Improving forecasts for rain-on-snow flooding

Many of the worst West Coast winter floods pack a double punch. Heavy rains and melting snow wash down the mountains together to breach riverbanks, wash out roads and flood buildings.

These events are unpredictable and difficult to forecast. Yet they will become more common as the planet warms and more winter precipitation falls as rain rather than snow.

University of Washington mountain hydrology experts are using the physics behind these events to better predict the risks.

"One of the main misconceptions is that either the rain falls and washes the snow away, or that heat from the rain is melting the snow," said Nicholas Wayand, a UW doctoral student in civil and environmental engineering. He will present his research Dec. 18 at the annual meeting of the American Geophysical Union.

Most of the largest floods on record in the western U.S. are associated with rain falling on snow. But it's not that the rain is melting or washing away the snow.

Instead, it's the warm, humid air surrounding the drops that is most to blame for the melting, Wayand said. Moisture in the air condenses on the cold snow just like water droplets form on a cold drink can. The energy released when the humid air condenses is absorbed by the snow. The other main reason is that rainstorms bring warmer air, and this air blows across the snow to melt its surface. His work support previous research showing that these processes provide 60 to 90 percent of the energy for melting.

Places that experience rain-on-snow flooding are cities on rivers that begin in the mountains, such as Sacramento, California, and Centralia, Washington. In the 1997 New Year's Day flood in Northern California, melting snow exacerbated flooding, which broke levees and caused millions of dollars in damage. The biggest recent rain-on-snow event in Washington was the 2009 flood in the Snoqualmie basin. And the Calgary flood in summer of 2013 included snow from the Canadian Rockies that caused rivers to overflow their banks.

The UW researchers developed a model by recreating the 10 worst rain-on-snow flooding events between 1980 and 2008 in three regions: the Snoqualmie basin in Washington state, the upper San Joaquin basin in central California and the East North Fork of the Feather River basin in southern California.

Their results allow them to gauge the risks for any basin and any incoming storm. The three factors that matter most, they found, are the shape of the basin, the elevation of the rain-to-snow transition before and during the storm, and the amount of tree cover. Basins most vulnerable to snowmelt are treeless basins with a lot of area within the rain-snow transition zone, where the precipitation can fall as snow and then rain.

Trees reduce the risk of flooding because they slow the storm's winds.

"If you've ever been in a forest on a windy day, it's a lot calmer," Wayand said. That slows the energy transferred from condensation and from contact with warm air to the snowpack.

Simulations also show that meltwater accounted for up to about a quarter of the total flooding. That supports earlier research showing that snow is not the main contributor to rain-on-snow floods, but cannot be neglected since it adds water to an already heavy winter rainstorm.

The complexity of mountain weather also plays a role.

"The increase in precipitation with elevation is much greater than usual for some of these storms," said Jessica Lundquist, a UW associate professor of civil and environmental engineering. "Higher flows can result from heavier rainfall rates at higher elevations, rather than from snowmelt."

In related work, Lundquist's group has developed a tennis-ball snow sensor and is measuring growth and melt of the snowpack in the foothills east of Seattle. The scientists aim to better understand how changes in climate and forestry practices might affect municipal water supplies and flood risks.

Wayand and another student in the group have developed a high school curriculum for Seattle teachers to explain rain-on-snow events and the physics behind why they occur. They hope to begin teaching the curriculum sometime next year.

The other collaborator on the work being presented in San Francisco is Martyn Clark at the National Center for Atmospheric Research in Colorado.

View the original article here

Accuweather forecasts extreme cold winter for Chicago (Reuters)

(Reuters) – Private forecaster Accuweather.com said on Wednesday that heavy snow and extreme cold should be expected in the north central United States, especially in the Chicago area, in the coming winter.

The East Coast faces average to slightly above average snowfall during the winter of 2011-2012 as a Pacific La Nina again drives weather patterns across the United States.

"People in Chicago are going to want to move after this winter," Accuweather Long-Range Meteorologist Josh Nagelberg said in a statement.

Accuweather's forecast projects Chicago will again be hit by extreme cold weather and several blizzards, as it endured last winter when more than 50 inches fell on the city including a single storm that dropped 20 inches.

Severe cold and heavy snow should expected from the Great Lakes across the Midwest and northern plains states, according to Accuweather.

La Nina, the name for a recurring phenomenon when sea surface temperatures across the central and eastern Pacific Ocean are below normal, will fuel weather across North America, including a persistent flow of tropical moisture that brings heavy rains to the West Coast called the "Pineapple Express."

The southeastern United States could again see flooding in the lower Mississippi Valley in late winter, Accuweather said.

Dry, mild weather is expected over most of the southwestern United States, with no relief seen for drought-stricken Texas.

The Mid-Atlantic states could see snow and ice with South Carolina and Georgia seeing rain. Florida should expect a mild, dry winter.

(Reporting by Erwin Seba in Houston; Editing by Gary Hill)

View the original article here

Forecasts, TV and luck eased tornado risk in Okla. (AP)

By KRISTI EATON and CHUCK BARTELS, Associated Press Kristi Eaton And Chuck Bartels, Associated Press – Wed May 25, 9:36 pm ET

PIEDMONT, Okla. – When three tornadoes marched toward Oklahoma City and its suburbs, thousands of people in the path benefited from good forecasts, luck and live television to avoid the kind of catastrophe that befell Tuscaloosa, Ala., and Joplin, Mo.

Although at least 15 people died in the latest round of violent weather that started Tuesday, schools and offices closed early, giving many families plenty of time to take shelter. And even stragglers were able to get to safety at the last minute because TV forecasters narrated the twisters' every turn.

"We live in Oklahoma and we don't mess around," Lori Jenkins of Guthrie said after emerging from a neighbor's storm shelter to find her carport crumpled and her home damaged.

The people of Oklahoma City, which has been struck by more tornadoes than any other U.S. city, knew the storms were coming. Anxiety was perhaps running higher than usual Tuesday after last month's twister outbreak in the South that killed more than 300 people and a Sunday storm that killed at least 125 in Joplin, Mo.

The Oklahoma twisters proved to be weaker than the other tornadoes. But the minute-by-minute accounts of the developing weather helped thousands of people stay abreast of the danger.

Television helicopters broadcast live footage while the system approached the metropolitan area of 1.2 million people — calling out to specific communities like Piedmont to "Take cover now!"

In Guthrie, about 30 miles north of the capital city, Ron Brooks was watching when he learned that a tornado was barreling toward him. He heeded the weatherman's warning, scooped up his two children and took cover with his wife in their laundry room.

"When they told us to get into the shelter or interior room, we did that," Brooks said. "The first year I moved to Oklahoma, in 1997, I saw a funnel drop out of a wall cloud. Since seeing one, I've always taken it pretty seriously." He emerged 20 minutes later, relieved to learn that the tornado passed just north of his home.

Another line of severe storms swept through the nation's midsection Wednesday, mainly east of Oklahoma. A tornado warning was briefly issued for downtown Kansas City, Mo., and at least two weak tornadoes touched down in or near the suburbs.

A few others were reported in Illinois. The storms moved later into Indiana, Kentucky and Tennessee.

In Joplin, the city manager said Wednesday that 125 people had died in the storm, raising by three the toll of the nation's deadliest single tornado since 1950. He said more than 900 people had been injured.

Rescue and recovery work continued, with crews repeating grid searches for any survivors who might still be buried in rubble. Structural engineers were sent inside the ruins of St. John's Medical Center, which was crippled by the twister, to see if the hospital could be saved.

Back in the Oklahoma City area, at least nine people were killed, despite broadcasters offering live coverage of the storms for two hours before the bad weather actually hit around the evening rush hour.

Across the border in Arkansas, people in the tiny hamlet of Denning didn't have the luxury of an early warning. A tornado killed at least one person there. Storms left three others dead elsewhere in Arkansas and killed two in Kansas.

The storms arrived in Denning in the darkness, with a warning posted only about 10 minutes before a tornado nearly obliterated the town of 270 shortly after midnight.

Troy Ellison didn't even have that much time.

He was watching a movie in his mobile home when he switched on the TV news. The tornado was four minutes away.

"We were going to take the work truck and get out," Ellison said. "I looked out the back door with my son and it was coming."

He dove under the kitchen table with his wife and two sons just before the tornado hit. "It got that growling sound and the windows popped," he said.

The tornado ripped the roof off his home and collapsed his workshop next door. Somehow, the family escaped unharmed.

Then Ellison went outside and saw the family dog, Jager, his paws splayed out on the ground. The animal "looked like someone stepped on him." Ellison assumed he was dead.

But the dog, a pit bull-boxer mix, turned out to be fine. By Wednesday, he was prancing around in the sun as the Ellisons moved belongings out of their home.

"He must have known to stay low to the ground," Ellison said.

Oklahoma City has been hit by tornadoes 146 times, according to the federal government's Storm Prediction Center. That history brings respect for severe storms and a simple rule for people who find themselves in a twister's path: Get out of the way or get underground.

"I think Oklahomans, simply because we're around it so much, take very seriously the threat of severe weather. It's something we live with year-round," said Michelann Ooten, a spokeswoman for the state Department of Emergency Management. "We have a genuine respect for the severe weather here."

Part of that comes from learning to deal with bad weather at a young age, Ooten said.

The long track of the storm in Piedmont gave Lynn Hartman's family time to take shelter and then run away. As warning sirens sounded, Hartman said, she huddled in the pantry of her Piedmont home with her two children and the family dog until her husband arrived home from work.

"We're there just crying and praying," Hartman said, and her daughter, Sierra, 10, was saying repeatedly, "I just don't want to die."

The family then decided to flee as the storm drew closer. They crossed the Oklahoma City area to Shawnee. Once there, sirens sounded again for a storm approaching from the south. The four drove around for three hours before returning to find their roof gone. The pantry was standing, but Hartman was not convinced the family would have survived.

Ooten said trying to outrun a tornado is dangerous.

"Find the sturdiest building you can gain access to," she said. "Unless you're an expert, I wouldn't try to outrun a tornado. You're not in charge. Mother Nature is the one in charge."

___

Bartels reported from Denning, Ark.

View the original article here