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December 30, 2018 – Cool nights, dry weather and some wind will wrap up 2018. New Years Eve will see another round of increased winds that may impact holiday festivities and mountain recreation. 2019 will also start dry, with he first rain/snow event for 2019 looking like next weekend.
A narrow band of high clouds associated with an upper level cold front is moving into NorCal from Oregon early this morning. The band of high clouds will be through the region this morning, otherwise clear skies will continue to prevail. Current temperatures are cooler across much of the region compared to 24 hours ago with the exception of the mountains and northern San Joaquin Valley. Patches of fog and frost will be more likely around sunrise as dew points are mainly in the lower to mid 30s through the Central Valley.
Sunshine and mild temperatures will continue today. The next system will dig southward through the Great Basin tonight and close off over the Desert Southwest on Monday. A few flurries may occur along the Sierra crest late tonight and early Monday.
The brunt of cold air will move southward to our east, but tightening surface gradients and strong cold-advection on the backside of the system will result in strengthening north wind late tonight into Tuesday. The strongest winds in the valley are expected Monday, then the focus of stronger winds will shift to the northern Sierra for Monday night into early New Year’s.
Lighter winds return by mid-week with mild daytime temperatures and cool nights.
Extended Discussion (Thursday through Sunday)
Broad upper ridging will extend over the southwestern U.S. through Friday afternoon which maintains the period of dry weather. Gradually a pattern shift should unfold as amplifying flow over the Pacific makes its way toward the West Coast. A lead shortwave is depicted by the 06Z/00Z GFS and 00Z ECMWF with the associated stronger lifting mechanisms remaining closer to the California/Oregon border.
In its wake, a more amplified second feature materializes off the coast which bolsters the opportunity for more widespread precipitation during the weekend. Timing differences continue to plague the forecast with the 06Z GFS a step ahead of the 00Z ECMWF.
There is reasonably high confidence in this pattern change which suggests an intermittently wet period Saturday afternoon into Sunday. It looks like a rather progressive pattern is setting up across the Pacific which would afford an additional chance for precipitation into the following week. Regarding the temperature forecast, highs should stay very near climatology with overnight lows a bit milder later in the week given increasing cloud cover across the region.