A low pressure system moving southward along the West Coast is bringing stronger onshore winds to the area which will last through Tuesday night. Widespread precipitation from the coast to the mountains for tonight into Tuesday night. Moderate to heavy rainfall rates will present a moderate flood risk, especially for northwestern valleys and coastal slopes. Gradual warming and drying with periods of weak offshore flow Wednesday through Friday. For this weekend, the warming continues inland while marine layer clouds and fog return to the coastal areas.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
An incoming low pressure system will contribute to strengthening onshore flow across Southern California today. This will reinforce the marine layer and allow for low clouds coverage to remain broken throughout the day across the coastal basin. Otherwise, a cool and breezy day with temperatures 5 to 15 degrees below average.
As the upper low slides down the West Coast this afternoon, winds will become elevated to strong for the mountains and deserts. Widespread wind gusts 30-45 mph from early this afternoon through late tonight. Gusts may exceed 60 mph along wind-prone mountain passes and canyons. Winds weaken slightly for the mountains and deserts by Tuesday morning, but should remain elevated throughout the day as the storm system passes. This system will also bring cooler temperatures, widespread rain and mountain snow above 7000-8000 ft. Precipitation will begin as orographic light rain along coastal slopes this afternoon before widespread light to moderate showers push into western areas late this evening.
The main frontal band will move across the area from northwest to southeast throughout the day on Tuesday. Widespread moderate rainfall rates expected from the coast to the mountains. Areas of heavy rainfall are likely, especially for Orange County, western Inland Empire and coastal slopes of the San Bernardino County Mountains where rates 0.4-0.8"/hr are expected. Isolated thunderstorms possible in these areas. Rainfall totals will range from over 2 inches along the western coastal slopes of the San Bernardino County Mtns to less than a tenth of an inch for most of the lower deserts. Widespread 0.5-1" for lower elevations from northern San Diego County and areas north. Higher amounts in surrounding mountains. A quarter to half an inch expected in the San Diego metro area with localized higher amounts. A similar story for the High Deserts. Accumulating mountain snow above 7000 feet with several inches expected along the highest peaks. A dusting of snow may occur down to Big Bear Lake. Chance for a few post-frontal convective showers in the evening and overnight Tuesday from the coast to the mountains, but these would be light and intermittent should they occur.
Quiet weather returns along with gradual warming from Wednesday onward as the trough exits the region and is slowly replaced by weak upper level ridging by the weekend. Surface high pressure will be in place over the intermountain west in the wake of the storm system, allowing for weak offshore pressure gradients. This may prevent marine layer clouds from reforming for a couple of days.
The warming trend continues this weekend as weak upper level high pressure moves over the region. Any marine layer clouds that form during the nights and mornings will mostly be restricted to the coastal areas due to the shallowness of the marine layer.
132100z, Low clouds continue to spin over the coastal basin this afternoon with BKN bases near near 4000-5000 feet MSL. High confidence SD County and Inland Empire regions staying mainly BKN this afternoon with partial clearing seen across OC. Any scattering that does take place will be replaced with BKN-OVC cigs after 00Z. Bases lowering after this time near 2500-3500 feet MSL and some higher terrain obscured. Increasing chances of -SHRA after 02Z, first in foothills, but later anywhere mountains westward into the evening. S/SW winds continuing to gust over 20 kts at times through the night at coastal TAF sites.
A Pacific storm system will bring areas of +RA/ISO TSRA 10-18Z near the LA Basin, SBD mts. -RA by 14Z-21Z in San Diego County. Areas of 3-5SM vis in +RA/ISO TSRA. As heaviest rain moves through, S/SW winds increasing with moderate confidence in wind gusts reaching 30 kts at all coastal TAF sites for a short period. Winds weakening on Tuesday afternoon from northern areas to southern areas.
Mtn Desert Slopes/Deserts. Gusty W/SW winds along desert facing slopes of the mountains into the deserts after 18Z continuing overnight into Tuesday. Wind gusts 25-40 kts leading to MOD UDDFS and local LLWS in lee of mountains such as vcnty KPSP. Local vis reduced 3-5SM in BLDU in deserts. Winds slightly weaker and more confined to desert slopes on Tuesday.
A robust eddy continues to bring brisk winds gusting to around 20 kts paralleling the coastline. Stronger southerly winds will develop late tonight and peak Tuesday morning into early afternoon and create conditions hazardous for small craft (check the Small Craft Advisory for details). A front with a distinct wind shift from southerly to westerly will move west to east across the coastal waters Tuesday afternoon. Additionally, there is a slight chance of thunderstorms Tuesday in the outer waters around San Clemente Island northeast toward Catalina. Any thunderstorm could produce briefly gusty, erratic winds, choppy seas, and lightning.
There is a slight chance of thunderstorms and lightning Tuesday morning and early afternoon, mainly in Orange County south to Camp Pendleton. Surf will also increase for Tuesday, 3 to 5 feet, with occasional sets to 6 feet, highest in San Diego County. This will produce a high rip current risk.
Ca, Flash Flood Watch from late tonight through Tuesday afternoon for Orange County Inland Areas-San Bernardino County Mountains-Santa Ana Mountains and Foothills.
Wind Advisory until 11 PM PDT this evening for Apple and Lucerne Valleys-San Gorgonio Pass near Banning.
PZ, Small Craft Advisory from 2 AM to 5 PM PDT Tuesday 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.
