Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 19 2024 08:20:26 FOUS30 KWBC 190820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 AM EST Mon Feb 19 2024 Day 1 Valid 12Z Mon Feb 19 2024 - 12Z Tue Feb 20 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SANTA BARBARA,VENTURA AND LOS ANGELES COUNTIES IN SOUTHERN CALIFORNIA... 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 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!_IgcZLTq3cjG2vMnWYoyEFc9gCGIhEzNYDRlyUGG-oAs= LD4Mh4qFn_nnNjWrRZU3RTNlSsH1zu8tlvS1ygjKEnHXd6E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_IgcZLTq3cjG2vMnWYoyEFc9gCGIhEzNYDRlyUGG-oAs= LD4Mh4qFn_nnNjWrRZU3RTNlSsH1zu8tlvS1ygjK0XYC-M0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_IgcZLTq3cjG2vMnWYoyEFc9gCGIhEzNYDRlyUGG-oAs= LD4Mh4qFn_nnNjWrRZU3RTNlSsH1zu8tlvS1ygjKnUhUVWo$=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) .