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On December 19, 1998, an arctic airmass began moving over Central California. The resulting cold air pool in the lowest levels of the atmosphere led to a devastating freeze to crops, especially citrus, in the Central and Southern San Joaquin Valley from December 20th through December 27th. Preliminary estimates of direct crop value loss are $555 million, with the total direct and indirect economic loss to the area estimated at $2.5 billion. The Central and Southern San Joaquin Valley counties account for $12 billion (Calendar Year 1997 gross value) of California's agricultural industry. The largest percentage of area crop losses are from damage to lemons and oranges. Several other unharvested fruit and vegetable crops, including avocados and broccoli, also suffered widespread damage. Many other crops, such as olives, may have been affected. This 1998 freeze event was eerily reminiscent of the freeze during the exact same time period in December 1990. In fact the coldest morning was December 23rd in both years. The 1990 freeze event destroyed 90% of the citrus crop, virtually all of the unharvested crop at the time of the freeze, and killed some mature groves. In the 1998 freeze, extended intervals of sub 27oF temperatures (a temperature threshold for damage) did extensive harm in citrus growing areas from Kern County northward to Madera County in the Central Valley. Meteorological Evolution Description Following early December frontal passages, high pressure established itself along the west coast of the United States during the 2nd week of the month. High pressure aloft effectively shielded Central California from any significant Pacific frontal passages and any further temperature-moderating, low-level moisture and stratus. Some minor frost occurred during this time period, but area agriculture was able to effectively protect against any citrus crop damage. Late on the 13th of December in Central California, very light precipitation occurred from a weakening cold front. By the 15th, a closed low aloft developed over the California-Arizona-Baja California border. The resulting offshore flow over Southern California brought downslope warming and drying conditions to the Central and Southern San Joaquin Valley by the 17th. Fresno reached a high temperature of 67oon that date while Bakersfield climbed to 71o. Both stations had low temperatures in the mid 30s before and after the relatively high daytime maximums. After sufficiently drying the lower atmosphere the closed, cut-off low filled and moved east by Friday the 18th. The coldest arctic air arrived in three steps between Saturday the 19th and Monday the 21st. By Saturday the 19th a high amplitude ridge along 145 West Longitude resulted in a high meridional (north-to-south) flow pattern linking arctic air to the Pacific Coast States and some cooler air entered the San Joaquin Valley. Brief temperature durations less than freezing were recorded in the Central San Joaquin Valley on the 19th. Despite some residual low level moisture that moderates radiational cooling, cold air advection put temperatures for the night of the 19th/morning of the 20th from 25oF to 28oF in the Central San Joaquin Valley. Frost sensitive plants were lost in Northern Merced County and caused citrus growers to struggle in their protection efforts throughout the Central and Southern San Joaquin Valley. The Southern San Joaquin Valley was spared the onset of freezing for one day due to upslope winds and residual cloudiness. Sunday the 20th had the arctic front along the West Coast. The flow pattern aloft tracked from Vancouver Island, British Columbia, slightly offshore before curving into Central California. The center of the surface high was well inland and not moderating to a large extent. Some minor modification of the arctic airmass aloft did occur from a brief East Pacific over-water trajectory before the colder air reached the Central and Southern San Joaquin Valley. This accounted for sea-level snowfall on the 20th similar to a "lake effect." Temperatures at 500 millibars had dropped to -25 C as depicted by the Oakland temperature sounding. Low-level moisture, with radar tops of no more than 10,000 to 12,000 feet, produced snow showers in the Central San Joaquin Valley with snow accumulations of over an inch on the Valley floor in the vicinity of Fresno. While cloud cover persisted in the Central and Southern San Joaquin Valley during the day, skies cleared shortly after sunset and strong radiational cooling rapidly lowered temperatures to below freezing. Overnight lows reached the lower to mid 20s by the morning of the 21st. The coldest morning was Wednesday the 23rd when minimum temperatures dropped to 16oF and durations of 16 hours or more of temperatures less than 28oF were observed in many locations. One more surge of arctic air moved over Central California on Monday December 21st, even though its passage was cloud-free. As depicted by the Oakland upper-air sounding, the lowest temperature at 500 millibars occurred on the morning of the 22nd when -30oC was observed. The freeze event was now well underway with cold, dry air in the Central and Southern San Joaquin Valley. The coldest morning of the event was Wednesday the 23rd when minimum temperatures dropped to 16oF and durations of temperatures less than 28oF of 16 hours or more were observed in many locations. (See the table at the end of this article). Radiational cooling was at a maximum with long periods of clear skies each night, cold, dry air trapped in the basin (dew points in the mid 20s), and light wind. Overnight freezes continued through the morning of December 30th, although the coldest nights most critical to agricultural ended, generally, on the 27th. Severity of the Freeze To place a historical perspective on the 1998 8-day Freeze event versus the 1990 10-day Freeze for the San Joaquin Valley, consider the following information about December 1990. In addition to the loss of the citrus and other freeze sensitive crops in 1990, mature orange groves were killed, eliminating production for several years in areas of Fresno, Tulare, and Kern Counties. Temperature durations less than 20oF were in excess of four hours in many locations. Lowest readings in the 11oF to 15oF range were common in the Southern San Joaquin Valley, with a couple of unofficial reports of single-digit minimum temperatures near the Kern-Tulare County Line. The range of reported minimum temperatures and durations of readings less than 28oF for this 1998 Freeze, where known, is provided on a day-to-day basis for the period from Sunday, December 20, through Sunday, December 27 (see table below).
CA Zone #20: San Joaquin Valley portion of Merced, Madera, and Fresno Counties CA Zone #21T: San Joaquin Valley portion of Tulare and Kings Counties CA Zone #21K: San Joaquin Valley portion of Kern County Dan Gudgel, Warning Coordination Meteorologist
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 The Weather Cockpit Get access to many live and historical weather data through this cockpit. From one single page, you can access numerous readings throughout California with a single mouse click. The Andean Weather Page
NWS Monterey Bay Office Yahoo Weather Accuweather Accuweather seems to be one of the few providers who has a true microclimate forecast according to zipcode. See for example Monterey Bay microclimates. The Weather Channel This site has easy to navigate graphics, including outlines of the jet stream, an item that should be watched closely throughout the winter months to predict the likelihood of freezes. |
