Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 10 2025 15:21:40 FOUS30 KWBC 101520 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1120 AM EDT Mon Mar 10 2025 Day 1 Valid 16Z Mon Mar 10 2025 - 12Z Tue Mar 11 2025 The probability of rainfall exceeding Flash Flood Guidance is less than 5 percent. Churchill/Chenard Day 2 Valid 12Z Tue Mar 11 2025 - 12Z Wed Mar 12 2025 The probability of rainfall exceeding Flash Flood Guidance is less than 5 percent. Chenard Day 3 Valid 12Z Wed Mar 12 2025 - 12Z Thu Mar 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... A deep mid level trough and cold front will move across CA Wednesday into Wednesday night bringing widespread rain and mountain snow to the region. The IVT axis is rather narrow and quick moving and only forecast to peak around 400-600 kg/m/s. When combining this modest strength and lack of longevity of these higher values...this event will likely only reach weak atmospheric river levels. This limitation will be partially offset by the strength of the cold front and deep layer forcing associated with the synoptic scale trough and upper jet. Even so, rainfall totals over much of the state are unlikely to be enough to cause much of a flood concern. Mainly looking at a beneficial rainfall, although some locally higher rates near the front could be enough to result in generally minor and localized flooding within urban and other prone low lying areas. More flood impacts are possible over southern CA, specifically portions of Santa Barbara, Ventura and Los Angeles counties. South to southwesterly upslope flow into the Transverse range will likely help enhance rainfall over this area late Wednesday night into Thursday morning. Also will likely have a bit better large scale forcing overrunning the front at the base of the mid level trough, helping steepen lapse rates and increasing rainfall rate potential. Most of the weak instability is currently forecast to be post- frontal, but could see just enough destabilization work into the front to produce some localized convective elements within the southward shifting rainfall axis. It does seem probable that some 0.5"/hr rainfall will accompany this front, and while any heavy rates will only last for an hour or two, this could be enough to locally increase the flash flood threat over the more sensitive burn scars and any flood prone urban areas. The quick shot of potential high rainfall rates, combined with the sensitive burn scars over this area, warrants a Slight risk upgrade, and localized significant impacts are possible. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8s29npwd1JOL8BtzH23x5OLQ-d7tlNfGfcRgzG8GSJSp= iZmYWR380v6YzWyMnPG40FHD22X74ZoCFhBMNgB5o4zcajg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8s29npwd1JOL8BtzH23x5OLQ-d7tlNfGfcRgzG8GSJSp= iZmYWR380v6YzWyMnPG40FHD22X74ZoCFhBMNgB5WPyXehI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8s29npwd1JOL8BtzH23x5OLQ-d7tlNfGfcRgzG8GSJSp= iZmYWR380v6YzWyMnPG40FHD22X74ZoCFhBMNgB5pbeb-pc$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .