Most of the higher coastal terrain, including the San Diego County mesas, was obscured by fog this morning, with visibility in several places reported to be less than a quarter-mile, according to Weather.Gov
Given the 10 degree Celsius marine inversion intensity and the low sun angle of December, the low clouds that accompanied it dissipated slowly and are unlikely to clear most beaches by the end of the day.
There will be cooling along the coast owing to some depth of the marine layer caused by some low clouds extending up over 1000 feet above sea level, and there will be cooling inland due to 2-3 degrees Celsius cooling above the marine layer due to the deepening of the marine layer.
With some moisture and instability below 700 MB, a few mid-level altocumuli will be seen today, but nothing major.
A pair of closed upper lows were over the Pacific to the west/southwest of San Diego, while a zonal jet stream was traveling over the US-Canada border at rates more than 150 knots, bringing the two countries together.
The low-pressure systems will slowly move eastward, with the first low-pressure system passing over San Diego late tonight. The primary consequences will be deepening the marine layer and colder temperatures on Friday.
The majority of the thick fog should be pushed eastward into the lowlands due to this. The second closed low is advancing southward, as seen by the ensemble spaghetti plots, with the majority of the ensemble members' variability occurring east-west rather than north-south, which suggests uncertainty in time.
The low passage to the south will most certainly happen around Saturday night, resulting in northeasterly flow aloft here from Saturday night through Sunday night or Monday morning, with surface flow from the northeast throughout that period.
It is expected that MSLP gradients will not be as strong as last year, with a stronger northerly versus northeasterly angle. As a result, the northern part of the Inland Empire and the area surrounding the Santa Ana Mountains will likely experience wind, with San Diego County being the least likely.
Currently, we have high-resolution WRF information, which indicates winds averaging 15-25 miles per hour, with gusts up to 40 miles per hour in the windiest regions.
As a result of the trajectories, the humidity will be reduced, perhaps 10-15 percent lower onshore. It's expected that inland temperatures will reach the 80s.
However, the intense sun angle, low humidity, and cold evenings in a wind-sheltered place will help minimize the heat and humidity.
Low clouds and fog will break away along the shore by this afternoon, and temperatures will begin to decrease in most regions by this evening.
The cooling trend will continue through Saturday as the marine layer deepens, resulting in low clouds and fog that will be particularly thick in the evenings and early mornings and spread into the valleys throughout the night and early mornings.
A light Santa Ana wind event will occur Sunday, bringing warmer and drier weather back to the region. However, a low-pressure trough moving through the area to the north will bring colder Weather with low clouds and fog by Tuesday, with a tiny possibility of a few showers on that day.