UPDATE2: All restrictions have been removed. (note this story in the Ukiah Daily Journal has a little more info on why the road was closed.)

UPDATE: Road open to vehicles with chains or 4 wheel drive with snow tires now.

According to the Caltrans site, Highway 101:

IS CLOSED FROM 3.7 MI NORTH OF THE EAST JCT OF SR 20 TO WILLITS

(MENDOCINO CO) - DUE TO SNOW - MOTORISTS ARE ADVISED TO USE AN ALTERNATE ROUTE

CHAINS ARE REQUIRED ON ALL VEHICLES EXCEPT 4-WHEEL-DRIVE VEHICLES WITH SNOW

TIRES ON ALL 4 WHEELS FROM 2.5 MI NORTH TO 6.2 MI NORTH OF WILLITS

(MENDOCINO CO)

CHAINS OR SNOW TIRES ARE REQUIRED FROM 4.6 MI NORTH TO 10.6 MI NORTH OF

LAYTONVILLE (MENDOCINO CO)

Check out the webcams for more up to date information. 
Here are the Humboldt ones.  Here are the cameras located on Hwy 101 at Willits. 

Photo from the Traffic cam located north of Willits.