| Day | Date | High/Low | Forecast |
| Mon | 25th | 78/66 | Mostly sunny remainder of day with some high clouds. Some evening high clouds. |
| Tue | 26th | 77/65 | Chance of some early morning low clouds; Otherwise, mostly sunny day with some high clouds. Mostly clear evening. |
| Wed | 27th | 77/65 | Chance of some early morning low clouds; Otherwise, sunny day. Mostly clear evening. |
| Thu | 28th | 78/66 | Chance of some early morning low clouds; Otherwise, sunny day. Some evening high clouds possible. |
| Fri | 29th | 80/67 | Mostly sunny day with scattered high clouds. Scattered evening high clouds. |
Synopsis
For most areas west of the mountains, this is the longest stretch of at or above normal temperatures this summer (started middle of last week). UCLA flirted with the 90 degree mark on Saturday (89 degree maximum that day). Temperatures lowered modestly since then, but the continued effects of a “monsoon” moisture incursion make it feel more uncomfortable for most people. The various computer models predict minor day to day changes in the wind flow pattern effecting the Southland (high pressure aloft over the desert Southwest and a very weak region of low pressure west of the state).
As of this writing, it got a little warmer on campus than I forecast this morning. I left the forecast unchanged for the next couple of days as subtle wind shifts could influence the temperature trend (up or down). However, it shouldn’t vary more than a couple degrees on a given day (applicable to most of the coastal plain). The flow of sub-tropical air into southern California may get cut off temporarily (driest days predicted for Wednesday/Thursday), but weak, surface on-shore flow is predicted to remain for much of this week. What marine layer exists is shallow and warmer than usual (some sub-tropical moisture mixed into the local, low cloud field off the coast). In short, I’m not expecting major weather changes this week, including the elevated, water vapor content (muggy for L.A. standards).
Thunderstorm activity in the Southland mountain/desert region picked up today (most active of the summer so far). One lone but strong thunderstorm formed mid-afternoon in the Moreno Valley as well. Unlike this past Saturday, no other rain clouds are likely to develop west of the mountains. Isolated thunderstorms may form over the L.A. County mountains, but winds should steer them into the desert. Deep layered moisture/instability forecast to stay to the east of L.A. County and keep the more significant storm clouds to the east. Most models show some decrease in moisture/instability after today.
There are some model solutions that show an uptick in moisture around Friday. Tropical storm Juliette was centered about 470 miles southwest of the southern tip of Baja California this afternoon. The storm may briefly become a hurricane tonight or early tomorrow, but as it travels northwestward, Juliette will gradually enter an atmosphere hostile to tropical cyclones (colder ocean and stable, low level air). By late this week, the storm should also get sheared apart. However, some of its moisture aloft could get directed at or near the Southland on Friday. Most model solutions keep things rainless west of the mountains, but it may get a bit muggier again for a day (sub-tropical air may also warm things up modestly).
The start of the Labor Day weekend could be slightly warmer than normal, depending on which model solution verifies. At least, most of the solutions favor a gradual drying trend (lessening of “monsoon” moisture in southern California). Some models show slightly cooler than normal weather in the Southland early next week (weak upper air trough to our northwest aiding a stronger on-shore flow). However, later next week, high pressure aloft should strengthen again. That should support a return to warmer than normal weather by the latter half of next week. Just how warm remains to be seen (model solutions vary greatly).
Next issued forecast/synopsis should be on Tuesday, 2 September.