OREGON STATE UNIVERSITY

OSU researchers create new precipitation maps for each state

12/23/1999

CORVALLIS, Ore. - Oregon State University researchers have completed new precipitation maps for each of the 48 states in the continental U.S. that are thought to be the most complete, accurate maps of their kind ever created.

The scientists are now working on final drafts of maps for Alaska and Hawaii. Their research during the project has uncovered some interesting new projections, including what may be a new candidate for the wettest place on Earth.

A 15,000-foot mountain range in southeast Alaska, that ironically includes Mt. Fairweather, may have an annual precipitation of nearly 500 inches. The reigning champ has been Cherrapunji, India, where monsoons contribute to a yearly rainfall of more than 460 inches.

"Unfortunately, we'll probably never be able to prove it because the site in Alaska has no gauges and if you were to develop a station there, you'd probably be dumping more than a foot of snow out of it most every day," said Chris Daly, an assistant research professor in OSU's Department of Geosciences. "The climate and conditions there are so prohibitive, it will probably never be monitored."

So how do scientists know how much rain such a region gets?

Like previous map-makers, they collect data from every weather station available. There are more than 9,000 climate monitoring stations in the U.S. alone that provide a sampling of precipitation from certain areas. In the past, that data led to the creation of a series of two-dimensional maps which have represented the state-of-the-art science in creating precipitation maps.

No longer.

The OSU researchers have created a new technology called PRISM, which stands for Parameter-Elevation Regressions on Independent Slopes Model. What it really means is that their model relies heavily on "orographics," or the effects of the terrain on weather.

Using orographics, these new PRISM maps take into account elevation, topography, rain shadows, temperature inversions, and coastal effects on the weather, as well as data from the rain gauges.

"Previous maps were very general and based on the reading of the closest weather station," Daly said. "If the station was on the west side of a mountain, there was a good chance that the east side just a few miles away would have a similar estimate, even if it was in a rain shadow. We know, of course, that it doesn't happen that way."

Daly developed the PRISM technology in the early 1990s while working with Ron Neilson of the U.S.D.A. Forest Service on global warming research, then joined forces with the Oregon Climate Service, based at OSU. Working with George Taylor, an OSU faculty member who serves as the state climatologist, he created a new PRISM map of Oregon.

The success of that map led to similar efforts for Utah, Nevada, Idaho and Montana. Eventually, the Natural Resources Conservation Service contracted with the OSU researchers to create maps for all 50 states. The process has not been without difficulty.

"I had to endure a lot of meetings where I was really on the hot seat," Daly said with a laugh. "A lot of other state climatologists weren't initially comfortable with us coming in and saying that we could make maps of equal or greater accuracy than their own. They wondered how researchers in Oregon could know more about their state than they did.

"But once they saw the science and technology that went into the creation of the maps, most of them have welcomed our work," he added.

The OSU researchers uncovered some interesting phenomena in the creation of new PRISM precipitation maps for the lower 48 states. Among them:

 

  • The wettest place in the lower 48 states is in the Olympic Mountains of Washington, where annual precipitation is estimated to be more than 260 inches. The driest is Death Valley, Calif., which receives less than two inches a year.

     

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  • Nevada is considered a desert state and that is true for the low-elevation valleys, where the annual precipitation is a meager four to eight inches. However, there are dozens of mountain ranges in the state that receive 20 to 45 inches annually, providing much-needed snow melt-water for the parched lowlands.

     

  • The bottom of the Grand Canyon picks up only about five inches a year, while the uplands above the north rim check in at about 30 inches.

     

  • The award for the least interesting state, precipitation-wise, would probably go to Delaware, where the total variation in mean annual precipitation across the entire state is less than five inches a year.

     

  • The award for the most precipitation variability in one state - outside of Alaska and Hawaii - goes to Washington, which varies from more than 260 inches in the Olympic Mountains to less than seven inches in the desert east of the Cascades. "This tremendous variation is caused by 'rain shadows,' or a blocking by mountains of moist oceanic air that brings rainfall as it moves inland from the Pacific," Daly said.

The OSU researchers say that the western United States has perhaps the most interesting weather outside of Alaska and Hawaii, but it doesn't have a monopoly on complex weather patterns. The Appalachians near Asheville, N.C. face the Gulf of Mexico and accumulate 90 to 120 inches a year, while less than 40 inches manage to reach Asheville itself.

Another large, unheralded rain shadow can be found in Arkansas, where the Ouachita Mountains - though generally less than 2,500 feet - block moisture traveling north from the gulf, causing the higher Boston Mountains to be 20 to 30 percent drier.

"Elevation is the key determinant to precipitation," Taylor said. "In Oregon, we've estimated that the annual rainfall about 100 miles off the coast is only 30 inches. At the coast, it's about 80 inches, in general. Go inland to the coast range, and there are places that it reaches 200 inches a year.

"As the air rises, the moisture drops," Taylor added. "It is that process of moving upward that is the key to precipitation."

State maps of the 48 continental U.S. states are complete and available for purchase through the Oregon Climate Service by calling the Oregon Climate Service at 541-737-4557. Or they can be ordered off the web.

A number of climate grids and other maps can be downloaded from the site for free.