Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 19 2024 20:29:37 FOUS30 KWBC 192029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EST Mon Feb 19 2024 Day 1 Valid 16Z Mon Feb 19 2024 - 12Z Tue Feb 20 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF VENTURA AND LOS ANGELES COUNTIES IN SOUTHERN CALIFORNIA... ....16Z Update... Some more substantive changes were made to today and tonight's Day 1 ERO were made following the latest trends in both radar and the high-resolution model guidance. The latest radar and satellite trends show that the plume of heaviest rainfall has largely cleared Santa Barbara County to the east, with a focus on Ventura County now, and soon to be more substantially impacting Los Angeles County. In coordination with LOX/Oxnard, CA forecast office, the Moderate Risk area was shifted out of Santa Barbara County entirely, and now extends to cover most of both Ventura and Los Angeles Counties. The Moderate Risk also now includes downtown Los Angeles. Rainfall rates to 1/2 inch per hour are ongoing across Ventura County, with good agreement in the high-resolution guidance that it will continue shifting eastward into Los Angeles County into this afternoon. By late afternoon and early evening, a brief "break" is forecast as the rain becomes more off-and-on showers for a time, but by mid to late evening, the rain will reintensify and become steady once again with rates potentially reaching as high as an inch per hour. This will continue through the overnight and the end of the day 1 period, as the steadiest rain gradually ends for Ventura County and points north and west, while it gets started south and east towards San Diego. The surrounding Slight Risk area was trimmed from Santa Barbara north to Monterey Bay, while it was extended east along the San Gabriel Mountains, and south down the coast almost to San Diego. There is more uncertainty in southern CA as the timing of the rain as to when it starts prior to 12Z will determine the flooding impacts. Regardless, the rain will continue into Day 2/Tuesday, with the worst impacts from this rainfall event occurring south and east of L.A. and San Diego on Tuesday. For central and northern CA, the changes were less significant. For the San Joaquin Valley, like the central CA coast, rainfall rates will diminish through the day and tonight in between forcing mechanisms (front/atmospheric river to the south, incoming low to the north), so as such, the Slight was trimmed to the immediate foothills of the Sierras south of Sacramento with a Marginal covering any isolated shower activity through the rest of today and into tonight. North of the Bay Area, the ERO risk areas remain largely unchanged. The approaching low has a fair bit of instability with it, resulting in a few thunderstorms making their way onshore. These storms will fortunately be moving fast enough to not have time to drop too much rain, but even short bursts of 1+ inch/hour rates have the potential to quickly cause flash flooding concerns as the storms interact with the terrain. CAMs guidance suggests the off-and-on heavy shower and occasional thunderstorm activity will continue for the rest of today and through tonight, with intermittent bursts of heavier rain in bands as pieces of upper level energy locally enhance rainfall rates. Wegman ....Previous Discussion... The vertically stacked low and associated surface fronts will stall which will result in moderate to heavy rain for the lower elevations of much of California. The initial surge of Pacific moisture will be directed into the Transverse Ranges which will been locally enhanced. The latest guidance depicted an uptick in QPF amounts and placement further east across Santa Barbara, Ventura and Los Angeles Counties with areal averages ranging from 1 to 5 inches with very isolated areas of higher amounts possible-- with total rainfall for this event with 4 to 8 inches in these favored upslope locations. Areas of significant flooding is expected. The Moderate Risk area for the Transverse Ranges was expanded into western Los Angeles County and the Slight Risk area was adjusted eastward into San Bernardino County. Hourly rainfall of 0.5"/hr is all but certain with this event, and 00z HREF data indicates a ~40% chance of 0.75"/hr. While not as extreme as the recent AR event, this should be enough rainfall, especially given the wet antecedent conditions, to produce some significant impacts. The most intense rainfall (when most of these totals will fall) is expected to be between 06z and 21z Monday. For northern and Central California additional rounds of rainfall is expected as the trough/low approaches. Along the coastal terrain from just north of San Francisco to Ferndale the duration of favorable upslope flow will be impressive and persisting through the entire period. Although the hour rates will likely be lower given there will be less forcing present the favorable orographic effects should still result in amounts approaching 1" per 3 hours. Thus over the 24 hour period we could be looking at 3-5" of rain with localized amounts of 5-7". Given that this will be falling on top of increasingly saturated soils and elevated streamflows, additional flooding is likely, and this is considered a higher end Slight risk for these areas. Sacramento and San Joaquin Valleys will both have continued rainfall and runoff concerns into areas getting less rainfall. For those latter locations, meaning areas getting much less rain (due to lack of upslope support), but the runoff from the nearby mountains greatly raises river levels, such that the amount of rain locally that bumps the area into "excessive" territory is much less. This is expected to become an increasingly common occurrence all throughout the Central Valley over the next few days. The area considered at the higher end of the Slight Risk category includes much of the coastal ranges north of the Bay Area through to about Crescent City, as well as downtown L.A. The Slight Risk includes much of the L.A. suburbs east/inland of downtown, including Hollywood, San Bernardino, and Riverside as well as all of the west-facing foothills of the Sierras. Areas east/lee of the Sierras, as well as the urban areas of Reno and Carson City, NV. may "Slop-over" precipitation continuing across all of northern Nevada, with the valleys likely staying plenty warm enough for all rain. Campbell/Wegman/Chenard Day 2 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... ....2030Z Update... The front/atmospheric river over Southern California will continue spreading rain from Los Angeles south along the coast through to the Mexican border, spreading northeastward from there into southern Nevada and the Las Vegas area to start the Day 2/Tuesday period. The rain will gradually weaken with time, but persist as late as midday through San Diego, and through the first half of the afternoon into southern NV. 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 A.R., expect steady rainfall for most areas. Much (though not all) of the CAMs guidance suggests there will be a lull in the steadiest rainfall activity from about 20Z/noon PST through the early evening/04Z/8pm PST. After that, a strong but fast moving shortwave trough will approach the coast. This will rapidly increase overall atmospheric lift resulting in widespread rainfall with embedded showers and a few storms along all of the central and southern CA coast. Once again, individual cells will be moving very quickly with the 150 kt jet still nearby, but the enhanced lift may allow for rates to approach 1 inch per hour in the stronger cells, with widespread 1/4 to 1/2 inch per hour rates likely. With only a short break in the rainfall, these enhanced rainfall rates should re-worsen ongoing flooding, especially in the L.A. Basin. The heavy rain will continue through the 12Z end of the Day 2 period. Given both antecedent rainfall conditions already causing flash flooding in the area, and the likelihood of 1-3 inches (with higher totals likely in the San Gabriel Mountains), in coordination with LOX/Oxnard, CA and SGX/San Diego, CA forecast offices, a Moderate Risk upgrade was issued for much of the Los Angeles Metro, generally from downtown L.A. south and east. 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 was expanded quite a bit from the lee of the San Gabriel Mountains northeast through the rest of San Bernardino County and into southern Nevada with this update. 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. Wegman 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!4dQJ_Ss41Rlu0kZeuFJE-Eqe6FukVFnurdOQ5n4j83iq= cApdR4AiFUU__R6mhV7EByvsffWQr7LRVeSu9ITe7LRxt-M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4dQJ_Ss41Rlu0kZeuFJE-Eqe6FukVFnurdOQ5n4j83iq= cApdR4AiFUU__R6mhV7EByvsffWQr7LRVeSu9ITenUmx5WE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4dQJ_Ss41Rlu0kZeuFJE-Eqe6FukVFnurdOQ5n4j83iq= cApdR4AiFUU__R6mhV7EByvsffWQr7LRVeSu9ITea5IOcv8$=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. 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