Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 23 2023 20:24:59 FOUS30 KWBC 232024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EST Thu Feb 23 2023 Day 1 Valid 16Z Thu Feb 23 2023 - 12Z Fri Feb 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Fri Feb 24 2023 - 12Z Sat Feb 25 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREATER LOS ANGELES METRO AREA OF SOUTHERN CALIFORNIA... Heavy to excessive rainfall is expected to develop over portions of Southern California on Friday as a deep and very anomalous low makes its way south along the California coast. The flow aloft becomes increasingly difluent aloft while low level winds become directed more normal to the coast which results in decent moisture transport on Friday and Friday night. This results in a ribbon of subtropical moisture to round the trough so that it impacts almost perpendicular to the Transverse Ranges and LA metro region of southern California for the bulk of the forecast period. The axis of heaviest rainfall and the potential for rainfall rates between 0.5-1.0" per hour is expected to gradually slide eastward by the latter half of Friday and peak on Friday night near the San Gabriel and San Bernardino mountains and down to the coastal areas of southern California before the upper low makes its way inland.=20 Most guidance depicts the potential for 3 to 5 inches of rainfall with isolated amounts approaching 7 inches throughout the elevations that remain below the freezing level during the entirety of the event. Snow levels will be quite low compared to normal, starting below 3000 feet but rising up to as high as 4000-5000 feet during the period with the most intense precipitation rates. Given few changes in the large-scale forcing and consistent placement/magnitude of the QPF...no changes were made to the excessive risk areas issued previously. Bann Day 3 Valid 12Z Sat Feb 25 2023 - 12Z Sun Feb 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... 2030 UTC Excessive Rainfall Discussion... The large-scale forcing remains largely unchanged...with comparable deterministic and ensemble-based QPF...from the overnight guidance. As a result...maintained the Marginal Risk area without any change.=20 Bann 0830 UTC Excessive Rainfall Discussion... A strong mid/upper low responsible for the landfalling atmospheric river during the D2/Feb 24 forecast period will still be west of the southern California coast at the beginning of D3/12Z Feb 25.=20 Models indicate that while the bulk of the strongest IVT will shift southeast of the area while weakening, continued onshore flow should result in several areas of convection that should be enhanced by upslope flow against the coastal ranges. Lingering areas of 0.25-0.5 inch/hr rainfall rates should persist beneath the stronger convection in this regime, which could promote flash flooding in low-lying areas and/or burn scars. An additional 1-2 inches of rainfall are possible especially across the Peninsular Ranges. A limited/conditional risk for flash flooding may develop across portions of western Arkansas through eastern Tennessee throughout the forecast period as showers and thunderstorms develop along a retreating warm front. The orientation of the convection (parallel to flow aloft and to the initiating boundary) may result in localized areas of training. Several mitigating factors for this scenario include 1) uncertainty regarding convective coverage/location and 2) lack of surface-based destabilization near the thunderstorms. The latest European depicts scattered areas of 1-1.5 inch precip totals across central Arkansas and eastern Tennessee, however, which could result in spots of excessive runoff in the event that the precip falls in a short-enough temporal window. A Marginal risk area may be needed in later outlooks to address this scenario. Cook Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_jU4fVVG-Nan_vZGlOtOC40rrlEq1l7sR_7CfRt8rBWm= zNMai4Kp1sNoHDpMjpa23DRwYM5mj323NVwQBEJOkg9Wq5E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_jU4fVVG-Nan_vZGlOtOC40rrlEq1l7sR_7CfRt8rBWm= zNMai4Kp1sNoHDpMjpa23DRwYM5mj323NVwQBEJOgNttzf0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_jU4fVVG-Nan_vZGlOtOC40rrlEq1l7sR_7CfRt8rBWm= zNMai4Kp1sNoHDpMjpa23DRwYM5mj323NVwQBEJONZJMTjY$=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) .