| Day | Date | High/Low | Forecast |
| Mon | 21st | 75/65 | Becoming sunny by afternoon. Good chance of evening low clouds. |
| Tue | 22nd | 77/67 | Morning low clouds likely but sunny by late morning. Chance of late evening low clouds. |
| Wed | 23rd | 81/68 | Chance of some early morning low clouds; Otherwise, sunny day. Some evening high clouds possible. |
| Thu | 24th | 82/69 | Mostly sunny day with some high clouds. Scattered evening high clouds. |
| Fri | 25th | 83/69 | Mostly sunny day with scattered high clouds. Scattered evening high clouds. |
Synopsis
Astronomically speaking, autumn will begin tomorrow (22nd) at 5:05 PM PDT. However, our recent spate of cooler than normal weather (for late September) is predicted to end soon. An upper, low pressure trough covering California is expected to move east of the state by tomorrow evening. High pressure aloft over Mexico is forecast to expand into the Southwest U.S. in the coming days. This pattern change should result in much weaker on-shore flow and attendant marine layer influence (more so after tomorrow). In short, a noticeable warming trend is likely for most of the state.
There remains some question about how warm it will get by midweek. Most computer model forecasts favor a marginal, off-shore flow on Wednesday, but the actual, low level wind flow should be pretty weak. The daily sea breezes may start up as usual for some areas near the coast (campus included). In that scenario, the warming trend for areas near the coast may be modest, at best (slightly warmer than normal). I leaned today’s forecast in that direction. Should the off-shore flow aloft mix down to the ground and greatly dilute whatever, shallow marine layer remains by Wednesday, the campus could warm into the mid/upper 80s (Wednesday-Friday period). Any ocean breezes may elevate water vapor content in the coastal plain (more than in recent days), but it shouldn’t get anywhere near as muggy as with some of this summer’s heat spells.
It’s rather atypical in southern California by this time in September, but some of the numerical models have forecast occasional, “monsoon” moisture incursions into the Southland. Some of the incursions would come from a flow of moist air from the Gulf of California. However, in most case, it would be too shallow in depth to extend beyond the lower Colorado River Basin. Southerly wind flow aloft may direct high clouds from the subtropics into the state (as happened yesterday). At least, for now, the vast number of model solutions aren’t showing a relevant, “monsoon” incursion into the Southland for the remainder of the month. Should tropical storm Odalys and/or hurricane Polo (current systems in the tropical, eastern Pacific) get caught up in southerly wind flow directed toward southern California, forecast problems may arise (not for a few days though). For the time being, the problem scenario remains a miniscule probability.
Regardless of how warm it gets in a couple of days, some minor cooling should occur by Sunday or Monday. How much cooling occurs is still uncertain. Model forecast vary on the strength of a predicted, upper air trough early next week. Another warming cycle is possible late next week, but model agreement remains poor on that too.
Next issued forecast/synopsis should be on Monday, 28 September.