Our new 'normal' weather: wetter and warmer

If you're among the many who feel Seattle has been getting wetter, guess what: You're right.

And — this year's chilly spring notwithstanding — it's been getting warmer, too.

Those two bits of weather wisdom come from an agency you may not even have heard of, NOAA's National Climatic Data Center.

It recently released a mountain of data all dealing with a common question:

What's normal?

Consider this: For years, the "normal" annual rainfall at Seattle-Tacoma International Airport has been listed as 37.07. Come Aug. 1, that will rise nearly a half-inch, to 37.49 inches.

At the same time, Seattle's year-round "normal" high temperature jumps half a degree, from 59.8 degrees to 60.3. The normal low temperature also rises, 44.8 to 45.

This is no idle chitchat — not just something for TV meteorologists to talk about.

It's something you might want to know before you head to the beach, or schedule your camping trip or decide what kind of tires you want on your car as you head over Snoqualmie Pass.

Utilities use the information to help determine how much power they'll need to generate, and what they may be able to charge for it. Farmers use it to help determine when to plant crops, and when to harvest them.

It can influence just about any business or leisure activity taking place in the great outdoors.

It starts with this: Weather conditions labeled "normal" in the U.S. are taken from weather readings recorded over a 30-year period.

Currently, the readings used are from the years 1971 through 2000.

But every 10 years, the "normal" period is moved up a decade, to reflect more recent weather conditions. That will happen Aug. 1, when readings shift to weather recorded from 1981 through 2010.

With the change, every state in the U.S. will have higher "normal" temperatures, said Thomas Karl, director of the National Climatic Data Center, who said the country is about 1.5 degrees warmer than it was in the 1970s.

So when the cooler years of the 1970s drop out of the "normal" pool and are replaced by the last 10 years, the temperature considered normal will rise — on average, by half a degree.

At Seattle-Tacoma International Airport, where the city's official weather observations are taken, the normal daily high temperature has risen for every month of the year, except October. In Seattle's warmest month, August, the "normal" daily high rises from 75.6 degrees to 76.3.

Seattle's precipitation picture is more complicated:

Six months — January, April, May, June, October and November — will see their normal monthly-precipitation figure rise, while the other six months will see a drop. But when they're added up, the annual normal precipitation will rise.

November saw the biggest increase in precipitation, from 5.9 inches to 6.57.

Johnny Burg, Weather Service meteorologist in Seattle, notes that the years added to the "normal" pool include a number of particularly wet Novembers, including a record 15.6 inches in November 2006.

On the temperature increase, Burg noted that the 1970s — the decade that now falls out of the normal pool — included some lengthy and severe La Niña events, associated with cooler temperatures.

There's no simple explanation for the overall changes in Seattle or nationally — or even a consensus as to whether they're a big deal, given the often polarizing debate surrounding global climate change.

Some scientists say the data provide further evidence that greenhouse gases are building up in the atmosphere, heating the planet.

But Tim Ball, chairman of the Natural Resources Stewardship Project, which is skeptical of global warming, told the Los Angeles Times he considers the half-degree rise in the normal U.S. temperature "essentially insignificant."

Jack Broom: 206-464-2222 or jbroom@seattletimes.com