A low pressure system will bring cloudy skies and periods of rain and showers through Thursday, with chances for thunderstorms for tonight through Thursday. The heaviest rain will likely fall late tonight through Thursday morning. A period of drier weather will occur on Friday. A series of weak but colder weather systems will bring more chances for light showers to occur over the region for this weekend into early next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This morning, Mid and high clouds cover the region as a low pressure system to the southwest brings warm, moist air from the tropics into SoCal. There is still a dry sub-cloud layer below about 10,000 ft as weak offshore flow continues at the surface. This layer is not as deep or dry as yesterday and stations in the mountains are beginning to report small accumulations of rain, although the lower elevations are still not getting measurable amounts of rainfall.
A frontal boundary with this system will move through the region tonight through Thursday morning when the heaviest rainfall will occur. This system will also bring stronger southerly winds this evening and overnight as well, mainly at the coast where wind gusts near 20 mph will occur at times.
The frontal band will lift over region on Thursday morning. This will be associated with increased instability as the upper level trough passes over SoCal. This forcing will produce a slight chance of thunderstorms for much of Thursday morning and afternoon for areas west of the mountains. Any thunderstorms that form will be capable of producing heavier rainfall, gusty winds and small hail. In terms of rainfall, the heaviest is expected along the coastal slopes of the San Bern Co Mountains with 2"-5" in total by Thursday night. Chances for 1"+ or more of rainfall is near 50-60% in Orange County and the western Inland Empire, slightly lower chances in northern SD County. Chances become closer to 15-30% for the eastern parts of the Inland Empire and the San Diego metro. The deserts will also receive rainfall, but most areas will see between 0.25"-0.75". Some hourly rain rates will be elevated in areas surrounding the LA Basin, leading to localized flooding on streets and near recent burn areas during this event. Snow levels will remain over 8,000 feet, with most mountain communities receiving only rain.
A brief drying period will occur by Friday as the weather system exits to the east and a weak ridge moves in from the west. While most of our region will likely remain cloudy, chances for sunshine and slightly warmer temperatures are in the forecast for the desert regions. Highs in the high desert will near 60 degrees with highs near 70 degrees across the Coachella Valley and Anza Borrego desert areas.
Another Pacific storm system currently in the Gulf of Alaska will move closer to SoCal by this weekend, providing another chance for showers on Saturday morning and continue through the weekend. While exact timing of the rain and rain totals remains uncertain, it is looking increasingly likely that this rain event will be lighter than the one we are about to see in the coming days. Snow levels may be slightly lower by this time period as well. This will continue the near average temperature and mostly cloudy weather pattern that we will be seeing this week. Global ensemble models start to diverge on what becomes of this weekend's weather system and how it evolves by the first half of next week. Periods of showers remain possible for this entire time period as some models suggest a series of weak disturbances rounding the southern base of the mean trough which will persist near the west coast. Though the exact details aren't in view too well for now, a cloudier, cooler and potentially wetter pattern looks to remain in place into the middle of next week.
311630z, VFR with occasional MVFR conditions expected to prevail through late this afternoon. BKN-OVC clouds 8000-12000 ft MSL this morning will gradually lower towards 5000-7000 ft MSL this afternoon. Light showers aloft continue to move over the region south to north today, and as precipitation increasingly manages to reach the surface late this morning into this afternoon, intermittent vis reductions 4-6 SM are possible from the coast to the mountains. South (along the coast) to southeast (mountains/coastal foothills) winds will be intermittently breezy today, with gusts up to 20-25 kts.
Widespread MVFR cigs/vis are likely starting this evening 03-06Z as SHRA increase in coverage and intensity, with intermittent IFR conditions with heavier showers. SSW winds increase for mountains westward late tonight through Thursday morning, gusts 20-30 kt for the coast and valleys and 30-50 kt along the mountains by Thurs morning. There is a 15-20% chance for TSRA for areas mountains westward (~10% chance vcnty KPSP) from about midnight tonight through late Thursday evening, highest chances 12-23Z Thurs. Coverage of TSRA will be iso/sct, and there is low confidence in timing.
Southeasterly winds strengthen this afternoon, with conditions hazardous to small craft this evening through Thursday afternoon, bringing south to southeast gusts around 25 knots and steep mixed seas 4 to 7 feet through the coastal waters. In addition, the incoming storm system will bring a chance for thunderstorms for Thursday, highest Thursday morning through the afternoon and for the nearshore coastal waters. Any thunderstorm will bring strong gusty winds, lightning, and brief heavy rain that will lead to poor visibility. There is also a chance for isolated waterspouts.
Winds diminish late Thursday afternoon. Then, no hazardous marine weather conditions expected through Sunday, though there will be periods of breezy south winds through the weekend.
Ca, Flood Watch from this evening through Thursday evening for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-Santa Ana Mountains and Foothills.
PZ, Small Craft Advisory from 7 PM this evening to 3 PM PST Thursday 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.
