Weather Synopsis – April 11, 2024

James Murakami

Thu11th71/54Sunny remainder of day. Chance of evening fog/low clouds developing.
Fri12th66/55Morning low clouds likely; Mostly sunny afternoon. Mostly cloudy evening.
Sat13th61/49Rain developing by late morning; Possibly breezy at times. Partly cloudy evening with a slight chance of showers.
Sun14th61/48Partly to mostly cloudy day with a chance of showers. Possibly breezy at times. Clearing in the evening.
Mon15th65/50Mostly sunny day. Clear evening.


High pressure aloft has shifted east some since yesterday. The surface, pressure pattern still indicated a slight off-shore flow early today. However, widespread fog/low clouds returned to the Southland coastline overnight. This has resulted in noticeable cooling for areas near the coast (compared with yesterday). It’s less noticeable farther inland, but most areas west of the mountains should be at least a bit cooler today than it got yesterday (sea breezes started earlier than recent, previous days). Dense fog should be less prevalent tonight/tomorrow as the marine layer deepens (more widespread, persistent low clouds though).

All the computer models continue to forecast a large, “cut-off” low pressure in the Gulf of Alaska moving toward California. Its center should be west of northern California by early Saturday. The predicted details of what weather will develop in southern California still needs ironing out (model agreement better now but far from unanimous in details). I am confident that a period of wet weather will occur across most of the Southland sometime on Saturday. For Bruin Day activities, rain, mostly light in intensity, should occur no later than the late morning (continuing well into the afternoon hours). It may also get breezy at times. The models generally agree that there will be a break in the wet weather in the overnight hours Saturday into Sunday. Showery weather is anticipated on Sunday, but it’s unclear (no pun intended) whether some of those showers will occur in the coastal plain of L.A. County (possibly mostly over/adjacent to the mountains in the afternoon). While some residual showers may remain early Monday in San Diego County, for the most part, that day should be a mostly sunny day (low pressure should be in Arizona by that point).

The models still don’t agree on how wet it be this weekend in L.A. County. It’s probably not going to be a significant storm in most areas. Most lowland areas away from the mountains should see storm totals under a third inch (some totals may be reach a half an inch west of L.A. County). The bulk of precipitation should occur on Saturday. Thunderstorm chances appear minimal with this predicted storm (isolated occurrence Sunday afternoon around the mountains, if it occurs). Snow levels should drop to about 5000 feet by late Saturday. Accumulations at resort level will probably be under 6 inches (some model solutions show only an inch or two).

All the longer range models show no significant high pressure building into the state next week. In fact, a large, new upper level trough is forecast somewhere in the Great Basin states by the middle of next week. Mostly sunny weather is anticipated early next week, but the predicted, new trough may promote a strong, on-shore flow pattern (widespread marine layer clouds west of the mountains and sunny but breezy weather in the interior). There is some chance that the trough will keep the air too “mixed out” for an organized blanket of coastal low clouds to form. In fact, some model scenarios include a chance for cold air instability showers over the mountains (mid-week onward). There are also some model solutions actually showing a marginal off-shore flow (a “cool” Santa Ana wind event…at or slightly cooler than normal levels). At this point, I’m leaning toward cooler than normal weather for most or all of next week, but confidence isn’t high at this point. Whatever occurs, the Great Basin trough should weaken and move eastward by the subsequent weekend. More typical conditions for April should prevail by then…I think.

Next issued forecast/synopsis may be on Tuesday, 16 April.