Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 19 2024 15:55:30 FOUS30 KWBC 191555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1054 AM 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... The multiday heavy rainfall event will be winding down during this period but the flood risk will remain elevated. The over-saturated soils will be very sensitive to any new precipitation. Model consensus depicts an additional 1 to 2 inches, locally 3 to 4 inches possible thus aggravating any flood recovery from the recent rain. The Slight Risk spans from southern California, up the coast near Eureka, across the Sacramento Valley and into portions of the Sierras. Again, the heaviest rainfall totals will be further south into the L.A. Basin and may lead to a few instances of significant flooding. Campbell Day 3 Valid 12Z Wed Feb 21 2024 - 12Z Thu Feb 22 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHERN CALIFORNIA... A Marginal Risk area remains over portions of southern California as the long duration rain event tapers off. Consensus show amounts generally less than 0.50 inch will fall over very sensitive, water-logged soils which will keep the threat for instances of flooding elevated. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_DWaH0s6GZBcScLqSnhvTKLXRceUnEFCQ-aOOgfcuNP3= 8SNXhKY9P0Iwk6hGPbkSqPAeaGSI8H7LNTOWkPvvfRA5XGE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_DWaH0s6GZBcScLqSnhvTKLXRceUnEFCQ-aOOgfcuNP3= 8SNXhKY9P0Iwk6hGPbkSqPAeaGSI8H7LNTOWkPvvmpsBx5Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_DWaH0s6GZBcScLqSnhvTKLXRceUnEFCQ-aOOgfcuNP3= 8SNXhKY9P0Iwk6hGPbkSqPAeaGSI8H7LNTOWkPvvCv40_vM$=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) .