| Day | Date | High/Low | Forecast |
| Thu | 17th | 67/57 | Possible partial afternoon clearing; Otherwise, mostly cloudy through mid-evening. Decreasing clouds thereafter. |
| Fri | 18th | 71/54 | Some clouds possible early morning; Otherwise, sunny day and possibly breezy at times. Clear evening and possibly breezy at times. |
| Sat | 19th | 74/55 | Sunny day. Clear evening. |
| Sun | 20th | 77/58 | Sunny day. Clear evening. |
| Mon | 21st | 76/58 | Mostly sunny day with some high clouds possible. Some high clouds possible in evening. |
Synopsis
I didn’t have a lot of time to peruse the weather maps today. So, forgive my forecast if I overlooked something relevant.
Today’s low cloud field is less solidly overcast in some areas. On-shore flow strengthened this morning, and the marine layer responded by deepening further. Infrequently, there seems a critical point at which the vertical air mixing within a deep marine layer disrupts the low cloud field. I just can’t figure out why it’s not more uniform in coverage. In any case, today should be the last day (for a while) dealing with a defined, marine layer. The passage of a “dry” cold front will scour away what exits today.
Prior to the cold frontal passage, weak dynamics associated with the front might induce areas of drizzle or light rain tonight (before midnight in L.A. County). The numerical models favor areas mainly to the east and south of L.A. County (predicted, low level winds should produce a “rain shadow” effect over most of the County). Still, any rainfall should be minor (at best, a few hundredths of an inch rain away from the mountains). Clearing skies should occur after the front passes.
All the models predict that an upper level trough, currently passing through the Pacific Northwest and northern California will develop a “closed” low pressure circulation (somewhere southern Nevada or northwest Arizona tomorrow). Deep layered northwest to north winds should promote breezy weather by tomorrow in many locales (strongest in Santa Ana wind prone areas). The models have wavered on the predicted trajectory of the low pressure across the Southwest. I’m less confident in breezy weather reaching the campus area tomorrow (reason for “possibly breezy” wording). There is some chance for campus wind gusts to exceed 35 mph for a time early Friday, but confidence is low. The usual Santa Ana wind prone areas, however, stand a much better chance for such weather (peak, higher mountain gusts could reach 60 mph in spots though it shouldn’t be persistent for a long period of time).
The wind in general should diminish tomorrow night. Breezy weather in wind prone areas should be more tolerable on Saturday (assuming the low pressure center isn’t close by still). For all practical purposes, the off-shore flow should be ending on Sunday (present but no significant wind gusts). A return to weak, on-shore flow is expected on Monday, but it should be fairly weak. It may not be till midweek that a better defined, on-shore flow pattern returns. Till then, most of the period should be at or slightly warmer than normal levels for well inland areas (chance of seasonable temperature near the coast early next week, especially if a diluted marine layer forms).
Most of the longer range models show wet weather in Pacific Northwest late next week (good soaker). Some showers may extend down to the northern third of the state, but it doesn’t appear likely to ever get down to southern California. There may be variable mid/high clouds though (related to a sub-tropical jet stream directing clouds this way). Low level on-shore flow should prevail for the second half of next week, but it’s unclear if widespread, marine layer clouds/fog will return at some point (as stated before, October isn’t known for widespread, coastal low clouds). Near seasonable temperatures are currently anticipated for the subsequent weekend.
Next issued forecast/synopsis may be on Monday, 21 October.