| Day | Date | High/Low | Forecast |
| Thu | 9th | 73/61 | Mostly sunny with some clouds. Becoming partly in the evening with a slight chance of light showers late. |
| Fri | 10th | 78/60 | Partly cloudy early with a slight chance of light showers; Becoming mostly sunny by afternoon. Mostly clear evening. |
| Sat | 11th | 74/56 | Mostly sunny day with some morning clouds possible. Clear evening. |
| Sun | 12th | 75/57 | Sunny day. Mostly clear evening. |
| Mon | 13th | 69/59 | Becoming partly to mostly cloudy. Cloudy evening with a chance of showers. |
Synopsis
Upper level moisture from weakening tropical storm Priscilla (center west of southern Baja California early this afternoon) has reached southern California. Radar imagery showed narrow bands of showers and brief-lived thunderstorms (mostly southern mountain/eastern desert region with a smattering Inland Empire valleys). A large, upper level trough off the Pacific Northwest coast should direct most of the tropical storm moisture into northwest Maxico and the U.S. desert Southwest (tonight through tomorrow).
Most of this tropical moisture lies well aloft (layer of dry air beneath over much of southern California). The computer models show the deeper layered moisture staying the east and south of L.A. County for this whole event. However, it’s possible that minor showers, including brief, “dry” thunderstorms (little rain reaching the ground) may occur in L.A. County (tonight through early Friday morning). Even where rain falls west of the mountains, accumulations should be under a tenth inch. The mountain/desert region could see totals up to about an inch, but that should be isolated in areal coverage (most total there probably staying well under a half inch).
Because the various models continue to predict very weak on-shore flow (maybe even a marginal, surface off-shore flow for a time), no defined marine layer is expected to return (got disrupted late yesterday). Temperatures should climb several degrees over today’s readings in many locales west of the mountains. If sub-tropical air aloft mixes down to ground, it could wind up warmer in places than I currently anticipate .
By Saturday, on-shore flow should resume a stronger stance. However, the aforementioned Pacific Northwest trough should prevent any widespread blanket of low clouds from forming. Sunny weather is expected for Sunday, but that trough should limit any warming potential that day, especially for areas near the coast.
Things starting turning more late autumn like early next week. A different, upper level trough (colder and stronger) is forecast to bring the first, widespread, relevant precipitation to California (late Monday- Wednesday morning window). There is disagreement amongst the models on weather details, but most solutions show a decent, October rain in the Southland (minor snowfall possible highest peaks). Currently, the model consensus favors storm totals in the lowland away form the mountains ranging from a quarter to three-quarters of an inch. The high end for coastal facing mountains could reach 1.5 inches. Since there are model solutions favoring less rainfall (coastal plain might stay under a quarter inch), my confidence in projected storm totals isn’t high at this time. In addition, the current timeline for most of the rain to fall on Tuesday in L.A. County is also subject to change.
Most models maintain some form of troughing over the state next week. The Southland should be dry for the second half of the week though. Temperatures should recover from the cold (for October) Tuesday, but at this point, no seasonable or warmer than normal weather is expected later next week. At one point, weak off-shore flow was favored by some models (favors warmer than normal weather), but that scenario is a minority solution (for now).
Next issued forecast/synopsis should be on Monday, 13 October.