Two Rare Snow Events in the San Joaquin Valley
Kevin Durfee, Meteorologist

San Joaquin Valley residents who hoped to see snow for Christmas had their wishes come true only five days before the holiday. Scattered snow showers during the late afternoon and early evening hours on the 20th caught many holiday shoppers by surprise as they walked out of the stores into a cold, wintry bluster that whipped large, wet snowflakes through the air and whitened car tops in the mall parking lots. Before the flurries diminished that Sunday evening, as much as a couple of inches of the white stuff had accumulated on some grassy areas in the Central Valley. Local television stations even captured video of neighborhood kids in the Fresno area throwing snowballs at each other that evening. For many other children and adolescents, it was probably the first time they had witnessed snowfall in the San Joaquin Valley.

According to weather archives, it had been 30 years since snow of this magnitude fell in the Fresno area. While trace amounts were observed in December 1975 and again in December 1990, the widespread nature of the snow that fell December 20th was more comparable to the snow that fell, ironically, thirty years earlier (December 20th, 1968).

Then, just when San Joaquin Valley residents were about ready to retire the thought that snow could ever fall again in one season, let alone in their lifetime, another, even LARGER snow event occurred only five weeks later.

This particular snowstorm, however, was exclusive to the southern San Joaquin valley, and produced a slushy 2-6 inch accumulation from southern Kern county to western Tulare county during the early morning hours of January 25th. The snowstorm forced many area businesses and schools to close for the day, caused nearly 100 fender-bender car accidents and local power outages. By all accounts, (weather observers, local media and state emergency management officials) the snowfall was unprecedented and historical. In Bakersfield, it was the first measurable snow in nearly 25 years and the heaviest snow since records began in 1928. In Visalia, where just under three inches of the white stuff fell, it was considered the heaviest snowfall this century. Only two other snows in Visalia's weather archives produced similar amounts –March of 1922 and December 11, 1932.

What weather parameters came together to produce these freakish snow events in the San Joaquin Valley? The synoptic summaries and the accompanying maps offer further explanation of these two events.

DECEMBER 20, 1998

A deep, upper air low pressure trough was centered over northern California. Arctic air originating over the Canadian Yukon was getting wrapped around the center of the upper air low, but not before traversing over the Pacific Ocean where upper air winds picked up some moisture. Temperatures were extremely cold aloft with freezing levels existing only several hundred feet above the Valley floor. The catalyst that produced a very unstable atmospheric environment over the San Joaquin Valley that afternoon was a vigorous upper disturbance off the northern California coast. As this disturbance tracked toward the central San Joaquin Valley, sunshine warmed the air in the lowest levels and caused rapidly developing cumulonimbus clouds. By late afternoon, scattered heavy showers produced gusty downdrafts of cold air and turned precipitation into large, wet snowflakes. In some of the heavier, longer-lasting showers, snow accumulated a couple of inches on grassy areas and car tops. By 9pm, the air had stabilized and skies cleared in most areas.

JANUARY 25, 1999

This time, a deep upper air low pressure system was centered off the California coast near Point Conception and was drawing up a rich supply of moisture in the mid and high levels of the atmosphere. A strong disturbance aloft associated with this low was producing plenty of convective precipitation across central and southern California. The air aloft was similarly cold to the December 20th storm. However, southerly winds that carried copious moisture up from Baja were descending into the Southern San Joaquin Valley, and strong convective downward currents that produced heavy precipitation also carried colder air down to the valley floor with its descent. As a result, rain had changed over to heavy, wet snow during the early morning hours and continued through the morning commute period.