Another system arrives today, delivering heavy rainfall tonight into Friday afternoon. Up to an inch of rainfall possible for coast/valleys, higher in the foothills and mountains. Periods of heavy snowfall in the mountains, especially early Friday morning, with snow levels around 6000-6500ft where 5-10+ inches of snow may accumulate. The slow moving system will allow precipitation to linger into Saturday. Drier weather with a very slow warming trend sets in for the end of the weekend into early next week, warming to near normal by Wednesday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Water vapor imagery shows a deepening low off the coast of Monterey, quickly moving southward where it is scheduled to set up off the coast of Southern California tonight. It will follow a very similar track to the last system, bringing cooler weather, ample moisture, and periods of gusty winds to the mountains and deserts. Ahead of this system, pre-frontal light showers should develop over the coastal waters, moving inland over the next few hours with generally less than 0.10" total expected. Upper levels remain quite chilly with limited time to warm in between systems, indicating snow level will be cooler with this event than previous. Snow levels begin today around 6500ft, falling to around 6000ft as the main moisture moves in late tonight/early Friday. Heavy rain bands with periods of 0.50"/hr rainfall rates expected this evening and into Friday morning, although there will also be around a 20-40% chance of thunderstorms region-wide that may result in briefly higher rates. While still looking at significant wetting rain, the lower rain rates with this system mean the flooding threat is somewhat less compared to what occurred over the weekend. Still, half an inch or greater with any thunderstorm in the vicinity of recent burn scars may allow moderate flooding to occur at times, primarily tonight into Friday morning. Always monitor the forecast and listen to local emergency managers in you live in a flood-prone region.
Rainfall totals haven't changed much from the previous forecast package, still looking at Thur-Fri totals of around 1-1.25" for coast/valleys and low-lying areas with around 2" for foothills and lower elevations of the mountains. With snow levels around 6000- 6500ft coinciding with the heaviest precipitation, elevations above 6000ft will likely see at least some snow accumulations, and those above 6500ft will see around 5-10". The greatest accumulations will be above 7500ft, which may be around 15-17" in total through early Saturday morning. The greatest uncertainty with precipitation remains on Saturday as the low pivots into Arizona. Even only 60 hours out, there is significant issues with model alignment for the center of the low and its progression, giving decent spread in amounts of wrap around moisture as the low moves east. As we enter the hi-res model window for Saturday morning, QPF amounts for Saturday range from 0.10"/6hr time frame to around 1"/6hr time frame, all dependent on the position of the low. For now, the wrap- around moisture looks to be most prevalent in San Diego County, in particular the mountains and deserts. Bands of 0.20-0.25"/hr rates won't be out of the question, especially before sunrise on Saturday. Further north, additional moisture being pushed into the Riverside and San Bernardino Mountains may lead to additional snowfall as those snow levels slowly rise from 6500ft to 7000ft, but chances are lower there than in San Diego County. Regardless of how the wrap around moisture shakes out, gusty winds as the low pivots across will begin Friday morning and spread across the High Deserts and higher terrain Friday afternoon into early Saturday morning. Gusts may exceed 30 mph at times, higher in mountain passes and along terrain, especially early Saturday morning.
As the low moves out Saturday, weak ridging will take its place and return us to a drier pattern with a slow warming trend through at least Thanksgiving. Highs will return to near normal by Wednesday, and slightly above by Thursday. The next system may arrive next week, but its a bit too early to tell if it will make it far enough south for us to benefit from. Stay tuned.
202130z, SCT-BKN low clouds 2500-4000 feet MSL through the coastal basin this afternoon. A wide band of moderate precipitation moves across the region from northwest to southeast from 00-12Z Fri. Expecting widespread MVFR cigs/vis through the coastal basin and deserts. Expecting brief periods of IFR conditions in the coastal basin (cigs below 1000 ft and VIS 1-3SM) as the heaviest precip passes through fro 03-09Z. Slight chance TSRA (10-20%) from the coast to the mtns. Mtns shrouded in FG through the night. Cigs/vis improving generally to VFR outside of the mountains after the band passes. Lingering SCT SHRA and ISO TSRA will create intermittent MVFR conditions for all areas on Friday. Easterly winds gusting 20- 30 knots along west facing slopes and downwind of mountain passes into the coastal foothills after 18Z Friday.
Strengthening southerly winds 10-20 knots this afternoon will turn westerly tonight with gusts to 25 knots, generating choppy seas. Check the Small Craft Advisory for details. There is also a slight chance of thunderstorms this evening through Friday afternoon. Any thunderstorm could bring lightning, gusty winds, choppy seas, and a waterspout.
Ca, Winter Storm Warning until 4 PM PST Friday for Riverside County Mountains-San Bernardino County Mountains.
PZ, Small Craft Advisory until 2 AM PST Friday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm- Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.
