January began with the central and southern San Joaquin Valley under a cold, dry airmass that moved into the region. An upper-level ridge over the east Pacific kept mostly clear skies over the central California interior, although an upper-level short-wave dropping into Nevada brought some high clouds over the region. This short-wave also brought gusty winds to the Kern County mountain areas, with gusts to around 50 mph recorded. Central and southern San Joaquin Valley lows for the morning of January 2nd fell into the mid to upper 20s in the coldest locations, and dense fog developed along the Highway 43 and 99 corridors from southern Fresno County to northern Kern County.
January 2nd saw an upper-level low drop south along the leading edge of the ridge. As the low moved to off Baja California, the ridge built into California. This created a stable airmass and resulted in the formation of dense fog in the central and southern San Joaquin Valley as well as patchy frost; the lows in the coldest parts of the central and southern San Joaquin Valley falling into the mid to upper 20s each day. Dense fog continued its reputation as the main winter weather hazard for the central and southern San Joaquin Valley, as a fatal collision occurred 3 miles southeast of Chowchilla in dense fog during the morning of January 4th. These conditions continued through the morning of the 5th, and then a strong upper-level low brought the first precipitation of the year that evening.
Heavy rain fell on the central San Joaquin Valley, with around an inch falling in parts of Merced and Madera Counties from the evening of January 5th into the 6th. Rainfall tapered off sharply to the south, with Bakersfield receiving only 0.15 inch. In the mountains and foothills, the snow level dropped to around 3500 feet, with snow falling as far south as the Grapevine. The system brought several inches of snow down to 5000 feet in the southern Sierra Nevada and up to a foot of snow at elevations above 7000 feet; a couple of spots had higher snowfalls, especially in the Yosemite National Park area. In the Tehachapi Mountains, 2.5 inches of snow fell at Bear Valley Springs.
The computer models had forecast a cold pool aloft to traverse the southern San Joaquin Valley during the afternoon of January 6th, but the pool tracked further south during the day and came inland over southern California, including southern Kern County. As a result, there was no convection over the central or southern San Joaquin Valley that afternoon.
The northwesterly flow aloft behind the storm kept upslope showers continuing over the Tehachapi Mountains into the morning hours of January 7th. An upper-level ridge built into the state the next day, bringing more patchy fog to the central and southern San Joaquin Valley.

Frost/Freeze in E CENTRAL S.J. VALLEY, CALIFORNIA
From 02-JAN-13 02:00:00 to 02-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in SW S.J. VALLEY, CALIFORNIA
From 02-JAN-13 02:00:00 to 02-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in SE S.J. VALLEY, CALIFORNIA
From 02-JAN-13 02:00:00 to 02-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in W CENTRAL S.J. VALLEY, CALIFORNIA
From 03-JAN-13 00:00:00 to 03-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in E CENTRAL S.J. VALLEY, CALIFORNIA
From 03-JAN-13 00:00:00 to 03-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in SW S.J. VALLEY, CALIFORNIA
From 03-JAN-13 00:00:00 to 03-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Frost/Freeze in SE S.J. VALLEY, CALIFORNIA
From 03-JAN-13 00:00:00 to 03-JAN-13 08:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Dense Fog in W CENTRAL S.J. VALLEY, CALIFORNIA
From 04-JAN-13 00:00:00 to 04-JAN-13 12:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Dense Fog in E CENTRAL S.J. VALLEY, CALIFORNIA
From 04-JAN-13 00:00:00 to 04-JAN-13 12:00:00 (PST-8)
Property damage: 500.00K
Crop damage: 0.00K
Reported by: Mesonet

Dense Fog in SW S.J. VALLEY, CALIFORNIA
From 04-JAN-13 00:00:00 to 04-JAN-13 12:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Dense Fog in SE S.J. VALLEY, CALIFORNIA
From 04-JAN-13 00:00:00 to 04-JAN-13 12:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Winter Weather in KERN CTY MTNS, CALIFORNIA
From 05-JAN-13 22:00:00 to 06-JAN-13 22:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Weather in S SIERRA MTNS, CALIFORNIA
From 05-JAN-13 22:00:00 to 06-JAN-13 22:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winter Weather in TULARE CTY MTNS, CALIFORNIA
From 05-JAN-13 22:00:00 to 06-JAN-13 22:00:00 (PST-8)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter