If you look beyond the three-digit heat wave the Southwest has experienced, you may get the first glimmerings of a disaster for food prices. But it hasn't happened yet, and the Department of Agriculture says it will be another 60 to 90 days before we know if it will.

Even though the drought has reduced the spring wheat harvest drastically, the winter wheat crop is so large it will offset spring wheat losses, according to Howard Hjort, director of Agricultural Economics for USDA.

And even though thousands and thousands of chickens have died as a result of the heat wave, Hjort says they represent only 0.3 percent of the total broiler stock.

Poultry, pork and beef prices are up, Hjort said, but weather had nothing to do with that. "We've now finally seen evidence that hog producers and poultry producers are taking steps to cut back production because they've been losing money hand over fist," Hjort said. "Cattle feeders have been having trouble because poultry and hog supplies have more than compensated for less beef. This will make for sharp increases for July and August, but little of it is due to weather."

As for fruits and vegetables, most of the country's supplies come either from California, which has not experienced a drought, or from irrigated land in areas hit by drought. Locally produced and marketed produce has been affected in some places, Hjort said. "We've had a lot of precipitation above normal and deficiencies just a few miles away. That makes it very difficult to generalize." For example, the corn in North Carolina is in fine shape but the corn in upper Mongomer County, Md., which should have been knee-high by the Fourth of July, was only calf-high.

"So far," Hjort said, "the small upward price effect is due to the drought," but if the drought continues for 60 to 90 days, and if it spreads, then we could be in for some steep increases in food prices. "If the adverse weather patterns that have been affecting things in the Southwest move further north and east into major corn and soybean producing areas, they would affect the supply of feeds for animals, which in turn would lead to higher meat and food prices for 1981 and 1982."