Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 29 2023 08:04:40 FOUS30 KWBC 290804 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 403 AM EDT Mon May 29 2023 Day 1 Valid 12Z Mon May 29 2023 - 12Z Tue May 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS... ....Near the California/Nevada Border... A low pressure system approaching from the West combined with ample moisture at elevation (PWs of 0.75"+) and CAPE that builds to 1000+ J/kg should set the stage for convection in and near portions of the Sierra Nevada, Shasta, and Siskiyou Ranges in and near northern and eastern CA. Some of these storms may spill out along a trough extending across portions of NV. There is a general signal in the guidance for local 1-2" amounts, which could occur in an hour or two, with the 00z HREF exceedance probabilities showing 15-25% of exceeding 3" amounts in far northern CA. This would be particularly problematic in any burn scars. ....Northern Plains... Precipitable water values of up to 1.25-1.50" with an area of decent low-level inflow ahead of an incoming mid-level shortwave should set the stage for showers and thunderstorms with heavy rainfall. The CAMs show local amounts near to above 2", which could fall within an hour or two should cell training/mergers occur. Within this region, there's a thin stripe where rainfall over the past week is 200-400% of average, aligned from roughly Bismarck east-northeast to Steele. The best signal for excessive rainfall has consolidated a bit over the MN/ND/SD border region, but the NAM-nest is an extreme outlier that is driving the extreme HREF signal (with 5" exceedance probabilities of 10-15%). A Slight Risk upgrade may be considered with subsequent updates, should better agreement occur between the CAMs. ....Oklahoma/Kansas and North Texas... Precipitable water values of ~1.25" are expected within a moisture plume extending north from the Gulf of Mexico ahead of an upper level trough. CAPE should rise to 1500+ J/kg across portions of the Plains, adding to the heavy rain potential. The 00z ARW2 and NAM-nest are most bullish with heavy rain potential (specifically into southern OK and possibly far North TX) though exact placement has shifted from run to run. Given the lack of consistent signal with the remainder of the CAMs, have held of on an introduction of a Slight Risk, though the 00z HREF 5" exceedance probabilities of 10-20% are duly noted. ....Southern Texas... There is still a more enhanced/agreeable signal in the guidance for somewhat more organized heavy rain in southern portions of TX where an organized convective area is expected to transit due to a progressive mid-level shortwave moving into the region from northern Mexico. The now adjusted Slight Risk in portions of southern TX covers where the heavy rain signal in the guidance best overlaps, though amounts have decreased a bit from prior cycles (though HREF PMM suggests localized totals of at least 2-3" still possible). Any locales that receive 5"+ (with 00z HREF 5" exceedance probabilities as high as 10-20%) would be due to either warm advection bands showing up in advance of any mesocyclones or backbuilding/training occurring on the southwest side/in the wake of any organized convective activity. ....Southern Mid Atlantic States... Vertically-stacked low near Upstate SC this morning will migrate slowly into eastern NC by the start of Day 2 (12Z Tues). Owing largely to the steepening mid level lapse rates (7-7.5 C/KM between 700-500 mb), mixed-layer CAPEs are likely to peak between 1000-1500 J/Kg during the peak heating hours Memorial Day afternoon into early evening. This along with PWs between 1.25-1.5" will support isolated hourly rainfall rates of 1.5+ inches, which could pose a localized flash flood threat (especially across areas that received appreciable rainfall yesterday, primarily from western VA northward into WV). Churchill/Roth Day 2 The Day 2 outlook will be updated by 0830Z. 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!-laOtCL4CAnc_EbVM3SlCOHQJhtBUtEw7ZEhuVuut787= uSuJZn2-dIdkxeg0l21FGP4sEhABWnL5_02ovinWFjIo6hk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-laOtCL4CAnc_EbVM3SlCOHQJhtBUtEw7ZEhuVuut787= uSuJZn2-dIdkxeg0l21FGP4sEhABWnL5_02ovinWf83c6Ew$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-laOtCL4CAnc_EbVM3SlCOHQJhtBUtEw7ZEhuVuut787= uSuJZn2-dIdkxeg0l21FGP4sEhABWnL5_02ovinW0e9dS0M$=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) .