| Day | Date | High/Low | Forecast |
| Mon | 9th | 95/69 | Sunny day. Clear evening. |
| Tue | 10th | 83/61 | Sunny day. Mostly clear evening but chance of low clouds late. |
| Wed | 11th | 74/63 | Good chance of early morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
| Thu | 12th | 73/62 | Morning low clouds likely but mostly sunny afternoon. Evening low clouds. |
| Fri | 13th | 73/62 | Morning low clouds likely but mostly sunny afternoon. Evening low clouds likely. |
Synopsis
This summer hadn’t featured many, long duration heat waves, but the current one may be the longest of this year. Today marks day five of UCLA recording temperatures of at least 90 degrees. The highest maximum of the year (so far) was 105 degrees on the 6th (Friday). That was the first triple digit reading at UCLA since 4 September, 2022 (104 degrees that day). The record maximum for the date was tied (first in 2020). Yesterday’s (8th) high of 98 degrees tied (with 1984) for the record maximum for the date. A new record maximum for today (9th) has occurred (102 degrees, as of this writing). The old record for the date was 98 degrees (1984),. Today’s minimum temperature was only 79 degrees, but the record high minimum for today is 81 degrees (1984,2022).
Today should be the final string of days with extreme heat for most of southern California. High pressure aloft has already begun to weaken and move eastward, but a predicted return of weak, on-shore flow should promote noticeable cooling tomorrow for areas near the coast (no more 80s at the beaches). Considering how warm it has gotten on campus today, it’s possible UCLA won’t be as cool tomorrow as I show in this forecast (maybe 86, 87 degrees?). However, I left the forecast alone as even a weak, sea breeze could limit the warming. [FYI– Since UCLA archives high/low temperatures in Greenwich Mean Time, there is a good chance that one more 90 degree day will occur if it stays in the 90s through 5 PM (Midnight, Greenwich Mean Time) and beyond. For the curious, the longest streak of 90 degree plus days at UCLA is 11 days. It occurred from 31 August through 10 September in 2022.]
All the computer models predict multiple, upper air troughs passing through or near by of southern California starting on Wednesday. This will help induce the return of a good, on-shore flow pattern. That, in turn, should promote a defined marine layer west of the mountains. Satellite imagery showed a thin band of low clouds west of the coastal islands this morning (more extensive, solid field of low clouds north of Point Conception). Even if a widespread, low cloud overcast doesn’t develop, the predicted on-shore flow and troughing aloft will ensure sizeable cooling west of the mountains (at first, more modest cooling in the interior). A return to cooler than normal weather is certain by the second half of the week. If some model forecasts are right showing a deep marine layer by Friday, there could be some spotty, morning mist/drizzle (probably not measurable though). Complete, daily clearing might also be limited in spots.
Today should be the last day of “monsoon” showers in the Southland mountain/desert regions (at least for the week). The predicted wind shift will help dry out the atmosphere in the interior. “Monsoon” moisture incursions into the Southland interior typically winds down by mid-September. Based on the today’s model consensus, no further moisture incursions from Mexico into the state may occur this year (moisture source from the tropical, eastern Pacific is another matter).
There is good model agreement that multiple, upper air troughs will promote a stretch of cooler than normal weather in the state (later this week through most or all of next week). A predicted trough early next week may be capable of widespread, marine layer drizzle or light rain. However, I’m being more cautious with such a forecast. A weaker than predicted trough should still promote cooler than normal weather, but widespread drizzle may be asking too much of the trough.
Next issued forecast/synopsis should be on Monday, 16 September.