| Day | Date | High/Low | Forecast |
| Tue | 2nd | 85/69 | Variable clouds but generally partly cloudy day. Slight chance of morning sprinkles. Partly to mostly cloudy evening. |
| Wed | 3rd | 85/67 | Variable clouds but generally sunny day. Scattered evening clouds. |
| Thu | 4th | 82/66 | Mostly sunny day with some mid/high clouds. Chance of evening low clouds. |
| Fri | 5th | 79/65 | Chance of early morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
| Sat | 6th | 78/64 | Chance of early morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
Synopsis
The weather forecast for today was a challenging one. High pressure aloft has been dominant over the last several days. The wind flow pattern has allowed for a “monsoon” moisture incursion into much of southern California (reason for recent, muggy weather). In addition there were a couple of upper air disturbances embedded within the flow field (reason for earlier uptick in mid-level clouds across the Southland). Temperatures today would depend partly on cloud cover and the strength of the surface on-shore flow. So far at UCLA, my temperature projection is close (84 degrees, as of this writing), but potentially, good mixing of sub-tropical air aloft downward to the ground could lead to a late afternoon heat up.
While most of today’s showers and isolated thunderstorms have been over the mountain/desert region (heaviest activity in the desert so far), some isolated cells did manage to develop west of the mountains. One such shower (a thunderstorm for a brief time) formed over the hills north of campus. It spread out over time to encompass part of the Westside, and measurable rain (0.01 inch) was recorded at UCLA by 10:12 AM. A second cell formed during the 1 PM hour, but it just missed passing over the campus (stayed a little to far to the east). For most of the L.A. Basin, however, there was no rain. It’s an example of how small scale features can produce a noticeably different, weather outcome in places.
All the computer models predict a decrease in “monsoon” moisture and instability after today (a gradual one). The focus of showers and isolated thunderstorms should return to just the mountain/desert region tomorrow (less areal coverage though). Muggy weather is predicted west of the mountains for tomorrow, but it should be a little less uncomfortable for most. The caveat might be if temperatures rise a little more than what most models predict.
The trend of decreasing “monsoon” moisture/instability should continue on Thursday (only threat of showers should be near the eastern state border). Some models show an uptick in moisture on Friday. This is due to a surge of humid Gulf of California air from tropical storm Lorena (currently several hundred miles south of Baja California but predicted to make landfall over somewhere in central Baja by Friday). Should Lorena take a different course or at least, travel more slowly, that moisture surge may not occur.
Most of the longer range models show more reasonable weather returning by this weekend. Temperatures in the coastal plain may even fall back to seasonable levels for a couple days (if on-shore flow can increase sufficiently). Some renewed warming is expected early next week, but most of the warming should be for well inland areas (shallow marine layer may protect areas near the coast from any noticeable warming).
Cooler than normal weather for the entire state is expected by the middle of next week when a large, upper level trough is predicted for the West Coast. If some model solutions are right, it could provide a small foretaste of autumn (fall officially starting on 22 September) for a few days. This pattern shift may also mark the end of this season’s “monsoon” for southern California (a rarity after mid-September).
Next issued forecast/synopsis should be on Monday, 8 September.