In an age of Doppler radars, satellite surveillance and 24-hour-a-day weather broadcasts on cable, the statistic stands out starkly - 18 out of more than 130 people killed in this spring's deadly tornado season got no warning before they heard the roar of the wind.

Two storms in this deadliest tornado season in two decades underscore the holes that still exist in the nation's warning network for severe weather.In one March disaster in Georgia, weather forecasters were caught off guard by a rare twister that sprang from a faltering line of thunderstorms and killed a dozen people before dawn near Gainesville in northwest Georgia.

Saturday night, most people in a South Dakota hamlet apparently didn't receive a warning issued 12 minutes before catastrophe struck.

A recently released internal assessment of the warning provided by the National Weather Service's Peachtree City, Ga., office in the hours before the March 20 twister concludes that the storm was virtually impossible to spot until just moments before it touched down. As a result, severe weather warnings came nearly a half-hour after the destruction started.

"This was a very difficult, rare type of storm that grew out of a system that everyone down the line agreed showed all signs of diminishing," said Stephen Cooper, acting chief of the meteorological services division at the NWS Southern Regional Headquarters in Fort Worth, Texas, who headed the assessment team.

After reviewing radar records, the team reported that the mesocyclone was visible to a forecaster at Peachtree City starting around 6:21 a.m., but that special alarms did not sound for several subsequent readings, until about the time damage reports from winds up to 206 miles per hour first started coming to the office by phone, at roughly 6:40 a.m.

"The storm spun up so rapidly and moved so fast, 50 to 60 mph, that it was over before the warning systems could kick in," Cooper said.

The report also indicates that an automatic range adjustment feature on the radar system may have inhibited detection of the tornado by computerized systems, and that communications with local emergency officials and the weather office were poor - the forecaster had to compete with 911 calls to contact the storm area.

Cooper said the storm may have developed along a weak weather boundary cutting across Hall County, Ga., whose upper third had been soaked by rain and cooled markedly during the night, with the rest of the county staying warm and dry.

"This storm is going to be a case study for our radar school of (the) kind of storm that can arise suddenly from a weak system," Cooper added.

The NWS had Spencer, S.D., under a tornado watch for more than four hours before Saturday night's twisters wiped the town off the map and had issued a tornado warning some 12 minutes before disaster struck.

Accounts from survivors to various news organizations indicate that few, if any, heard broadcasts of the warning. With power knocked out by advancing thunderstorms, warning sirens didn't sound, and most residents reported just barely having time to take shelter in basements.

South Dakota Gov. Bill Janklow, surveying the aftermath of the storm, suggested that given the power of such an event, "a warning system's irrelevant."

Just last week, Vice President Al Gore launched a series of public service announcements urging more Americans to buy and use special radio receivers tuned to pick up severe weather alerts broadcast by the National Oceanic and Atmospheric Administration's Weather Radio system.