The Weather Channel - Everything Weather/TheWeatherChannel-EverythingWeather-Win31.iso/twcia/winters/winters.dxr/00090_WSBAST20.TXT.txt

Text File | 1995-08-22 | 5.3 KB | 127 lines

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.  The general availability of both cold and warm air in the
troposphere makes winter storms in middle latitudes a common
occurrence.

When the jet stream dips to the south in the Northern Hemisphere, it
creates what meteorologists call a trough.  A trough is a dip to the
south in the jet stream caused by the advancement of colder air from
north to south. This allows cold polar air masses to invade the United
States.  Simultaneously, the Gulf of Mexico and/or the Atlantic Ocean
often supplies the heat and moisture needed to fuel winter storms.

It's not an uncommon winter weather pattern for a low pressure
system located as far away as the western Gulf coast of the United
States to spread high cirrus clouds across the Great Lakes, causing
wintry precipitation to fall in the Ohio Valley, hundreds of miles to the
northeast.

How can a low pressure system located in the western Gulf of Mexico
affect the weather hundreds of miles to the northeast? In this scenario,
warm moist air from the Gulf rises over a dome of cold air in the
Northeast, resulting in wintry precipitation.  Meteorologists refer to this
occurrence as overrunning.  This overriding pattern of warm air over
cold can cause steady snow, sleet, or freezing rain over a wide area
for as long as 12 to 24 hours at a time.

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.

In places like New York City and Boston, for instance, if the 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 accompanying the
storm, Boston and New York will typically get snow or a mixture of
precipitation types.

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 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 degrees Fahrenheit.
.
To produce a significant winter storm in the Deep South, not only must
temperatures be cold enough to produce snow, sleet or freezing rain,
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 precipitation.

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.  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 of
the blue norther 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 a blue norther.

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 Rocky Mountain 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 wide-open spaces and lack of
vegetation found in the Great Plains, the winter winds can blow even
light amounts of snowfall on the ground into large drifts.