Dense Fog in California, November 23 to 27, 2012

A weak upper-level disturbance brushed the northern fringes of the WFO Hanford warning/ forecast area on November 20th-21st. This disturbance brought 0.21 inch of rain to Yosemite Valley, with only around a hundredth as far south as Shaver Lake and into the Southern Sierra Nevada foothills near Mariposa and Oakhurst. On the San Joaquin Valley floor, no measurable rain was reported. The Merced Regional Airport did report a trace of rain, and this was the southern extent of the Valley rain from this storm.

Despite the lack of rainfall from the storm on November 20th-21st on the San Joaquin Valley floor, patchy dense fog developed during the morning of the 22nd. Initially the fog formed in the Los Banos area, but this patch of fog eroded from the north. Later, a larger patch of fog developed in the Reedley-Visalia area, and then spread northward and westward, lingering through much of the morning. At the same time, clouds banked up over the south end of the San Joaquin Valley. The combination of low clouds and fog kept central and south Valley highs in the lower to mid 60s, except for the Madera Regional Airport which only had a high of 58 degrees.

Dense fog redeveloped in the central and southern San Joaquin Valley during the evening of November 22nd, forming a ribbon along the Highway 99 corridor. This narrow band of fog drifted over the Valley floor, causing visibilities at most airports to fluctuate from near zero to a couple of miles. The fog became more widespread toward daybreak, persisting through the late morning of November 23rd.

The 24th saw fog develop later in the night, mainly after midnight. The fog lasted through daybreak, then began to lift by 8 AM in some locations; the fog had completely dissipated by 11 AM.

Widespread dense fog continued through November 27th across most of the San Joaquin Valley. On the morning of the 27th, dense fog contributed to a chain reaction crash involving two big rigs and one vehicle. Three people were killed in this crash. The crash occurred on Highway 152 about two miles south of Chowchilla.