December 19, 2017 – Dry weather this week except for a fast moving storm system bringing Valley rain and mountain snow through the area tonight into Wednesday. Breezy northerly winds Wednesday night and Thursday. Winter Weather Advisory from 8 PM this evening to 1 PM PST Wednesday for West Slope Northern Sierra Nevada-Western Plumas County/Lassen Park.
Some high clouds spreading across the far northern portion of California, otherwise skies are clear across interior NorCal early this morning. Winds are generally light and most areas are running a few degrees cooler compared to 24 hours ago. Current temperatures range from the upper teens and lower 20s in the mountain valleys to the 30s across the Central Valley. Foothill and mountain thermal belts are in the 40s.
The strong ridge that has provided dry weather for much of December will flatten and shift east today as the cold trough off the BC coast moves southward. Precip will spread into NW California by midday, then a strong cold front will move rapidly southeast this evening spreading precip into the southern Sacramento Valley and across the northern Sierra Nevada.
This is a fast moving system and will bring a quick shot of precip to the area as the cold front and strong upper short-wave move through. Showers are expected to end across most of the area on Wednesday morning, but will likely linger into early evening along the northern Sierra crest. QPF amounts approaching an inch will be possible in the northern Sierra while most of the valley will see less than a quarter inch.
A Winter Weather Advisory has been issued for the northern Sierra, mountains of western Plumas County and Lassen Park from 8 pm Tuesday to 1 pm Wednesday. Snowfall amounts of 3 to 6 inches above 4000 feet with up to 10 inches over higher peaks. Locally gusty southwest winds are also expected, up to around 50-60 mph.
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Dry weather returns beginning Wednesday night 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.
Extended discussion (Saturday through Tuesday)
Dry weather will continue to for the weekend as area becomes sandwiched between ridging across the Pacific and a slider dropping south into the Great Basin. Models have backed off a bit on the pressure gradient across the area so have lowered wind forecast slightly, but still could see some breezy north winds across the area given the pattern. High temperatures will be several degrees above normal. Clear skies will allow for plenty of radiational cooling with cool overnight lows.
Attention then focuses toward Christmas Day and beyond. Models have come into better alignment over the past few runs with a trough approaching the West Coast sometime on Monday and moving across NorCal Monday night into Tuesday. As of now, system doesn’t look overly strong, but still plenty of uncertainty a week out. Regardless, have added a chance of precipitation to the forecast for Monday night into Tuesday, mainly across higher elevations. If this solution verifies, this may lead to some holiday travel impacts in the mountains. Something to keep an eye going forward.