Dry weather expected today with areas of morning valley fog. Another weak weather system will bring a chance of light precipitation later Thursday into early Friday. Dry and milder weather with night and morning valley fog expected over Christmas weekend, then a pattern change to wetter weather next week.

Discussion

Satellite imagery shows partly to mostly cloudy skies north of Sacramento while skies are clear to the south. IR difference imagery shows the footprint of fog in the San Joaquin Valley surging northward. The visibility dropped to 1/4 mile in Modesto around 2 AM, and it’s going to drop shortly in the Stockton area. Visibilities will likely lower significantly in the Sacramento area as well by 5 AM affecting the morning commute. A Dense Fog Advisory has been posted until 11 AM.

The amplified ridge over the eastern Pacific will move overhead today into Thursday providing dry weather with milder temperatures across most of the region. Quite a bit of mid and high clouds are forecast to move through the ridge into NorCal, however forecast soundings indicate enough mixing for the valley that all day fog/stratus is not likely.

Some valley fog will likely again tonight, but probably not as much as this morning given the increased cloud cover. Another weak system, similar to yesterday’s, is forecast to move across the area Thursday night into Friday morning with light QPF. The cold airmass from earlier in the week has modified and snow levels will likely be above 6k ft, so little to no impact to travel is expected.

Strong ridge still forecast over NorCal during the Christmas weekend resulting in unseasonably mild temperatures and areas of night and morning valley fog. Some light warm-advection precipitation will be possible Saturday across the far northern portion of the state as a system rides over the top of the building ridge.

Extended Discussion (Sunday through Wednesday)

Upper level ridge will be in place for the start of the extended period. This will bring quiet conditions for the Christmas Holiday. We will see the chance for morning fog and with a strong inversion (warm air aloft) in place we may see the fog lift into a stratus layer for the afternoon. The upper level ridge will begin to push east on Monday as deep troughing begins to dig out of the Gulf of Alaska. This will bring a pattern change for NorCal next week. High confidence in rain and snow returning but low confidence in exact details such as timing and amounts.

At this point Tuesday is looking wet with unsettled weather continuing for the week. Highs will be mild for the period but will remain cooler in any areas that see stratus develop on Sunday.