A jet on the windward side of a mountain barrier, blowing parallel to the barrier. The jet is produced when stable synoptic flow at low levels approaches the barrier and is blocked (see blocking) for a significant fraction of a day or longer. This often occurs, for example, when a cold front approaches the barrier. The component of the large-scale flow perpendicular to the ridge forces the flow to ascend the barrier. Because the air column is stable, the air layer near the surface is potentially colder (by definition) than the air layer above it, and the stratification opposes and retards the upslope flow. As the colder air ascends, it produces higher pressure along the slope than at the same level over the plain, and consequently also a pressure-gradient force directed away from the mountains. If this pressure configuration lasts for several hours or more, Coriolis deflection accelerates the flow with a component perpendicular to the pressure gradient, that is, in the along-barrier direction. At timescales greater than a pendulum day.