| Day | Date | High/Low | Forecast |
| Mon | 21st | 65/52 | Mostly sunny remainder of day with some high clouds. Chance of evening low clouds. |
| Tue | 22nd | 65/53 | Mostly sunny day after low clouds clear late morning. Evening low clouds likely. |
| Wed | 23rd | 64/53 | Morning low clouds but mostly sunny afternoon. Evening low clouds likely. |
| Thu | 24th | 65/52 | Morning low clouds but mostly sunny by afternoon. Chance of evening low clouds. |
| Fri | 25th | 64/51 | Partly cloudy. Some evening low clouds possible. |
Synopsis
An upper level trough occupies much of the interior West today. The southern end of the trough is rather weak, but it is promoting an on-shore flow pattern in southern California. The computer models predict that the trough will strengthen slightly, which should induce stronger on-shore flow for the next couple of days. That, in turn, should favor a deeper marine layer and more extensive low clouds west of the mountains. It should also become breezier in the interior (notably during afternoon hours). Subtle day to day shifts in the low level, wind flow pattern will probably dictate how persistent low clouds will be on a given day. For this forecast, I’m expecting daily clearing in the campus area will occur by midday, but that may not necessarily apply for the entire coastal plain (maybe, just partial clearing in some areas). In addition, there’s some chance at spotty, early morning mist with the expected, deeper marine layer (best chance Wednesday).
The aforementioned trough should move eastward on Thursday, but a different trough should quickly follow. The marine layer should clear more readily on Thursday compared with what’s expected Wednesday. By Friday, the second and colder trough could mix out the air enough to disrupt the low cloud field (partly cloudy instead of mostly cloudy west of the mountains). Friday may also be the breeziest day of this week. On Saturday, with the upper trough more squarely over the state, there could be some cold air instability showers (isolated, afternoon thunderstorms possible). This should be mostly over the higher mountains (from the Sierras down to our local mountains). If any marine layer induced showers occur (???), the best chance of that should be down in San Diego County (most favorable wind flow in that area). None of the showers or even thunderstorms should be significant as the predicted trough is forecast to be moisture starved, relatively speaking (rainfall mostly under a quarter inch).
Temperatures for this week shouldn’t vary much from day to day. Below normal temperatures are expected. The coolest days of the week should be on Friday, Saturday. With the predicted departure of the cold trough on Sunday, a warming trend should develop. Assuming the marine layer gets disrupted, it may take at least a few days for the local, low cloud field to return. The longer range models are at odds on a definitive, weather trend next week. Most show high pressure aloft strengthening over the Southland early in the week. Some show the high pressure sticking around for the rest of the next week. That scenario would favor a period of warmer than normal weather (temperatures possibly reaching the low 80s inland coastal plain). However, there are model solutions with a more temporary high pressure. In that scenario, typical on-shore flow should keep temperatures near seasonal normals. How extensive marine layer clouds gets remains a question mark to me (not expecting any “May Gray” episode, for now).
Next issued forecast/synopsis may be on Friday, 25 April.