| Day | Date | High/Low | Forecast |
| Mon | 9th | 67/54 | Mostly sunny remainder of day with some high clouds. Mostly clear evening. |
| Tue | 10th | 72/55 | Mostly sunny day with variable high clouds. Partly cloudy evening with high clouds. |
| Wed | 11th | 71/51 | Partly cloudy with high clouds through the evening. |
| Thu | 12th | 65/50 | Chance of some morning low clouds; Otherwise, variable high clouds through the evening. |
| Fri | 13th | 65/50 | Chance of morning low clouds; Otherwise, variable high clouds through the evening. |
Synopsis
An “inside slider” type trough is passing through the Great Basin states today. It won’t come close enough to pose a threat of instability showers around some of the local mountains, but its passage should promote a period of gusty Santa Ana winds (already showing up in some wind prone locales at higher elevations). Peak winds should occur late tonight through early tomorrow. This off-shore flow episode should end completely late Wednesday evening or early Thursday.
All the computer models show a strong off-shore gradient developing at the surface, but good upper air support is forecast to be limited to the lowest 4000 feet of the atmosphere. The predicted, deep layered, wind support should also favor the west-east oriented canyons and passes over the north-south oriented ones (at least, for most of the event). The implications should be that highest wind gusts in the mountains should occur a little lower in elevation than many other off-shore flow events. In addition, areal coverage of significant wind (25 mph or more) should be more restricted (mostly favoring areas near west-east oriented passes/canyons). The campus area typically doesn’t experience 25 mph or higher wind gusts in these situations (doesn’t mean it’ll be perfectly calm though).
The predicted high pressure aloft is strong enough to promote widespread 80 degree weather. For today, a lingering though now diluted marine layer is preventing significant warming. More noticeable warming should prevail tomorrow (much lower humidity too). However, the models insist that an influx of polar air into the Mohave Desert will counter some of the warming effects of subsidence by the off-shore flow (west of the mountains). The models have infrequently under estimated the warming potential of previous off-shore flows, but in this case, 80 degree weather may not occur. However, I wouldn’t be surprised if some locales warm into the mid-70s tomorrow and Wednesday (near immediate coast tomorrow; inland coastal plain and coastal valleys on Wednesday).
While off-shore flow is expected on Wednesday, it should be on the decline (helped along by the passage of a weak, upper level trough). A passing trough on Thursday is expected to bring areas of wet weather to northern California. Its passage through southern California may bring some light showers Thursday evening (marine layer induced, assuming a well defined one forms by that time). The predicted, low level winds favor showers in areas to the east and south of L.A. County. However, any rainfall should be well under a tenth inch away from coastal facing mountains.
A weak on-shore flow pattern is expected to continue into the weekend. So, any warming cycle after Thursday should be minor (temperatures at or slightly below normal levels). A passing storm well to the north over the weekend should bring variable mid/high clouds (in addition to possible marine layer clouds). Some model solutions bring isolated showers as far south as western Santa Barbara County (late Saturday), but dry weather should prevail across most of southern California this weekend.
Some model solutions show potential for widespread, wet weather reaching the Southland around Tuesday of next week, but that scenario doesn’t occur with any consistency amongst the numerical models. So, for now, I’m skeptical of wet weather for that latter period. Most solutions that favor the wet weather don’t show any significant storm (lowland storm totals under a third inch, for the most part). Model agreement is fairly poor for later next week, but a fair number show dry weather for most of the state (temperatures at or slightly above normal levels).
Next issued forecast/synopsis should be on Friday, 13 December.