Arizona weather model aids lettuce growers

with better forecasting of icing conditions


A weather model developed by the University of Arizona (UA) at Tucson is providing timely weather forecasts and monitoring information to improve crop efficiency in winter lettuce production.

“The goal of the Lettuce Ice Forecast System is to provide high resolution temperature and ice forecasts and monitoring to help growers better manage and schedule harvest crews when ice forms on lettuce during nighttime freezing conditions,” says Paul Brown, UA biometeorologist and director of the Arizona Meteorological Network.

The UA’s Atmospheric Science Department, under the leadership of Mike Leuthold, created the Lettuce Ice Forecast System model for use in Yuma County, Ariz.

“Weather models are the nuts and bolts behind forecast maps,” Brown says.

About 90 percent of the U.S. winter vegetable supply is grown in Yuma County and neighboring Imperial County, Calif., in this low desert region. Harvested iceberg, romaine, and leaf lettuce in Yuma County totaled about 56,000 acres during the 2008-2009 crop year, according to the National Agricultural Statistics Service.

Ice formation in winter lettuce is common, occurring on average about five times during December and about 10 times each in January and February, says Kurt Nolte, UA’s Cooperative Extension agriculture agent and director in Yuma County, who has worked closely with Brown on the project.

“Growers want to know beforehand about the predicted location and severity of a freeze,” he says.

If ice forms on lettuce, the plant can’t be harvested until the ice thaws in the later morning hours, Brown says. Harvesting iced lettuce can damage cell tissue and create quality issues.

The new UA lettuce ice system has a two-pronged approach. The first is a 48-hour weather forecast available online, which creates a hourly forecast and a “movie” illustrating the predicted hour-by-hour weather change for the next two days. 

“Every hour, you can watch the sequence of cold air development and the cool temperatures draining down into the river bottoms online,” Brown says. “During the day you can watch the temperatures warm as the sun comes up.”

The second component focuses on critical real time information gathered from 10 weather monitors located across the county that capture current temperature, humidity, dew point, and wind speed information, which is immediately available online.

While many think of Arizona as a place to escape extreme winter temperatures, nighttime temperatures can drop below the freezing mark.

The just-ended 2010-2011 lettuce season included three significant cold snaps. The coldest recorded ground temperature of 18 degrees occurred Feb. 3 in the South Gila Valley and the South Yuma Valley. The deep freeze caused significant damage to the leaf lettuce crop; in some fields, the entire crop was lost.