Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 23 2023 15:37:01 FOUS30 KWBC 231536 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1032 AM 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... A very anomalous and deep upper-level trough over the western U.S. this week is expected to spawn a cut-off upper-level low by early Friday as it slides down the California coast. The sub 540 dam closed low will then slide eastward towards southern California by Saturday. The result of this storm track will allow for a ribbon of subtropical moisture to round the trough and eventually reorient so that it impacts almost perpendicular to the Transverse Ranges and LA metro region of southern California for the bulk of the forecast period (12Z Fri through 12Z Sat). Impressive upper-level diffluence is also forecast over much of southern California due to the depth and location of the system. 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. PWATs are forecast to rise to around 0.75", which is around the 75th-90th percentile compared to climatology. IVT values up to 700 kg/m*s from the ECMWF highlight the increasing atmospheric moisture content as well. Instability will be limited, but MUCAPE of 250 J/kg may reach just inland per the 12z NAM, supporting the potential for excessive rainfall rates. As far as actual rainfall amounts, most guidance depicts the potential for 2 to 4 inches of rainfall 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. Locally higher rainfall amounts are possible. Most of the recent burn scars should be protected and within the heavy snowfall, but urban flooding and rapid runoff will be likely in the lower elevations of Ventura and Los Angeles counties. After coordination with WFOs Los Angeles (LOX) and San Diego (SGX), an eastward expansion of the Moderate Risk area was made to include more of Los Angeles and Orange Counties. A few limiting factors for excessive rainfall include the low snow levels and the relatively dry antecedent conditions. Snow in the elevated terrain will limit runoff to the lower elevations and coastal regions, bottling up the 5-7" QPF amounts within the mountain ranges. NASA SPoRT-LIS also depicts normal to below normal moisture percentiles in southern California, so soils may be able to soak in the initial bursts of rainfall (this is somewhat different from the previous atmospheric river to impact the region in early January). A broader Slight Risk incorporates more of the Transverse Ranges and coastal regions of southern California. Snell/Cook 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... 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!6e_6vyw0KOgXdwrokqKJ_n_JsKqMd8EwTtesGSF9QwV7= BO43aPvAifG5wYCdtko84W_dVhWFdVfxlcPPrCWMYl5MeFQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6e_6vyw0KOgXdwrokqKJ_n_JsKqMd8EwTtesGSF9QwV7= BO43aPvAifG5wYCdtko84W_dVhWFdVfxlcPPrCWMvG8N-I4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6e_6vyw0KOgXdwrokqKJ_n_JsKqMd8EwTtesGSF9QwV7= BO43aPvAifG5wYCdtko84W_dVhWFdVfxlcPPrCWMasUq4FM$=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) .