Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 20 2024 19:59:34 FOUS30 KWBC 201959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM EST Tue Feb 20 2024 Day 1 Valid 16Z 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... ....16Z Update... The previous forecast reasoning still holds, with the heaviest rainfall forecast along the Transverse Ranges in southern California where onshore flow will be orthogonal to the mountain face. The Moderate Risk covering this area was expanded southwards a small amount, just touching the northern San Diego metro area, to account for a southward trend in hi-res model QPF. HREF EAS probabilities of at least one inch of rain remain high (60-90%) along the Transverse Ranges, but have lowered over the Los Angeles and northern San Diego metro areas due to increased forecast spread among the hi-res guidance. Even though spread has increased, enough of the model guidance continues to suggest heavier QPF over the metro areas that the Moderate Risk is still justified. Elsewhere, only minor adjustments were made to the Slight and Marginal Risk areas. The Slight Risk includes most of coastal California and much of the Central Valley where thunderstorms with locally heavy rainfall will be possible this afternoon. The Marginal Risk area extends from the Southern Oregon Coast into the northern California Coast, down through the San Joaquin Valley, and into the desert regions of southern California and Nevada, including the Las Vegas metro area. Dolan ....Previous Discussion... 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 Valid 12Z Wed Feb 21 2024 - 12Z Thu Feb 22 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHERN CALIFORNIA... ....21Z Update... The afternoon forecast maintained continuity with only a Marginal Risk area over portions of Southern California. The slow-moving frontal system that produced rain in this region over the previous few days will be moving inland, and the heavy rainfall threat will decrease quickly Wednesday morning as the cold front pushes south and east of California. Additional rainfall totals look to remain fairly low, but the soils will be saturated so any additional rainfall could pose a flooding threat. Dolan ....Previous Discussion... The last of the heavy rain will be during the morning hours as the strong, fast-moving shortwave sweeps through from west to east. New accumulations will generally be less than 0.50 inch before there will be a sharp cut off of the rain and clearing of skies across southern California. Given the highly saturated soils and new accumulations expected, a Marginal Risk for excessive rainfall and flash flooding remains in effect. The latest trends in the QPF and placement support a small northward/eastward expansion for San Bernardino, Riverside and San Diego Counties. With the rainfall ending very early in the period this will likely be dropped with the mid-morning update, finally ending this multi-day atmospheric river event. Expect dry conditions throughout California for the rest of the day Wednesday. Campbell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7IMIX3ENZZisW8agT-YXifRX9igclpPpQ5lH7y1Ly0nm= BR2sGffXXTXjpC2mnhxbm8Xo0pi86AWDjGrhdmSbitD4lRs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7IMIX3ENZZisW8agT-YXifRX9igclpPpQ5lH7y1Ly0nm= BR2sGffXXTXjpC2mnhxbm8Xo0pi86AWDjGrhdmSbQaQkzb4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7IMIX3ENZZisW8agT-YXifRX9igclpPpQ5lH7y1Ly0nm= BR2sGffXXTXjpC2mnhxbm8Xo0pi86AWDjGrhdmSb27rlxzI$=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) .