Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 25 2023 20:10:58 FOUS30 KWBC 252010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 310 PM 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 currently over the Southwest is expected to eject into the southern High Plains by Sunday night while opening up into a sharp negatively-tilted trough. This will result in lee side cyclogenesis and the strengthening potent surface low to race northeastward toward Iowa by Monday morning. CAMs show convective initiation by around 23z Sunday near the TX/OK Panhandle, which should organize into a QLCS within a few hours and move east. While the individual storms will certainly be capable of locally heavy rain, they will be fast moving and start out scattered along the line. Heavy rain should be more prevalent along the northern part of the convection and near the surface low, where storms become more elevated and into a comma head structure just to the north of the low center. This is also the typical location of a bookend vortex should bowing segments develop, which are also know to have locally higher rainfall. Tropospheric moisture content will be plentiful (tapping into both the eastern Pacific and western Gulf of Mexico), with PWATS into the 1-1.5" range. These moisture values are 4 standard deviations above the climatological mean and above the 95th percentile according to the 12z GEFS. However, the fast-moving nature of the developing storm system will limit excessive rainfall potential. Total rainfall amounts are currently forecast to be in the 1-2" range, with CAMs only showing a maximum of 3" possible. The Marginal Risk area from central/eastern Kansas to far southwest Wisconsin covers areas that are most likely to be affected by both the initial line of storms and the comma-head region of the low; therefore, running the greatest risk of localized flooding concerns. There are several factors working against this becoming more of a widespread flooding threat. First, the storms and the developing low will all be moving rapidly towards the northeast as the entire system swings 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, most of the 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. However, the Marginal Risk was expanded during this update to include more of eastern Iowa, far southwest Wisconsin, and northwest Illinois to account for the more susceptible soil conditions and lingering snow cover that could add to the fresh water runoff. Otherwise, the Marginal Risk was also expanded to include more of south-central Kansas to account for the northern portion of the initial QLCS development and where 12z HREF probabilties for QPF greater than 1 inch in an hour exceed 30%. Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-WV7c-cVGeu0ZerEkxP_9fxtw1vF6U2YS8a3tIbmqF0N= Ky8E6Prq7XOhCjQ-19oWIfYbsFFzC_La0iqZuKhaD2ec2tE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-WV7c-cVGeu0ZerEkxP_9fxtw1vF6U2YS8a3tIbmqF0N= Ky8E6Prq7XOhCjQ-19oWIfYbsFFzC_La0iqZuKhaM6BwRTI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-WV7c-cVGeu0ZerEkxP_9fxtw1vF6U2YS8a3tIbmqF0N= Ky8E6Prq7XOhCjQ-19oWIfYbsFFzC_La0iqZuKha9GRvyFE$=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) .