NOTE: THE TEMPERATURE/HUMIDITY SENSOR SHOWS PERIODIC, SPURIOUS VALUES, INCLUDING IN THE EVENING NOW. THE CAUSE IS YET UNKNOWN, BUT RESOLUTION TO THE PROBLEM MAY NOT COME FOR SOME TIME.
A weak but full, fledged, low level, on-shore flow pattern has returned to southern California today. That has led to better than expected cooling in the coastal (at least, from what I forecast). The smoke cloud from the southern Sierras wildfires may have contributed to the cooling (FYI--wasn't sure this morning whether the smoke cloud would thin out as it approached the L.A. Basin; Hence, I worded the sky forecast with just increasing haze). None of today's computer models show any appreciable change to the on-shore flow tomorrow (Saturday too). So, while it may have turned out cooler than originally expected, it shouldn't cool off much more in the next couple of days. The weather wildcard for the coastal plain, however, would be the marine layer. Should it make a greater comeback than anticipated, a few more additional degrees of cooling (beyond what I show in today's forecast) is possible.
The return of on-shore flow comes as a weak, upper level trough of low pressure develops just to our east. The various models show a "cut-off" low pressure forming by tomorrow (somewhere around Colorado River Valley). Its center may wind up as far west as the waters just west of northern Baja California (Saturday), but model solutions continue to vary on its predicted location. Today, the trough is promoting some isolated thunderstorms over the southern mountains and east desert (of southern California). Depending on where the low pressure center develops, thunderstorm activity could increase or decrease (latter if center forms in Arizona). The same would apply on Saturday (farthest west position for low pressure potentially could permit any mountain/desert showers to migrate into the coastal plain (especially down in San Diego County). For this forecast, I've maintained a conservative approach on potential, wet weather (i.e. staying close to what's "normal").
One way or another, the aforementioned low pressure should move east on Sunday in response to an approaching, new Pacific trough. Weak high pressure should develop over the Southland on Monday and give way to the next trough on Tuesday. The resultant weather should be an end to any mountain/desert showers and thunderstorms by late Sunday, and some minor to modest warming on Monday (minor warming in coastal plain if marine layer persists). General cooling should resume on Tuesday/Wednesday (a little better cooling than what's anticipated in the next couple of days...including the interior).
Some of the longer range models show a minor rain threat with the trough next week. I wouldn't rule out marine layer induced drizzle/light rain, but I confidence in this scenario is low at this time (this scenario also includes blustery weather in the interior). Model forecasts vary greatly for the latter half of next week. Some maintain a cooler than normal weather pattern (a bit warmer than first half of week though). Other solutions include a warm, off-shore flow pattern. For now, it's a coin toss forecast.
Next issued forecast/synopsis should be on Tuesday, 28 September.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-09-23