Near Dulles International Airport yesterday, Larry Wenzel launched a 15-foot-long balloon that measured temperature and humidity every six seconds before it burst.

Deep inside the World Weather Building in Prince George's County, James Kemper checked satellite photos.

And from his home in Cumberland, Md., Timothy Thomas was on the lookout for tornadoes.

As Mother Nature socked the Washington area during the weekend with temperatures that on Saturday tied the 1936 record of 99 and with lightning that silenced televisions and dishwashers, the National Weather Service went to work.

The Weather Service has perhaps the toughest job of all: predicting the unpredictable. On off days, it advises people to carry umbrellas when not a cloud is in sight. On the best days, it evacuates Midwestern towns before tornadoes rip through them.

"This is exciting for us," said forecaster Kemper. Pointing to his computer screen, where a white glob moved across an outline of Virginia, he said, "These are not what we call garden variety showers; they are unusually strong thunderstorms."

Seconds later, Kemper issued a warning relayed to television and radio stations that read: "Weather conditions are unusually ripe for scattered thunderstorms." Two hours later, there was clapping in the sky.

With a $344 million budget and a staff of 4,800, the Weather Service has come a long way from the days when the Farmer's Almanac was the weather gospel.

A satellite 22,500 miles above the Equator constantly sends photographs of white puffs and spinning air to computers in Prince George's. A bank of computers there disperses the photos and other data to weather stations in England, India and a host of other countries.

David Weinbrenner, a medium-range forecaster, said technological advances mean that the agency can now predict weather three days in advance with the same accuracy as it did 36 hours in advance 15 years ago.

Yesterday, Weinbrenner and two other meteorologists -- the three-member team that compiled the official extended forecast for the entire country yesterday -- drew spaghetti-like squiggles across maps of the United States on the fourth floor of the World Weather Building in Camp Springs.

Marking "H" for the hot spots and "L" for the low pressure points, the forecasters looked as if they could have been conducting a kindergarten sketch class. Yet, depending on their information, ships drop anchor, military maneuvers are canceled, flights are delayed and picnics are postponed.

The recipe for a weather forecast calls for four main ingredients: satellite and radar readings, computer models, balloon-relayed upper air readings, and ground observations.

Since World War II, helium and hydrogen balloons have been launched all over the world twice a day, every day, at precisely the same hour: noon and midnight Greenwich time (7 a.m. and 7 p.m. Eastern Standard Time).

At 100 locations in the United States and hundreds more around the world, weather experts such as Wenzel and Peter Ahnert near Dulles send weather-reading balloons into the heavens.

As the $100 balloon ascends, it sends back readings on temperature, humidity and air pressure. At about 90,000 feet, "it's expanded to the size of a small house," and explodes, Ahnert said. Then, a tiny parachute guides the attached radiosonde, as the meteorological and radio-transmitting equipment is called, back to earth.

"They fall on doorsteps and in fields," Ahnert said. "There is a little note attached that encourages people to mail the radiosonde back to the Weather Service." A spokesman said about 30 percent are returned and that as far as he knew, nobody has been hit by one.

Despite billion-dollar satellites and computers, one of the service's best sources of information is a little-known cadre of volunteer weather watchers known as the "cooperative observers." The 200 volunteers in the Washington area tell the agency exactly how much it rained in their back yards and whether they have observed a flood, a twister or high winds.

Yesterday, Thomas of Cumberland was the first cooperative observer who called in about local tornado reports. Thomas, a 911 dispatcher for the Allegany County Civil Service, said he does it because "we've had weather in the family for 40 years." Before him, his grandfather phoned the Weather Service every day.

Forecaster David Caldwell, who took Thomas' call about noon, said ground observations are still hard to beat: "The science of weather forecasting is relatively new. We still don't understand everything that happens in the atmosphere."