Weather Synopsis – October 11, 2024

James Murakami

DayDateHigh/LowForecast
Fri11th74/58Variable high clouds but mostly sunny remainder of day. Chance of late evening low clouds/fog.
Sat12th72/57Chance of early morning low clouds/fog; Otherwise, mostly sunny day with some high clouds possible. Good chance of evening low clouds.
Sun13th70/58Hazy sunshine after low clouds clear late morning. Low clouds likely in the evening.
Mon14th68/59Morning low clouds likely but mostly sunny afternoon. Low clouds likely returning in the evening.
Tue15th67/59Morning low clouds likely but mostly sunny by afternoon. Chance of evening low clouds.

Synopsis

Weather more reminiscent of summer in southern California continues (at least, it gets reasonably cool in the evenings unlike many summer situations). High pressure aloft continues its residence in the Southwest (promoting a warmer than normal air mass aloft). Near the ground, a shallow but effective marine layer has promoted widespread, low clouds/fog over most of the coastal plain late night/early morning hours. Subtle day to day changes in the wind flow pattern have produced mostly, minor weather changes this week. Predicting daytime temperatures near the coast has been a challenge (for me) even though the consequences (of being wrong) haven’t been significant.

A weak, upper level trough will move inland over northern California tomorrow (same trough responsible for variable high clouds passing through the Southland today). It should weaken the Southwest ridge of high pressure slightly, and low level, on-shore flow should increase some. The aforementioned trough is forecast to take up temporary residence somewhere in the Southwest later this weekend and early next week (Nevada/Arizona area before moving eastward). All the computer models are predicting an increase in marine layer depth through most of next week (minor increase at first). This should result in more widespread, persistent low clouds by Sunday (a little hard to believe since current satellite imagery showed large clear zones in the off-shore, low cloud field). A deeper marine layer should lead to much less fog near the coast. Patchy mist/drizzle could develop by Sunday (mainly early mornings near coastal facing foothills/mountains), especially if a coastal eddy circulation forms as some models show. Little, if any measurable rain is expected though.

If the current model consensus holds up, a relatively deep, marine layer should exist for most of next week. Typically, this type of pattern results in “June Gloom” like weather west of the mountains. Cooler than normal weather is anticipated (more like autumn as most people expect by this time of year). Since this type of weather is uncommon in October (statistically, the least likely time of year to have persistent, marine layer clouds), I’m unsure if multiple days of mostly cloudy skies will happen. It’s possible that an unforeseen, deep mixing of the air mass could disrupt the low cloud field (partly cloudy skies at worst). We’ll see.

Most model solutions show an “inside slider” type trough passing through the state on Thursday. Some model solutions favored marine layer induced drizzle/light rain. While this could happen in some locales, it appears unlikely in lowland, L.A. County (predicted wind flow pattern favors a “rain shadow” effect). As with many, “inside slider” troughs, a period of breezy weather in the interior sections is expected. The passage of the trough should clear away the marine layer clouds unless the trough winds up very weak (in that case, cancel any rain threat, strong wind in the interior, and any significant cooling).

It’s still far from certain, but a number of model solutions show the first Santa Ana wind event of the autumn (beginning Thursday night or early Friday next week). Most show a weak event (peak wind gusts under 50 mph…higher elevations). The wind should diminish quite a bit over the subsequent weekend, but some model solutions show a big warm up. Widespread 90 degree weather west of the mountains may occur. Many beaches could reach 80 degrees for a day or two. Again, this scenario is still far from certain. I’m reasonably certain of warmer than normal weather developing, but how warm it ultimately gets….???

Next issued forecast/synopsis should be on Monday, 14 October.