The strength of the on-shore flow didn't change much (slightly stronger), but the coastal low cloud field became more solidly overcast overnight (not just along and off of San Diego County as had been the case for a few days). Satellite imagery still showed a tattered appearance to the blanket of low clouds (some clear or mostly clear zones early today), but the clouds did reach the lower mountain slopes on the coastal side valleys (deep marine layer...near 4000 feet depth in the L.A. Basin). Only minor changes are anticipated with the marine layer this weekend. Like today, it's unclear where low clouds may persist into the afternoon hours (no pun intended). It's often a futile task trying to predict (accurately) the "bahavior" of the marine layer when day to day changes in the wind flow pattern are minor.
Temperatures through mid-day were running about the same or somewhat lower than yesterday in many areas west of the mountains (a few well inland areas noticeably cooler today). Generally, it's warmer today in the interior (cooling trend expected there starting on Sunday). The computer models predict modest warming in most valleys tomorrow, but the marine layer should prevent any warmer than normal weather. Like the coastal plain, I wouldn't be surprised if the warming there turned out minor (dependent on how quickly low clouds clear). It may turn out a little cooler on Sunday as a new upper low pressure strengthens over the state. Daytime temperature changes in the coastal plain should be negligible, but that could depend partly on how complete daily clearing is.
There is good agreement on an upper low pressure entering the Southland late Monday . Its center should occur somewhere over the Mohave Desert on Tuesday. The predicted pattern should help maintain a deep marine layer (possibly a little deeper than over the weekend) The occasionally breezy weather in the interior should continue into early next week, but a substantial increase in wind speeds isn't forecast at this time. Daytime temperatures over the Southland should bottom out on Tuesday (coastal plain shouldn't be much different from today though).
The aforementioned, low pressure may be strong enough to trigger some cold air instability shower late Monday into Tuesday (mainly mountain region). Even an isolated, mountain thunderstorm on Tuesday afternoon can't be ruled out. There is, as often the case with such low pressures, the possibility for marine layer induced drizzle or light rain (could occur as early as Sunday morning near foothills/mountains). Based on today's model consensus, that appears remote for L.A. County (slightly better chance for the western Inland Empire southward through San Diego County). I have greater concerns that the low pressure may end up disrupting the low cloud field by Monday (just partly cloudy skies most areas early next week).
The low pressure that passes through the Southland should linger somewhere over the Southwest for a couple days next, but it will weaken over time. Thus, the chance for instability showers or marine layer drizzle shouldn't extend beyond Tuesday. Daytime temperatures should also recover (i,e, warming trend) by the middle of next week. West of the mountains, the predicted on-shore flow and marine layer should limit that warming trend (just seasonable temperature toward the end of next week). A number of model solutions today do show high pressure building across the Southland late next week and into the Memorial Day weekend. If this occurs, warmer than normal weather would develop over the holiday weekend (especially well inland areas). Since this appears to be a new trend (didn't show up in with most, past model runs), I'm not particularly confident with the weather trend for the holiday weekend (more to come next week).
UCLA Forecasts and analyses are written by James Murakami
– posted: 2018-05-18