| Day | Date | High/Low | Forecast |
| Mon | 22nd | 64/55 | Mostly cloudy day with mainly high clouds. Mostly cloudy evening. |
| Tue | 23rd | 63/57 | Mostly cloudy through the evening. Slight chance of morning showers; Chance of showers in the afternoon. Light rain developing in the evening. |
| Wed | 24th | 62/54 | Rain, intense at times, mainly in the morning; Breezy at times. Evening showers and possibly breezy at times. |
| Thu | 25th | 61/53 | Showers likely through the evening, possibly heavy at times. Possibly breezy at times. |
| Fri | 26th | 59/48 | Good chance of showers, mainly in the morning. Decreasing chance of showers in the evening. |
Synopsis
Other than bouts of dense fog producing wet surfaces, it’s been a dry December up to this point. That will change by tomorrow evening. All the computer models, while differing on small details, predict a major storm for southern California. This storm will have a good tap into an atmospheric river. Atmospheric dynamics with the predicted storm should be plentiful on Wednesday (Christmas Eve). Cessation of widespread, wet weather should come by the weekend, if the current model consensus holds up.
Temperatures today got a bit warmer than I anticipated in many areas. A marginal, surface off-shore flow kept the sea breeze weaker than forecast (despite presence of a shallow, marine layer). As the aforementioned storm starts taking shape off the coast tomorrow, some minor, daytime cooling should occur relative to what occurred today (left preliminary temperature forecast as is). However, due to the influx of sub-tropical air, it shouldn’t get as cold as with most, typical winter storms (at least through Christmas Day). Some additional cooling should occur around the end of the week when the cold core of the storm should move ashore (most noticeable for overnight low temperatures).
Wind in most areas should increase tomorrow (from a southerly direction during height of storm midweek). Some higher mountains may see infrequent gusts exceeding 55 mph. Even some lowland locales may experience wind gusts over 25 mph for a few hours (less widespread after Wednesday). Although atmospheric instability should be marginal on Wednesday, isolated, brief-lived, severe thunderstorms can’t be ruled out (not restricted to near the mountains). Atmospheric instability should increase on on Thursday, but deep layered, strong winds should diminish (relative to what should occur on Wednesday). So, peak wind gusts on the latter day should be lower.
Storm totals (tomorrow afternoon through Friday) should be in the 3 – 6 inch range in the lowlands away from the mountains (potentially wetter than a couple of the significant storms last month). The wettest day, total-wise, should be Wednesday when more than half of the storm total should occur in L.A. County. Should the atmospheric river stall out over one area, totals should be enhanced there and taper off quite a bit on either side of the atmospheric river axis. Because of the sub-tropical moisture, snow levels should stay above 7500 feet for a good chunk of the storm. Snow levels should lower by Thursday, but it may not lower below 6500 feet till Friday (minor snowfall by then). Except for the highest peaks, resort level snowfall with this storm should be minor (at best, a few inches of “wet, sticky” snow).
Model agreement isn’t the best, but the current consensus leans toward dry weather returning for the weekend (mostly sunny with modest warming trend…seasonable to slightly warmer than normal by Sunday). There are some model solutions showing a weak, upper level low pressure hanging back over or just southwest of the region this weekend into early next week. That would keep a slight risk of showers going (mostly affecting mountain locales). Other model solutions depict the low pressure sinking well to the southwest of the our region. In some scenarios, a weak, off-shore flow sets up, which would favor slightly warmer than normal weather for the last days of the month. I would lean toward the dry weather scenario, but much could change between now and the weekend. FYI– A few model solutions show a minor chance of showers for New Year’s Day but not until the afternoon hours. Most favor seasonable, mostly sunny weather though…the typical Chamber of Commerce weather for New Year’s Day in the Southland.
Wishing all a pleasant holiday and good start to the upcoming, new year!
Next issued forecast/synopsis should be on Monday, 5 January.