Weather Synopsis – April 9, 2024

James Murakami

Tue9th77/63Sunny day. Clear evening.
Wed10th78/60Sunny day. Clear evening.
Thu11th76/57Sunny day. Mostly clear evening.
Fri12th67/55Chance of morning low clouds; Otherwise, mostly sunny day. Evening low clouds likely.
Sat13th63/51Mostly cloudy day with a chance of light showers; Possibly breezy at times in the afternoon. Mostly cloudy evening with a chance of showers.


A moderate strength, upper level high pressure exists just west of the state today. A weak, off-shore flow prevails over most of southern California. This began yesterday when peak wind gusts in the higher mountains exceeded 50 mph in a few spots (rather impressive for April). At UCLA, late afternoon, northerly winds of relevance helped push the campus temperature to 74 degrees late yesterday (5:35 PM). Those winds (some gusts as high as 22 mph) helped keep the overnight low temperature from falling below the low 60s. Winds in Santa Ana wind prone areas are generally weaker this afternoon than it got yesterday (lessening upper air support), but subsidence (sinking air motion) warming is promoting good warming today in most locales west of the mountains (some coastal areas in Ventura County briefly reached 80 degrees).

The computer models predict a further degradation in the off-shore flow (i.e. lessening winds in Santa Ana wind prone locales). However, high pressure aloft should continue to promote an additional day of warm weather. Most areas should get at least as warm as it gets today (inland areas probably will be a few degrees warmer tomorrow). A cooling trend should begin on Thursday as a weak, on-shore flow should develop. Best cooling will be along the coast, and most other areas west of the mountains should see only minor cooling (relative to what occurs tomorrow). Much better cooling should develop on Friday as on-shore flow picks up strength more rapidly and a Pacific storm approaches.

This storm will be a semi-“cut-off” low pressure (predicted centered a few hundred miles west of northern California on Friday). In southern California, marine layer clouds should make a rapid comeback during the day on Friday. By Friday night, some areas of marine layer induced showers are possible (mainly by the mountains and to the west of L.A. County). Wet weather is anticipated for this weekend (not continuously for entire period). Since it’s predicted to be a “cut-off” low pressure (hasn’t formed yet), the various numerical models offer varied details for the weekend weather (commonality being cool and occasionally wet at some point). For this forecast, I kept the wording uncertain (“chance of showers”). Some models such as the usually reliable ECMWF favor a mostly Saturday morning rain in the lowlands (snow level would fall to around 5000 feet by late afternoon). A secondary disturbance would bring a second round of showers early Sunday. Other parts of the day could see additional showers but it would occur mostly around the mountains or down toward San Diego County. Other models such as the GFS favor a later time frame (Saturday evening, late Sunday into early Monday). Atmospheric instability is forecast to remain rather low. So, no thunderstorms are currently in the forecast (subject to chance though, depending on how the models change in coming days).

I am currently leaning toward the wet Saturday morning scenario (ECMWF solution) with decreasing chances of rain by mid-afternoon. Rain could briefly be moderate to heavy at times, and it could get a little breezy at times (less than ideal conditions for Bruin Day activities set for this Saturday). However, confidence remains low at this time. If the latest GFS scenario plays out, much of the wet weather may get delayed till late afternoon or early evening (showery weather for a part of Sunday and early Monday). Hopefully, model consensus will become good for my next issued forecast (Thursday).

The predicted storm may be a little wetter than last weekend’s storm. A number of model solutions show a range of a quarter to half inch rain in the lowlands away from the mountains (mostly under two tenths inch in L.A. County with the prior storm). Local resorts may see 3 to 6 inches of snow this weekend. Of course, depending on which model solution actually verifies, amounts could be higher or lower.

One way or another, clearing and warmer weather will occur sometime early next week (starting early Monday or as late as Monday evening). Unlike this week, the expected warming trend next week should be modest at best (at or slightly warmer than normal on Tuesday/Wednesday). A large, upper level trough in the Great Basin states is forecast for later next week. That may promote a good on-shore flow pattern (abundant low clouds west of the mountains and cooler than normal temperatures). Some renewed warming may take place over the subsequent weekend when the aforementioned trough weakens and moves east.

Next issued forecast/synopsis should be on Thursday, 11 April.