Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 01 2023 16:01:26 FOUS30 KWBC 011601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Thu Jun 01 2023 Day 1 Valid 16Z Thu Jun 01 2023 - 12Z Fri Jun 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN-CENTRAL PLAINS AND MUCH OF EAST-CENTRAL MONTANA INTO NORTH-CENTRAL WYOMING... ....Central and Southern Plains... Mid-level flow between a longwave trough across the Intermountain West and a high centered over the Upper Midwest will force moisture confluence from the Gulf of Mexico and Pacific Ocean into the Southern Plains today. This will result in 850mb winds that will emerge out of the Gulf and surge up the RGV and into the Southern High Plains at 20-40 kts, resulting in anomalous PWs of 0.75-1.25", around +1.5 standard deviations according to the NAEFS ensemble tables. This anomalous moisture will combine with MUCAPE that could reach 1000-2000 J/kg this aftn/eve despite morning convection/cloud cover, to result in another day of scattered thunderstorms with rain rates of 1-2"/hr at times. There is considerable uncertainty into how the convection later today will evolve, and the incoming 12z high-res models are slightly less aggressive than earlier, possibly due to being under-representative of the morning activity, or more reflective of faster shortwave progression this morning followed by aftn subsidence. This could slow or somewhat inhibit re-destabilization later today. However, the area from eastern NM through the TX Panhandle and even the OK received as much as 2-5" of rainfall yesterday according to MRMS, and this was on top of soils already saturated from 14-day rainfall that has been widespread 300-600% of normal leading to much above normal streamflows according to USGS. This suggests that any additional heavy rain could rapidly lead to runoff, and HREF probabilities for 3-hr FFG exceedance reach as high as 40% in a few locations this aftn. If any cells can train over the same areas that received heavy rain yesterday, more significant flash flooding could occur, prompting a targeted MDT risk area. However, confidence in placement is not sufficient for a MDT risk area, and the SLGT was just adjusted cosmetically for the newest guidance, with the MRGL expanded downstream into IA. ....Northern Great Basin through the Northern High Plains... The active pattern will persist across the Northern Great Basin through the High Plains today as a broad longwave trough centered over the region moves little. Downstream of this trough, increasingly impressive meridional flow transports moisture out of the Gulf of Mexico and around the trough from the Pacific, surging PWs to as high as +2 standard deviations above the climo mean according to the NAEFS ensemble tables. During this evolution, shortwaves and associated vorticity lobes will periodically rotate northward within this flow, interacting with this higher column moisture and MUCAPE that is progged to surge to 1000-2000 J/kg, highest across MT. Within this environment, convection is likely to develop and become widespread during the aftn/eve as reflected by most of the high-res guidance, with rain rates exceeding 1"/hr at times as progged by the HREF. As the 850mb inflow increases through the evening, storm motions should increase to 15-20 kts, but aligned Corfidi vectors indicate that training, or repeated rounds, of storms are likely in many areas. This could result in rainfall exceeding 3 inches in some areas, with the greatest potential occurring across MT where subtly backed flow around the trough will intersect with a cold front to drive locally more pronounced ascent and better training potential. This area has been saturated recently, both by 24-hr rainfall that has been 2-4 inches according to MRMS, and 14-day rainfall that has been as much as 400% of normal, highest in MT. This has resulted in compromised FFG as low as 0.5-1"/3hrs, which the HREF indicates has a 30-50% chance of being exceeded this aftn/eve. The inherited SLGT risk was extended just a bit southeast into northeast WY to better reflect the PQPF and CSU first guess fields, with additional subtle changes made to the expansive MRGL risk where brief heavy rain rates may occur through the day. ....Florida Peninsula... A closed mid-level low and accompanying surface low will meander in the northeast Gulf of Mexico today, embedded within a larger scale trough extending from the Gulf into the Atlantic Ocean. This area of low pressure currently has a 50% chance of tropical development according to NHC, and while currently direct impacts from this wave are not anticipated across the Peninsula, PWs in the vicinity are progged to reach 1.5 - 1.75 inches, within which convergent flow should result in scattered slow moving thunderstorms today. The greatest risk for these slow moving storms is likely to be along a stationary front in the southern Peninsula as well as along the W/SW sea breeze front due to enhanced ascent aiding instability that will eclipse 1000 J/kg. 0-6km mean winds are progged to be just around 5-10 kts, and with bulk shear values forecast to be weak, this should result in pulse type storms that may merge or redevelop on outflows resulting in very slow and at times chaotic motion. With rain rates possibly exceeding 2"/hr at times as reflected by the HREF probabilities, where storms crawl or repeat, falling atop pre-conditioned soils from 14-day rainfall has been as much as 400% of normal in some areas. This could produce instances of flash flooding today, with the greatest chance across any urban, more impermeable, areas. Weiss Day 2 Valid 12Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF MONTANA-WYOMING AS WELL AS THE SOUTHERN-CENTRAL PLAINS... ....Central and Southern Plains... Overall pattern again shows little change given the degree of mid-upper level blocking. One notable difference is that by Day 2 (Fri-Fri night), the models show the main southern stream shortwave (north of the weakening subtropical jet streak) dropping a bit farther south across the southern Plains -- i.e. south of the Red River across North-Central TX. This will result in a small southward expansion of the more widespread, organized convection and heavier areal-average rainfall totals. Only minimal changes were made to the Slight and Marginal Risk areas inherited from yesterday's Day 3 ERO. Given the overlap of Slight Risk areas on Days 1 and 2 across parts of the TX-OK Panhandles into north-central OK and south-central KS, an upgrade to a Moderate Risk may be needed at some point somewhere within this region given the amount and coverage of heavy rainfall from the previous day(s). At this point, all current CSU First-Guess fields depict at most a Slight Risk. ....Northern/Central Rockies and Plains... Stagnant, blocky upper-level pattern also supports an excessive rainfall risk on Day 2 very similar to that on Day 1, as lingering DPVA and diffluence aloft look to persist from the mid-upper low being forced northward due to the ridging. The biggest difference compared to Day 1 is across northern WY, as the risk looks to shift west into the terrain a bit more (covering more of northwestern WY). In addition, the CSU First-Guess fields depict a Slight Risk as well. ....Central and Southern Florida... Models show the mid-upper trough across the eastern Gulf of Mexico flattening a little upon traversing the FL Peninsula and Cuba on Day 2. This would likely lead to a weakening trend with the low-level inflow and moisture transport/flux convergence, although continued favorable thermodynamic parameters (deep-layer moisture and instability) along with slow storm motions will likely maintain an isolated/localized flash flood threat, especially over urban areas. Therefore, will continue to maintain a Marginal Risk across Central and South FL. ....Northern New England... 'Breaking' northern stream shortwave will drop south across Quebec, east of the closed mid-upper high, and reach far northern New England by the end of the forecast period (late Fri night). Given the amplification aloft, strong along-stream upper divergence will pivot across northern New England during Day 2, which along with the uptick in deep-layer instability (mixed layer CAPEs climbing between 1000-1500 J/Kg with the steepening 700-500 mb lapse rates coinciding with the diurnal heating Fri afternoon and evening), will lead to pockets of 1-3+" rainfall totals within a 3-6 hour period across northern New England. This may lead to localized short-term runoff issues, and as such have maintained the Marginal Risk area inherited from yesterday's Day 3 ERO. Churchill/Pereira/Hurley Day 3 Valid 12Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE PORTIONS OF THE NORTHERN/CENTRAL ROCKIES, THE HIGH PLAINS, AND THE CENTRAL/SOUTHERN PLAINS... As the blocky synoptic pattern continues, there will likely be a broad area of the western and central CONUS under the risk of isolated/localized flash flooding into Day 3. While it is difficult to pinpoint a Slight Risk at this range with less discernible synoptic features, there is still a relatively high likelihood of a targeted Slight Risk being introduced in future updates as any mesoscale forcing elements become more clear. At this stage, a very broad Marginal Risk area was inherited and maintained (with some very minor northward expansion into northwest MT) from the prior Day 4 ERO. Both the GEFS and ECENS depict splotchy probabilities for 1" exceedance, and the downscaled versions of the deterministic models depict localized totals of 1-3". Additionally, the CSU First-Guess fields also support a broad Marginal Risk area. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9L4N6GWI-Lqj3FZerkLpBvV2g5yTye_jaMHZeUa225r1= XQLOyoYqE3FEiHTstq4XiMxahPK5zH4iF09afS5k1fkDlkw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9L4N6GWI-Lqj3FZerkLpBvV2g5yTye_jaMHZeUa225r1= XQLOyoYqE3FEiHTstq4XiMxahPK5zH4iF09afS5kRJBNz7A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9L4N6GWI-Lqj3FZerkLpBvV2g5yTye_jaMHZeUa225r1= XQLOyoYqE3FEiHTstq4XiMxahPK5zH4iF09afS5kYyy2zc8$=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) .