| Day | Date | High/Low | Forecast |
| Fri | 25th | 75/58 | Mostly sunny remainder of day. Mostly clear evening. |
| Sat | 26th | 77/59 | Chance of early morning low clouds/fog; Otherwise, mostly sunny day with scattered high clouds. Variable evening high clouds. |
| Sun | 27th | 73/58 | Partly to mostly cloudy day. Mostly cloudy evening. |
| Mon | 28th | 66/55 | Partly to mostly cloudy day; Possibly breezy at times in the afternoon. Partly cloudy evening and possibly breezy at times. |
| Tue | 29th | 68/53 | Mostly sunny day and possibly breezy at times in the morning. Mostly clear evening. |
Synopsis
High pressure aloft continues to dominate the weather in the Southwest and most of California. It has, however, weakened over the last couple of days. That has allowed for a weak, on-shore flow to re-establish a shallow but effective marine layer along the Southland coast (reason for low clouds/fog for most of the coast at times). Warmer than normal weather continues for well inland locales, but temperatures have fallen off compared with a few days ago.
All the computer models predict that high pressure aloft will strengthen slightly tomorrow. That should result in a weaker on-shore flow or even a marginal, albeit brief, off-shore flow (too weak for relevant wind in Santa Ana wind prone areas). Areal coverage of low clouds/fog late tonight into tomorrow morning may decrease some (already limited coverage seen on satellite imagery this afternoon). A few to several degrees daytime warming is probable tomorrow for well inland areas (i.e. inland coastal plain and most valleys). It’s less certain about warming nearer the coast (wild card being how much marine layer will be present tomorrow). I’ve leaned the campus forecast toward minor warming, but confidence is low (diluted or absent marine layer should result in greater warming).
Currently, hurricane Kristy churns in the open, tropical Pacific (about 1100 miles west of the southern tip of Baja California). The various models show the hurricane getting sheared apart. High clouds from the storm are expected to reach southern California over the weekend (already some nearing San Diego). A separate, weak, upper level trough will contribute its high clouds toward the Southland. While cloud cover tomorrow should be thin enough for “mostly sunny” skies, partly to mostly cloudy skies are anticipated for Sunday (may get that way as early as tomorrow evening). Some past model forecasts even showed some chance at sprinkles or light showers reaching parts of southern California on Sunday (mostly south of L.A. County). The model trend has been to keep the Southland rainless that day, but any undetected, upper air disturbance could upset my forecast (decided to omit any shower threat for Sunday).
By Monday, high clouds from Kristy (or its remains) should get shunted southward. It gets shunted by an approaching, cold upper level trough from the Gulf of Alaska. That trough should be an “inside slider” type. It should promote a deep marine layer capable of spotty, morning mist/drizzle (mostly up against coastal facing foothills/mountains). I left out chances for drizzle in the campus forecast as a recent, similar “inside slider” did not bring wet weather to UCLA (famous last words?). Breezy weather should prevail in the interior with the approaching trough (afternoon sea breeze may also blow briskly). Unlike a past, “inside slider”, this one may produce some cold air instability showers Monday evening or early Tuesday. This would occur over/adjacent to the higher mountains as well as some coastal valleys in San Diego County. Most of these showers shouldn’t produce more than a tenth inch rain. If some model forecasts are right, the snow level could fall to near 5000 feet for a time (only a dusting).
The predicted trough should be east of our area later Tuesday morning. Initial, breezy west to northwest winds should become northerly upon passage of the trough. Peak wind gusts in the higher mountains may exceed 65 mph. Peak gusts in the lowlands should stay under 50 mph (windiest in the usual, Santa Ana wind prone areas). The wind should diminish noticeably by the afternoon hours. Marginal, on-shore flow could return by Wednesday.
Although temperatures should recover from their lows on Monday, the predicted wind flow pattern may not produce significant warming next week. High pressure aloft is not predicted to get strong at all. So, any subsidence warming may lead to only seasonable to slightly warmer than normal levels. Other predicted troughs later next week should also prevent any significant warming trend. Today’s model consensus favors cooler than normal weather for all of next week (only minor warming after Monday). At this point, it’s unclear whether any trough late next week and beyond will produce wet weather (besides marine layer induced drizzle). Some model solutions show more weak, “inside sliders”. Some show a shower threat, but most solutions show only minor precipitation. At this point, anything is possible (almost November…typical start to the rainy season in southern California).
Next issued forecast/synopsis should be on Monday, 28 October.