and west through the central Gulf Coast where a stationary front lingers. Ongoing scattered/semi-organized activity will continue into the overnight in the warm and unstable air mass over Georgia and the Carolinas into VA before progressing east per recent HRRR runs. There is a repeating activity threat farther north over the central Mid-Atlantic along the progressing warm front, so the Marginal Risk was expanded north through Baltimore.=20 ....Plains to Upper Texas Gulf Coast... Low level jet strengthens up the Plains overnight providing an expansive warm/moist sector with ongoing isolated activity over the southern Plains likely to persist with new development possible. Supercells over northeast NM are moving east into a favorable environment and with areas of the TX/OK Panhandles primed from recent rains, a Slight Risk was added. Farther north, the low level jet encounters a stationary front over Neb into WY where ongoing heavy activity is over/west of the Sandhills to southeast WY which will continue from the southerly influx of moisture/instability in the LLJ. Decaying activity over eastern SD allowed the Slight to be retracted there. Jackson Day 2 Valid 12Z Fri Jun 23 2023 - 12Z Sat Jun 24 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE HIGH PLAINS AND THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....Plains... The risk of excessive rainfall spreads out over the northern Plains as a strengthening shortwave intensifies into a closed low by Friday night. Given highly divergent with dual left exit and right entrance upper jet support between 2 separate jet streaks...the expectation is for increasing coverage of showers and thunderstorms capable of producing 1+ inch per hour rainfall rates with a corresponding risk of flooding. It helps that precipitable water values are pretty modest...but plenty above average for late June. Thus many ingredients are coming together to support a heavy rainfall event over portions of Montana, Wyoming and the Dakotas. The magnitude of the flash flood threat is likely going to come down to the placement of the low level front and higher instability, and how progressive things end up being. This does look like a pretty quick moving system, which should put an upper bound on total rainfall potential. The guidance is hinting at portions of south central MT, near Billings, where leeside troughing may locally amplify the forcing and allow for more persistent heavy rains in the broad southwesterly flow. The Dakotas will have the benefit of more moisture, but being somewhat displaced from the best upper level divergence, may see lesser amounts of rain, though any storms that form over that area still have the potential to produce 2 inch/hour rainfall rates. For the southern Plains, the typical afternoon showers and thunderstorms will redevelop again Friday afternoon, but since they will be a bit more isolated and less organized as the upper level flow diminishes, the flash flooding risk is expected to be more isolated as compared with previous days. ....Eastern U.S... Some heavy rainfall will be lingering over portions of the Southeast U.S. within a corridor of moisture along and south of a quasi-stationary front. Winds on the moisture-laden/stormy side of the front should allow the storms to produce locally intense downpours although northeastward storm motion should result in less overall rainfall over this area despite the tendency for storms to backbuild. Nevertheless, given the incredibly wet conditions already present over this area, and the potential for another 1-3 inches of rain, with those higher values falling in a short time remaining very capable of resulting in additional flash flooding, think the potential is high enough for a Slight Risk area. Farther north across the Mid-Atlantic, the upper level low will be moving northward as it weakens and opens up to a trough that eventually bets absorbed into the broader scale flow over the Ohio Valley/eastern Great Lakes. Storms which form in the moisture should be fed by a broad southerly flow on the east side of the low/trough and supported by divergent upper level winds...leading to some cells which produce 1 to 2 inches per hour rainfall rates. Given the still overall dry conditions over much of the Mid-Atlantic from a few months of abnormally dry conditions, think any instances of flash flooding will be localized to urbanized and poor drainage areas.=20 Bann Day 3 Valid 12Z Sat Jun 24 2023 - 12Z Sun Jun 25 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR MUCH OF NORTH DAKOTA AND MINNESOTA... ....ND/MN area... A strengthening shortwave trough over south-central MT Saturday morning will track east to western Minnesota by Sunday morning. This feature will allow the widespread area of rain, with some embedded thunderstorms, to track east across the Slight Risk area on Saturday. A slowly strengthening surface low will remain out ahead of the shortwave, which will both draw moisture from the ever-present Low Level Jet northward, while also dragging the Low Level Jet eastward ahead of its cold front. The front will set off additional showers and thunderstorms across Iowa/Missouri and Wisconsin, but since the front will be progressive, so too will the storms along the front, so only isolated flash flooding is possible, namely in urbanized areas. North Dakota and Minnesota should remain in the best forcing as the low wraps up, training storms should develop along the track of the low. While soil conditions are dry from eastern North Dakota and even drier across northern Minnesota, it's important to note that the abnormally high atmospheric moisture (characterized by PWATs over 1.5 inches which would rank 1.5 to 2.0 standard deviations greater than climatology for late June) and favorable flow for training storms may allow for rainfall rates that overcome the antecedent dry conditions and result in flash flooding from the heaviest storms. Expect this to be the case in the Slight Risk area Saturday. ....Northeast... The upper level trough will continue to ever so slowly northeast on Saturday, starting the day in the Ohio Valley and moving to Central NY by early Sunday morning. Maintained the Marginal risk area in parts of New York and New England. In particular....flash flood guidance has been lowered over portions of western Maine over the past two weeks by rainfall totals of 200 to 300 percent of normal and extended into northwest New York. In addition...there will be some modest enhancement afforded by the equator ward side of a mid- and upper-level jet. Trimmed out areas farther south as there should be weakening ageostrophic component to the mid- and upper-level flow as the low begins to fill. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8OzW4SGo8ev_yvC8Fb5Bjd2wLCBWfd7v-kmhJK9sZ994= KS38fXcQKFQQ3ohsjleuT_PQvElhgmyNhKXniU5T4LYAjNE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8OzW4SGo8ev_yvC8Fb5Bjd2wLCBWfd7v-kmhJK9sZ994= KS38fXcQKFQQ3ohsjleuT_PQvElhgmyNhKXniU5Tr8P8S2A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8OzW4SGo8ev_yvC8Fb5Bjd2wLCBWfd7v-kmhJK9sZ994= KS38fXcQKFQQ3ohsjleuT_PQvElhgmyNhKXniU5T_oP5v9w$=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) .