experience snow and Arctic temperatures. However, the forecast would be quite
different further to the south, where there would be a round of severe thunder-
storms followed by the passage of the cold front.
MLC’s travel beneath the flow of the middle and upper troposphere. In general,
their centers move along at about 40 km per hour and can traverse thousands of
kilometers during their life cycles. At any onelocation, the disturbance takes
about two days to pass. Because the associated clouds andprecipitationspread
hundreds of kilometers in all directions from the center, huge areas are affected
at one time. Considering their motion, there are some large MLCs that affect
virtually all of the continental United States.
Middle latitude cyclones occur in families, and it is a common wintertime
occurrence to have a cyclone crossing North America while another three cyclones
are over the Pacific Ocean in various stages of development. As these storms swing
onshore there will be dramatic changes of weather every few days.
There is a definite seasonality to MLCs. In that they are energized by the winds
aloft, they are strongest and most plentiful in the winter. In winter, the polar front
jet stream brings occasional storm centers to the subtropics. In summer, MLCs are
fewer, weaker, and have an average position of about 50°latitude. Additionally,
the Earth’s surface geography plays a role in defining average storm tracks. For in-
stance, winter MLCs frequently form just downwind (east) of the Rocky Mountains
because the air has been forced to bend to the right to compensate for the presence
of the mountains; this effectively becomes a disturbance aloft that produces a
MLC at the surface.
MLCs can sometimes achieve the strength of hurricanes. In 1921, the Pacific
Northwest experienced the “Great Blowdown,” a MLC with winds exceeding
160 kph toppling seven billion board feet of valuable old growth timber and killing
hundreds of farm animals and elk. As winds become strong, MLCs are able to
move significant quantities of dust and snow off of the surface. Strong disturb-
ances passing through without much precipitation are sometimes labeled “dust
storms.” A blizzard, defined in the United States as winds of greater than 46 kph
with blowing snow, is a serious social concern with loss of life and property likely.
The largest blizzards have been responsible for massive delays in air and ground
traffic and the deaths of grazing livestock. Urbanized areas are not immune to
these large storms. In 1888, a blizzard dumped over 60 cm of drifting snow in
the New York area. Over 400 people lost their lives and commerce was at a stand-
still for over a week. The “Storm of the Century” was a cyclone affecting Central
America to Ontario in March 1993. Its immense intensity and scope cost econo-
mies several billion dollars while ruining properties with tornadoes and snowdrifts
over 10 m.
Middle Latitude Cyclones 227