| Day | Date | High/Low | Forecast |
| Tue | 19th | 70/53 | Mostly sunny day but some afternoon clouds possible. Mostly clear evening. |
| Wed | 20th | 68/54 | Mostly sunny day except chance of early morning low clouds. Mostly clear evening. |
| Thu | 21st | 68/54 | Chance of early morning low clouds; Otherwise, mostly sunny day with some high clouds. Some high clouds in the evening. |
| Fri | 22nd | 68/54 | Chance of early morning low clouds; Otherwise, mostly sunny day with scattered high clouds. Partly cloudy evening. |
| Sat | 23rd | 64/50 | Amended…Mostly cloudy morning with a good chance of light rain; Becoming mostly sunny afternoon and breezy at times. Partly cloudy evening and possibly breezy at times. |
Synopsis
A “cut-off” low pressure has meandered between the California desert and western Arizona for several days now. The low pressure has promoted showers and brief-lived thunderstorms in parts of the Southland (mostly late afternoon hours in L.A. County). Although the low pressure has weakened steadily since late last week (contributed to the strong Santa Ana winds last week), atmospheric instability in the afternoon hours was greater than I anticipated (resulted in some brief, heavy rain in places, including in the coastal plains of L.A./Orange Counties). So far today, shower activity is far less than the previous few days. The numerical models still predict some further mountain showers later today, but it should be more on the tame side (little in the way of downpours expected). There could be one more afternoon (tomorrow) of light, afternoon showers in some mountains (just enough left over moisture and instability), but I think the currently, exiting “cut-off” low pressure should finally be done with the Southland, weather-wise.
High pressure aloft, which was centered over northern California early this afternoon, should become better established over southern California tomorrow (lasting through most of Friday). This should result in generally sunny days across the Southland for the coming days, and slightly warmer than normal temperatures should prevail in most areas. The exception might be near the coast where a return of weak, on-shore flow should promote a shallow marine layer. Depending on whether an organized, low cloud field develops (developing now west of the coastal islands), the coastal plain may end up slightly cooler than normal (as early as tomorrow). This could even include the coastal valleys later this week if the marine layer deepens more than currently expected.
All the models predict a Pacific trough moving through the state over the weekend. There is a growing consensus (compared with previous days) for widespread wet weather reaching L.A. County. Timing issues remain, but most model solutions favor Saturday morning for steady, lowland rain (snow levels mostly no lower than 6000 feet). Due to a better model consensus, I amended the Saturday portion of the forecast from my preliminary, late morning issuance.
It should be a fairly minor rain event (no thunderstorms anticipated this far south). Rainfall totals should be mostly under a third inch (possibly well under that figure for lowland areas away from the mountains). Snow accumulations at the local resorts shouldn’t amount to more than a couple inches or so. After the cold front passes through (late morning or early afternoon in L.A. County), most areas should see a break in the weather (may actually be mostly sunny in spots for the afternoon). Brisk winds following the front, however, should make it feel a little nippy. A secondary batch of showers (associated with the main body of the upper level trough) are expected on Sunday. However, by that point, a number of model solutions show low level, northwesterly wind flow. That would favor a “rain shadow” effect across most of the L.A. Basin (would include the campus area). So, the chance for showers on Sunday is less certain than with Saturday’s cold front. Where wet weather does occur, mostly light precipitation is expected. However, colder air should lower the snow level to 5000 feet or slightly lower. Atmospheric instability could also become high enough for brief-lived thunderstorms Sunday afternoon.
A northwest wind flow pattern is predicted for early next week. A couple of “inside slider” troughs could bring breezy weather to the interior sections. Depending on the path and strength of each trough, there could be some chance for mountain showers. Temperatures early next week should hover near seasonal normals early on. It may get slightly warmer than normal if some model solutions for stronger high pressure verify. While it’s far from certain, there is some chance at wet weather again late next week (nothing of significance currently indicated).
Next issued forecast/synopsis should be on Thursday, 21 March.