NOTE: THE TEMPERATURE/HUMIDITY SENSORS HAVE SHOWN PERIODIC, SPURIOUS VALUES, INCLUDING IN THE EVENING. THE CAUSE IS YET UNKNOWN, BUT RESOLUTION TO THE PROBLEM MAY NOT COME FOR SOME TIME.
A weak, marginal off-shore flow exists today. It's similar in pattern to a couple days ago, but the air mass preceding this one was colder. So, daytime temperatures on Tuesday didn't get as high as it has gotten today (widespread 80 degree weather in most coastal valleys; mid to upper 70s inland, coastal plain; briefly reaching 70 degrees many beach areas). On-shore flow should return tomorrow, and that should permit widespread low clouds to reach the coast south of Point Conception. It was already along the coast north of Point Conception but one patch was nearing Ventura County coast (most still lying west of the coastal islands). The computer models don't show much inland penetration tomorrow morning, but I wouldn't be surprised if most of the coastal plain is shrouded early tomorrow. A further increase in on-shore flow throughout the day should lead to more widespread low clouds tomorrow night (gradually reaching the coastal valleys).
With the return of on-shore flow tomorrow, it should get noticeably cooler in most areas (relative to today). For the most part, weekend temperatures in southern California should stay under 70 degrees (low to mid-60s coastal plain). In addition to more marine layer clouds, there should be a general increase in high clouds as well. So, cloudier weather is anticipated by the weekend (through Monday morning). All these changes will come as a few, upper level trough of low pressure pass through the Southland. One such trough will pass tomorrow (currently causing precipitation over parts of northern California). A weaker trough should pass on Saturday morning. Some model forecasts show the marine layer being deep enough to support patchy, early morning drizzle or light rain. One again, I decided to omit any rain threat for the campus area. What model solutions that show wet weather favor areas to the east and south of L.A. County. This predicted trough also is weaker than one that surprised me with light rain this past Monday.
The third trough, however, looks to produce widespread precipitation (includes probable, first good snow dump in the northern Sierras). Decent storm dynamics are forecast to reach the southern California Monday morning, and there will be a brief connection to an atmospheric river when the storm arrives here. The timing for the steady rain period may change between now and the actual event, but today's model consensus favors Monday morning (starting with the early morning commute). A brief period of moderate to heavy intensity rain is likely for most areas west of the mountains. No thunderstorms are currently forecast with this storm (predicted to be too stable over the Southland), but I wouldn't rule out a brief thunderstorm or two. However, there has been a model trend toward less rainfall than what was forecast a couple days ago. Storm totals in the lowlands away from the mountains should range in the quarter to three quarters of an inch range. Still, there is chance that earlier projections of 0.75 to 1.5 inches may verify. Snow levels should fall to near 5000 feet by Monday night, but by then the bulk of precipitation should have passed. Hence, the local resorts probably won't get more than a few inches of snow.
As the storm sweeps through the Southland, breezy conditions should prevail in most areas. The storm should be east of L.A. County by sunset Monday. So, clearing and less wind should occur by then. A return to mostly sunny weather is anticipated for the remainder of next week. A predicted, weak off-shore flow should promote warmer than normal weather by the second half of the week. Widespread 80 degree weather is expected, and there is some potential for 90 degree weather for a day or two (end of the week). Cooler weather should prevail over the subsequent weekend (on-shore flow returning).
Next issued forecast/synopsis should be on Tuesday, 26 October.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-10-21