Dirty ridge of high pressure keeps temperatures above normal

December 23, 2017 – Dry weather into the weekend with chilly overnight lows. A weak weather system will bring a few showers, mainly to the mountains, Sunday into Sunday night.

Discussion

Extensive cloud cover has blanketed northern California under a dirty ridge of high pressure. This cloud cover has aided in preventing strong radiational cooling; temperatures across the region are running upwards of 15 degrees warmer than 24 hours ago. Additionally, a bit of mist is present across much of the Valley, but visibilities are for the most part in decent shape at this hour.

High pressure will remain in control across the region through early Sunday, bringing dry weather and mostly light winds. High temperatures today will be in the upper 50s across the Valley, with 40s and 50s over the mountains.

The upper ridge will flatten on Sunday as a disturbance moves into the Pacific Northwest. Both the GFS and NAM have been hinting at a few showers developing on Sunday, mainly along the northern mountains but may also drift into the northern Sac Valley.

This system shifts eastward on Sunday night, with dry weather expected across the region on Christmas Day and Tuesday. Temperatures will warm a few degrees as high pressure rebuilds over Northern California.

Extended discussion (Wednesday through Saturday)

Fair skies with daytime temperatures a few to several degrees above normal on Wednesday as upper level high pressure rebuilds over the west coast. Overnight lows should come in near normal for this time of year.

Upper ridge slips inland over the west coast Thursday and Friday bringing even a bit more warm up of daytime high temperatures. Generally light northerly winds are expected for the second half of the week. Extended models show a northern Pacific system moving through the Pacific Northwest on Friday night. GFS keeps any precipitation north of the forecast area while ECMWF drops light precipitation across the northern CWA. Will keep extended forecast dry for now and wait for better model consistence but should at least see a little cooling going into next weekend.