Day | Date | High/Low | Forecast |
Tue | 21st | 71/55 | Mostly sunny day with some high clouds. Some high clouds in the evening. |
Wed | 22nd | 77/57 | Mostly sunny day with some high clouds. Some high clouds in the evening and possibly breezy at times. |
Thu | 23rd | 78/54 | Sunny day and possibly breezy at times in the morning. Mostly clear evening. |
Fri | 24th | 72/52 | Mostly sunny day with some high clouds. Variable evening clouds. |
Sat | 25th | 58/46 | Mostly cloudy day. Slight chance of morning light showers. Increasing chance of showers by late afternoon. Good chance of evening showers. |
Synopsis
High pressure aloft continues to dominate the weather across much of the West (been mostly that this month…so far). An “inside slider” type trough passed on the eastern flank of the aforementioned high pressure yesterday (reason for moderate strength, Santa Ana winds in places…as high as 88 mph yesterday). Another, weaker, “inside slider” should pass nearby Wednesday night or Thursday morning. While the Santa Ana winds have generally decreased today, there should be a modest uptick in winds once the next “inside slider” passes (interim period will feature less off-shore flow but no complete cessation is likely in most wind prone areas).
Aside from the Santa Ana winds, daytime temperatures (west of the mountains) have been increasing over successive days (nights have remained on the chilly side…more so than I anticipated for the coastal plain). This trend should continue for a couple more days as less polar air is expected to filter into the Southland (subsidence/sinking air warming should get more effective in warming the lower atmosphere). Thursday should be the warmest day this week for most areas west of the mountains. Some readings into the low 80s are possible though upper 70s away from the coast should be more common. Cooler weather should begin on Friday (warmer than normal still but noticeably cooler than what’s expected on Thursday).
Much colder, cloudier weather along with a return to more normal humidity is expected this weekend. The numerical models aren’t in complete agreement on details, but the model consensus favors widespread, occasional, wet weather. Scattered showers could start from an approaching, cold, upper air trough as early as Saturday morning (mid-morning in L.A. County). If the model consensus holds true, the period with more consistent showers or even steady precipitation should happen Saturday evening through early Sunday. Scattered showers would prevail Sundasy afternoon onward (more so near/over the mountains than along the coast in L.A. County). Decreasing showers would occur beginning Sunday evening. Lingering showers would be possible Monday morning, but that should be to the east and south of L.A. County. By Monday evening, the cold trough should be exiting the state.
The trough that reaches the Southland this weekend is what I describe as an “outside slider” (term I coined…not a National Weather Service descriptor). Instead of traveling south over the Great Basin states (“inside slider” pathway), this one is predicted to pass over the ocean once its circulation reaches the state (accessing a ready source of water vapor). The predicted, wind flow pattern isn’t conducive for tapping into any sub-tropical moisture, let alone an atmospheric river. Available water vapor will be limited, and storm totals should be mostly under a half inch, especially away from the mountains. Also, this storm should be more showery in nature (rather than possessing a band of steady precipitation). So, storm totals may vary more than usual (as little as a few hundredths of an inch in spots to over an inch along some coastal facing, mountain slopes). For L.A. County, average rainfall around a quarter inch appears reasonable. Snow levels potentially could drop to as low as 2500 feet (Sunday morning) although 3500 feet may be more probable (several inches of snow at resort level expected).
There is still some chance that the aforementioned trough doesn’t take much of a coastal route. In that case, precipitation may occur mainly around/over the mountains in L.A. County (coastal plain possible southern Orange County southward). At this point, the chance for brief-lived thunderstorms appear low, but the cold nature of the storm could support some isolated activity on Sunday (not restricted to around the mountains). Any thunderstorms could be capable of short duration, heavy rain. If the trough moves through the Southland more slowly than currently forecast, onset of wet weather may get delayed by 12- 24 hours (comparable delay to the end of the storm period).
Based on the current model consensus, this may be the one and only storm reaching southern California over the next ten day period (northern California might get an additional, minor storm before the month ends). Another round of off-shore flow is expected by the middle of next week (weak to moderate Santa Ana winds again). A period of warmer than normal weather is probable, but it may not get as warm as what’s expected later this week. If any more wet weather does occur in the Southland, it may not occur before the 4th of February.
Next issued forecast/synopsis may be on Friday, 24 January.