On-shore flow strengthened overnight, and that led to a slightly deeper marine layer. Low clouds still cleared by late morning in the coastal valleys, but some areas in the coastal plain (particularly L.A. County) remained under an overcast as of this writing. Curiously, the San Diego area cleared out as well as a part of the coastal waters around Catalina Island (unsure why). At least, some partial clearing is anticipated for the currently shrouded, coastal plain, but confidence isn't high (almost never is when predicting the behavior of marine layer clouds). Subtle day to day changes are forecast by the numerical models, but classic (more or less) "May Gray" weather is likely to continue for a couple of more days.
Today's more persistent low clouds could be attributed to a weak, diffuse, upper level trough just off the coast. It's embedded within an overall region of high pressure aloft. A better defined and stronger trough several hundred miles west of the Pacific Northwest states is forecast to head our way this weekend. Currently, these is little to mark it on satellite imagery, but it's forecast to induce some instability showers over the Sierras as the trough travels through the state (from northwest to southeast). For now, the various models aren't predicting much instability reaching southern California, but depending on the path/timing of the trough passage through the Southland, isolated, convective showers (including brief-lived thunderstorms) can't be totally ruled out (valid period would be on Sunday). Some model solutions show marine layer induced drizzle/light rain sometime late Saturday evening through Sunday morning (no more than a tenth inch rainfall). I also wouldn't rule some spotty drizzle occurring as early as Saturday morning (probably wouldn't be measurable most locales that day). While the best chances for such weather should occur adjacent to coastal facing mountains/foothills, some model scenarios show more widespread activity. Hence, I decided to include a slight risk of wet weather for the campus area Sunday morning.
There is one scenario that could negate the marine layer drizzle on Sunday. There is some possibility of the incoming trough disrupting the low cloud field (enough vertical air motion to mix drier air aloft down into the marine layer). In that case, Sunday could end up sunnier than currently forecast (today's campus forecast does show partly cloudy skies for the afternoon though). Cold air instability would have to greater than currently forecast to still produce some mountain showers. In any case, an overall clearing should take place late Sunday afternoon or evening (upper level trough should exit the state).
Temperatures at UCLA didn't climb as predicted recently (applicable to other areas near the coast). Cooler than normal weather should continue through the weekend even if it winds up mostly sunny on Sunday (cold air associated with the trough should maintain cooler than normal weather...even in the deserts over the weekend). As is often the case by mid-spring, the magnitude of the cooling trend is least noticeable for areas near the coast and most in the valleys and interior.
Should the low cloud field get disrupted on Sunday, it may take a day or two for it to "heal". Widespread low clouds are expected for much of next week (no off-shore flow predicted). How far low clouds extend inland is still uncertain. It's likely to reach all of the coastal plain on most days next week, but there are some model solutions showing high pressure strong enough to limit low cloud coverage. Fairly ho-hum weather is anticipated next week (and beyond), but "May Gray" weather is possible for some areas for at least a day (maybe late in the week).
Next scheduled synopsis should be on Tuesday, 18 May. However, I am "on call" for jury duty that week.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-05-13