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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 coast weather, while under the in-fluence of the same general air mass, varies con-siderably as a result of different trajectories of mP air over the Pacific. Thus knowledge of tra-jectories 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 4-1-8 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 connectively unstable. This instability is released when the air is lifted by the coastal mountain ranges. Showers and squalls are com-mon 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 visibility is low. Icing conditions, generally quite severe, are present in the clouds. After this mP air has been over land for several days, it has stabi-lized and weather conditions improve significantly.

TRAJECTORY PATHS B AND C (CY-CLONIC).— Trajectory paths B and C air with a longer overwater trajectory dominate the west coast of the United States during winter months. When there is rapid west-to-east motion

Figure 4-1-8.—Trajectories of mP air over the Pacific Coast in winter.

and small north-to-south motion of pressure systems, mP air may influence the weather over most of the United States. Because of a longer overwater trajectory, this mP air is heated to greater heights, and convective instability is present up to about 10,000 feet.

This air has typical k characteristics turbulent gusty winds, steep lapse rate, good visibility at ground except 0 to 3 miles in pre-cipitation, as well as cumulus and cumulonimbus clouds with showers. These showers are not as intense as those produced in the shorter trajec-tory mP air, but the total amount of precipita-tion is greater.

TRAJECTORY PATH D (ANTICY-CLONIC).— This trajectory usually is over water long enough to permit modifications to reach equilibrium at all levels. When the air reaches the coast, it is very stable with one or two subsidence inversions. Stratus or stratocumulus clouds are frequently found. Ceilings are usually 500 to 1,500 feet and the tops of clouds are generally less than 4,000 feet. Visibility is fair except during the early morning hours when haze and smoke reduce the visibility to less than 1 mile. This type of air is found over the entire Pacific coast. It is incorrectly referred to as mT air, since it follows the northern boundary of the Pacific anticyclone. However, mT air does on rare occasions move into California along this path.

Gradually mP air drifts eastward with the prevailing west-east circulation. In crossing the coastal ranges and the Rocky Mountains, much of the moisture in the lower layers is condensed out; the heat of condensation liberated is absorbed by the intermediate layers of air. On the eastern slopes of the mountains, the air is warmed as it descends dry-adiabatically. As it flows over the cold and often snow-covered land surface east of the mountains, the warm mP air becomes stable in the lower layers. 

The flying conditions in mP air east of the Rocky Mountains are in general the best that are experienced in winter. Relatively large diurnal temperature ranges are observed. Turbulence is almost absent and visibility is good, except for the smoke and haze in industrial areas. Ceilings are generally unlimited, since either no clouds or only a few high clouds are present. This type of mild winter weather occasionally spreads eastward to the Atlantic coast.

When mP air crosses the Rocky Mountains and encounters a deep, dense dome of cP air, it is forced to overrun it and results in storm con-ditions that produce blizzards over the plains states.

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