As a stable air column rises to cross over a ridge, it shrinks in the vertical, therefore diverging in the horizontal. This gives it a negative relative vorticity. Accordingly, in the Northern Hemisphere, the flow is deflected anticyclonically: to the right. Since low pressure always lies to the left of an airstream in the Northern Hemisphere, the pressure is lower on the lee of the mountain. See Weisman (1990) Monthly Weather Rev. 118, 4.