A large, upper level low pressure was centered around the Oregon/Nevada border area at midday. This low pressure started a good onshore flow pattern in southern California over the weekend, and it appears to have peaked today. A strong on-shore flow and coastal eddy have promoted widespread, persistent low clouds west of the mountains. There were also areas of marine layer induced, light showers earlier (mostly before sunrise). Measurable rain fell in some locales up against the mountains (0.03 inch in Pasadena, for instance). Down in San Diego County, some measurable rain even fell in the coastal plain (away from the mountains). Still, rainfall was under a tenth inch in most locales that got wet weather (maximum of about a quarter inch at some mountains).
As is often the case, predicting the daily clearing pattern of marine layer clouds is a challenge (futile to accurately predict in some cases). Early this afternoon, partial clearing occurred in spots (Long Beach area in L.A. County...no general pattern...some clear zones occur over the ocean while many inland areas remain overcast). The aforementioned, upper low pressure isn't forecast to make much more movement to the south. In fact, it should translate toward the east as early as tomorrow. This should lead to a weakening on-shore flow. In theory, the marine layer should begin to get shallower, and clearing of low clouds tomorrow should be more definitive in more areas than today. At the least, today's blustery weather in portions of the interior and passes leading to the desert will diminish (less wind but far from calm...better as the week progresses).
If most computer model forecasts are right, weak on-shore should result in limited marine layer clouds by Friday (mainly near the immediate coast and possibly no widespread overcast). Most model solutions show a marginal, off-shore flow by Saturday night or Sunday. Some show a full fledged, off-shore flow with gusty winds in most Santa Ana wind prone areas on Sunday. Other model solutions restrict any significant wind to the higher mountains only (peak gusts up to 45 mph). At this point, the latter scenario is in the majority amongst the models, but it's far from unanimous. In any case, a noticeable warm up is likely over the weekend, especially in the valleys (60s today to near 90 degrees possible on Sunday). Unless the off-shore flow fails to materialize or high pressure aloft ends up weaker than forecast, widespread 80 degree weather is likely early next week (some 90s in some valley locales too).
The weather trend for the rest of next week is still uncertain. A cooling trend is almost certain. High pressure aloft will break down and allow a Pacific, upper trough to reach the state (midweek or slightly later). While most model solutions favor just a return to a healthy, on-shore flow pattern (not as robust as today, however), some model solutions show an actual storm reaching southern California. Depending on the track and strength of the storm, widespread, wet weather is possible for sometime next week. This would be an "ordinary", April rain, should it occur. However, infrequently, the models have shown some wet weather in the latter portion of the model run. As the event nears,the model solutions back track to a nothing event. So, the latest, wet forecast may be yet another illusion. We'll see.
Next scheduled synopsis should be on Thursday, 15 April.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-04-13