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2004-5 FOR IMMEDIATE RELEASE: March 3, 2004

Colorado’s Blizzard of 2003, One Year Later—
The Prediction Power of Fine-Scale Models


David Hosansky
UCAR Communications
Telephone: (303) 497-8611
E-mail: hosansky@ucar.edu

Doug Wesley, UCAR/COMET

BOULDER —Weather forecasters may soon do a better job handling snowstorms like the one that paralyzed Colorado’s Front Range last March, according to an analysis by the University Corporation for Atmospheric Research (UCAR), the National Oceanic and Atmospheric Administration (NOAA), and colleagues in the private sector.

According to the study team, the fine detail from a new generation of computer forecast models may help forecasters convey important town-to-town differences in snowfall timing and intensity, like those observed during the historic storm on March 16–20, 2003. These new tools—such as the Weather Research and Forecasting Model, to be implemented later this year by the National Weather Service (NWS)—will help meteorologists include more detail in public forecasts.

Overall, forecasters did a good job alerting the public to last year’s storm, according to the study's leader, Douglas Wesley (UCAR Cooperative Program for Operational Meteorology, Education and Training). Computer models indicated the storm’s general approach up to a week in advance. Then, within two days of its arrival, they showed the potential for over four feet of snow along Colorado’s Front Range. “The accuracy of these model projections, and the public forecasts issued by the NWS, were perhaps unprecedented,” says Wesley.

But neither human nor virtual forecasters predicted how dramatically the storm would vary across the Front Range. “We found differences of several feet of snowfall at similar elevations in a span of 15 miles or less,” says Wesley. Just west of Longmont and Loveland, for instance, only three to six inches of wet snow accumulated. Meanwhile, at the same elevation on the south and west sides of Boulder and Denver, three to four feet of snow piled up.

The huge disparities occurred in part because of subtle temperature differences. As the massive storm spun in a counterclockwise direction, says Wesley, “relatively warm air descended from the canyons of Boulder and Larimer County, while cold northerly surface winds were blocked by the terrain.” This helped keep readings at or just above freezing downstream from the canyons, limiting the snowfall accumulations there.

In large-scale computer models currently used by the NWS, the atmosphere is depicted at points separated horizontally by more than 10 miles. This is much too great a distance to capture the small-scale variations in temperature and precipitation that shaped the March 2003 snowstorm. To get a better view, the UCAR-NOAA team put two finer-scale research models to the task, each with less than 2 miles between grid points. The team fed actual data from the storm into these two models, then watched how well the models depicted the storm’s evolution.

One of the key elements in heavy Front Range snowfall is upslope flow, where moist winds blow from east to west. Along with their sharper depiction of the mountains themselves, the finer-scale computer models did a better job depicting the upslope flow that slammed into the Front Range, says Wesley. About a mile above Denver, the standard computer model showed east-to-west flow of 15–20 miles per hour during the height of the storm. The two finer-scale models showed speeds of 20–25 mph, closer to the observed speed of 30–35 mph.

“The models’ abilities to capture the depth and strength of the upslope flow are likely critical to their forecasts of low-level temperatures and precipitation,” says Wesley. However, he notes, both models failed to predict the full extent of the warmer air at ground level, where light downslope winds kept some areas at or above freezing, even as howling upslope winds fueled snowfall just above the surface.

“With the ongoing advances in computing speed that we are currently seeing,” says Wesley, “forecast models with horizontal grid spacing of less than five miles are becoming a realistic expectation in a 5- to 10-year timetable. These enhanced models will become even more important tools for forecasters in complex terrain anywhere in the world.”

high resolution image: blizzard.jpg (960 KB, 3072 x 2048)

The Front Range snowstorm of March 16-20, 2003, dropped more than six feet of snow at some foothill locations. (Photo courtesy Carlye Calvin.)

high resolution image: snowsat.jpg (1 MB, 2750 x 2125)

This visible satellite image of Colorado was taken on March 22, 2003, a couple of days after the Front Range snowstorm ended. Several pockets of snow-free ground are visible just to the east of the foothills north of Denver, where downslope canyon winds kept some areas near or above freezing. (Image courtesy COMET and NOAA.)

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The National Center for Atmospheric Research and UCAR Office of Programs are operated by UCAR under the sponsorship of the National Science Foundation and other agencies. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors. UCAR is an Equal Opportunity/Affirmative Action employer.

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Prepared for the web by Carlye Calvin
Last revised: Wednesday, August 30, 2006 11:41 AM