On the leeward side of the Great Lakes and on the
windward side of the Appalachians, you can expect a
rather low, broken to overcast sky condition with
frequent and widespread snow squalls. Stratocumulus
and cumulus clouds with bases at 500 to 1,000 feet and
tops at 7,000 to 10,000 feet form on the leeward side of
the Great Lakes. Over the mountains, their tops extend
to about 14,000 feet. Visibility ranges from 1 to 5 miles
during rain or snow showers and occasionally lowers to
zero in snow flurries.
Severe aircraft icing conditions may be expected
over the mountains and light to moderate aircraft icing
on the leeward side of the lakes. Moderate to severe
flying conditions are the rule as long as the outflow of
cold air continues.
East of the Appalachians, skies are relatively clear
except for scattered stratocumulus clouds. Visibility is
unrestricted and the surface temperature is relatively
moderate because of turbulent mixing. In the Middle
West, clouds associated with this type of air mass
continue for 24 to 48 hours after the arrival of the cold
mass, while along the Atlantic Coast rapid passage of
the leading edge of the air mass produces almost
immediate clearing.
TRAJECTORY PATHS C AND D (ANTI-
CYCLONIC).The weather conditions experienced
over the central United States under the influence of
trajectories similar to C and D (fig. 4-5) are quite
different. Unusually smooth flying conditions are
found in this region, except near the surface where a
turbulence layer results in a steep lapse rate and some
bumpiness. Low stratus or stratocumulus clouds in may
form at the top of the turbulence layer. As the cold air
stagnates and subsides under the influence of the
anticyclonic trajectory, marked the haze layers develop
indicating the presence of subsidence inversions. The
surface visibility also deteriorates because of an
accumulation of smoke and dust as the air stagnates and
subsides. This is especially noticeable during the early
morning hours when the stability in the surface layers is
most pronounced. In the afternoon, when surface
heating has reached a maximum, the visibility usually
improves because of the steep lapse rate and resultant
turbulence.
Movement of cPk and cAk air westward over the
Rocky Mountains to the Pacific coast is infrequent.
However, when successive outbreaks of cold air build
up a deep layer of cP air on the eastern slopes of the
Rocky Mountains, relatively cold air can flow toward
the Pacific coast.
TRAJECTORY PATH E.When the trajectory
of the cold air is similar to E in figure 4-5, rather mild
temperatures and low humidities result on the Pacific
coast because adiabatic warming of the air flowing
down the mountain slopes produces clear skies and
good visibility.
TRAJECTORY
PATHS
F
AND
G.
Occasionally, the trajectory passes out over the Pacific
Ocean (see fig. 4-5). The air then arrives over central
and southern California as cold, convectively unstable
air. This type is characterized by squalls and showers,
cumulus and cumulonimbus clouds, visibility of 1 to 5
miles during squalls and showers, and snow even as far
south as southern California.
Maritime Polar (mP) Air Pacific in Winter
Maritime polar air from the Pacific dominates the
weather conditions of the west coast during the winter
months. In fact, this air often influences the weather
over most of the United States. Pacific coastal weather,
while under the influence of the same general air mass,
varies considerably as a result of different trajectories
of mP air over the Pacific. Thus knowledge of
trajectories is of paramount importance in forecasting
west coast weather.
When an outbreak of polar air moves over only a
small part of the Pacific Ocean before reaching the
United States, it usually resembles maritime arctic cold
(mAk). If its path has been far to the south, it is typically
mP. Figure 4-8 shows some of the trajectories (A, B, C,
D) by which mP air reaches the North American coast
during the winter.
TRAJECTORY
PATH
A
(CYCLONIC).
Trajectory path A air originates in Alaska or northern
Canada and is pulled out over the Pacific Ocean by a
low center close to British Columbia in the Gulf of
Alaska. This air has a relatively short overwater path
and brings very cold weather to the Pacific Northwest.
When the air reaches the coast of British Columbia and
Washington after 2 to 3 days over the water, it is
convectively unstable. This instability is released when
the air is lifted by the coastal mountain ranges. Showers
and squalls are common with this condition. Ceilings
are generally on the order of 1,000 to 3,000 feet along
the coast and generally 0 over the coastal mountain
ranges.
Cumulus
and
cumulonimbus
are
the
predominating cloud types, and they generally extend
to very high levels. Visibility is generally good because
of turbulence and high winds commonly found with
this trajectory. Of course, in areas of precipitation, the
4-9