Weather Synopsis – October 4, 2024

James Murakami

DayDateHigh/LowForecast
Fri4th77/62Variable mid/high clouds but mostly sunny remainder of day. Scattered evening mid/high clouds.
Sat5th79/63Chance of some early morning low clouds/fog; Otherwise, mostly sunny day with some mid/high clouds. Some evening high clouds possible.
Sun6th80/63Sunny day. Mostly clear evening.
Mon7th80/62Mostly sunny day with some high clouds possible. Chance of late evening low clouds/fog.
Tue8th77/61Mostly sunny day except chance of early morning low clouds/fog. Chance of evening low clouds/fog.

Synopsis

Accurately forecasting the marine layer effects or non-effects (in today’s case) remains a challenge for me. In a previous forecast, I amended the campus temperature forecast when it got warmer than expected on day one. That turned out to be a mistake (wound up cooler than the amended forecast). A shallow but effective marine layer promoted a moderating affect near the coast on an otherwise, warming weather pattern (plenty of sizzle in the western San Fernando Valley).

Today, it once again became several degrees warmer than expected at UCLA (similar warm up in other areas of the Westside). A flow of subtropical air (reason for variable mid/high clouds and even sprinkle clouds west of Ventura County) appears to have induced some vertical air mixing down to the ground. Satellite imagery showed clearing of low clouds from much of Santa Monica Bay (for a while). Sometime near the noon hour, the campus temperature jumped, peaking at 83 degrees (12:28 PM). Water vapor content also climbed (evidence of sub-tropical air from aloft). By 1:30, things were back more in line with my morning issue. Curiously, the unexpected, brief warming didn’t occur for coastal areas outside of L.A. County (at least, as far as I’m aware). The effect was also appeared absent for most, well inland locales.

I decided not to amend the campus forecast this time. All the computer models do show high pressure aloft in the Southwest strengthening a little over the weekend. That should help to decrease the surface, on-shore flow, which should result in less marine air influence in the coastal plain (i.e. warmer weather). However, as was the case earlier this week, a shallow but effective marine layer might promote less warming than what the models forecast. My forecast may still be a bit too cool for the weekend, but as long as mixing of sub-tropical air doesn’t repeat, the forecast numbers shouldn’t be too far off. I hope. By the way, assuming less marine air influence verifies, areal coverage of low clouds/fog should be at a minimum on Sunday and possibly Monday too (any low clouds/fog restricted to immediate coast).

The various models show a couple of weak, upper level troughs affecting southern California next week. This should help induce modest cooling in most areas early next week and late next week (includes subsequent weekend). If some model solutions are right, the late week trough may induce cooler than normal weather (all of the Southland for at least a day). At this point, neither trough should pose a threat of wet weather in the state (troughs too weak and no predicted tap into sub-tropical moisture). There seems to an overall trend of lower temperature in the longer range forecasts, but that’s not unusual as we get further into the autumn season.

Next issued forecast/synopsis should be on Monday, 7 October.