OVERVIEW Describe how fronts are modified by their movement, orographic features, and underlying surfaces.

The typical fronts we have just covered can and do undergo modifications that strengthen or weaken them. Such things as frontal movement, orographic effects, and the type of surface the fronts encounter contribute to the modification of fronts.

Learning Objective: Describe how fronts are modified by their movement, oro-graphic features, and underlying surfaces.

The weather is greatly affected by the move-ment of frontal systems. From the time the front develops until it passes out of the weather picture, it is watched closely. The speed of the movement of frontal systems is an important determining factor of weather conditions. Rapidly moving fronts usually cause more severe weather than slower moving fronts. Fast-moving cold fronts often cause severe prefrontal squall lines that are extremely hazardous to flying. The fast-moving front does have the advantage of moving across the area rapidly, permitting the particular locality to enjoy a quick return of good weather. Slow-moving fronts, on the other hand, may cause extended periods of unfavorable weather. A stationary front may bring bad weather and can disrupt flight operations for several days if the frontal weather is sitting over your station. Knowledge of the speed of the frontal system is necessary for accurate forecasting. If the front has a somewhat constant speed, it makes your job and the forecasters job comparatively easy. However, if the speed is erratic or unpredictable, you may err as far as time and severity are con-cerned. If a front was ultimately forecast to pass through your station and instead becomes sta-tionary or dissipates, the station forecast will be a total bust.

Mountain ranges affect the speed, slope, and weather associated with a front. The height and horizontal distance of the mountain range along with the angle of the front along the mountain range are the influencing factors. Mountain ranges can affect cold fronts, warm fronts, and occluded fronts differently.

As a cold front approaches a mountain range, the surface portion of the front is retarded and the upper portion pushes up and over the moun-tain. On the windward side of the mountain, warm air is pushed up along the mountain slope because of the additional lift of a now steeper frontal slope and the mountain itself (view A of fig. 4-7-1). After the front passes the crest of the mountain, the air behind the front commences to flow down the leeward side of the range. The warmer air on the leeward side of the mountain is displaced by the colder air mass. As this cold air descends the leeward side of the mountain, the air warms adiabatically (view A of fig. 4-7-1) and clearing occurs within it. However, since the cold air is displacing warm air, typical cold frontal clouds and precipitation may occur within the warm air if the warm air is sufficiently moist and conditionally unstable. In some cases maritime polar air that has crossed the Rockies is less dense than maritime tropical air from the Gulf of Mex-ico that may lie just east of the mountains. If the maritime polar air is moving with a strong westerly wind flow and the maritime tropical air is moving with a strong southerly wind flow, the maritime polar air may overrun the maritime tropical air. This results in extremely heavy showers, violent thunderstorms, and possible tornadoes.

If COLDER stagnant air lies to the leeward side of the mountain range, the cold front pass-ing over the mountain range does not reach the surface but travels as an upper cold front (view B of fig. 4-7-1). Under this condition, frontal ac-tivity is at a minimum. This situation does not continue indefinitely; either the stagnant air below mixes with the air above or the upper cold front breaks through to the ground when the stagnant surface air has warmed sufficiently. Then the front returns to a normal classic front and begins to lift the now warm air. This ultimately results in the development of thunderstorms and squall

Figure 4-7-1.Orographic effects on a cold front.

lines (view C of fig. 4-7-2). In the summer, this occurs frequently in one form along the eastern United States. When a cold sea breeze occurs and a cold front crosses the Appalachian Mountains, the associated cold wedge of on-shore flow forces the warm air in advance of the cold front aloft, producing intense thunderstorm activity.

Generally, the area of precipitation is widened as a cold front approaches a mountain range. There is an increase in the intensity of the precipitation and cloud area on the wind-ward side of the range and a decrease on the leeward side.

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