| Day | Date | High/Low | Forecast |
| Mon | 22nd | 80/65 | Sunny remainder of day. Chance of evening low clouds. |
| Tue | 23rd | 82/66 | Partly cloudy day. Mostly cloudy evening with a slight chance of light showers. |
| Wed | 24th | 79/64 | Mostly cloudy morning but mostly sunny afternoon. Chance of evening low clouds. |
| Thu | 25th | 77/63 | Chance of morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
| Fri | 26th | 75/63 | Partly cloudy day. Partly to mostly cloudy evening. |
Astronomically speaking, autumn began at 11:19 AM today. However, the recent weather with “monsoon” moisture entering southern California is more typical of summer than autumn. A “cut-off” low pressure several hundred miles southwest of San Diego this morning can be blamed for the current, Southland weather. It continues to funnel modest amounts of upper level moisture into southern California/Arizona (much less than last week’s moisture incursion though). Both yesterday and today, there have been scattered showers to the east and south of L.A. County (under a quarter inch each day…under a tenth inch being common).
Often times, the wind flow pattern that promotes “monsoon” showers ends up disrupting the local, low cloud field. However, at least for the last couple of days, disruption of the marine layer has been minor. Whether it stays this way is uncertain (aah, the complexities of marine layer physics). My cloud forecast for this week is of the low confidence type. The total cloud cover will be comprised of low and higher elevation clouds. The computer models infrequently mishandle marine layer clouds as well as “monsoon” related clouds poorly (especially when a “cut-off” low pressure is “in the neighborhood”). Depending on what actually transpires, temperatures on a given day may end up much warmer than expected (local marine layer has been running warmer than usual lately).
All the models show the aforementioned, low pressure migrating north. It is forecast to move ashore somewhere between Monterey and San Luis Obispo. An arc of showers is forecast to form by tomorrow, but it should remain off the coast for most of the day. A few model solutions show some showers beginning in L.A. County late in the afternoon, but most of the solutions delay any showers to the evening hours. There is a distinct chance that most of the showers will avoid L.A. County (showers staying west of the County). Even where it rains, amounts should be minor in most locales (tenth inch or less away from mountains). At this point, I’m expecting nothing worse than sprinkles on the Westside.
In most situations, once a “cut-off” low pressure moves inland, it continues to trek eastward while weakening further. However, the model consensus today moves the low pressure inland only as far as the Death Valley area. The low pressure may then meander somewhere in the Southland this weekend (possibly through early next week, if some model solutions are right). Depending on strength/position of the low pressure, there may be numerous mountain/desert showers and brief-lived thunderstorms (mainly afternoon hours). Most solutions, however, don’t include the coastal plain or most coastal valleys.
Eventually (by midweek), the “cut-off” should finally move off east of the state and allow high pressure aloft to build back into the Southland. Most models show a return of widespread, warmer than normal weather. How warm remains to be seen. Some only show modest warming followed by modest cooling by the subsequent weekend. At least, the predicted wind flow pattern should dry out the air mass appreciably later next week.
Next issued forecast/synopsis should be on Monday, 29 September unless I end up serving as a juror on a trial.