Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 10 2025 19:21:55 FOUS30 KWBC 101920 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 PM 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 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....2030z Update... Introduced a Marginal risk area for portions of Southern CA, as a deep layer cut-off low looks to bring a risk of moderate to heavy showers to the San Diego metro area (and possibly as far north as portions of greater LA metro area). While the bulk of the IVT in association with the low will likely remain just offshore and to the south of populated areas, PWATs will be near 0.75" (about the 90th percentile at NKX, per SPC sounding climatology) with the 12z HREF indicating 20-30% probabilities for hourly 0.5" exceedance (using a 40-km neighborhood method, though these probabilities are likely a bit overdone due to the high biased NAM-nest members). While the flash flood risk is conditionally dependent on the development of stronger showers, the risk appears high enough to necessitate a Marginal risk for the potential for localized flash flooding (particularly so if these heavier showers are co-located with more sensitive terrain, such as burn scars). Churchill 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... ....2030z Update... Very little adjustment to the inherited outlook, as thinking is unchanged from the prior discussion regarding the upcoming trough and associated atmospheric river. While the expected timeframe for peak rates (00z-12z Thurs) remains just outside of CAM range, the odds for 0.5"/hr (or locally even higher) rates are relatively high based on the strong ensemble signal for 1-2" QPF combined with IVT peaking around 500 kg/m/s. Only needed to expand the Slight and Marginal risks a tad to the east based on very minor timing differences in the new guidance.=20 Churchill ....Previous Discussion... 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!7dR9hzCKfbmtROHN5jDX3dP4e7Nyyn-LXTy5N2wBFTW7= 6qzGVyawnPZs6u3ns_psH4UiLak2wekSAz8Bf62uWPglb9s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7dR9hzCKfbmtROHN5jDX3dP4e7Nyyn-LXTy5N2wBFTW7= 6qzGVyawnPZs6u3ns_psH4UiLak2wekSAz8Bf62ul7afe2o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7dR9hzCKfbmtROHN5jDX3dP4e7Nyyn-LXTy5N2wBFTW7= 6qzGVyawnPZs6u3ns_psH4UiLak2wekSAz8Bf62upTyQ_mY$=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) .