When towering thunderstorms threaten, Lloyd Huffman fires up the Doppler radar he built from military surplus parts.

Unlike conventional radar, Doppler can detect the speed and direction of raindrops within a storm, allowing Huffman to spot rotation that could indicate the formation of a tornado and warn the weather service and local authorities."It just kind of grew over the years," Huffman said of the radar he's been tuning up in anticipation of the April-through-June tornado season.

With a few differences - cost being a big one - Huffman's S-band Doppler is like the NEXRAD system being tested at the National Severe Storms Laboratory in Norman, Okla., and planned for nationwide installation in the coming decade.

Huffman asked the laboratory for the plans to build his Doppler. "After they believed us, they gave us the information we needed," he said.

Huffman built his first radar station in 1976, using World War II surplus Army and Navy equipment and putting the antenna on the local hospital. He began working in 1979 on the present station, which became Doppler-capable in 1985.

"I'm very interested in thunderstorm structure," he said.

Huffman, 42, has a degree in physics from North Texas State University. After working as a communications systems designer for Motorola in Dallas, he returned to his hometown in 1974 and started his own communications business.

Huffman and his crew staff the station whenever severe weather is possible. He's in microwave link with WFAA-TV in Dallas and talks by radio with National Weather Service meteorologists in Fort Worth.

The radar can detect rain intensity for several hundred miles but can process Doppler information for only about 150 miles. The radar pulses about 1,000 times a second, sending a beam about 11/2 degrees wide. The reflected data is sorted into "range bins," each 150 meters deep.

A computer remembers and compares the location of raindrops, allowing it to determine the speed and direction. "The speed of rotation is the rate of change of phase angle - that's the velocity," Huffman explained.

The computer assigns a color to each velocity - red indicates movement toward the station, green is away - painting a picture of the storm. The right combination of colors on the southwest corner of a thunderstorm provides a "meso-cyclone vortex signature," indicating rotation.

After several minutes of observation and tilting the radar beam up and down, Huffman might spot elongation into a cylinder that could mean a tornado. He then makes sure the National Weather Service knows and uses police-band radios to notify authorities in surrounding counties.

Huffman has the money to upgrade his system by speeding up the computer and adding memory but said, "It takes a lot of time. It's a mechanical, electrical, electronic and computer project - a little of everything involved in in it."

He calls the project "a good community thing to do."

It is funded by the Navarro Community Foundation, money from wealthy families that profited from two oil booms. Operating expenses are covered by WFAA-TV in Dallas, which shows the radar during weathercasts. The station sits on an acre provided by the Corsicana Independent School District. A donated 80-foot oil derrick supports a 25-foot-diameter Fiberglas dome containing the 161/2-foot dish antenna. At the base of the derrick is a building housing the radar and the computer equipment that makes it different from conventional systems.