The forecast for Aug. 19, 2009, called for cloudy and unseasonably cool conditions, with the possibility of thunderstorms. Nothing like the steamy swirl that can spawn tornadoes.

So when one dipped out of the sky over Minneapolis, 19 minutes passed before a fire official's report triggered warning sirens. By then, the twister had already torn up a south Minneapolis neighborhood and damaged a landmark record store, a Lutheran church and the Minneapolis Convention Center.

Several months from now, such airborne ambushes may be less likely. The first significant upgrade to U.S. weather radar in nearly two decades, coming to the Twin Cities office of the National Weather Service (NWS) in September, promises to give forecasters a way to accurately identify tornadoes in progress even when existing radar can't.

The enhancements may also prove valuable every day, not just when violent tornadoes are appearing. By identifying the size, shape and variety of bits of precipitation in clouds, they'll allow forecasters to determine far more precisely than they can now whether it's rain, hail or snow soaring toward us, and how much. And that's expected to improve warnings, emergency planning and even workers' commuting plans.

"Doppler on steroids" is how meteorologist Paul Douglas described the improved version.

"It's significant in that it removes a lot of the uncertainty that is normally associated with looking at radar," said Paul Schlatter, Washington, D.C.-based lead trainer for the Weather Service's switch to the new radar.

Doubling Doppler

Dual-polarization radar, as it's called, won't replace Doppler radar, which was installed at the NWS Chanhassen office in 1995 and is a long-standing promotional tool for television stations. But it will basically double what Doppler can do, allowing forecasters to see what's in the clouds in two dimensions rather than just one.

That could lead to better forecasts for snowfall and flash-flooding and, by extension, improve public safety and transportation during storms, said Dan Luna, meteorologist in charge of the Twin Cities NWS office.

"This should create an order of magnitude improvement estimating the size of hail stones from a given thunderstorm, and also rainfall and snowfall amounts, based on the ability to estimate raindrop/snowflake size more accurately," Douglas said in an e-mail. "It's a much more sensitive filter, giving us greater confidence that a specific storm is producing ice versus snow versus a cold rain."

Dual-polarization can also sort out radar "clutter" such as birds and clouds of insects -- indeed, "more than 99 percent of non-weather targets," according to the Weather Service.

That ability will likewise enable the radar to detect flying debris high above the ground -- a key identifier of a tornado on the ground, even when it might be hidden from eyewitnesses by darkness or rain. Doppler radar's "hook echo," long regarded as accurate enough to trigger tornado warnings even without a ground sighting, only indicates rotation that might be a tornado. In that respect, "dual-pol" radar has the potential to reduce the number of false tornado warnings the weather service issues, Douglas and Luna said.

Nationwide project

The upgrade is part of a $50 million project to install the radar enhancement at 150 of the Weather Service's major forecasting stations across the country. Installation at the office in Chanhassen is scheduled for September, after upgrades at La Crosse, Wis., in late April, Duluth in June, Grand Forks, N.D., and Aberdeen, S.D., in July and Sioux Falls, S.D., in early September. Des Moines is in line for an upgrade in October. The equipment will be housed in the same white spheres at weather stations that now house Doppler radar dishes.

Each location will lose its radar operations for up to two weeks to accommodate the upgrade. The schedule was designed to avoid causing major gaps in radar coverage across the United States and to occur before or after each location's usual severe weather season, Schlatter said.

Radar displays aren't expected to look much different from what the public can now see on weather websites. But once dual-polarization is up and running, the public will find several dozen applications available. Users will be able to see the rates at which rain or snow are falling over a given area, as well as at what altitude frozen, falling precipitation is melting -- an important consideration for pilots, in particular.

The Weather Service expects it may be a year before forecasters have a full understanding of what the new radar can do.

But Luna is confident dual-pol will quickly become a valuable new set of eyes on approaching storms.

"We believe it will be better than having a spotter there," he said. "I wish we had it now."

Bill McAuliffe • 612-673-7646