Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 25 2023 15:52:28 FOUS30 KWBC 251552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1051 AM EST Sat Feb 25 2023 Day 1 Valid 16Z Sat Feb 25 2023 - 12Z Sun Feb 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOS ANGELES AND SAN DIEGO BASINS... The deep upper-level low impacting the Southwest is beginning to make its push inland and will aid in diminishing the widespread heavy rainfall threat across southern California. Rainfall rates have dropped behind a cold front to below 0.5"/hr, but the broad post-frontal precipitation shield could still exacerbate any lingering flooding concerns through at least 21z, thus the Slight Risk was maintained for now. Farther north and along the central California and regions just inland, an isolated heavy rainfall threat is expected to linger through this evening as the closed low moves over the region. Sufficient mid-level lapse rates above 8 degrees C/km are likely to lead to the development of scattered showers capable of containing brief, but locally heavy rates that could pass over regions already saturated from previous heavy rain. This potential resulted in an expansion of the Marginal Risk area to include more of the central California coast and Santa Lucia Range. The conveyor belt of moisture moving into the Desert Southwest could support locally heavy rain at time from southern Nevada to the upslope region of the Mogollon Rim in Arizona. However, low snow levels and the eventual progressiveness of the system as it picks up forward speed should limit the flash flooding potential. Snell Day 2 Valid 12Z Sun Feb 26 2023 - 12Z Mon Feb 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER MISSISSIPPI RIVER VALLEY... The upper level low causing the heavy rain currently across Southern California will eject eastward across the Plains on Sunday. As it does so, the low will open up into a trough, which will become negatively tilted by Sunday evening. Ahead of the trough, CAMs show strong to severe thunderstorms erupting over western OK and western KS. While the individual storms will certainly be capable of locally heavy rain, they will be fast moving and start out scattered along the line. As the evening goes on, the storms will develop upscale into a solid line, which will further develop into a comma-shaped low by Monday morning. The Marginal Risk area from eastern KS through northeastern IA covers areas that will be affected by both the initial line of storms and the comma-head region of the low. This will make for a small area of higher storm total rainfall, which the Marginal Risk highlights. There are several factors working against this being a large flooding threat. First, the storms and the developing low will all be moving quickly towards the northeast as the entire system moves along with the aforementioned trough as it joins the 110 kt southwesterly jet. Thus, no one area will see very heavy rainfall for very long. Second, instability is moderate, with MUCAPE values peaking near 1,000 J/kg. The relative lack of instability should hinder how strong the cells can get, which will limit how heavy the heaviest rainfall will be. Finally, the entire Marginal Risk area has dry antecedent conditions, with soil percentiles on average at 10-20% saturated. Any areas that pick up heavy rain may see that rainwater drain off into the local river basin with little flooding to note. Therefore the Marginal Risk for isolated flash flooding appears to be the right risk category for this area. The Marginal Risk area was trimmed quite a bit from the inherited area to highlight only those areas where there is good agreement from the guidance that will get the heaviest rainfall. The largest trimming was on the southeastern side in MO, which will likely enter into the dry slot behind the initial line of storms. Wegman Day 3 Valid 12Z Mon Feb 27 2023 - 12Z Tue Feb 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-eHprZa0ZX9FtWO9zf5Qj9tH0gqPGIMYNEVSrpOF9Pew= WoKmebHpWd-4Prisy8Kz3B9TA_LDlHZh-KaiNvI1eRo9LQM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-eHprZa0ZX9FtWO9zf5Qj9tH0gqPGIMYNEVSrpOF9Pew= WoKmebHpWd-4Prisy8Kz3B9TA_LDlHZh-KaiNvI1M3K2IZo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-eHprZa0ZX9FtWO9zf5Qj9tH0gqPGIMYNEVSrpOF9Pew= WoKmebHpWd-4Prisy8Kz3B9TA_LDlHZh-KaiNvI1Pw34ktA$=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) .