| Day | Date | High/Low | Forecast |
| Mon | 19th | 79/65 | Sunny day. Clear evening. |
| Tue | 20th | 81/65 | Sunny day. Clear evening. |
| Wed | 21st | 79/64 | Sunny day. Chance of late evening low clouds. |
| Thu | 22nd | 75/62 | Good chance of morning low clouds; Otherwise, mostly sunny day. Evening low clouds likely. |
| Fri | 23rd | 73/61 | Morning low clouds likely; Otherwise, mostly sunny day. Chance of evening low clouds. |
Synopsis
High pressure aloft occupies the Southwest (center in New Mexico). There is also a low pressure aloft just west of Washington state, and the accompanying trough extends southwestward from the low pressure. Last week, most of the computer models predicted that the low pressure trough would be closer to the West Coast than it actually is. Thus, southern California wound up being more influenced by high pressure than the low pressure trough. The net weather result…sunny and warmer than normal, including the coast today. In fact, many locales this afternoon are warmer than I anticipated this morning (UCLA maximum 82 degrees, as of this writing). Since areas near the coast can be sensitive to when ocean breezes begin on a given day, I left the preliminary forecast (valid for tomorrow) unchanged. At worst, it may end up more a couple degrees above today’s maximum readings (assumes today’s model consensus is correct).
All of the model forecasts show today’s Pacific trough moving inland by midweek. That weakens the current, Southwest high pressure as well as displacing it eastward (only slightly at first). A second trough, however, is expected to drop south by Thursday. This second trough is forecast to be stronger and larger than today’s trough. The predicted wind flow pattern should enhance the Southland sea breeze pattern greatly by Thursday. While marine layer clouds were patchy in coverage early today (mostly occupying a narrow zone of coastal waters off of western Santa Barbara County and San Diego County), widespread coverage is anticipated Thursday and Friday (increase in coverage expected to begin Wednesday). If the low pressure develops as predicted, the marine layer should be deep enough for low clouds to reach the coastal valleys. Inland low clouds should still clear by the afternoon each day.
A noticeable cooling trend west of the mountains is likely (a little less magnitude cooling in the deserts). Cooler than normal weather is likely if the forecast low pressure winds up as strong as predicted. Patchy, early morning mist/drizzle can’t be ruled out either (best chances up against coastal facing foothills/lower mountain slopes). The northern third of the state should see widespread, instability showers on Friday (unusual for this time of year). The northern Sierras may get additional, instability showers (includes thunderstorms) on Saturday. If any showers fall early Saturday in the Sierras, a brief snow flurry on the highest peaks is a possibility (no accumulation though).
Lastly, the predicted trough should induce a moderately, strong on-shore flow in southern California (beginning Thursday and diminishing Saturday). That should result in widespread, breezy weather in parts of the interior. There should also be breezy weather in some of the north-south, oriented canyons/passes by the end of the week (mostly at higher elevations). In the coastal plain, the afternoon sea breeze should blow briskly on Friday.
The almost autumn like weather in most of the state should be short-lived. High pressure aloft is forecast to build back into the Southwest beginning on Sunday. It should hold sway for most or all of next week (potentially including the subsequent, Labor Day weekend). So, a rapid return to seasonable or slightly warmer than normal weather is probable (as early as Sunday). It’s still unclear how warm it may ultimately get next week, but at this point, widespread, triple digit heat for the coastal valleys isn’t foreseen.
Next issued forecast/synopsis should be on Monday, 26 August.