Cooling will continue to spread inland today and Thursday with greater cooling on Friday. Periods of precipitation are expected for Thursday night through the weekend. The precipitation could be locally heavy at times for late Friday into Sunday. Another round of more widespread precipitation is possible for Monday into Tuesday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
.Morning Update,
A ridge over Mexico and deepening trough off the California coast continues to help funnel in subtropical moisture in the form of high clouds this morning, which will continue into Thursday. We are watching the developing storm offshore that is expected to move toward the area bringing heavier rain by Friday into the weekend. Confidence is growing that light showers may try to move over the area by Thursday afternoon out ahead of the approaching frontal system. The onset of the main band of rain will begin sometime early to mid Friday morning across the LA basin, starting into San Diego County by late morning into Friday afternoon. The WRF model shows the band potentially stopping somewhere along the SD Co coast, pointing to a scenario where the SD metro area only receives minor amounts of rain. The low will become cut off on Friday, where moisture from the south will funnel into a greater part of the region, providing light to moderate showers across SoCal, including the deserts, Friday night into Saturday around the north side of the low.
There will be an increasing risk for minor flooding, including near burn scar areas with this storm. Increased traffic incidents may result in the morning and evening commutes on Friday as well. Rain rate confidence is moderate with this event. Areas west of the mountains are increasingly likely to see rates near 0.50"/hr or slightly more with the main frontal band on Friday. Confidence lowers on rain rates for Saturday as the low's position will be key to determine where the heaviest rain sets up. The deserts have the potential to see the heaviest rain on Saturday.
.Previous Discussion (347 AM Wednesday),
(today through Friday),
High temperatures will continue to cool for today and Thursday with Thursday high temperatures within a few degrees of average for the coast and valleys and around 4 to 8 degrees above average for the deserts. There will be greater cooling on Friday with high temperatures for the coast and valleys as much as 5 to 10 degrees below average with the deserts near to around 5 degrees below average.
There continue to be model differences with the evolution and details of what is expected to be a cooler and wetter pattern for late this week into next week. Model guidance is now showing the chance for generally lighter precipitation for Thursday night into Friday morning with increasing chances for heavier rainfall for later on Friday into Saturday.
Snow levels will mostly remain above 10000 feet through Thursday, then gradually fall to around 9500 feet on Friday and and 7500 feet on Saturday.
(Saturday through Tuesday), Through the weekend, NBM deterministic rainfall has continued to increase. For Orange and southwestern San Bernardino Counties, it now ranges from around 2.5 inches near the coast to 3 to 5 inches for the mountains with locally greater amounts. For San Diego County it now ranges from 2 to 2.5 inches near the coast to 3 to 4 inches for mountains with 1 to 2 inches for the high desert and around 1 inch for the lower deserts. Chances for greater rainfall amounts are highest on Saturday, followed by Friday and Friday night. Chances for thunder are greatest for Saturday morning through evening.
The ECMWF Extreme Forecast Index for rainfall for late Friday afternoon to late Saturday afternoon reaches the 70th to 80th percentile for much of southwestern California and for late Saturday afternoon through late Sunday afternoon mostly the 80th to 90th percentile.
The model spread grows for next week with another round of relatively more significantly precipitation possible for late Monday into Tuesday. Beyond Tuesday, cluster analysis shows ensemble members with high pressure off the West Coast and northwest flow aloft over California are the drier solutions while those ensemble members slower to move low pressure to the east of California wetter.
121700z, Coasts/Western Valleys, Low clouds have largely cleared out this morning, with some remnant FEW-SCT clouds along the coast near 500-800ft MSL and localized VIS to 3-5 SM in BR/HZ for the coasts and inland valleys. Lingering clouds and VIS restrictions this morning should clear out over the next few hours, giving way to VFR conditions beneath SCT-BKN high clouds near 20 kft through the afternoon. Low clouds expected to redevelop after 01z Thursday, but will likely be more patchy and random in coverage initially, becoming more uniform around 09-12z. Bases will be generally be 700- 1100 ft MSL to start, eventually lifting and favoring 900-1300ft MSL for 12-16z. VIS restrictions to 0-4 SM for inland valleys and coastal mesas, with better VIS near 4-6+ SM for lower elevations and the immediate coast.
Otherwise, VFR conditions will continue through Thursday morning. BKN high clouds AOA 20 kft will gradually lower to around 15 kft by Thursday.
A storm system will approach Friday and move through during the weekend. This will boost southerly winds and seas some, but currently they do not appear to rise to hazardous levels. Widespread rainfall is expected Friday through Sunday, which may be moderate to heavy at times. There is a slight chance of embedded thunderstorms Friday afternoon through Saturday.
Ca, None. PZ, None.
