| Day | Date | High/Low | Forecast |
| Mon | 23rd | 71/59 | Low clouds/fog clearing to hazy sunshine late morning. Chance of evening low clouds/fog. |
| Tue | 24th | 73/60 | Chance of morning low clouds/fog; Otherwise, hazy sunshine. Chance of evening low clouds/fog. |
| Wed | 25th | 71/60 | Chance of morning low clouds/fog; Otherwise, hazy sunshine. Chance of evening low clouds/fog. |
| Thu | 26th | 71/60 | Chance of morning low clouds/fog; Otherwise, hazy sunshine. Chance of evening low clouds/fog. |
| Fri | 27th | 70/60 | Chance of morning low clouds/fog; Otherwise, hazy sunshine. Chance of evening low clouds/fog. |
Synopsis
Last week, an usually strong (for September) low pressure aloft brought some of coolest weather since late spring. It also helped trigger some showers in places (mainly higher terrain region). This included some brief-lived thunderstorms in parts of the Southland mountain/desert region (Thursday/Friday).
Today high pressure aloft was nosing into the Pacific Northwest states as well as the interior Southwest. It’s expected to take up residence somewhere in the Southwest this week. Early last week, most of the longer range, computer models predicted the aforementioned high pressure would induce significant warming across the Southwest. For well inland areas, this has happened (widespread 90 degree weather returning most valley locales…a few spots flirting with triple digits). In the coastal plain, however, a shallow but effective marine layer has stifled or limited the warming trend (so far). There is a weak, upper level low pressure a few hundred miles west of the Southland, but it’s unclear to me if it’s having any relevant contribution maintaining the current, marine layer.
The various models predict only modest day to day changes this week in the wind flow pattern affecting southern California. None of the models show the marine layer getting purged. There may be minor day to day changes in the on-shore flow strength. The subtle, wind flow shifts from changes to the on-shore flow may influence the areal coverage/persistence of the low clouds. Even though astronomical autumn has begun, the current weather is more typical of summer. I’ve leaned today’s forecast toward persistence, but any unexpected dilution of the marine layer may result in sunnier, warmer weather than I show in the forecast. Any noticeable warming, however, will be more probable for the inland, coastal plain than areas closer to the coast (beaches probably won’t see any warming trend this week).
Although the model consensus shows a minor cooling trend for well inland locales this weekend, a number of model solutions show high pressure making a move to strengthen over the state. If thie scenario verifies, any cooling this weekend will be reversed early next week. Potentially, warmer than normal weather may include areas near the coast (wildcard being any possible marine layer). Later next week, the model consensus favors a large scale, upper level trough along the West Coast. At this point, no relevant chances for wet weather are foreseen in the state (related this predicted trough). However, cooler than normal weather should develop by late next week (comparable to the recent, cooler than normal weather).
Next issued forecast/synopsis should be on Monday, 30 September.