Dense Fog in California, December 17 to 18, 2009

Heavy Tule fog returned to the central and southern San Joaquin Valley on December 14th, and increased in coverage each day through the 17th. The fog lifted into a stratus deck during the afternoon of the 17th and persisted through the afternoon of the 19th, as a ridge of high pressure became established over the area. The CHP Report that the stratus deck had penetrated the lower foothill elevations causing visibilities near zero in dense fog, especially in the Fresno County foothills. The stratus started eroding from the southern San Joaquin during the late morning of the 19th, then kept clearing toward the central part of the valley by the afternoon. Some patchy fog was observed on the night on the 19th-20th, and again on the night of the 20th-21st.