December 20, 2017 – Lingering showers this morning, then dry weather through the remainder of the week. Breezy northerly winds tonight and Thursday. Winter Weather Advisory until 1 PM PST this afternoon for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
Discussion
Surface cold front is moving southeastward around 20 mph through NorCal early this morning and is presently just to the north of the Sacramento area. Precipitation has been on the light side for the most part with most of the valley seeing less than a tenth of an inch while the mountains have generally seen around a third of an inch or less. The exception has been eastern Butte County where persistent showers have left some areas with a half an inch to around an inch.
Showers will clear most of the remainder of interior NorCal this morning while a few showers are expected to linger at the higher elevations of the northern Sierra into the afternoon and early evening as the upper trough shifts from the PacNW into the Great Basin.
Dry weather returns tonight with breezy northerly winds and cold temperatures developing. Most Valley locations are expected to see overnight minimum temperatures in the upper 20s and lower 30s later this week. The coldest night is expected to be Thursday night into Friday morning with clear skies and a cold/dry airmass in place along with light winds. A little milder readings are expected Friday night as some high clouds begin to spill through the flattening West Coast ridge.
Extended discussion (Sunday through Wednesday)
Dry weather with light north winds is expected for Sunday and Christmas Day across NorCal as high pressure is in control. Daytime highs will be a few degrees above normal with chilly overnight lows.
Models continue to have little consistency past Christmas Day leading to a low confidence forecast for Christmas night into Wednesday. GFS continues to show a weak cold front moving across the area Tuesday, but has slowed from previous run. Looks somewhat similar to last night’s system, with precipitation mainly over the mountains, snow levels 4000-5000 feet, and only light amounts across the Valley. ECMWF has now trended dry during this timeframe with ridging remaining in control for most of the week. Given vastly different (choose your own adventure) model solutions, have stayed close to going forecast which hedges more toward somewhat consistent GFS. This means precipitation chances on Tuesday but mostly confined to higher elevations. Will continue to watch trends going forward given potential holiday travel impacts.