Low clouds and patchy fog will fill much of the coastal basin this evening into Wednesday morning as a trough of low pressure moves closer to the area. This will also lead to slightly cooler conditions for inland valleys along with windier weather across the mountains and deserts by Wednesday and Thursday. An area of high pressure will briefly move into the area by Friday through the weekend leading to a warming trend. A low pressure system will move closer to our region by early to the middle part of next week, leading to cooler weather and the potential for rain showers.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Evening update,
Marine layer clouds have been gradually filling in over the last few hours this evening, already stretching into inland valleys of San Diego County. Expecting these clouds to eventually fill nearly all of the coastal basin by Wednesday morning. Aided by the trough swinging into northern California tonight, the marine layer will deepen and pockets of drizzle or light rain is possible late tonight, primarily over the nearshore waters and coastal areas.
From previous discussion,
Mostly clear skies will continue through the afternoon as high temperatures stay near average west of the mountains and around 5 degrees above average in the mts/deserts. A trough of low pressure will continue to move closer to Northern California and the Pacific Northwest into Wednesday. This will help deepen the marine layer further tonight into Wednesday morning, filling much of the coastal basin with low clouds and fog across higher terrain into the Inland Empire. The trough axis will push through Northern California on Wednesday, bringing further cooling into the inland valleys with highs mostly in the 70s and 80s across the lower deserts. The passing system will also tighten the pressure gradient, leading to elevated west/southwest winds across the mountains and deserts. There is high confidence in widespread wind gusts 25 to locally over 45 mph across desert slope passes. Upper level flow aloft becomes more zonal by Thursday, which will help ease the winds, but maintain a similar temperature picture to Wednesday.
Spectacular model agreement is seen across global models on high pressure to our south today beginning to build across the Desert Southwest by Friday into the weekend. Models project the pressure gradient turning weakly offshore by this time period as the ridge and associated surface high amplify over the Great Basin by Sunday. This will bring in light Santa Ana winds mainly out of the east w through the mountain passes and western valleys. The forecast was slightly modified to increase temperatures a few degrees above NBM guidance for Saturday and Sunday for western valleys as the offshore flow becomes strongest. Chances are around 30-50% to see highs over 90 degrees across the Inland Empire and foothills of San Diego County.
Models are in better agreement of the an amplified trough developing offshore by early next week, where a cutoff low will form somewhere over the Pacific due west of California. Confidence becomes lower on where the exact path of system will go and if/when it will bring precipitation to our region, especially how fast the system moves. The forecast looks dry through at least next Tuesday where temperatures will begin to cool off as the system moves closer to our area. The wrinkle in this story is that another trough to north will likely move into the West Coast by the middle of next week. This will bring us a cooler weather pattern change but how far south this goes will determine on if we can see any rainfall. Ensembles are starting to hone in on Wednesday being the earliest day to see rain, while some do not point to any rain until late Thursday or Friday of next week.
050330z, Coasts/Valleys, Patchy low clouds based 1500-2000 ft MSL have developed from the coast to the inland valleys. Bases should rise a few hundred feet throughout the night, then rise mostly above 2000 ft MSL around 13z. VIS reductions 0-5SM for far inland valleys, including the Inland Empire, and coastal slopes with localized patchy DZ. Slight VIS reductions elsewhere, 4-6SM, in BR/HZ. Clouds begin to clear east to west, approaching the coastline around 17z. SCT-BKN cigs start to develop over inland areas between 00-02z Thu with bases 2000-3000 ft MSL.
Mountains/Deserts, Mostly clear and VFR conditions today and tonight. Increasing high clouds AOA 20,000ft MSL on Wednesday. Southwest winds in the deserts and over the mountains with surface gusts 35 to 40 knots late Wednesday morning and into the afternoon.
Onshore winds may approach 20 knots in the outer coastal waters near San Clemente Island Thursday afternoon and evening. Otherwise, no hazardous marine conditions are expected through Saturday.
Ca, None. PZ, None.
