Weather Synopsis – February 20, 2024

James Murakami

Tue20th58/53Periods of rain, possibly heavy at times through the evening.
Wed21st62/50Overnight showers tapering to a slight chance of morning showers; Otherwise, partial clearing. Partly cloudy evening.
Thu22nd64/50Variable clouds but generally sunny day. Partly cloudy evening.
Fri23rd69/52Partly cloudy with high clouds through the evening.
Sat24th67/53Variable, mostly high clouds, but generally sunny day. Becoming mostly cloudy in the evening.


Another, major storm is over southern California today. Episodes of heavy intensity rainfall have varied more than with a major storm a couple weeks ago (downpours not sticking around any one spot for too long a period with current system). Yesterday, the emphasis of heavy rain was to the west of L.A. County, but the combination of good storm dynamics and tap of an atmospheric river have shifted eastward now. The main body of the upper level reflection of the storm should move inland tomorrow morning. With that, most of the wet weather should end by sunrise in L.A. County (may take till tomorrow evening for areas to the south and east, especially mountain locales).

Storm totals in most areas have been noticeably lower compared with the “historic” storm a couple weeks ago, but many two inch totals away from the mountains have been reported so far (a good soaker by any definition). The UCLA auto-gauge reported a storm total of 3.70 inches (12:35 PM), but I caution that it’s consistently over reported rainfall (15%, on average). The weather station at the Bel Air Hotel recorded over 3 inches by 4 AM. So, the campus total might around 3 inches in this case (official measurement won’t occur till Saturday…don’t like to deal with heavy traffic whenever possible).

Weak high pressure aloft will develop over the Southland on Thursday/Friday. The numerical models predict a weak, low level, off-shore flow will develop Thursday night into Friday (transitioning back to on-shore flow Saturday afternoon). Weak off-shore flow was predicted late last week, but it failed to materialize (pressure pattern barely met requirements). The one predicted later this week appears to have better, upper air support, but I’ve taken a more cautious stance in today’s forecast. If deep layered, low level easterly, wind flow develops (no widespread, significant wind in Santa Ana wind prone areas expected though), daytime temperatures should reach the low/mid-70s in some locales west of the mountains. Of course, this assumes no more than a diluted, marine layer remains late in the week. Also, variable high clouds may play some role (dense high clouds could limit daytime warming).

Most of the longer range models show a “cut-off” low pressure approaching southern California sometime this weekend. Numerical models infrequently handle the future movements of “cut-off” lows poorly. In this case, there’s another complication. A separate system (this one, an “inside slider” trough) is expected early next week. A number of plausible scenarios are on the table currently. Most model solutions favor wet weather for L.A. County at some point (as early as Saturday evening or as late as Sunday evening). Of those solutions, most don’t predict major rainfall (favoring well under half an inch away from the mountains…a “run-of-the-mill” storm). There are some solutions that have the “cut-off” passing too far to the south of L.A. County for much, if any showers locally. A big unknown for now is how much interaction will occur between the “cut-off” low pressure and predicted “inside slider”. There is one scenario where the “inside slider” actually develops off the state coast (“outside slider”?). It then directs the “cut-off” low pressure more directly over the Southland. That’s followed by the other trough. That scenario, while unlikely at present, would lead to a wetter weather outcome (wet Sunday-Monday). Hopefully, more clarity shows up in the coming days.

Next issued forecast/synopsis should be on Thursday, 22 February.