Winter Storms

Beautiful and threatening at the same time, winter storms can throw society into chaos. A winter storm's patchwork of snow, rain, freezing rain, and sleet may range over many hundreds of miles. Among the forecast challenges being addressed at NCAR and elsewhere are small-scale variations in the swath of a winter storm that make a huge difference to the storm's impact on people and property.

Some winter weather is driven by geography. As winds pass up and over mountain ranges, they can dump huge amounts of snow. Large bodies of water (the Great Lakes in particular) can feed snow bands that hold stationary, pummeling positions near the shore, resulting in gigantic accumulations.

Most other areas get their worst winter weather as a result of extratropical cyclones. These are large areas of low pressure that form in the midlatitudes, swirling counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Driven by the polar jet stream, they typically move east while curving northward over time.

Many extratropical cyclones strengthen sharply as they move from land toward a coastline, where they can ingest warm, moist air. While the U.S. Great Plains and Midwest can see raging blizzards with bitter cold and high wind, the heaviest U.S. snowfalls from extratropical cyclones are more likely to fall from the mid-Atlantic toward New England. In the great extratropical storm of March 1993, records for low pressure were set along the East Coast, and snow fell in every state from Florida to Maine.

Extratropical cyclones can maintain their identity as they cross parts of the Atlantic or Pacific, sometimes "bombing out"—intensifying rapidly over less than 24 hours. Because of their large scope, these cyclones can influence weather far downstream over several days.

Research

In the Denver area’s heaviest winter storm in 90 years, some foothill locations got upwards of 79 inches (200 centimeters) of snow during March 17–19, 2003. With several field projects in the Atlantic and Pacific over the last few years, NCAR scientists and collaborators have probed extratropical cyclones and their downstream impacts. These projects have lent support to the notion of targeted observing and modeling. For example, U.S. forecasts can be improved when extra data is gathered over small, well-defined parts of the Pacific. In a nested computer model, such areas are followed in detail within a larger-scale model that is less detailed overall.

Every piece of data is important when a complex winter storm threatens. NCAR scientists have examined the dramatic impact of missing data on model projections of an East Coast storm in January 2000. Just before that storm arrived, the models omitted a report from a single radiosonde (weather balloon) launched from Tallahassee, Florida. Due to this and other factors, computer models erroneously predicted the storm would stay farther offshore. Instead, Washington, D.C., got nearly a foot of snow with less than 24 hours' notice.

One of the hardest details to predict in a winter storm is the line separating rain from snow. Most forecast models can only discern such details roughly every 20 miles (32 kilometers). In the March 2000 storm, forecasts for Raleigh, North Carolina, called for mostly rain, with flurries toward the storm's end. The rain-snow line ended up parking a few miles eastward, and Raleigh got over 20 inches (50 cm) in its biggest snowfall on record.

NCAR is part of a team developing the Weather Research and Forecasting computer model (WRF). With a resolution as fine as 2.5 mi (4 km), WRF should be able to pin down the location of rain-snow transitions more accurately. Since October 2004, WRF has been used by the National Weather Service in crafting its public forecasts.

thunderstorms • hail • lightning • tornadoes • floods • hurricanes • winter storms