Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 20 2024 08:23:52 FOUS30 KWBC 200823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 322 AM EST Tue Feb 20 2024 Day 1 Valid 12Z Tue Feb 20 2024 - 12Z Wed Feb 21 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOS ANGELES METRO AREA... Rain associated with the atmospheric river and surface front will continue spreading across California, south to the Mexico, and northeastward from there into southern Nevada and the Las Vegas area during the morning hours. As the day progresses the rain will lessen for portions of northern and central California while continuing across southern California into midday and adjacent southern Nevada/northwest Arizona through the afternoon. As this will be the second half of the next bout of heavy rainfall which starts late this evening, flash flooding that begins tonight will continue to worsen through the morning hours as the rain continues. A 150 kt southwesterly jet will be flowing in parallel with the rainfall, keeping the overall atmospheric flow unidirectional. Thus, while individual showers and any embedded storms will be moving very quickly, training will be constant. In the corridor of the Atmospheric River, expect steady rainfall for most areas. A relative lull in the rain is expected during the afternoon hours before a more robust round of widespread rainfall with embedded showers and a few storms track along all of the central and southern CA coast. The swift motion of the shortwave trough and accompanied strong upper level jet will be supportive of yielding rates of 1 inch/hr in some of the embedded cells while elsewhere hour rates will be int he 0.25 to 0.50 inch/hr range. These accumulations over the water logged L.A. basin will further amplify the impact of flooding across the area. A Moderate Risk area was adjusted and maintained for parts of Los Angeles, San Bernardino, Orange, Riverside and San Diego Counties to reflect the current threat level for excessive rainfall and flooding impacts. Despite lesser amounts of rain further north west up the coast leaving the area in the Slight, the widespread rain from as far north as Monterey all the way to the Mexican border should still make for widespread impacts from new flash flooding. For central and northern CA, the heaviest rain is expected to occur roughly during the same time period southern CA is having a break, generally from 18Z through 05Z. The same shortwave trough that will enhance rainfall overnight Tuesday night in southern CA will hit northern areas first, with local rates to 3/4 inch per hour. This area has had multiple days of rainfall, so here too all new rain will convert to runoff and worsen ongoing flash flooding. No significant changes were made to the ongoing Slight Risk area. The rain will be heavy at times and moving slower than further south, but the axis of heaviest rain will be oriented south to north, as the plume of moisture advances east, which will overall shorten the duration any one area is getting rain. Thus, the Slight Risk remains in effect. The Marginal Risk area encompassing the lee of the San Gabriel Mountains northeast through the rest of San Bernardino County and into southern Nevada was maintained. Upper level lift will support light rain continuing northeastward in a plume from the L.A. Basin, and the desert conditions may not support the long-duration of generally light rain, so flood prone arroyos may flash flood on isolated occasions through the High Desert. Campbell/Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-dQol2axEs0HY1MltkX-vPwTA6ea4Bo1MEVC2cesJh6x= dunQL8Uu2Z8GbCPp7ys54UgaUIux7KxUWzk4d7aGWddf4S8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-dQol2axEs0HY1MltkX-vPwTA6ea4Bo1MEVC2cesJh6x= dunQL8Uu2Z8GbCPp7ys54UgaUIux7KxUWzk4d7aGiCLHgAY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-dQol2axEs0HY1MltkX-vPwTA6ea4Bo1MEVC2cesJh6x= dunQL8Uu2Z8GbCPp7ys54UgaUIux7KxUWzk4d7aGkpOKUTg$=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) .