Predicting winter weather: the weather never sleeps, inconsistency is the only constant

Flying Safety, Oct, 2003

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Reprinted from Flight Training, December 1995

Pilots from Boston, Massachusetts, to Birmingham, Alabama, had no problem making go/no-go weather decisions 13 March 1993. For the first time since commercial aviation began, a blizzard had closed every airport from the Southeast to New England. Called the "Storm of the Century," it dumped more than a normal decade's worth of snow--17 inches--in Birmingham, Alabama. It gave Syracuse, New York, 43 inches. Snowfall rates of two to three inches an hour were common.

While some larger airports were closed less than a day, others needed days to clear the snow. Even if pilots had been able to get a clear runway, they wouldn't have wanted to take off. The weather station atop Mount Washington, New Hampshire, at 6,200 feet above sea level, recorded a gust of 144 mph. The wind at New York City's LaGuardia Airport was gusting to 71 mph.

The "Storm of the Century" also included "thundersnow." Anyone in the air would have faced the hazards of thunderstorms hidden in the clouds and snow. Aircraft also would have encountered heavy airframe ice. Central New Jersey reported 2.5 inches of sleet on the ground. Sleet on the ground means supercooled raindrops, or freezing rain, is somewhere above, and freezing rain creates the most dangerous kind of airframe icing.

While a "Storm of the Century" is rare, ordinary weather storms make life harder for pilots in many ways. While some weaker storms make it clear that flying isn't a good idea, most require pilots to make difficult choices without clear-cut information. Winter brings the strongest and biggest storms, because mid-latitude storms, those that form outside the tropics, draw energy from temperature contrasts. The greater the temperature differences between large air masses, the stronger a storm is likely to be.

Arctic temperatures begin plunging as the days grow shorter during fall and into winter. The tropics, however, stay warm because the days stay nearly the same length all year. Most of the contiguous 48 states become winter's battleground between frigid Arctic air and the balmy air of the tropics. The resulting weather can range from mild systems that cloud the skies and produce a little rain or snow to full-fledged blizzards. A blizzard, by the way, is a storm with snow falling while the wind blows at sustained speeds of 35 mph or faster near the ground and the visibility stays at or below .25 miles for an extended time.

As with any kind of dangerous weather, information is the pilot's first defense. Big storms don't appear by magic. Today's forecasts do a generally good job of saying when something big is brewing. But even with the best technology, forecasters have difficulty pinning down the details of winter storms.

In March 1993, the computerized forecast models in the U.S. and Europe pointed to a major East Coast storm six days before it began. Two days before the storm began, the computer forecasts agreed totally, and the National Weather Service began issuing storm warnings. The forecasts did not, however, point to some important details, such as snow as far south as the Florida Panhandle. The lesson for pilots? Stay abreast of the general weather picture even when you aren't flying.

The "Storm of the Century" was an extratropical storm with a low-pressure center and warm and cold fronts, as shown in figure 1. Such storms account for a good share of the nation's bad winter weather, but not all of it. Many storms track across the U.S. from the Pacific to the Atlantic with their characteristics changing on the way. The storm's exact path also makes a big difference in the weather it causes.

Figure 2 shows some of the common storm tracks. A storm that moved across the United States from 13-16 February 1990 is a good example. It's one of the best-documented cross-country storms because it moved across regions, each with different scientific groups studying winter weather. Effects of the 1990 storm included:

* Heavy snow in the West, northern Plains, and New England, including snow that shut down Chicago's O'Hare Airport.

* Freezing rain from Oklahoma to New England.

* Severe thunderstorms with tornadoes and flooding in the South and along the Ohio River.

* Frost damage to citrus in California and Arizona.

* Damaging winds in California, New Mexico, Texas and Wyoming.

* Aircraft icing over much of the U.S. east of the Rockies, and north of the Ohio River and North Carolina.

Scientists are still studying the detailed observations made by researchers in Denver, Kansas City, Missouri, Champaign, Illinois, and western New York. The observations show that winter storms are complex and composed of layers of air at different temperatures. Bands of precipitation will bury some areas in snow and deposit freezing rain, rain, or almost nothing on nearby areas. Here's a very general picture of what to expect from winter storms in different parts of the country.

The West Coast to the Rockies. As storms move inland from the Pacific Ocean, they normally bring heavy rain to low-elevation coastal areas and snow to the mountains. Pilots used to Eastern or Midwestern weather must be prepared for huge differences over short distances. For example: Sacramento, California, averages a trace of snow in a year. South Lake Tahoe, California, less than 100 miles away in the mountains, averages 58 inches in January alone. Heavy snow also falls eastward to the Rockies. Another danger is turbulence as high winds flow through valleys and canyons and over mountains.