December 17, 2020 – A storm system will continue to impact Northern California today today; however, precipitation will eventually become more showery in nature. Drier weather fills in behind this system by the afternoon and evening hours, which will then continue through the weekend. Patchy dense fog will be possible during the overnight and morning hours Friday into the weekend.
Discussion
The main front has moved through most of interior northern CA this morning, with the precipitation band currently spanning from Gibsonville down to Newman. Behind this, post frontal showers are occurring which will linger through the rest of the morning and early afternoon hours. As the trough exits the region, it will slide down towards the Four Corners and allow for high pressure to build in behind it.
Northerly flow will develop during this transition phase which will make the fog forecast for Friday morning tricky. The north winds will likely limit the fog on the west side of the Sacramento Valley and also make it a bit more patchy across the rest of the Sacramento Valley. Where there is less north flow, such as in the western portion of the San Joaquin Valley and near the Motherlode, fog will likely be more widespread.
A system passing to the north will bring some high clouds to area Friday afternoon, otherwise quiet conditions can be expected. North winds look to be weaker Saturday morning and the fog will likely be a bit more widespread.
Highs throughout the period will generally be within a few degrees of average and chilly overnight lows will return tomorrow night.
Extended discussion (Monday through Thursday)
Some light precipitation will be possible across the northern mountains Monday night into early Tuesday as the tail end of a weather system moves across the area. Following that, ensemble guidance is in good agreement with high pressure yielding dry and mild weather with occasionally breezy north winds in the Sacramento Valley through through the remainder of the week.