A cooling trend will develop from Friday into the weekend, most notably for the mountains and western valleys, as high pressure weakens over the region. Low clouds and fog will also return to the coastal areas and inland valleys as onshore flow replaces the Santa Ana winds. By the middle of next week, an area of low pressure from the north will move over the Desert Southwest. Though the path of this system remains uncertain, chances of precipitation are in the forecast.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This evening, sfc pressure gradients are still weakly offshore but are trending onshore. The upper level ridge has been replaced by troughing aloft. The height falls and cyclonic flow aloft will likely cause the marine layer to deepen as onshore flow returns. Patches of low clouds have formed over the coastal waters and have begun to move onshore in northern San Diego County. High clouds and drainage winds could still disrupt the formation of low clouds so there isn't much confidence that we'll see much low cloud coverage or fog tonight, although high resolution models indicate significant cloud cover by sunrise Friday.
From previous discussion, A weather system currently in the Pacific Northwest will continue to push inland over the western part of the country, weakening and displacing the area of high pressure over our region through the weekend. This will lead to cooler weather with stronger onshore flow. By the weekend, temperatures in most areas will be in the 60s and 70s, with 50s across areas above 5,000 feet. Chances for low clouds and fog will increase over coastal areas into the weekend as the marine layer deepens.
A low pressure system will move across the Great Basin by Sunday and Monday, which will lead to a slight increase in offshore winds for Monday. Depending on the strength of this pressure gradient, temperatures may climb into the mid or even upper 70s for valleys west of the mountains. A second weather system will move in from the north around Wednesday of next week. Model projections show quite a varied array of where the system will go. Some projections point to a more inland scenario, where we would remain dry with Santa Ana winds. Others point to a wetter scenario, where the system moves closer to our area or off the ocean. Nbm shows increasing chances for precipitation around 15-30% by Wednesday and Thursday of next week. Continue to keep an eye on the forecast as our confidence grows on the path of this weather system and how it could impact your area.
280345z. Coasts/Valleys, SCT-BKN high clouds AOA 20,000ft MSL along with patchy cloud cover, mostly offshore, this evening. Low cloud cover based 400-700 ft MSL will gradually spread into parts of the coast overnight, with a 40-60% chance for CIGs at coastal sites overnight (highest chances after 08Z KSAN, 10Z KCRQ, and 14Z KSNA). Localized VIS reductions down to 3-5SM possible during this time as well, 0-2 SM for elevated coastal terrain impacted by clouds. Any low clouds clearing to the coast 15-17Z, except for parts of northern Orange Co where clouds could persist through 18Z. More widespread low clouds 600-900 ft MSL spreading into coastal areas up to 15 miles from shore after Sat 02Z.
.Mountains/Deserts, VFR conditions expected through the TAF period. High clouds SCT-BKN AOA 20,000ft MSL through tonight.
No hazardous marine conditions are expected through Tuesday.
Ca, None. PZ, None.
