Published Sunday July 19, 2009

Vicki Price laughed when she recalled her son's outdoor wedding. Though sunny skies had been forecast, it rained all day.

The wedding party took refuge in a pavilion and was promptly joined by a muddy, pregnant dog that seemed intent on leaving paw prints on the bride's dress. The bride's attendants swatted the dog with roses until they shooed it away.

“It was,” Price said, “a memorable wedding.”

In spite of what happened that day, Price said she routinely checks weather forecasts. “I don't dismiss forecasts as always being wrong,” she said. “I don't think we're as accurate in Omaha as in other places because the weather can be so changeable.”

Statistics on the accuracy of weather forecasts are hard to come by. But Price is right about the unpredictable nature of Nebraska's weather.

Because Nebraska is in the central Great Plains, it is one of the toughest places in the nation for which to forecast weather, meteorologists say.

And though opinions vary, some of the state's meteorologists say spring and summer are about the hardest times to get Nebraska forecasts correct.

“ ‘Challenging' is the best word,” said Daniel Nietfeld, science and operations officer for the National Weather Service in Valley.

“There aren't very many (geographic features) that can control our weather, which means we're vulnerable to about anything that can blow through,” he said.

California's weather is moderated by the Pacific Ocean. Florida's is rendered reliably humid by the Atlantic Ocean on one side and the Gulf of Mexico on the other. Denver's weather is more predictable because of the dominating Rocky Mountains.

But Nebraska is far from any large bodies of water that might moderate its weather. And the state's relatively wide-open terrain leaves it vulnerable to competing and powerful air masses from many directions.

Cold air can stream down from the Arctic, humid air up from the Gulf of Mexico, and dry air from out of the Rockies.

As these air masses jockey for dominance, conditions can change rapidly.

“We in Nebraska don't have a climate of our own,” said Ken Dewey, an applied climatologist at the University of Nebraska-Lincoln.

“We are in a battle zone where there is a conflict of all these climates. Based on who is stronger at the moment, it can be wet or dry, hot or cold,” Dewey said.

Forecast errors generally are largest in the middle of the country by virtue of the fact that there is no single, persistent weather pattern, Dewey said.

“It doesn't take much of a shift in air masses to suddenly move us into another climate,” he said.

Iowa is buffeted by the same conflicting air masses, meteorologists say; but because it's farther east, forecasters there generally have a half day or more warning about storms coming from the west.

Steve Kisner has been forecasting Nebraska weather for about 20 years from the National Weather Service office in Hastings. In addition to the flatness of the Great Plains, Kisner said a number of other factors influence the unpredictability of the state's weather:

Ÿ Nebraska is often at the heart of collisions between cold air from the north and warm, moist air from the Gulf of Mexico. These moisture-laden storms can result in intense snow or rainfall amounts.

Ÿ The Rocky Mountains act as a wall that prevents air masses from the Arctic or Gulf from spreading further westward, which intensifies their effects over the Central Plains.

Ÿ Nebraska's gentle rise from east to west — eastern Nebraska is about 1,000 feet above sea level while western Nebraska is about 4,000 feet in elevation — makes it harder to predict when and where clouds will form when the wind blows from the east, as it often does in spring and winter. That elevation change also makes it harder to predict where summer storms will occur when winds are blowing from the west.

Why does a rising grade in the land affect weather?

When winds are blowing from the east, the slope pushes air up. As the air rises, it cools, and as it cools, the moisture in it condenses into clouds. Likewise, when winds blow down the Rockies and descend across the Plains, the air compresses. It becomes a ground-hugging wedge that pushes warm, moist air from the Gulf of Mexico into the upper atmosphere, creating storms.

But why does that make clouds and storms so hard to predict?

A sharp upslope such as a mountain range sets a definite boundary for storms to occur. But Nebraska's slope is so gradual that it's hard to predict when and where enough moisture will condense to generate a storm.

To produce forecasts, scientists with the National Oceanic

and Atmospheric Administration plug vast amounts of weather data into sophisticated computer programs, called models, and then run those models on some of the world's most powerful computers. Humans interpret the results. .

The timing and location of severe storms, which is information that people need most, are some of the hardest things for computer models to predict, said Gary Lackmann, associate professor of atmospheric sciences at North Carolina State University, who was in Omaha recently for a conference on forecasting.

That's because the atmosphere is extraordinarily chaotic. Winds blow at different speeds in different directions at different levels. Like falling dominoes, a change in one spot in the atmosphere triggers changes elsewhere, which in turn trigger more changes, creating escalating uncertainty.

And a thunderstorm or tornado, while large on a human scale, is a small occurrence on an atmospheric scale, Lackmann said.

By contrast, hurricanes, which can last for days, are much larger storms, and that makes it easier to project their path.

John Ferree, national severe storm leader for the atmospheric administration's National Weather Service, said tornadoes are hard to predict because scientists still aren't certain how and why they form — something they hope to change with research currently under way, including an extensive study of tornadoes that took place this summer in Nebraska and other Plains states.

In addition, radar sweeps the sky at about four-minute intervals, so a funnel can drop to the ground and get a head start of a couple of minutes before radar picks it up. Some tornadoes form at an altitude too low for radar to detect them. And others are so wrapped in rain, human weather spotters can't see them.

Climatologists say local factors influence the weather, too.

Vast acres of irrigated cornfields contribute to increased humidity across Nebraska, especially at night. The impact of that increased humidity on the weather is a topic of research.

“The irony is that irrigation adds to the humidity and makes it more uncomfortable at night,” said Dewey, the UNL climatologist. “People say they used to be able to leave their windows open in the summer, and they can't do that anymore because it doesn't get as cool at night.”

Nebraska's weather is so variable that meteorologists even differ on which season is the trickiest to forecast.

Dewey, Nietfeld of the Omaha Weather Service and Kenny Roberg, who is stationed with the Weather Service in North Platte, say they find summer forecasts the toughest to get right.

“The patterns are more subtle in the summer,” Dewey said. “Our enemy is well-defined in January, but much more subtle in the summer.”

For his part, Kisner said he finds winter the most challenging because that weather has the widest impact.

Snowstorms tend to be widespread, while devastating summer events, such as hail storms or tornadoes, strike a relatively small area.

Snow can have a more dramatic impact than rain on people's lives and, as a result, the effects of a missed snowfall forecast are more glaring.

“An inch of rain in the spring will have some effect, but not everybody will notice,” Kisner said. “You take that one inch of moisture in the winter and everyone will notice: It could fall as 10 inches of snow or 20 inches, depending upon the temperature.”

Intense snowfalls can occur along relatively narrow bands, making it harder for meteorologists to provide warnings to school and highway officials, Kisner said.

Mark Anderson, an associate professor of meteorology at UNL, said forecasters in Nebraska also have less time than those farther east to warn the public about impending storms.

Storm systems from the west get blocked by the Rocky Mountains and then spin out anew on the east side of the mountains.

Once a storm resets, forecasters can begin calculating where it will head. Folks on the East Coast will have two to three days' warning, while those in Nebraska sometimes receive only six to 12 hours' notice, Anderson said. This is particularly true with winter storms.

“We know these storms will move through, but we don't know where their center will be,” Anderson said.

Anderson said he likes to use a rain-snow forecast as an example of how a subtle difference in the atmosphere can make a huge difference on the ground — and how a lack of data can make discerning that difference difficult.

One-half degree in temperature can make a difference in whether it rains or snows, he said. And that one-half degree can occur at any point from the surface to 3,000 feet up.

But how do meteorologists know what the temperature is at those upper levels? They don't. Instead, they rely on the twice-daily launch of weather balloons in Omaha and North Platte. The balloons record the air temperature as they rise and sail across Nebraska, or another state.

“Think about it: I'm trying to determine a half-degree difference (going 3,000 feet into the atmosphere) with two balloons launched about 300 miles apart,” Anderson said. “It makes for a challenge.”

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