Winter Stormsand Blizzards

 

Situated between the Blue Ridge Mountains to the west, and the Chesapeake Bay and Atlantic Ocean to the east, the Washington metropolitan area is located in a classic “meteorological battle zone” in the winter. The battle pits dry, Arctic air which plunges south out of Canada against relatively warm, moist air that streams in from the Atlantic Ocean and the Gulf of Mexico. This often results in forecasts for the area which include the phrase “wintry mix,” referring to a combination of snow, sleet, and freezing rain. In fact, it’s not unusual for a winter forecast to call for four to eight inches of snow in places like Leesburg, Virginia and Damascus, Maryland, while 30 miles to the southeast in Washington the forecast calls for “snow changing to rain” with an accumulation of only one to three inches. When it comes to winter storms, a temperature fluctuation of just a couple degrees can turn a sluggish commute on the Beltway into an icy gridlock.

 

 

Wintry Precipitation Types

 

To fully understand why one day you may have snow falling with a surface temperature of 40°F, while on another day freezing rain may be falling with a temperature of only 25°F, you have to look at the bigger picture. Knowing how the temperature of the air changes above the ground is crucial in determining precipitation type. When temperatures throughout the atmosphere are at or below freezing (32°F), snow will fall. In contrast, when temperatures throughout the atmosphere are above the freezing mark, rain will fall. However, the scenario is often more complicated than this. For example, when a storm moves to the northwest of Washington – let’s say just west of the Appalachians – warm air will usually be swept in by winds from the south and southeast aloft. The inflow of warm air from the south erodes the deep layer of cold air, producing what is called a temperature inversion. In this case, the inversion refers to a warm layer of air somewhere between the surface and an altitude of 5,000 feet. The precipitation begins falling as snow in the cold layer above the inversion, but then the snow melts to rain as it encounters the warm layer. Depending on the depth of the subfreezing air below the inversion, the precipitation may refreeze into ice pellets (sleet) or fall as rain that freezes on contact (freezing rain) with objects near the ground that are at or below freezing.

Freezing rain and sleet occur about as often as snow in the Washington area during the winter. In fact, aside from central Pennsylvania and some of the deeper valleys in upstate New York, western parts of our region see more icy weather than just about anywhere else in the country. Parts of the Shenandoah Valley, and areas just east of the Blue Ridge in Maryland and Virginia, average between 30 and 40 hours of freezing rain each winter.

On January 14-15, 1999, a crippling ice storm struck the nation’s capital and its surrounding suburbs. While temperatures aloft were too warm for snow, a dry, arctic air mass was in place near the ground. As the rain fell into the dry, cold air, it initially evaporated, further cooling and reinforcing the arctic air. As rain fell through the night, it increased in intensity and froze on everything. Trees and power lines were no match for the 1 inch thick layer of ice that coated them. Hundreds of thousands of people in the Washington area lost power, thousands of trees were toppled, and many roads were impassable for days!  Yet, just south of the District, in places like Waldorf and Upper Marlboro, the freezing rain event did not occur; temperatures were a few degrees warmer and only rain fell.

 

 

The “Alberta Clipper”

 

Often producing what may be termed “nuisance snow,” the “Alberta Clippers” are fast-moving, low-pressure systems which are enhanced on the lee side of the Canadian Rockies in south-western Canada (Alberta). They usu-ally track south-easterly into the Northern Plains, through the Upper Midwest, and then zip across the Northeast or Middle Atlantic Region. Due to their quick movement and great distance from a moisture source (like the Gulf of Mexico), clippers usually result in only light snow, followed by a blast of colder air. However, there are exceptions to every rule. Just a few days after the great “Blizzard of 1996” struck the Middle Atlantic region, a rather vigorous Alberta Clipper moved through the Washington area. Accompanied by strong winds in the upper atmosphere which helped to lift the air, this Clipper added to the misery by dumping up to five inches of new snow on roads that were still being cleared of nearly two feet of snow that had just fallen with the blizzard.

 

 

Nor’easters: Winter’s White Hurricanes

 

One of the first weather watchers to gain a true glimpse into the nature of nor’easters was Benjamin Franklin. In 1743, while staying up late one night to watch a lunar eclipse in Philadelphia, the weather turned stormy and prevented him from viewing it. Later, he learned that his brother in Boston had seen the eclipse as scheduled, but had noted that he was hit by the same storm later that night. Franklin was puzzled as to why the storm would move against its prevailing winds, which blew from the northeast. After thinking about it, he concluded that the storm’s winds must circulate counterclockwise. This explained why the prevailing winds in Philadelphia blew from the northeast, while the storm itself moved toward the northeast.

Today, we know a lot more about the type of storm that spoiled Franklin’s view of the eclipse. In fact, Washington’s biggest and most famous winter storms are those that form along the coast. These storms are called northeasters, or as they are more commonly known, nor’easters. Their name is derived from the strong northeast winds that are generated ahead of the storm as air circulates in a counterclockwise direction around the storm center. Many of the strong nor’easters that bring significant snow to Washington form in the northern Gulf of Mexico or along the southeast U.S. coast and then move up the eastern seaboard; some that originate in the Gulf of Mexico track into the Ohio Valley before redeveloping east of the Carolinas.

During winter, a fierce coastal storm can produce snowfall rates up to 4 inches per hour, with thunder and lightning, while 30-40 mph winds can pile snow in five to ten foot drifts. Winds are usually much stronger near the coast, often exceeding 60 mph. Nor’easters are also notorious for creating relentless, pounding waves that can demolish oceanfront homes and wash away miles of beach. The “Ash Wednesday Storm” of 1962, arguably one of the worst nor’easters of the 20^th Century, assaulted the East Coast for five days in early March!  While 20-25 foot waves and 60 mph winds pummeled places like Ocean City, Maryland and Atlantic City, New Jersey, a blinding snowstorm raged in the mountains. Big Meadows, southeast of Luray, Virginia, was buried by 42 inches of snow, a state record that still stands today.

 

 

Nor’easters: Setting the Stage with

Cold High Pressure to the North

 

The classic setup for a nor’easter begins as a cold dome of high pressure builds over New England and/or Quebec, Canada. The importance of the high is two-fold. First, it serves to impede the northward movement of the developing storm. A slow-moving storm results in a long-duration precipitation event and produces strong winds blowing in the same direction over several tide cycles. This creates large, destructive waves along the coast. Second, the high acts as a conduit for the cold air. As the cold air flows around high pressure in a clockwise direction from New England toward the Appalachian Mountains, it is too dense to make it over the high terrain. As a result, it takes the path of “least resistance,” which results in the cold air funneling southward through Pennsylvania, into Maryland, Virginia, and the Carolinas. This is called cold air damming.

 

 

Nor’easters: A Favorable Jet Stream

 

Once a supply of cold air is established, a sequence of events begins to take place high in the atmosphere to spark cyclogenesis (storm formation). A favorable jet stream pattern for big nor’easters to hit Washington is called a split-flow pattern. This is when the jet stream splits after reaching the west coast of the U.S. The northern branch travels across Canada toward New England. It helps maintain a dome of cold air over the Northeast and Middle Atlantic region. The southern branch crosses the Rockies, dives southward toward the Gulf Coast, and then makes a sharp turn to the northeast across the Carolinas and then off the New England coast. As a disturbance moves through the southern branch of the jet stream toward the Gulf Coast, the winds become stronger and air is swept away aloft. This is referred to as divergence. This forces warm, moist air to rise up from the surface to replace it. You have probably witnessed this at home while sitting in front of a roaring fire in the fireplace. As the wind blows across the top of the chimney outside, air is forced to rise up through the chimney.

 

 

Nor’easters:

A Storm is Born!

 

In nature, divergence aloft can cause the atmospheric pressure to fall at the surface, depending on other factors. Once the pressure begins to fall, air spirals in to the center of the low-pressure area, and the storm intensifies. The Gulf Coast and the offshore waters of North and South Carolina are a prime breeding ground for nor’easters. The storms usually form along coastal fronts that develop due to the large temperature contrast between the cold land and the warm water just offshore. January water temperatures about 50 miles offshore are often a balmy 68°F to 78°F in the Gulf of Mexico and in the Gulf Stream east of the Carolinas; meanwhile, land temperatures can be in the 20’s or 30’s!  A coastal storm fueled by very warm, moist air over the Gulf Stream, coupled with a powerful upper air disturbance, can result in explosive development, leading to a very deep, rapidly intensifying low-pressure system called a Bomb.