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 forecaster s 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. 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. 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. 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.