| Day | Date | High/Low | Forecast |
| Mon | 18th | 65/50 | Some clouds but mostly sunny day. Scattered evening high clouds and possibly breezy at times. |
| Tue | 19th | 69/52 | Mostly sunny day with variable high clouds. Variable evening high clouds. |
| Wed | 20th | 73/53 | Sunny day. Mostly clear evening. |
| Thu | 21st | 76/54 | Mostly sunny day with some afternoon high clouds. Scattered evening high clouds. |
| Fri | 22nd | 72/54 | Partly cloudy day with high clouds. Partly cloudy evening. |
Synopsis
An “inside slider” type trough was passing through the Great Basin states today. Little precipitation accompanied this trough. Its circulation is also a little too far north for strong winds in southern California. It’s breezy this afternoon in the interior, but peak speeds are less than some recent, other “inside slider” passages. Northerly winds should increase tonight in the typical, Santa Ana wind prone areas (some breezy conditions already noted in some higher mountain locales). Peak mountain wind gusts should largely remain under 45 mph (lowland areas prone to Santa Ana winds shouldn’t see gusts exceed 35 mph).
An off-shore flow should be fully in effect tomorrow through late this week, but the computer models predict a rather shallow off-shore flow (lowest 2000 feet, at best). So, the warming effects by the off-shore flow should be more limited than many other off-shore flow events. While high pressure aloft is forecast to get moderately strong over the Southwest (Wednesday through Friday), the predicted lack of off-shore flow depth should keep temperatures from reaching the 80s, for the most part (wouldn’t rule out isolated readings into the low 80s in some coastal valleys). Some locales may reach the mid/upper 70s on Wednesday/Thursday (assumes shallow off-shore flow can delay daily, ocean breezes till mid-afternoon). At one time I had concerns of sub-tropical air mixing down to the ground and warming things well. However, variable high clouds (related to sub-tropical jet stream) and the expected shallow off-shore flow should make this scenario less probable, or so the numerical models insist.
Although the details of the weather remain to be hammered out, all the models predict a couple of storms tapping into an atmospheric river later this week. Significant precipitation is expected over southern Oregon and the northern third of California starting on Wednesday (on-off wet weather extending into the weekend). The model consensus shows the first cold front getting into southern California by sometime Saturday. A secondary front is expected to reach the Southland sometime Sunday or Monday. After that, today’s consensus leans toward multiple “inside slider” troughs for the remainder of Thanksgiving week (possibly through the holiday weekend). The implication would be periodic episodes of winds, especially in the interior. Some cold air instability showers (mainly higher mountains) would also be possible.
As I said, the details of weather remain to be discerned. It appears likely that widespread, wet weather will occur in the Southland sometime in the Saturday-Monday (23rd-25th) period. However, possible solutions run the gamut from a minor event to the first, significant, widespread precipitation event for southern California (this autumn). The middle of the road solution favors a couple of modestly, wet systems. The main uncertainty (for now) is whether storm dynamics with the predicted storms (Saturday and the one later Sunday or Monday) will stay mostly to the north of our region. The water vapor from the atmospheric river will remain just that if storm dynamics are lacking. The middle of the road solutions favor some storm dynamics, but rainfall (combined two storm total) would be mostly under half an inch in the lowlands away from the mountains. Assuming that “inside sliders” dominate later next week, little precipitation would accompany those systems. However, the predicted wind flow pattern would promote cooler than normal weather (any Santa Ana winds following any “inside slider” wouldn’t promote strong warming…too much polar air expected).
Next issued forecast/synopsis should be on Friday, 22 November.