High pressure aloft will bring warmer weather to the region today. This will be followed by gentle cooling Sunday through Tuesday as a trough passing by to the north weakens the ridge. Another ridge will build along the West Coast late next week for more warm, dry weather Wednesday through Friday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Today is the first day of astronomical fall but temperatures will be more summery and above normal in many locations.
Coastal stratus is becoming widespread along the San Diego County Coast but struggling in Orange County. Onshore gradients are moderate holding around 5 mb from San Diego to the lower deserts. Low clouds will continue to push inland to some of the western valleys through daybreak. Should see fairly quick diminishing of the low cloud decks after sunrise due to slightly weaker onshore flow. Some patchy fog will be noted this morning along the mesas.
Weak ridging aloft and slight warm air advection will bring a warmup today to all areas, but most noticeable will be warming of about 4 to 6 degrees for most inland areas and 2 to 4 degrees near the coast. Ridging will mainly be across the south and the airmass over the Mojave Desert will be rather stagnant thus not much change in temperatures over the high desert.
Residual moisture over the mountains and weak instability could cause a rogue thunderstorm to develop over the highest peaks during the afternoon therefore have left a slight chance mention in there. The high resolution models are focusing the best chance near San Gorgonio Peak where the highest relative instability should be found.
A weak trough inside slider will move across Northern California and into the Great Basin Sunday through Monday. This feature will crumble as it approaches SoCal but will nonetheless bring some decrease to the 500 mb heights and therefore should bring a slight cooldown and deepening of the marine layer. Temperatures will settle back to near normal levels during this period.
A weak upper level ridge will develop over California Tuesday through the end of the work week, sandwiched between a cutoff low to the west around 35N and 140W and a very broad trough to the east over the middle of the USA. During this period temperatures will warmup a bit to above normal levels once again, but as has been the modus operandi of late, none of this will be especially dramatic; instead it should be just a subtle warmup and some outdoor enthusiasts may not even note the change. Coastal conditions will likely remain rather stagnant with late night and morning low clouds and light fog.
220915z, Coast/Valleys, Low clouds are increasing in coverage at this hour. Bases are 800-1100 ft MSL with tops to 1500 ft MSL. KSAN and KCRQ are currently impacted and KSNA will likely be impacted by 12Z. Clouds are likely to spread up to 15 miles inland by 14Z, with areas of fog reducing vis to 1-3 miles on higher coastal terrain. Clearing to the coast is expected between 15Z and 17Z.
Mountains, SCT-BKN at or abv 8000 ft MSL, layered to 40000 ft in the afternoon/evening. Isolated showers/thunderstorms are possible in the afternoon/evening and could produce localized gusty and erratic winds near storms.
No hazardous marine weather is forecast through Wednesday.
Ca, NONE. PZ, NONE.