| Day | Date | High/Low | Forecast |
| Mon | 16th | 69/57 | Partly cloudy remainder of morning; Mostly sunny afternoon. Some evening clouds possible |
| Tue | 17th | 71/58 | Partly cloudy morning; Mostly sunny afternoon. Mostly clear evening. |
| Wed | 18th | 70/59 | Partly cloudy morning; Mostly sunny afternoon. Becoming mostly cloudy in the evening. |
| Thu | 19th | 68/57 | Chance of early morning showers; Otherwise, partly cloudy morning; Mostly sunny afternoon. Mostly clear evening. |
| Fri | 20th | 72/59 | Sunny day. Clear evening. |
Synopsis
A large, cold (for mid-September in California) low pressure aloft was over the Bay area early today. It triggered some showers in parts of the state (particularly around the mountains). This included southern California (mainly lower, mountain slopes from the Inland Empire southward into San Diego County). Early morning rainfall in the Southland was mostly under a tenth inch. Although the numerical models had predicted some rain for L.A. County, none fell to my knowledge.
The current low pressure should be east of the state by early tomorrow. Only a little, cold air instability exists this afternoon in southern California, and that should be gone later tonight (ending any, minor threat of light showers). Since the various models maintain a deep, moist layer of air (modified, marine layer), there should be low clouds over the next two mornings. However, satellite imagery showed only a disorganized, low cloud field. So, most areas probably won’t experience any overcast, low cloud cover. At least, that’s what I’ve shown in today’s forecast. It should be sunnier tomorrow where low clouds have been fairly persistent. The afternoon sea breeze should also be a little weaker than it is today in most areas. Even with an expected, sunnier day tomorrow, it shouldn’t be much warmer than it gets today (left overs of the cold air brought by today’s low pressure).
A new, upper low pressure will move through the state late Wednesday through Thursday. It is forecast to be a little weaker than the today’s low pressure. However, a number of model solutions show the low pressure center passing through the Southland. If this forecast holds true (?), there could be better storm dynamics over the Southland than today’s system. More widespread, showers may occur in the first half of Thursday (not restricted to areas near coastal facing foothills/mountains). There could even be sufficient, atmospheric instability for isolated, brief-lived thunderstorms over/adjacent to the higher mountains (valid Thursday afternoon). Since this hinges on the low pressure center coming through the Southland, confidence isn’t the best at this time (event is still a few days off…plenty of time for the models to “change their minds”). Even if the wet weather scenario verifies, most rainfall should still be under a tenth inch away from the mountains (possibly up to half an inch rain around some mountain locales, especially with a thunderstorm).
While the second low pressure should be passing east of L.A. County by late Thursday, some model solutions show a slow exit from the state. Under that scenario, there may be one additional day (Friday afternoon) of mountain showers. Also, daytime temperatures may not rise as much as I’ve shown in this forecast (lingering marine layer may keep temperature similar to Thursday readings in the coastal plain).
All the models predict high pressure developing just west of the state by the weekend. It will gradually strengthen and expand its weather influence (i.e. warming atmosphere) into next week. There are varied, model solutions on how quickly the warm up occurs. Some show decent warming on Saturday followed by leveling off on Sunday/Monday (passing, “inside slider” type trough). Other solutions favor a steady, warming trend (widespread 80 degree weather most of the coastal plain by Sunday….70s by the coast). There would be a chance for widespread 90 degree weather for the inland coastal plain and valleys in the first half of next week. At this point, I don’t see areas near the coast (campus included) will get that hot. Warmer than normal weather, however, could stick around for all of next week (some cooling toward the subsequent weekend though). Since the models are far from unanimous, the ultimate peak temperatures remains a question mark. All I can say for the moment is that the arrival of autumn (astronomically speaking, it begins this Sunday at 5:44 AM) is likely to be a warm one.
Next issued forecast/synopsis should be on Monday, 25 September.