Weather Synopsis – March 12, 2024

James Murakami

Tue12th63/51Partly cloudy morning; Mostly sunny afternoon. Mostly clear evening.
Wed13th69/51Sunny day and possibly breezy at times. Clear evening and breezy at times.
Thu14th69/53Mostly sunny day and breezy at times. Some evening clouds and breezy at times.
Fri15th67/53Mostly sunny day with variable clouds; Possibly breezy at times. Some evening clouds and possibly breezy at times.
Sat16th68/54Scattered clouds but generally sunny day. Scattered evening clouds.


Well, that rain last Thursday (7th) afternoon was more significant that I expected. The numerical models showed an upper air disturbance passing through southern California late in the afternoon. Most of the models predicted sufficient moisture/instability would develop for isolated thunderstorms (starting near/over the higher mountains). That happened, but the shower activity was more widespread than the models predicted (longer lasting too even after thunderstorms dissipated). Because the rain cells moved relatively quickly, and there was a somewhat dry layer of air beneath the thunderstorms, rainfall that afternoon wasn’t high in most locales. UCLA received 0.16 inch rain from the late afternoon event (1.62 inches for entire period going back to the 6th). For the month so far, the campus tally has risen to 2.57 inches (March normal is 2.73). For the current water year (began last October 1st and concludes end of September), the total stands at 28.01 inches. For the time being, that ranks as the 13th wettest season at UCLA (last year was the 2nd wettest with 41.97 inches). Weather records at UCLA date back to 1932.

A passing “inside slider” type trough helped increase the on-shore flow greatly overnight. That resulted in a noticeable increase in winds in the interior sections of southern California (brisk sea breeze this afternoon too). Marine layer clouds didn’t last long for most of L.A./Orange Counties, however (also Ventura County). Clouds remain abundant from the western Inland Empire southward into San Diego County (possible spotty drizzle earlier in the day though no confirmed reports, to my knowledge). There will be a nearby pass of a developing “cut-off” low pressure tomorrow. Its passage will shift winds over the Southland to a northerly direction.

As with many “cut-off” low pressures, a correct weather forecast lies with the predicted details in wind flow and sometimes vertical moisture distribution. Numerical models infrequently don’t predict the smaller scale details accurately (at least, in some areas within the “sphere of influence” of the low pressure). Most of the models predict a moderately strong Santa Ana wind event for late tomorrow into Thursday. However, where the strongest, most persistent winds occur varies, depending on which model solution one accepts (current model consensus favors Inland Empire). Peak wind gusts at higher elevations could exceed 65 mph for a time (45 mph at lower elevations). Winds in the campus area may reach up to 35 mph early Thursday, but confidence isn’t the best (some scenarios don’t show winds exceeding about 20 mph). Winds in all areas should diminish after Thursday, but potentially, occasional gusty winds may continue through the weekend (“cut-off” low pressure predicted to move very slowly eastward after a reaching a closest approach around the Colorado River Valley).

The temperature forecast with this developing event is also somewhat in doubt. Most areas west of the mountains should get noticeably warmer than today (interior sections should be cooler than normal due to proximity to “cut-off” low pressure center). However, just how warm it gets will depend on how close the “cut-off” center gets as well as how long it lingers in place (closer position favors less warming due to influx of polar air from “cut-off” low). For this forecast, I’ve leaned toward a “cool” Santa Ana wind event (at best, temperatures getting slightly warmer than normal). No 80 degree readings in the Southland for the next several days are currently forecast (predicted center of high pressure aloft staying too far away for such warmth…at least through the weekend).

One other weather scenario includes increasing shower activity by Friday. The current model consensus shows the “cut-off” low pressure getting close enough for areas of showers and isolated thunderstorms over parts of the Inland Empire and areas southward (includes coastal San Diego County). While it’s a minority solution (at present), there is a scenario where afternoon showers and isolated thunderstorms develop over the mountains farther west (includes Ventura/L.A. Counties). This could occur Friday afternoon as well as the weekend afternoon hours. For this forecast, I omitted a mention of such weather. A lot depends on the “shape” the low pressure develops into. I may have to amend the forecast on Thursday, however, if the wetter scenarios gain traction with the models.

No matter which scenario verifies, the aforementioned “cut-off” low pressure will weaken and eventually move off to the east. It could happen as early as Monday or as late as next Wednesday. High pressure will build over the Southland at that time, but low level on-shore flow is also expected to return. Hence, any warming trend next week may be fairly minor in the coastal plain (temperatures may even lower a little from later this week, if a shallow marine layer develops). There are some model solutions showing a weakening storm approaching the Southland over the subsequent weekend (potential for some wet weather). A number of model solutions keep that storm well away, however. It’s not unusual for the longer range models to fluctuate wildly as the spring season nears (astronomically speaking, spring begins on the 19th at 8:06 PM). So, I’m not going to say anything more for now.

Next issued forecast/synopsis should be on Thursday, 14 March.