UCLA Weather

from the Department of Atmospheric and Oceanic Sciences.

Thursday - 9:14am

Fog

52°

High - 53°

Low - 48°

Today: Areas of dense fog before 10am. Otherwise, cloudy, then gradually becoming mostly sunny, with a high near 68. Calm wind becoming southwest around 5 mph in the afternoon. Patchy dense fog after 10pm. Otherwise, mostly cloudy, with a low around 50. Calm wind. *NWS LA

Five Day Forecast

Latest Average Hourly Temperatures

NOTE: THE TEMPERATURE/HUMIDITY SENSORS HAVE SHOWN PERIODIC, SPURIOUS VALUES, INCLUDING IN THE EVENING. THE CAUSE IS YET UNKNOWN, BUT RESOLUTION TO THE PROBLEM MAY NOT COME FOR SOME TIME.

High pressure aloft continues to dominate over most of the West (even Pacific Northwest states had enjoyed a brief respite from wet weather). In southern California, the off-shore flow has weakened compared with past days, but it's not gone. Breezy weather at higher elevations in Santa Ana wind prone areas persist although wind speeds are much lower than a few days ago (now wind gusts mostly under 30 mph). The computer models are predicting a modest increase in winds again tonight through early tomorrow (likely higher elevations only). They should taper off later tomorrow. The off-shore flow should transition back to a weak, on-shore flow by late Thursday (lasting through Saturday morning).

Technically, a marginal, surface on-shore flow exists today (on-shore flow gradient toward low desert, albeit pretty weak). That probably explains the recent increase in coastal low clouds/fog over the coastal waters south of Point Conception. Curiously, the only area that low clouds/fog penetrated well into the coastal plain early today was in L.A. and northern Orange Counties (restricted to near the coast elsewhere). Although the surface off-shore flow is predicted to increase again for a time, I wouldn't be surprised if dense fog prevails tonight in some parts of the coastal plain (less extensive than this morning though). Fog should get restricted to the coast tomorrow morning, however. At least, it should be that way in theory, but marine layer physics is quite complex (don't bet the family farm on a marine layer purge by the off-shore flow).

That same, shallow marine layer appears to have wreaked havoc with today's temperature forecast in the coastal plain. Temperatures warmed well into the 80s again in the coastal valleys (another 90 degree day in Woodland Hills too). However, the coastal plain stayed mostly cooler than what the models predicted (some beaches didn't even reach 60 degrees). A lack of vertical air mixing is the likely cause. There is still some chance for a late afternoon warming at UCLA, provided better mixing occurs, but as of this writing, it hasn't gotten past 69 degrees (6 degrees lower than forecast). I left the forecast unchanged, but a continued lack of vertical air mixing tomorrow could torpedo the temperature forecast (i.e. tomorrow could wind up about the same as today).

There is a weak, upper level low pressure along the central Baja California coastline. In summer, such a circulation often would direct "monsoon" moisture into the Southland. In this instance, only some high clouds are expected to waft into the region. The low pressure is forecast to head north and approach San Diego before it subsequently heads inland over Mexico. Its approach is what should help trigger a return to a weak but full fledged, on-shore flow. When that happens, there should be an increase in marine layer clouds as well as a general cooling trend (more pronounced near the coast than well inland areas...marine layer related).

A separate, weak, upper level low pressure will pass well to our south on Saturday. Like its sister low pressure, no inclement weather is expected (just a continuation of on-shore flow for a little longer). High pressure aloft, however, is predicted to build back into the state Saturday night into Sunday. Another round of off-shore flow is predicted at that time. Some breezy weather is anticipated for Santa Ana wind prone area, but mostly at higher elevations only. Peak wind gusts should remain under 45 mph. Temperatures with this next off-shore flow should be a little less than the current situation. The exception might be in the coastal plain if the marine layer were to get purged. Then, temperature potentially could get a little higher (as high as upper 70s inland coastal plain on Sunday/Monday).

Most of the longer range models show an "inside slider" type trough passing through the state sometime on Tuesday (a few solutions delay that to early Wednesday). Depending on which solution verifies, there could be some wet weather reaching southern California. The predicted, cold air instability showers would mainly affect the mountains (snow levels potentially as low as 5000 feet). The better chances in the coastal plain would be down toward San Diego County ("rain shadow" effect farther north, based on today's consensus for low level wind flow). None of the wet solutions show a significant storm (limited water vapor and moving too quickly). At the least, a decent cooling over the Southland should take place.

It could get breezy with the predicted trough (almost certain for the interior), but there could also be a moderate strength, Santa Ana wind event following the trough passage (around the middle of next week). It would be brief in duration (about 12 hours maximum), and the predicted influx of polar air would keep this off-shore flow on the "cool" side (maximum warming in the low/mid-70s...possibly barely reaching 70 degrees most areas west of the mountains). A number of model solutions show another, potentially wetter storm toward the end of next week. This is far from certain considering that it's more than a week into the future. At least for northern California, it could mean a return to occasional wet weather (been dry for most of November). Even if the wet scenario that include the Southland occurs, it's not likely to be major storm (just "run of the mill"). As always, stay tuned...

Next issued forecast/synopsis should be on Thursday, 2 December.

UCLA Forecasts and analyses are written by James Murakami

– posted: 2021-11-30

Data and technology is provided by UCLA's Department of Atmospheric and Oceanic Sciences

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