| Day | Date | High/Low | Forecast |
| Tue | 26th | 67/55 | Partial afternoon clearing and possibly breezy at times. Partly to mostly cloudy evening. |
| Wed | 27th | 66/54 | Partly to mostly cloudy through the evening with a slight chance of light showers, mainly in the morning. |
| Thu | 28th | 67/55 | Partly to mostly cloudy through the evening. Slight chance of morning light showers. |
| Fri | 29th | 69/57 | Morning low clouds likely but mostly sunny afternoon. Chance of evening low clouds. |
| Sat | 30th | 72/58 | Morning low clouds likely but sunny by afternoon. Chance of late evening low clouds. |
Synopsis
An elongated (oriented north-south today), upper level, low pressure (“cut-off” type) was centered over coastal southern Oregon this morning. Its center is forecast to be south of Lake Tahoe tomorrow morning. The cloudy at times and cooler than normal weather promoted by this low pressure should continue through Thursday. The low pressure center is expected to be exiting the state Friday morning. Upper level high pressure should slowly build into the state over the weekend and be the dominant weather force next week.
A deep marine layer exists west of the mountains today. A passing, weak cold front (still west of L.A. County early this afternoon) is helping to disrupt the coastal low cloud field. However, small scale wind flow continues to support mostly overcast conditions in some areas (better areas of clearing over the ocean compared with over land, as of this writing). The aforementioned, upper low pressure is also promoting breezy weather in the interior (some coastal areas too this afternoon).
Although I didn’t see any reports of drizzle or light rain, the marine layer was certainly deep enough to support such weather, especially up against coastal facing foothills/mountains. With some occasional dynamics induced by disturbances pinwheeling about the upper low pressure, there is some chance at areas of light rain showers (again, mainly near coastal facing foothills/mountains). Chances, as it were, should be highest tomorrow morning (upper low pressure predicted to weaken after tomorrow). Cold air instability is expected to be marginal over the Southland northern mountains, but an isolated, brief-lived thunderstorm can’t be ruled out tomorrow and Thursday afternoons. In any case, any rainfall that should be under a tenth inch (quarter inch where thunderstorms occur).
I worded the campus forecast with “partly to mostly cloudy” through Thursday, but there is a chance the upper low pressure will stir up the atmosphere enough to dilute the marine layer. In that case (coastal plain in particular), things may wind up much less cloudy than my forecast shows. Regardless, the current cool air mass will limit daytime warming through Thursday. A return to a more typical marine layer is expected by Friday, however (assumes low pressure exits region as computer models predict).
Slowly building high pressure aloft will start a general warming trend this weekend (minor warming starting well inland areas Friday). By Sunday, most areas should be at least slightly warmer than normal. A decreasing marine layer and weak on-shore flow should even permit modest warming at the coast early next week. If high pressure builds over the Southwest as predicted, the Southland low deserts should get toasty by the middle of next week (approaching 110 degrees in places). Widespread 90 degree weather in the valleys should occur by early next week (lasting much or all of next week, depending on which model scenario verifies). As usual, how warm areas near the coast gets will depend on the marine layer and on-shore flow strength. If a shallow but defined marine layer occurs along with a moderate strength, on-shore flow, warming next week will be more muted (UCLA staying well under 80 degrees).
Next issued forecast/synopsis should be on Friday, 29 May.