Weather Synopsis – March 21, 2024

James Murakami

Thu21st66/54Mostly sunny remainder of day with some high clouds. Low clouds returning in the evening.
Fri22nd66/54Morning low clouds likely but generally sunny afternoon with scattered high clouds. Mostly cloudy evening.
Sat23rd61/50Mostly cloudy morning with light rain likely; Becoming mostly sunny and breezy in the afternoon. Partly cloudy evening with a slight chance of showers.
Sun24th62/49Partly cloudy day. Chance of showers or a brief thunderstorm late afternoon. Clearing in the evening.
Mon25th65/50Sunny day. Mostly clear evening.


A weak ridge of high pressure aloft lies over the state today. Near the surface, an intermittent, eddy exists over coastal southern California. The eddy helped deepen the marine layer overnight (low clouds reached many, coastal valley locales early morning hours). Temperatures today are about the same as yesterday near the coast, but most other locales west of the mountains are a little cooler than it got yesterday (still mild for March). This weather pattern should change little tomorrow. However, subtle changes in the low level, wind flow pattern could lead to more persistent low clouds in places relative to today’s weather. At the least, there should be more high clouds tomorrow than today (sub-tropical high clouds projected toward the Southland ahead of an approaching, Pacific trough).

For days, the numerical models have forecast a large trough of low pressure reaching the state by the weekend. It’s forecast to have decent storm dynamics over northern California. In addition, the accompanying cold front possesses above average water vapor content. So, a good snow dump (for an early spring storm) is expected across most of the Sierras. Down in southern California, widespread wet weather is anticipated west of the mountains. However, storm dynamics should be weaker than up north. Unless storm dynamics wind up stronger than the models predict, this it should be a fairly minor storm down here.

In L.A. County, a period of mostly light rain should fall sometime Saturday morning (some moderate intensity rain possible up against the coastal facing mountains). Just when that occurs is still a little uncertain. Some models favor a mostly early morning event. Other model solutions delay that near mid-day. In any case, the steady, wet weather shouldn’t last more than a few hours (at worst, rain ending on the Westside mid-afternoon). Once the cold front passes (moving generally from west to east), there should be some clearing along with a noticeable increase in winds. A number of model solutions show additional, post-frontal showers developing late in the afternoon or early evening. Most models show developing, west-northwest winds by that time. If true, a “rain shadow” effect may limit the areal coverage of showers (leaving much of the southern half of L.A. County dry). I kept a “slight chance” of showers in the campus forecast for Saturday evening just in case low level winds stay more westerly (i.e. no “rain shadow” development).

On Sunday, the main core of the aforementioned, Pacific trough will move inland. With predicted northwest winds across much of the Southland, most shower activity in the morning should occur to the east and south of L.A. County (“rain shadow” again). In the late afternoon, an embedded disturbance is forecast to pass through the County. Similar to a situation a couple weeks ago, atmospheric instability may increase rapidly (going from just a few clouds to widespread, rain clouds in just a couple of hours). Some of these “home grown” showers may become brief-lived but strong thunderstorms. The model consensus on this wet weather scenario is far from unanimous. Some model solutions keep this inclement weather to the western Inland Empire and points south (i.e. San Diego County). Since the usually reliable ECMWF model favors the wetter scenario (includes L.A. County in the wet weather coverage), I decided to word the forecast at “chance” (not confident enough to go “showers likely” as a small error in predicted wind flow could change the forecast altogether).

Expected rainfall Saturday through early Sunday (including evening showers) should be mostly under a third inch (possibly well under that for areas away from the mountains). If Sunday afternoon showers and thunderstorms occur, rainfall in that period could range from a few hundredths to about a half inch (variable in areal coverage due to showery nature of event). Snow levels should stay above 6000 feet through early Saturday afternoon. So, snowfall during the day will probably be no more than a couple inches at resort level. Potentially, an additional few inches could fall Saturday evening into early Sunday (snow levels falling to 5000 feet or slightly lower by early Sunday). Up to several more inches snow are probable in places with the Sunday afternoon showers (snow levels possibly down to 4000 feet for a time).

Except for a few lingering showers early Monday, sunny weather should be the rule that day. After a cool morning (down to the mid/upper 40s in many locales west of the mountains), afternoon temperatures should recover nicely (approaching seasonal normals in the lowlands). Depending on which model solution verifies, a bit more warming may occur on Tuesday (models disagree on how strong high pressure aloft gets). Uncertainty abounds on weather for the Wednesday-Thursday period. A new Pacific trough is forecast to pass through the state. Some model solutions show a weak storm providing a minor threat of wet weather in the Southland (sometime Wednesday or early Thursday). Looking farther out in time, there is unusual agreement (for a long range forecast) on a storm for the subsequent weekend (last days of March). Some model scenarios show a decent strength storm reaching southern California. While it’s still too early to gauge much confidence in the wet weather scenario, I’m relatively confident that the rest of this month will not feature warm weather west of the mountains (no widespread 80 degree weather). At best, some days could get slightly warmer than normal.

Next issued forecast/synopsis may be on Tuesday, 26 March