A difference in air pressure and temperature occurs. While weather conditions CAN affect air mass movement, a mixture of cold and warm air masses (which have differences in air pressure and temp) cause jet streams and more directly affect the movement of air masses. As a cold air mass builds, the temperature gradient between the cold air mass and the warmer air outside the air mass will increase. This increases the upper level winds which in turn helps steer an air mass away from its source region.
