Forgot I was supposed to write a synopsis...
Only minor changes occurred overnight with the wind flow pattern, but the marine layer deepened a little more this morning. Some early morning drizzle was reported in some areas up against coastal foothills. Low clouds this afternoon were slow to clear across L.A. County, but it was more quick in other Counties (not sure why...mysteries of the marine layer). All the computer models maintain a deep marine layer through the weekend. That would argue for little daytime clearing of low clouds each of the next few days. However, as is so often the case, the daily clearing pattern of marine layer clouds can be difficult to predict accurately. So, slow clearing today may occur in some other areas tomorrow or the next day. In any case, less sunshine is still anticipated for the next couple of days because increasing high clouds are forecast on top of the low clouds (weak, upper low pressure trough approaching...currently over 600 miles to the west).
A large, upper low pressure is developing over the Great Basin states (separate from the low pressure well to our west). This low pressure should help increase the on-shore flow substantially over the weekend (approaching low pressure from the west providing minor help). The afternoon breezy weather in the interior should become more persistent throughout the day over the weekend (probably rather blustery out there by Sunday). The afternoon sea breeze in the coastal plan should also gain strength in the coming days, but any coastal eddy circulation may stifle its magnitude, depending on how large the eddy gets.
The recent change to cooler than normal weather will continue into the weekend. This trend affects daytime temperatures more than overnight low temperatures (marine layer clouds helping to limit temperature drops...more apparent in the interior though. A warming trend is expected starting on Monday when the passage of a "dry" cold front should be followed by a Santa Ana wind event. A return to warmer than normal weather is likely by Tuesday. Widespread 80 degree weather should prevail starting on Wednesday (potentially lasting into the subsequent, Halloween weekend). At this point, no widespread 90 degree weather is foreseen next week.
With the approach of a low pressure trough from the west and north on Saturday, there may be sufficient dynamics to trigger marine layer drizzle/light rain (more so than the isolated occurrence early today). I wrote for a slight chance of wet weather beginning Saturday evening. The best shot at widespread, marine layer induced rain should be Sunday morning. However, most of that should be to the east and south of L.A. County (most favorable wind flow there). Most areas that get wet weather should see no more than a tenth inch rain, but some spots up against coastal facing foothills/mountains may get as much as a third inch rain (nothing widespread in areal coverage though). I suspect that for the L.A. Basin, amounts, should it occur, won't be more than trace amounts. On Sunday evening or very early on Monday, the predicted trough from the north may be sufficiently cold and strong enough to trigger snow flurries on north facing mountains (mainly Tehachapis above 5000 feet elevation...less than an inch accumulation).
Most of the models show a moderately strong Santa Ana wind event early next week. The influx of polar air should also bring a night or two of sub-freezing weather to the high deserts (a few hours duration). The polar air incursion should help provide some muscle to the off-shore flow. Peak wind gusts in the higher mountains could exceed 65 mph for a time. For Santa Ana wind prone areas at low elevations, peak gusts could approach 50 mph in a few locales. This may be the first, widespread wind event of the season (breezy even in non-traditional locales on Monday due to post-cold front winds). The off-shore flow should quickly lose upper air support on Wednesday, but a complete end to the off-shore flow isn't expected before the end of next week.
Next scheduled synopsis should be on Tuesday, 27 October.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2020-10-22