Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 23 2023 08:15:38 FOUS30 KWBC 230815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Sun Jul 23 2023 Day 1 Valid 12Z Sun Jul 23 2023 - 12Z Mon Jul 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, SOUTHEAST AND MID ATLANTIC... ....Southeast into the Mid-Atlantic... An area of heavy rainfall appears likely this morning and afternoon across portions of northwest FL, with ingredients in place to potentially support an organized area of training/backbuilding convection. An uptick in 850mb moisture transport and convergence, and a more favorable position of the upper jet to the north would support organized convective development. Meanwhile unidirectional flow parallel to the front could support a training and backbuilding threat. Most of the 00z HREF members do show organized convection with some training this morning. HREF neighborhood probabilities of exceeding 3" and over 80%, with 5" probabilities more in the 30-50% range. These numbers seem reasonable, and would expect a swath of 2-4" of rainfall with very localized totals of 4-7" possible. Contemplated a targeted Slight risk upgrade for this area, but opted against for now. Most of the high res guidance indicate the organized convection will eventually develop a southward push that should gradually accelerate with time...and this progression may make it difficult to exceed the very high FFG that is in place. Even with the impressive rainfall the HREF is outputting, HREF FFG exceedance probabilities don't get much above 5-10%. Thus while localized flash flooding is possible, tend to think the coverage will remain localized enough to keep the risk at a marginal level. By this afternoon we should see additional convective development over portions of the FL Panhandle into southern AL/GA closer to the actual surface front. Mean flow parallel to the front may result in some training here, but also expecting this convection to gradually push off to the southeast. Recent rainfall has lowered FFG to some extent across portions of this area, so do think at least a few instances of FFG exceedance are probable. Another area of locally heavy rainfall is expected across the Carolinas into VA. Central and eastern SC looks to be one area of focus near the stationary front draped across the area. This boundary should aid in the development of convective clusters, with local mergers potentially resulting in some higher rainfall totals. Enough shear is present to support some maintenance of convective cells, and with PWs forecast around 2" heavy rainfall rates are likely. Did give some consideration to a Slight risk here as well, but there's some placement differences amongst the models, and overall tend to think the coverage of flash flooding will stay rather localized, thus think a Marginal risk should suffice. Convection is also likely across NC into VA with some development focusing along the terrain of the western Carolinas into southwest VA. As we go into Sunday night a weak wave of low pressure will move northeast along the stationary front, which may help nocturnal convection persist across portions of the central/eastern Carolinas into southeast VA. This activity may also pose an isolated flash flood risk. ....Southwest... The strong mid/upper ridging shifts a bit eastward compared to Saturday, which should allow the better moisture to migrate a bit westward. This lowering of moisture and increasing heights over NM should lower the flash flood risk there. Marginally above average PWs over AZ will persist and nose northward into southern NV, with another day of isolated to scattered diurnally driven convection. Any storms could drop a quick inch of rain (00Z HREF probs for 1" amounts over the day 1 period peak around 25-40%), which could drive a localized flash flood risk within any more susceptible basins. Chenard 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!6RT_Sd3aNmLR05nuich_OO6WyrQuPmv7UQRIvTSCaExi= qYXz5FxZmzTE_lb1j8EqTMbRWZO2CavyTsNvAlT55Lou6eE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6RT_Sd3aNmLR05nuich_OO6WyrQuPmv7UQRIvTSCaExi= qYXz5FxZmzTE_lb1j8EqTMbRWZO2CavyTsNvAlT57rjis0U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6RT_Sd3aNmLR05nuich_OO6WyrQuPmv7UQRIvTSCaExi= qYXz5FxZmzTE_lb1j8EqTMbRWZO2CavyTsNvAlT5NMyKzhg$=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) .