Types Of Winter Storms
The type of precipitation that comes from a winter storm situation often depends on the storm's path. Since cold air is usually found on the north side of a storm and milder air on the south side, wintry precipitation generally falls in areas north of the storm center's track.

Various regional storms, such as a nor'easter or an ice storm in the southeast, can dominate an area for days.

Snowstorms in the eastern Rockies or a "blue norther" in the Southern Plains can cause havoc as well.

Winter weather also means low cloudiness and wintry precipitation that is experienced with upsloping and overrunning.

Blizzards Back to Top
Blizzards are characterized by low temperatures (usually below 20°F) and accompanied by winds that are at least 35 mph or greater, there must also be sufficient falling and/or blowing snow in the air that will frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours.

A severe blizzard is considered to have temperatures near or below 10°F, winds exceeding 45 mph, and visibility reduced by snow to near zero.

Blizzard conditions often develop on the northwest side of an intense storm system The difference between the lower pressure in the storm and the higher pressure to the west creates a tight pressure gradient, which in turn results in very strong winds.

These winds combine with snow and blowing snow to produce extreme conditions. Storm systems powerful enough to cause blizzards usually form when the jet stream dips far to the south, allowing cold air from the north to clash with warm air from the south.

With the colder and drier polar air comes atmospheric temperatures cold enough for the development of snow, sleet, or freezing rain.

The Dakotas and parts of Minnesota were pounded by a succession of blizzards in the winter of 1996-1997. With few trees or other obstructions to reduce wind and blowing snow, this part of the country is particularly vulnerable to blizzards.

A continuation of the broad central plains that run from the Gulf of Mexico into Canada, this area often experiences severe blizzard conditions, with wind chills of 40°F below or more and whiteout conditions.

Another scenario occurs when a cold core low over the Hudson Bay area is displaced southward over southeastern Canada, the Great Lakes, and New England. As rapidly moving short waves, or areas of upward vertical motion and energy, move around the low, they collide with warmer air coming north from the Gulf of Mexico. This produces strong surface winds, lots of cold air advection, and extensive wintry precipitation.

In the southern central Great Plains, rapidly intensifying low pressure systems moving out of the Rocky Mountains can cause heavy snows and strong winds to the north, while to the south and east are thunderstorms and rain.

When cold, moist air from the Pacific Ocean makes it over the Rockies and into the Plains and warmer, moist air moves north from the Gulf of Mexico, all that is needed is a cold closed low aloft and a strong polar jet for potential blizzard conditions that may extend from the panhandle of Texas to the Great Lakes.

Eastern Rockies Back to Top
A nearly continuous trough in the western United States often allows enough moisture and cold air to provide the Rockies with ample snowfall in winter.

The world record for the most snowfall in a 24-hour period was set in the town of Silver Lake, Colorado, where 76 inches of snow fell on April 14 and 15, 1921. Eighty-seven inches of snow were recorded for the entire storm which lasted less than 28 hours.

Strong areas of low pressure often develop on the east side of the Rocky Mountains and may bring blizzard conditions to the Northern Plains states. Because of the lack of natural barriers found in the Great Plains, the winter winds can blow even light amounts of snowfall on the ground into large drifts.

Great Plains Back to Top
Texas and the Southern Plains also receive their fair share of winter storms. A typical scenario begins with a strong cold front moving down the eastern slopes of the Rocky Mountains. As this front moves into the Texas panhandle, winds switch to the north and temperatures drop markedly.

Locals in the panhandle call this weather event a blue norther, referring to the blue-black sky that often accompanies these storms.

A blue norther brings cold air to Texas. The situation is complicated when moist air is present farther south in Texas, accompanied by upper winds out of the southwest or west. As the surface cold front continues to push southward, it undercuts the warm, moist air near the earth's surface.

The cold front slows or perhaps remains in a stationary position. This can result in an extended period of wintry precipitation, usually in the form of freezing rain or sleet caused by a shallow cold air dome near the ground.

The effects of a blue norther could be seen on Thanksgiving Day, 1993, when the Dallas Cowboys played football at Texas Stadium in the midst of a sleet storm. In January 1985, San Antonio experienced unusual weather in the form of snow and sleet, spurred on by the effects of the cold front.

Ice Storms Back to Top
The Deep South only experiences winter storms a few times during a typical winter.

Southeastern snow or ice storms often form when an area of low pressure moves eastward across the northern Gulf of Mexico. When this happens, cities like Jackson, Mississippi, Birmingham, Alabama, and Atlanta, Georgia, find themselves on the cold side of the storm.

On January 7 and 8, 1973, Atlanta and areas of north Georgia were hard hit by one to four inches of ice that closed schools and left 300,000 people without power for up to a week. Over 2.25 inches of liquid equivalent in the form of freezing rain, sleet, and snow fell on Atlanta between 7:00 and 9:00 p.m. on January 7, while temperatures remained at 32°F.

To produce a significant winter storm in the Deep South, not only must temperatures be cold enough, but there must also be enough moisture in the atmosphere to produce adequate precipitation.

Though it may be cold enough in Jackson, Birmingham, or Atlanta for frozen precipitation to fall, if the low pressure system passes too far south of these cities, there will not be enough moisture to produce wintry precipitation.

Lake Effect Back to Top
Lake effect snows occur when a mass of sufficiently cold air moves over a body of warmer water, creating an unstable temperature profile in the atmosphere.

As a result, clouds build over the lake and eventually develop into snow showers and squalls as they move downwind. The intensity of lake effect snow is increased when higher elevations downwind of the lake force the cold, snow-producing air to rise even further.

The most likely setting for this localized type of snowfall is when very cold Arctic air rushes over warmer water on the heels of a passing cold front, as often happens in the Great Lakes region during winter.

Winds accompanying Arctic air masses generally blow from a west or northwest direction, causing lake effect snow to fall on the east or southeast sides of the lakes.

Whether an area gets a large amount of snow from lake effect is dependent on the direction of the winds, the duration they blow from a particular direction, and the magnitude of the temperature difference between the water and air.

Since cold air can hold very little moisture and the low level of the atmosphere is quite unstable, clouds form very rapidly, condensation occurs and snow begins to fall. Lake effect snow is lighter than snow that forms from frontal stratus or nimbostratus.

Areas of relatively high elevation downwind of the Great Lakes generally receive heavier amounts of lake effect snow than do other locations in this region.

For example, residents of the Tug Hill Plateau in New York State east of Lake Ontario can spend the winter months digging out of anywhere from 200 to 300 inches of snow. Likewise, the mountains of West Virginia can receive over 200 inches of snow in a winter, helped by the lake effect.

The only other lake that produces significant lake effect snow in the United States is the Great Salt Lake in Utah.

Cape Cod Bay in Massachusetts and Chesapeake Bay in Maryland and Virginia, on occasion, produce what is called bay effect snow. Bay effect snow forms in the same manner as lake effect snow, only over the ocean.

Nor'easters Back to Top
Nor'easters are among winter's most ferocious storms. These strong areas of low pressure often form either in the Gulf of Mexico or off the East Coast in the Atlantic Ocean. The low will then either move up the East Coast into New England and the Atlantic provinces of Canada or out to sea.

In places like New York City and Boston, for instance, if the wintertime low tracks up to the west of these cities, wintry precipitation will often change to rain. However, if the low moves slightly off the coast to the east of these cities, assuming there is enough moisture and cold air accompanying the storm, Boston and New York will typically get snow or a mixture of precipitation types.

A nor'easter gets its name from its continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

These winter weather events are notorious for producing heavy snow, rain, and oversized waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity.

Detailed studies taken from satellite imagery and other readings suggest that some low pressure systems associated with nor'easters may develop tropical storm characteristics such as an eye in the center of the low.

However, the presence of fronts and drop in temperature at higher levels of the troposphere keep the storm from being classified as tropical.

Some notable nor'easters have wreaked havoc upon major metropolitan areas of the Northeast. The Halloween Storm of 1991 damaged or destroyed more than 1,000 homes from Maine to the Carolinas. The February 1979 Presidents Day Storm brought the nation's capital to a standstill.

The Ash Wednesday Storm of 1962 battered the northeast coastline with rain, snow, sleet, coastal flooding, and wind for five days.

View the stories of some of these storms and other information about nor'easters through our Special Report area.

Overrunning Back to Top
Because the United States is located in the middle latitudes of the Northern Hemisphere, an ample amount of both cold polar air and warm tropical air is available in the troposphere during the winter months.

Overrunning occurs when moist, warmer air is directed up and over a mass of colder air at the surface of the earth. The warm air cools as it rises, and its moisture condenses into precipitation-producing clouds.

This type of weather pattern typically occurs ahead of a trough in the upper level of the atmosphere. When the jet stream dips to the south in the Northern Hemisphere, it creates a trough of low pressure, allowing cold polar air masses to invade the United States.

Simultaneously, southwesterly flow ahead of the trough at high levels of the atmosphere directs warmer, moist air from the Gulf of Mexico over the low-level cold air.

This overriding pattern of warm air over cold can cause steady precipitation over a wide area for as long as 12 to 24 hours at a time. If low level air is cold enough, the precipitation can fall in frozen form.

It's not an uncommon winter weather pattern for southwesterly winds aloft to spread clouds all the way from the Gulf Coast to the Great Lakes, with rain falling across much of the south and wintry precipitation glazing areas farther north.

Pacific Coast Winters Back to Top
The West Coast of the United States receives the lion's share of its yearly precipitation during the winter months.

Unlike many other areas of the country where precipitation is relatively abundant throughout the year, the West Coast, particularly California, depends on winter rain and snow to replenish its water supply. If precipitation is light, especially for more than one winter in a row, water resources can become dangerously low.

In winter, the jet stream generally dips to the south, allowing storm systems from the Pacific to affect the West Coast. Big dips in the jet stream allow low pressure systems to move into the southern California area as well.

Sometimes California can receive a deluge of precipitation when significant subtropical moisture makes its way across the Pacific and interacts with a West Coast trough. Meteorologists often dub this occurrence "the Pineapple Express."

During some winter seasons, El Niño also plays a significant and sometimes disastrous role in southern California weather. During El Niñntilde;o years, the eastern tropical Pacific Ocean sea water temperatures are above normal. Higher water temperatures create more heat and moisture in the air than is usual over this generally cool body of water.

The anomaly is generally just a few degrees, but that's enough to enhance the amount of subtropical moisture that southern California receives in winter. This moisture, especially when it interacts with the polar jet stream, can produce heavy downpours, resulting in mud slides, rock slides, and severe flooding and flash flooding conditions.

While California sometimes feels the force of too much precipitation, there are years when the state does not receive an adequate amount of winter rain and snow. In these years, the jet stream moves away from the Golden State.

In a winter storm track situation where Washington and Oregon receive most of the winter precipitation, if the low pressure system is strong enough powerful upsloping winds may accompany the storm and heavy snow may fall in the Cascade Mountain range, particularly on the western-facing slopes. This situation can also occur in California with the Sierra Nevada Mountains.

Westerly winds often follow behind a West Coast winter low pressure system. Because the wind flows eastward from the Pacific Ocean, it brings with it additional moisture. When the air mass aloft behind the front is cold enough, thunderstorms frequently occur, often producing small-sized hail.

Upsloping Back to Top
Usually when heavier precipitation is expected, abundant moisture is present in the troposphere. Sometimes, however, substantial amounts of precipitation can fall with less moisture available in the air if adequate lifting is present.

In an upslope flow, air rises and cools as it is forced to ascend to higher terrain. It then condenses and forms into clouds if it is cooled to its dew point. The eastern Rockies is an area that often experiences upslope conditions during the winter. Denver, Colorado, may have lots of wintry precipitation, even if there is not a substantial amount of moisture present in the troposphere.

With an area of low pressure to the south, east to northeast winds blow in from the eastern Colorado plains forcing air to rise upward. This is known as an upslope flow.

Even during the winter months, precipitation will not fall as anything but rain unless temperatures in the vertical profile of the lower troposphere are cold enough to produce freezing or frozen precipitation.

Freezing rain, sleet, and snow generally cannot fall unless a cold polar air mass is present.

In the Southeast, station elevation is often a key factor in determining precipitation type, since the air generally is colder at higher altitudes.

For instance, temperatures may be cold enough to produce snow in the Georgia mountains, but not cold enough to produce anything more than rain at lower altitudes just 20 miles down the road. Upslope is also common along the eastern slopes of the Appalachian Mountains when east to southeast winds are present.

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