A new forecasting technique using multiple Doppler weather radars to track water vapor could produce forecasts of severe weather just moments before it starts pouring.

A project at the National Center for Atmospheric Research in Colorado measures changes in the speed of radar signals caused by refraction of the signal by water vapor.

If the technique proves successful, scientists said today, it could be adapted in the next few years by the national network of Doppler radars.

Regular Doppler radar spots raindrops or hailstones, but not the moisture that leads to precipitation.

The new setup can detect strong contrasts in the density of water vapor, which often indicates imminent intense storms.

"Nobody's ever seen such high-resolution data on moisture before," said NCAR scientist Rita Roberts, who heads the project. "We believe this could greatly help forecasters predict where heavy rains might develop."

The method could "help forecasters pin down the locations and timing of storms that might rage a few minutes to a few hours later," according to an NCAR statement.

The project is named REFRACTT (Refractivity Experiment for H2O Research and Collaborative Operational Technology Transfer). It will be tested through Aug. 11 in Denver.

"Low-level moisture is the key to our weather here, especially during the summertime," said Larry Mooney, meteorologist in charge at the Denver office of the National Weather Service. "We're really excited about the REFRACTT data."

Along with four overlapping radar stations, REFRACTT relies on computer models, satellites and weather balloons. It's funded by the National Science Foundation and is the brainchild of Frederic Fabry of McGill University.

Copyright © 2006 Imaginova Corp. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.