Yesterday the computer models predicted that widespread, wet weather would occur today from northwest L.A. County westward. That was correct, but widespread showers also prevailed along and adjacent to the mountains to the east (also much of the high desert, albeit very light showers). For a brief time, rain did fall over most of L.A/Orange Counties. Most of the heavy rain has been so far been limited to near foothills/mountains or to the west of L.A. County. Storm totals have already exceeded an inch in some areas with more to come.
The rain in the L.A. Basin (southeast corner dry as of this writing) should come to a temporary end by late afternoon or early evening. The axis of deep layered, moisture is forecast to shift westward for a time tonight (wet weather should continue farther to the west though much of Ventura County may also dry out for a time). By early tomorrow, the upper level trough of low pressure that is guiding an atmospheric river through southern California will move eastward. That will lead to a mostly wet day tomorrow in L.A. County (spreading across most areas to the east and south as the day progresses).
While there have been some pockets of heavy rain today in L.A. County, most of that has happened farther to the west. That should translate more east tomorrow (L.A. County more squarely in the middle). The most consistent, heavy rain should fall around south facing mountains (potentially several hours of on/off heavy rain). The rain in the L.A. Basin should taper off late tomorrow afternoon and become more showery in nature tomorrow night (some areas may actually be dry for part of the evening). By Friday sunrise, most of wet weather should be over (some lingering, mountain showers as well as down in San Diego County in the morning hours).
I'm sticking with widespread storm totals in the two to three inch range away from the mountains (L.A. County westward...less for most of Orange/San Diego Counties). Maximum totals from six to twelve inches of rain are possible for areas around most south facing mountains from L.A. County westward (possibly western Inland Empire too). Three to five inches, however, should be more common. Since this is a "warm" storm (moisture from an atmospheric river comes from the tropics), snow levels for most of the event should remain at or above 9000 feet. One to two feet of "wet" snow should fall above that elevation though. The snow level will fall to around 6000 feet by Friday, but by that time, only lingering showers will occur (at most, a couple inches of snow at resort level but nothing widespread).
Upper level low pressure will remain along the West Coast on Saturday (moving inland on Sunday). More showers are expected over northern California on Saturday, and some light showers should reach San Luis Obispo/western Santa Barrbara County in the afternoon or evening. There is some chance that scattered showers will occur in the northern mountains too. At this point, nothing is forecast for the L.A. County, coastal plain (just clouds and some wind). If mountain showers do occur, snow levels should be around 5000 feet (slightly lower in the evening), but no relevant snowfall is likely (limited, available moisture).
Some model solutions extend the threat of cold air instability showers to Sunday. At this point, I've kept the forecast dry (just partly cloudy skies), but I may need to reconsider the forecast tomorrow. In any case, cooler than normal weather should prevail this weekend.
The weather trend for next week remains uncertain. More model solutions show a large, upper low pressure somewhere in the Southwest early next week (actually the same low pressure currently off the West Coast). Depending on how close it is to the Southland, cooler than normal weather may last through early next week (could be occasionally breezy). After that time, most models show some warming (a few solutions show strong warming). There are some model solutions showing marine layer weather late next week, which would promote near seasonable temperatures. However, it's possible that warmer than normal weather may occur. Hopefully, clarity in the forecast comes soon.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2018-03-21