| Day | Date | High/Low | Forecast |
| Fri | 22nd | 71/53 | Sunny day. Mostly clear evening. |
| Sat | 23rd | 62/52 | Mostly cloudy through the evening. Chance of occasional light showers late morning through early evening. Slight chance thereafter. |
| Sun | 24th | 63/52 | Amended slightly…Partly to mostly cloudy through the evening. Slight chance of light showers. |
| Mon | 25th | 63/53 | Partly to mostly cloudy through the evening. Chance of occasional showers through the evening. |
| Tue | 26th | 62/53 | Partly to mostly cloudy through the evening. Chance of morning showers. Slight chance thereafter. |
Synopsis
Let me state from the outset that the forecast for the period after today is of low confidence.
A large, “cut-off” low pressure occupies the area west of the Pacific Northwest states and northern California. The circulation is actually comprised of multiple, smaller circulations pinwheeling about a common center. Draped on its southern flank is an atmospheric river. The various interactions of the smaller circulations and the atmospheric river have been responsible for the recent, stormy weather in the western sections of the Pacific Northwest and parts of northern California. A sub-tropical ridge of high pressure has so far protected southern California from the inclement weather up north, but all the computer models predict wet weather reaching the Southland over the weekend.
While I’m confident that widespread, wet weather (west of the mountains) will take place in the Southland, I have low confidence in the details of what weather will transpire. There have been wavering forecasts by the models regarding details of the low level, wind flow pattern over southern California. Adding to the confusion (for weather forecasters) is that run to run model consistency has been lacking so far. One model may predict only scattered showers one run only to go with a period of heavy rain on a subsequent model run. Even the usually reliable ECMWF has been flip flopping every few runs. Today’s forecast is a consensus of what the models have generally agreed on (consensus in the broadest sense today).
An atmospheric river should be over the Southland tomorrow (spigot getting shut off though). So, there will be no shortage of available water vapor. However, most of the models have shown little storm dynamics getting this far south with the first cold front tomorrow. Some moderate intensity rain may fall as far south as Santa Barbara County, but for most of southern California, only light showers are expected tomorrow (starting in L.A. County late morning hours or early afternoon). In L.A County the first “wave” should exit by early or mid-evening Saturday (chance it might be dry for football game in Pasadena). Rainfall from the first “storm” should be mostly under a quarter inch (could be a little higher parts of Santa Barbara County). I wouldn’t be surprised if most areas away from the mountains received under a tenth inch rainfall (latest ECMWF run doing this).
After looking at various model output, I decided to spread the “slight chance” for showers throughout the Sunday period (originally bumped up probabilities slightly for Sunday evening). A weak but “flat” ridge of high pressure aloft may cover the Southland for that day. A secondary storm front is predicted to reach southern California sometime on Monday (as early as the daylight morning or as late as late evening). This storm should be connected to different tributary of the atmospheric river, but the models disagree on how long and where the axis of the atmospheric river will run through the Southland (model consensus slightly favoring through San Luis Obispo/Santa Barbara Counties). This storm is expected to exit the region by late Tuesday or even as late as Wednesday morning (midday Tuesday through L.A. County if model solution average holds true). There is potential for storm totals to range between 0.30 to 0.80 inch (lowland areas away from the mountains…over 2 inches possible coastal facing foothills where the atmospheric river axis runs).
At some point, the atmospheric river shifts away from the Southland, and the effects of the aforementioned “cut-off” low pressure fades (low pressure weakens and migrates east). High pressure should develop by late Wednesday and stick around through the Thanksgiving weekend. It’s not expected to be a strong high pressure, but a predicted, weak off-shore flow may eventually promote slightly warmer than normal weather (one scenario supports enough warming to be similar to recent warm up). However, this amount of warming isn’t likely before the latter half of the holiday weekend (Turkey day probably will be no warmer than “normal”…upper 60s at UCLA).
I should point out the many of the longer range models predict a split flow pattern in the eastern north Pacific next week (polar jet stream splits into two separate streams). The southern stream should be the weaker of the two, but it may allow for an upper low pressure to approach southern California sometime over the subsequent weekend (30 November- 1 December). Some model scenarios include wet weather for much of the Southland (potentially much wetter than what is expected this weekend and early next week). Since the current model consensus keeps this wet weather scenario on the low probability side, I’m not seriously entertaining this idea for now. It looks odd anyway. The weak southern jet stream shouldn’t be intensifying the predicted storm as much as some models show. Also, the predicted weakness in the southern stream shouldn’t propagate the storm eastward so quickly. Anyway, more on this weather feature next week.
Next issued forecast/synopsis should be on Monday, 25 November.