| Day | Date | High/Low | Forecast |
| Fri | 18th | 63/52 | Possible, partial afternoon clearing. Partly cloudy evening. |
| Sat | 19th | 69/54 | Chance of some early morning low clouds; Otherwise, sunny day. Mostly clear evening. |
| Sun | 20th | 70/55 | Mostly sunny sunny day with some clouds possible. Some evening high clouds. |
| Mon | 21st | 68/55 | Chance of early morning low clouds; Otherwise, mostly sunny day with some high clouds. Chance of late evening low clouds. |
| Tue | 22nd | 67/55 | Chance of morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
Synopsis
A large, upper level trough exists over much of the West today (center over Utah). The trough has promoted strong on-shore flow with breezy conditions in places (strongest in the interior). In addition, isolated light showers have occurred around the higher mountains as well as San Diego County and parts of the high desert. Like yesterday, a deep marine layer (fostered by the healthy on-shore flow) has promoted extensive low clouds west of the mountains. At least partial clearing occurred by midday for some areas near the coast. The reason for clearing in places remains a mystery to me (marine layer physics, in general). Despite the clearing, the presence of the upper air trough hasn’t permitted significant warming today.
The aforementioned trough will move eastward overnight. Most of the computer models predict a marginal off-shore flow developing (NAM model favors a full fledged one, albeit only for Saturday morning). Due to the deep marine layer today, I have some reservations about whether the marine layer can get scoured away or, at the least, get diluted significantly by tomorrow morning (as the various models predict). I wouldn’t be surprised if low clouds survive the night, especially for areas to the east and south of L.A. County (marginal off-shore flow less effective there). Since high pressure aloft is expected to build into the state, better clearing of low clouds (where they occur) should happen tomorrow. Noticeable, daytime warming should also occur tomorrow (general subsidence warming by the high pressure aloft). If off-shore flow winds can reach lowland areas, slightly warmer than normal temperatures may occur. Most areas, however, shouldn’t exceed seasonal normals. If a shallow but well defined marine layer remains, only modest warming (compared with today) is likely (same may apply for Sunday). The off-shore flow should weaken on Sunday, but general subsidence warming may permit a little more warming for most inland areas.
By Monday, high pressure aloft is expected to weaken over the Southwest (some weak troughing develops). This should promote an on-shore flow pattern. It should get progressively stronger later in the week. Extensive, coastal low clouds should return (assuming diminishment over the weekend). At this point, I’m not expecting another period of “May Gray” like weather west of the mountains, but I wouldn’t rule it out (especially for second half of next week). At the least, temperatures should lower some for next week (probably not as much as recent days though).
Some of the longer range models have shown a trough capable of wet weather reaching the state over the subsequent weekend. Northern California stands a better chance at wet weather than in southern California, however. Still, this projected storm would be a minor one. Later on (final days of April), high pressure aloft may begin to strengthen over the state. A period of sunny, warmer than normal weather may follow by the beginning of May. Of course, this assumes the building high pressure scenario verifies.
Next issued forecast/synopsis may occur on Monday, 21 April.