Competing for companies and consumers seeking reliable weather information, commercial forecasters are sharply expanding the boundaries of their predictions.

Not long ago, five, six or seven days was considered the practical limit for accurately forecasting temperature and precipitation in a particular place at a particular time.

But the Weather Channel, on its Web site, and its archrival,, two of the largest commercial purveyors of weather information, have been leaping into the forecasting future.

Last fall, pushed its ZIP-code-by-ZIP- code predictions out from 7 days to 10. The move closely followed a substantial expansion of the National Weather Service's medium-range forecasts to include average conditions 8 to 14 days in the future. These forecasts are the foundation on which the commercial products are built.

Last spring, countered by offering detailed forecasts up to 15 days ahead. decided not to parry Accuweather's move. "If we felt really comfortable going there, we would've gone there," said Alex Kaminsky, vice president of marketing for

AccuWeather's decision raised the eyebrows of many seasoned meteorologists, who have long presumed that any detail in a forecast that far ahead owes more to imagination than science.

For someone planning a business trip, outdoor wedding or golf weekend two weeks in advance, do such efforts offer much of anything beyond a dice throw? AccuWeather adamantly says yes.

Dr. Joel N. Myers, the founder and president of the 39- year-old company, said it had dozens of meteorologists who constantly assessed the findings of the most advanced American, Canadian and European weather models. They add their own dose of local information about the effect of soil moisture, lakes, mountains and other influences around particular places, then produce detailed forecasts for tens of thousands of spots around the world.

He said they often compared their predictions with competing forecasts and "climatology," which is what history says is the most likely condition in some place on a certain date. "There is definitely value and skill in these forecasts," he said.

He said AccuWeather had not yet done detailed checks of the accuracy of its new 15-day forecasts.

There is also no independent organization that tracks the performance of commercial forecasts, so it is up to people using the services to judge how, say, today's two-week forecast for Cleveland matches up with what conditions actually occur there two weeks from now. Last March, noting the lengthening commercial forecasts, the American Meteorological Society — the largest organization of American atmospheric scientists — issued a statement essentially repudiating the idea that local weather could be accurately forecast two weeks ahead.

"There is no scientific basis for the deterministic prediction of day-to- day weather beyond a week or two," the statement read. "Claims of skillful predictions of day-to-day weather changes beyond this limit have no scientific basis and are either misinformed or calculated misrepresentations of true capabilities."

While there have been substantial advances in forecasting the general conditions one might expect 10 to 15 days from now, many meteorologists agree that there has been little progress in zeroing in on conditions that will prevail locally on a particular day.

"We don't think the skill is sufficient beyond Day 7 to put out what we call deterministic forecasts Day 8 and beyond," said Dr. James D. Laver, the acting director of the federal Climate Prediction Center in Camp Springs, Md., which generates the government's forecasts. "In the private sector, everybody wants to stretch the envelope a little bit."

Although many meteorologists are criticizing AccuWeather's detailed day-by-day forecasts, they say there has in fact been substantial improvement in forecasts of the likely conditions over a range of days two weeks ahead.

A rapid rise in computing power, improvements in equations used in computer models to simulate the physics of the atmosphere and more satellites and radar tracking conditions over the oceans and in other places lacking weather stations are fueling forecasting progress, experts say.

Computer power has allowed forecasters to sidestep one of the most significant roadblocks limiting the accuracy of predictions: the lack of a perfect snapshot of "initial conditions," the state of the atmosphere at the moment when the computer starts calculating how it will behave days from now.

Typically in a computer simulation, little errors in that starting snapshot can produce big inaccuracies as time flows and the atmosphere swirls. The result is a predicted future that strays far from reality.

But now climatologists using supercomputers can run their models over and over again, slightly tweaking the initial conditions each time. The computers generate maps showing the outlines of each run's predicted zones for certain temperatures, atmospheric pressures or precipitation. The resulting ensembles of possible outcomes, when averaged, point to the most probable future.