Yesterday, a weak, upper level trough of low pressure passed through the Southwest. It helped promote a good, on-shore flow. That in turn, helped promote widespread, marine layer clouds in the coastal plain (into some coastal, valley locales too, albeit briefly). Yesterday's daytime temperatures were confined to the 60s. Today, a weak off-shore flow exists (some breezy conditions in Santa Ana wind prone areas at higher elevations...under 30 mph gusts, for the most part). While it's warmer today than yesterday, subsidence warming is relatively weak. Afternoon temperatures are still running at or slightly lower than seasonal normals (warmest spots in the valleys barely up to the mid-70s).
This off-shore flow will end quickly overnight. Another, "inside slider" type trough will approach tomorrow. The trough will produce some light precipitation over the northern most part of the state, and some isolated showers should occur over the Sierras, but for most of the state, it'll be just a wind storm. Down in southern California, the brief return of on-shore flow and an expected, coastal eddy should promote coastal low clouds tomorrow morning. That should be the extent of clouds with this system into the Southland (maybe some high clouds too for some areas). After early Thanksgiving morning, marine layer clouds should be absent from the weather scene for several days.
The approaching trough will promote breezy weather in the interior beginning late tomorrow. If most computer model forecasts are right, widespread breezy weather should prevail on Thanksgiving Day (particularly Santa Ana wind prone areas). There is potential for peak wind gusts in the higher mountains to reach 60 mph (nothing widespread nor of long duration though). Even the Santa Ana wind prone areas at low elevations may see some gusts over 35 mph (mainly morning hours). There could be a minor lull in the afternoon, but it should pick up strength again Thursday evening into early Friday. By that point, however, decreasing upper air support may lead to less wind in the lowlands (far from calm, however). The off-shore flow should weaken over the latter half of the holiday weekend, but it's not expected to go away completely.
The "inside slider" will usher in polar air on Thanksgiving Day. So, subsidence warming from the off-shore flow won't be able to warm the air significantly at first (a "cool" Santa Ana wind). Unless the polar incursion ends up minor, Thursday temperatures should be cooler than normal (a little higher most areas than what occurs tomorrow though). By the way, a minor incursion would also result in a less windy event (polar air boundary will help provide the necessary, upper air support for windy weather). As the polar air incursion ceases later in the week, temperatures west of the mountain should increase more noticeably. None of the various models show high pressure getting strong enough to promote widespread 80 degree weather this weekend.
Depending on which model forecast is right, another, weaker, "inside slider" may pass nearby on Monday or Tuesday next week. Slightly cooler weather should result, but it should still remain warmer than normal. If today's model consensus is right, renewed warming will occur thereafter (for a couple of days). There is some potential for widespread 80 degree weather for a day next week. Toward the end of next week, some model solutions show a weak, upper level trough reaching southern California. It should be too weak for a relevant shower threat, but temperatures should fall back to seasonable levels by the subsequent weekend, assuming this scenario verifies (???).
Happy Thanksgiving to all!
Next scheduled synopsis should be on Tuesday, 1 December (none this Thursday, 26 November).
UCLA Forecasts and analyses are written by James Murakami
– posted: 2020-11-24