Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 25 2023 08:12:59 FOUS30 KWBC 250812 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 312 AM EST Sat Feb 25 2023 Day 1 Valid 12Z 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 upper level low bringing heavy rainfall across much of Southern California currently will begin to move eastward a bit more quickly today. The low's eastward progress will begin its process of rejoining the jet stream. The eastward movement of the upper level forcing is causing the surface cold front to also move eastward into the coast. Thus, the axis of heaviest rainfall will move into the Los Angeles and San Diego basins this morning. The rainfall into the Peninsular Ranges won't be nearly as heavy as the rain ongoing across the Transverse Ranges. Nonetheless, widespread 2-4 inch storm total rainfall is likely to cause flooding problems in the areas south and east of Los Angeles, which haven't gotten much rain thus far. Meanwhile, flash flooding is ongoing north and west of Los Angeles as heavy rainfall continues. Fortunately, once the cold front moves through, the steadiest rains will shift eastward, leaving this area to contend with scattered showers. Those showers will still be capable of producing locally heavy rain, which may keep any localized flooding going for longer, but most areas will be on a drying trend going forward. Thus, expect the risk categories to lower this afternoon as the rain coverage and overall intensity diminish. No changes were made to the inherited forecast risk areas, as there's good agreement that the rain shifting east will end the threat for areas north and west earlier, and much of the Slight Risk area got relatively little rainfall yesterday. Wegman 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 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!4jbv_0vk0XrrSWtrAw5DfuN29ISaJtrap8SegQ3svsCs= 0L2wEZaEdHW4Z_js0fAZFcRjChZOzaMg8h44VG7qfIuZL3I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4jbv_0vk0XrrSWtrAw5DfuN29ISaJtrap8SegQ3svsCs= 0L2wEZaEdHW4Z_js0fAZFcRjChZOzaMg8h44VG7q_B_JHLQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4jbv_0vk0XrrSWtrAw5DfuN29ISaJtrap8SegQ3svsCs= 0L2wEZaEdHW4Z_js0fAZFcRjChZOzaMg8h44VG7qzsHaU0Y$=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) .