Dense Fog in California, January 3 to 4, 2012

The year 2012 began with warmer than average temperatures due to a dominating upper-level ridge of high pressure. Temperatures were well above average in the Southern Sierra Nevada foothills where highs reached into the upper 70s. However, temperatures elsewhere were generally milder in terms of highs; reaching into the 60s in the Kern County deserts and the central and southern San Joaquin Valley, with the exception of the south end. Maximum temperatures reached into the lower 70s in this part of the Valley, including in Bakersfield, as downslope winds allowed for further warming in this area. Rather cool overnight lows continued over much of the valley, and patchy dense fog briefly developed during the early morning hours. An upper-level short-wave trough brought high clouds to the region during the night of January 1st-2nd, mostly suppressing fog formation in the San Joaquin Valley, although some fog did form in Merced County during the evening of the 1st.

At the surface, a strong offshore flow developed over the Tehachapi Mountains. The winds funneled through the Tehachapi and Tejon Passes, gusting to around 45 mph and warming the south end of the San Joaquin Valley to well above normal temperatures. The high at Bakersfield on January 2nd was 82 degrees, not only setting a record for the date but also tying the warmest day on record for January (January 16th, 1923 and January 31st, 1984).

Highs at many locations in the south end of the San Joaquin Valley reached well into the 70s. Representative highs for the south end of the San Joaquin Valley are listed below (including airports and NWS cooperative program sites):|Taft: 78 Wasco: 77|Porterville: 75 Arvin: 75|Delano: 74 Avenal: 74|Visalia: 70|However, locations in the central part of the San Joaquin Valley, such as Fresno, Madera, and Merced, only reached into the 60s during the 2nd due to high cloud cover that occurred during much of the day.

Dense fog returned to the central San Joaquin Valley on January 3rd, extending south from Merced to northern Tulare County. Visalia remained on the edge of the fog, and saw visibilities fluctuate through the early morning. On the 3rd and 4th, temperatures moderated behind the previous upper-level disturbance and the upper-level ridge once again began to take control. Another upper-level trough passed over the area late on the 3rd; it was at least enough to prevent widespread fog development, except dense fog managed to formed in the low-lying center of the valley, including Hanford, Selma, and areas west of Fresno (such as Kerman).

Dry cold fronts moved through the central California interior on January 5th and 6th. While lowering temperatures, these systems did nothing to break the dry spell which began on November 20th, 2011, the last day most of the central and southern San Joaquin Valley had seen measurable rain.