Scattered light showers this evening will increase in coverage and intensity Thursday morning, with chances for thunderstorms Thursday morning and afternoon. Showers taper off late Thursday. A period of drier weather will occur on Friday. A series of weak but colder weather systems will bring more chances for light to moderate showers over the region for this weekend into the middle of next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Evening update, The large swath of light stratiform precipitation is lifting north out of the area with additional light showers beginning to move in from the south. Expect on and off showers tonight, though precipitation along the south slopes of the San Bernardino/San Gabriel mountains will be a bit more persistent. So far there has been around one-tenth to one-half inch across the region, including the deserts, with around one-half to one inch in the mountains and foothills with isolated amounts near 1.5 inches.
A warm front will lift north across the area overnight, bringing breezy southerly winds, especially over the coastal waters and San Bernardino Mountains and desert foothills. Latest CAMs now show heavier precipitation ahead of the cold front not moving into the area until around 6-8 am Thursday morning. Only change in the forecast was to modify low temperatures in the Big Bear area each night as NBM was far too cold given the moisture, cloud cover, and high snow levels.
Previous discussion, The frontal band will move 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.
010330z, BKN-OVC clouds based 2500-4000 ft MSL will continue to lower to 1500-3000 ft MSL for most areas overnight. SCT SHRA will slowly increase in coverage and intensity overnight with intermittent MVFR vis becoming more widespread by 12Z. Southerly winds increasing overnight with gusts 20-30 kt for the coast and valleys and 30-50 kt along the northern slopes of the mtns by 12- 15Z. SHRA becoming numerous and intermittently heavy after 12Z from the coast to the mtns with intermittent IFR cigs/vis. Precipitation may form into an organized band of heavy rain which would move across the region from west to east 18Z Thu-03Z Fri bringing IFR cigs/vis with its passage. There is a 15-20% chance for TSRA for areas mountains westward (~10% chance vcnty KPSP) from 12Z through late Thursday evening, highest chances 15-23Z.
Winds turning westerly and weakening with the passage of the heaviest rain 21Z Thu-03Z Fri. Additionally, coverage of SHRA diminishes from west to east 00-06Z Fri with cigs lifting mostly above 3000 ft MSL after 06Z.
Southeasterly winds continue to increase this evening with intermittent gusts 20 to 25 knots becoming more frequent overnight. 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 that develops will bring strong gusty winds, lightning, and brief heavy rain that will lead to poor visibility. There is also a slight 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.
There is a slight chance for thunderstorms Thursday, highest chances in the morning and early afternoon, which could bring dangerous lightning. If you see lightning or hear thunder, seek a sturdy, enclosed shelter. Additionally, starting Thursday morning and continuing through the weekend, large high tides of 6.5 to 7 feet could lead to minor coastal overflow/flooding for beaches and flood- prone beach lots and walkways.
Ca, Beach Hazards Statement through Sunday morning for Orange County Coastal Areas-San Diego County Coastal Areas.
Flood Watch 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 until 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.
