| Day | Date | High/Low | Forecast |
| Mon | 15th | 78/66 | Sunny remainder of day. Some evening high clouds possible. |
| Tue | 16th | 83/68 | Mostly sunny day with scattered mid/high clouds. Scattered evening mid/high clouds. |
| Wed | 17th | 81/69 | Becoming partly cloudy with mid/high clouds. Mostly cloudy evening with a slight chance of showers late. |
| Thu | 18th | 79/70 | Mostly cloudy through the evening. Chance of occasional showers, mainly in the morning. |
| Fri | 19th | 79/67 | Mostly cloudy morning with a slight chance of showers; Partly cloudy afternoon through evening. |
Synopsis
High pressure aloft covers the state today. It is responsible for the recent warm up in the state. All the computer models predict that the high pressure will strengthen a little more tonight through tomorrow. That should continue to promote weaker on-shore flow (already fairly weak). That in turn, should result in noticeable warming for most areas tomorrow (some question mark on how much warming occurs at the coast…uncertainty about whether any defined, shallow marine layer will persist). Some varying cloudiness in the form of mid/high clouds, however, are expected to decorate the sky tomorrow.
That decoration stems from a weak, upper level low pressure a few hundred miles west of northern Baja California. Its circulation extends down into the tropics. A “monsoon” moisture incursion is predicted for southern California beginning tomorrow. It’ll start out minor at first, but it’s predicted to pick up steam by Wednesday. The current, somewhat muggy weather in the coastal plain comes from the local marine layer (warmer than normal ocean in the inner coastal waters aiding the cause). If the models are right, it’s going to get much more muggy in the coming days (peak day Thursday, based on today’s model consensus). Cloud cover is also expected into increase substantially.
While the models agree on the general changes in the wind flow pattern affecting the Southland, there is disagreement on details of the expected weather (nothing unusual). First off, the marine layer should become diluted for the remainder of the week as subtropical air mixes down to the ground (evidence already seen on satellite imagery). If subsidence warming gets stronger than expected, tomorrow could be much warmer than currently anticipated. UCLA may flirt with the 90 degree mark in this scenario. The temperature forecast gets more complicated for the rest of the work week. If less cloud cover prevails than what the model consensus favors, it may be several degrees higher by day (compared with my current forecast).
The biggest weather challenge (for me) is the chance for widespread, wet weather sometime Wednesday evening through Friday morning (includes chance of brief-lived thunderstorms anywhere in the Southland). The aforementioned, upper low pressure is predicted to direct moisture from current, tropical storm Mario (a few hundred miles west-southwest of the southern tip of Baja California this afternoon) into southern California (parts of Nevada/Arizona too). The low level, storm itself will die off well before it can near the local waters (rapid weakening predicted tomorrow). However, most models predict its upper level circulation will pass through the Southland late in the week. The big unknown (at this time) is where will the upper circulation cross the coast. Model solutions run the gamut Santa Barbara County down to San Diego County. The usually reliable ECMWF model (most recent run) favors San Diego County (quarter to half inch rains away from the mountains…over an inch possible around the mountains and isolated amounts in the desert region). There are other, plausible solutions favoring L.A./Ventura Counties instead (similar rainfall totals). Due to all the uncertainties at this time, I chose the “chance” wording for showers in the campus area.
Wherever the widespread, relevant rains fall, the threat should diminish greatly by Friday afternoon. High pressure aloft is forecast to rebuild back into the Southland over the weekend (potentially for most of next week too). “Monsoon” moisture should decrease (just a small threat of mountain/desert thunderstorms afternoon hours). While mugginess should decrease too west of the mountains, predicted warming (slightly warmer than normal) will likely make it feel more uncomfortable than usual. Drier air is expected next week, but some models predict a heat spell for much of next week (UCLA upper 80s at its peak). Since some models show only minor warming followed by slightly cooler than normal weather late next week, confidence in temperatures next week are of low confidence (for now).
Next issued forecast/synopsis may be on Monday, 22 September, but I will be on call for jury duty (re-assigned week from original August date).