Day | Date | High/Low | Forecast |
Mon | 3rd | 67/52 | Low clouds clearing late morning to scattered high clouds. Mostly cloudy evening. |
Tue | 4th | 61/54 | Mostly cloudy through the evening. Slight chance of afternoon light showers. Increasing chance of light rain in the evening. |
Wed | 5th | 60/54 | Rain likely in the morning; Chance of afternoon showers. Mostly cloudy evening with a slight chance of showers. |
Thu | 6th | 61/53 | Mostly cloudy through the evening. Chance of morning light rain; Slight chance in the afternoon. Chance of evening rain. |
Fri | 7th | 60/48 | Chance of morning rain, mainly early; Partly to mostly cloudy remainder of day. Decreasing clouds in the evening. |
Synopsis
The ridge of high pressure aloft that promoted warmer than normal weather (more so than I thought last week) is slowly receding into the sub-tropics. A shallow marine layer returned yesterday, but today, it’s been more effective in keeping temperatures down near the coast today (within the marine layer…still reaching 70 degrees above the marine layer in the Santa Monica Mountains and Hollywood Hills…flirting with 80 degrees in some valley locales). For the remainder of the week, temperatures in most areas should struggle to reach the low 60s.
An upper level trough off of the Pacific Northwest (center around Vancouver Island early today) continues to tap into an atmospheric river (axis has temporarily drifted north since the weekend but still remains over northern California…several inches of rainfall common in parts of northern California since Friday). It’s nearly a certainty that most of southern California will see wet weather (desert rainfall should be spotty at best), but it’ll pale in comparison to the multi-day totals up north (6 – 10 inches in places). Since my previous synopsis, the computer model consensus has drifted downward on storm totals for the Southland. The tributary of the atmospheric river reaching the coast is forecast to dissipate as it reaches the Southland. In addition, the aforementioned upper low pressure isn’t forecast to move much farther south. Storm dynamics are expected to be minimal by the time it reaches L.A. County.
Mostly minor, upper air disturbances will ripple toward the coast (southern branch of a “split flow” pattern). These disturbances should provide the main forcing mechanisms (for the upcoming wet weather). Aircraft reconnaissance flights into the storm are planned to continue this week (once a day). Model runs should benefit from the additional data, but the large size of the storm may prevent significantly greater model consensus (planes can’t cover all of the storm and surrounding area in a single flight).
There continues some uncertainty about when steady rain will reach L.A. County. Some scattered, light showers may begin by the afternoon tomorrow (wet weather will proceed generally from west to east). I’m expecting enough storm dynamics to induce a steady, light rain in the evening, but some model solutions hold that off till near midnight. The bulk of rain in L.A. County (snow levels mostly above 8000 feet for this storm) should fall during the overnight hours (wee, early hours of Wednesday). It should taper off to scattered showers by mid-morning Wednesday (maybe a little earlier, if some models are right).
A separate disturbance is forecast to approach the Southland from the west on Thursday. Most model solutions show a chance for marine layer induced drizzle/light rain Thursday morning. The main thrust of this storm, however, should come Thursday night or early Friday morning. The threat of wet weather is expected to end by sunset Friday, if not several hours earlier in L.A. County. This second storm (not identified in model runs last week) potentially could be a little stronger than what comes through tomorrow night through Wednesday (not much though…rapid weakening still predicted by most of the models).
Last week, I thought a half to one inch rain away from the mountains (L.A. County) for the first storm was reasonable. If today’s consensus holds up, a quarter to half inch rainfall looks reasonable now. Areas in L.A. County around coastal facing foothills/mountains may see up to double those figures. Higher totals are also expected west of L.A. County (parts of Santa Barbara County may receive around two inches). Totals are expected to be mostly under a quarter inch away from the mountains to the east and south of L.A. County (predicted storm dissipation). No relevant snowfall is expected with the first storm. For the second storm, totals in L.A. County may increase a little (a tenth to quarter inch). Snow levels with the second storm should also remain mostly above 8000 feet, but may ultimately fall to near 6500 feet (couple inches or so resort level).
Dry, mostly sunny weather should return for the weekend. Some model solutions show a marginal, off-shore flow development (short duration though). So, there is a chance for seasonable to slightly warmer than normal weather by Sunday (not likely to get as warm as most recent warm up, however). Cooler weather should re-develop early next week when a new, weak, upper level trough develops over the state. At this time, most models show little chance of widespread, wet weather from this projected system (isolated mountain showers possible). Next week, for the most part, looks to be dry (at least, that’s what today’s model consensus shows).
Next issued forecast/synopsis may be on Friday, 7 February.