Weather Synopsis – April 16, 2024

James Murakami

Tue16th71/53Sunny day. Mostly clear evening.
Wed17th70/54Chance of early morning low clouds; Otherwise, mostly sunny day with some high clouds. Some high clouds in the evening.
Thu18th68/54Chance of morning low clouds; Otherwise, mostly sunny day with variable high clouds. Chance of evening low clouds.
Fri19th67/54Morning low clouds but mostly sunny afternoon. Chance of evening low clouds.
Sat20th67/55Morning low clouds but mostly sunny afternoon. Chance of evening low clouds.


The past weekend storm wound up wetter in most areas than I expected. Although the onset of rain in L.A. County started later than originally predicted (good for Bruin Day), that portion of the storm behaved more in line with expectations. Sunday’s weather, however, was wetter than I anticipated (colder too). Thus, instead of most lowland areas (away from the mountains) receiving no more than a third inch rain, it was more like a half inch (UCLA received 0.52 inch). Some valley locales received over an inch of rain. Some computer model solutions did lean in this direction, but I was skeptical (bias of it being April). Oh, well….

Something changed after my forecast last week that brought about warmer than expected weather this week (as I saw it). High pressure aloft following the weekend storm turned out stronger than what most model solutions showed last week. That led to better warming today than what I imagined last week (for this week). In addition, a predicted, large, cold trough won’t form in the Great Basin states this week (a trough in the northern Rockies but too far north to influence state weather much). So, it looks like weather concerns this week will be more typical of spring in southern California (the comings and goings of on-shore flow).

A marginal, surface off-shore flow gradient existed this morning. That, along with moderately, strong high pressure aloft just to the west of the state, has promoted good warming today (warmer than normal conditions). Surface on-shore flow is predicted to increase tomorrow, but high pressure aloft should come right overhead. So, while areas near the coast should cool off a little (compared with what occurs today), more inland areas should be as warm, if not slightly warmer than it gets today (some valley locales may warm into the upper 80s tomorrow). Modest but more general cooling should develop on Thursday when a weak, sub-tropical, upper level trough approaches southern California.

There were some patchy low clouds in Santa Monica Bay early today. With a predicted increase in on-shore flow, low clouds should increase in areal coverage tomorrow morning. However, the models aren’t predicted any significant marine layer deepening. So, it’s unclear to me how widespread low clouds/fog will get (probably reaching parts of the coastal plain though not necessarily as a widespread overcast). Subtropical mid/high clouds should also be on the increase tomorrow afternoon. It’s possible that Thursday will be mostly cloudy at times but not from marine layer low clouds. Satellite imagery currently shows considerable high cloud cover associated with the aforementioned, subtropical trough. There are even some showers near its center (predicted weakening of trough should lead to dissipation of those shower clouds).

The trough should be east of the Southland by Saturday, and high pressure aloft should build back into the Southwest over the weekend. This should, at the least, result in warmer weather for well inland areas again (after the expected, modest cooling later this week). The models predict a decent marine layer for Saturday. So, areas near the coast may not see warmer weather till Sunday (low clouds should be prevalent in the coastal plain Saturday morning). That warming trend should be similar to the current warming trend unless a shallow but effective marine layer persists (minor warming at most, with this scenario).

More weak troughing along the West Coast is anticipated as next week progresses. So, much of next week should be marine layer influenced west of the mountains (temperatures sticking close to seasonal normals). There are some weather scenarios that include a trough capable of widespread, wet weather late next week. That scenario stands a better chance at verification up north than down here (assumes persistent troughing actually develops next week). Even if wet weather reaches southern California, the storm should be no wetter than what occurred this past weekend.

Next issued forecast/synopsis may occur on Thursday, 18 April.