Weather Synopsis – April 25, 2024

James Murakami

Thu25th64/53Possible partial afternoon through early evening clearing; Otherwise, mostly cloudy through the evening.
Fri26th68/53Partly cloudy morning; Mostly sunny afternoon and possibly breezy at times. Mostly clear evening and possibly breezy at times.
Sat27th69/52Some morning clouds possible; Otherwise, sunny day. Mostly clear evening.
Sun28th68/53Chance of early morning low clouds; Otherwise, mostly sunny day. Mostly clear evening.
Mon29th70/54Chance of some early morning low clouds; Otherwise, sunny day. Clear evening.


The passage of an upper level trough (such as last night) commonly leads to less marine layer clouds. However, in this case, low level on-shore flow changed little after the trough passage (weaker but not by much). Also, the ridge of high pressure following this trough is pretty weak. An approaching, “inside slider” trough is preventing the ridge from strengthening over southern California. After the “inside slider” moves off to the east, high pressure is forecast to build into the Southland, but that won’t be till the weekend (lasting into early next week).

The deep marine layer promoted another morning of drizzle/light rain in places. It was most prevalent from the western Inland Empire southward through San Diego County. Rainfall this morning was mostly well under a tenth inch. The various computer models predict one more morning of drizzle/light rain (triggered being the “inside slider”), but areal coverage should be less then it was this morning (assuming “inside slider” doesn’t pass through farther to the west than currently forecast). One difference from the last couple of days is that the models predict the drizzle may continue through the day in spots (up against coastal facing mountains, mainly in San Diego County). In addition, marginal atmospheric instability could trigger some briefly heavy showers in the San Bernardino Mountains late tomorrow afternoon (wouldn’t rule out a brief thunderstorm though that isn’t typical of “inside sliders”. Regardless, rapid clearing should occur in the wee, early morning of Saturday.

Low clouds have been persistent this afternoon in some areas but not in other areas (clearing in some spots of the coastal plain). The complexities of marine layer physics infrequently make it a futile effort to forecast clearing patterns accurately (I gave up years ago). The wind flow pattern associated with the approaching “inside slider” may disrupt the low cloud field from L.A. County westward, but confidence is low (reason for “partly cloudy” forecast tomorrow morning). The current breezy weather in the interior should peak in intensity tomorrow afternoon. The sea breeze tomorrow should also blow briskly as early as late morning. The wind should diminish tomorrow evening in most locales. More tranquil conditions should prevail over the weekend.

None of the models predict a complete purge of the marine layer in the coming days. No full fledged, off-shore flow is predicted to develop (not common by this time of year anyway). At best, a shallow but diluted marine layer may occur. This would prevent any widespread low cloud development in the coastal plain. However, any warming trend promoted by high pressure aloft would be tempered in the coastal plain. At most, temperatures there may get slightly above normal early next week (better warming for the valleys and interior…common spring pattern). Most of the longer range models show weak, upper level low pressure setting up camp along the West Coast late next week. This may lead to an episode of “May Gray” weather over the first weekend of May (lasting into first full week of next month).

Next issued forecast/synopsis should be on Monday, 29 April…busy on Tuesday