Cooler and dry this weekend. Patchy fog along the coast this morning, then increasing coverage tonight into Sunday morning. Weak Santa Ana winds develop Monday and weaken Tuesday for warmer and drier weather. There is a 20-35% chance of light precipitation Wednesday into Thursday, though confidence remains low. Another round of offshore winds will follow this system late in the week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Morning Update,
While lingering marine layer clouds are present in Orange County this morning, these should continue to clear out over the next hour or so as the clouds across coastal San Diego County have. Other than some high clouds this afternoon, mostly sunny skies expected across the region today with temperatures right around normal for late November. For more details about the forecast through mid-next week, refer to the previous discussion.
Previous Discussion,
An upper level trough will drop out of the Pac NW and into the Great Basin late today into Sunday. This will bring cooling with highs returning closer to seasonal normals. Gusty west winds will also develop as onshore pressure gradients strengthen, with gusts around 35-45 mph in the wind prone passes and the desert slopes of the mountains. Gradients quickly turn offshore on Monday as high pressure settles into the Great Basin behind the trough. There will be modest upper level support in the form or northerly winds behind the digging trough (850 mb winds of 15-20 kt), albeit briefly. Winds are currently forecast to peak on Monday when gusts of 30-45 mph are likely below the passes and canyons and into the valleys, with isolated gusts around 50 mph near the Cajon Pass and Santa Ana Mountains. Otherwise warmer west of the mountains but cooler elsewhere on Monday. Offshore flow weakens and turns back onshore on Tuesday ahead of the next system for cooling and lighter winds.
The upper level pattern is still questionable beyond Wednesday with the next trough dropping down from the north. Ensembles remain inconsistent with how far southwestward the trough digs, which impacts how much (if any) precipitation we'll see. For Wednesday, around 58% of the ensemble members bring the low far enough west for at least light precipitation late the evening. By Thursday, 32% keep a weak positively tilted trough over the area (light precipitation), 25% have the track inside (dry), 22% develop a closed low off the coast (relatively heavier precipitation), and 21% have a weaker low closer to the Colorado River Valley (lighter precipitation at the coast but heavier inland). Even into Friday, 48% of the members keep a closed low somewhere in the vicinity of So Cal while the remaining 52% have a weaker open wave inland. With the closed low solution, we could continue to see cooler weather and light precipitation if the low is close enough, or with the more progressive/inside track it'll be another round of Santa Ana winds and warmer and drier weather. For what it's worth, the ECMWF AIFS ensemble, which has been performing fairly well, is much drier on Friday than its EPS and deterministic counterparts. For now just have the slight chance to chance (15-35%) PoPs limited to Wednesday morning through Thursday evening, then slightly warmer and dry weather Friday and Saturday.
291600z. Coasts/Valleys, Very patchy areas of low clouds based 300-700ft MSL remain across coastal regions with VIS between 1/4-2SM but are quickly clearing, with VFR everywhere expected beyond 18z. Very patchy low clouds with similar bases and VIS will redevelop and move ashore after 02Z Sunday with similar to slightly more inland extent. A weak eddy may allow bases to rise a bit and clear out quicker Sunday morning, possibly by 13z.
.Mountains/Deserts, VFR conditions expected through the TAF period.
No hazardous marine conditions are expected through Thursday.
Ca, None. PZ, None.
