Weather Synopsis – October 18, 2023

James Murakami

NOTE: Weather Conditions at sites below–
Current UCLA Data at a Glance
Current UCLA Data Summary

Notice: The AOS department has decided on not updating the weather page (been broken for some time now), This includes the five day, campus forecast and synopsis. The page with current UCLA weather conditions will continue, however (it’s a Davis Instruments site).

I am continuing to assess whether there is sufficient demand to continue writing the weekly synopses/blogs. Please let me know (James Murakami: if you would like to see the synopses continue (would likely get moved to a separate page within the AOS revamped site). In the meantime, I will continue the synopsis for at least the remainder of this month.

High pressure aloft was centered over the central Sierras this afternoon. A weak, upper low pressure was also circulating west of central Baja California. That, along with a marginal, low level, off-shore flow, typically results in a noticeable warming trend. So far, that is the case for the coastal valleys (a few triple digit readings as of this writing). However, a surface, on-shore gradient toward the low desert has supported a strong enough sea breeze that the coastal plain is actually a bit cooler today than it got yesterday (aah, the complexities of marine layer physics). I left the campus forecast unchanged as I thought a dilution of the shallow marine layer might yet occur (no sign of it, as of this writing though).

The computer models continue to insist that the off-shore flow will ultimately dilute or even purge the shallow but effective marine layer. The off-shore flow is predicted to reach maximum strength early tomorrow (breezier than currently for Santa Ana wind prone areas at higher elevations…max wind gusts under 45 mph). Thus, I couldn’t ignore forecasts of noticeable warming in the coastal plain tomorrow. Satellite imagery did show some dissipation of the low cloud field in places. That may be an indication of better mixing of warm air aloft down into the marine layer. However, if this dilution doesn’t continue into tomorrow (i.e. no return of fog near the coast), the campus will probably stay cooler than I show in the forecast today (may huff and puff to reach 80 degrees).

Regardless of the outcome tomorrow, a slight cooling is expected on Friday (aforementioned, Baja low pressure starts a move toward the Southwest…displacing high pressure eastward). The cooling trend in southern California should pick up steam over the weekend (especially Sunday when interior sections included in cooling). Depending on how quickly the on-shore flow develops, breezy weather in the interior could start as early as Saturday afternoon (brisk sea breeze along the coast too). The marine layer may get sufficiently deep on Sunday for spotty drizzle (mainly up against coastal facing mountains/foothills).

The passage of an “inside slider” type trough may temporarily disrupt the low cloud field on Monday morning, but an influx of cooler air should still promote cooler then normal weather throughout the Southland. Additional, “inside slider” troughs are forecast for later next week (such as one around midweek). So, most of next week should feature waxing/waning alternations in the low cloud pattern west of the mountains (analogous increases/decreases in the wind strength). Other than minor day to day changes, cooler than normal weather should prevail for most of next week. Some model solutions show high pressure aloft taking hold by the subsequent weekend, but some model solutions keep low pressure “in the neighborhood”. That would favor additional, “inside sliders”. At this point, none of the models show a relevant chance at wet weather in southern California for the rest of the month.

Another synopsis around Wednesday, 25 October is currently planned.

UCLA Forecasts and analyses are written by James Murakami