Why It Sometimes Pours When It Rains

Dr. Sandra Yuter got used to the rain while living in the Pacific Northwest for several years. “It’s mostly a duration issue. It might rain lightly for 18 hours instead of pouring down for an hour,” she says. Still, Yuter was always curious why one storm could cause localized flooding while a similar system left no damage. Now an associate professor in NC State’s Department of Marine, Earth and Atmospheric Sciences, she and her research team are looking at how heavy precipitation storms develop and move, which could lead to better forecast models in the future.

Yuter and researchers at Stony Brook University have analyzed five years of data from National Weather Service (NWS) stations in Washington state and Oregon — measurements are taken every six minutes — to determine factors in heavy precipitation events. Strong winds perpendicular to the Cascade Mountains and relatively warm surface temperatures provide the best conditions for plenty of rain, and the research team determined that the amount of rain that falls in valleys plays a major role in determining the potential for flooding. When rain falls on the mountainside, cooler air moves down into the valleys, creating a wedge of cold air along the foothills. “Air rides up and over the wedge, effectively widening the mountains, which increases the area where rain falls,” she says. “The precipitation that flows down the mountainside is added to what falls in the valley, and flooding results.”

With a three-year, $346,000 Recovery Act grant from the National Science Foundation, Yuter is paying graduate and undergraduate students to examine NWS data and help develop algorithms to track intense storm cells within systems to determine where they start and how long they last. “These storms aren’t uniform sheets of rain,” she says. “Even if we have flooding, we’ll have some areas worse or better than others.”

Yuter says her team’s work should help other meteorologists develop better short- and long-term precipitation forecasting models for mountainous areas, such as western North Carolina, which is prone to flash flooding. Improved forecasts also would help utilities know if and when to release water from hydroelectric dams and when winter resorts should plan for snowfall. “So much of our fresh water comes from mountain precipitation,” she says. “We really need to understand the basic physics of the storms better.”

 

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Dr. Sandra Yuter is developing models to better forecast flooding in mountainous areas.