HELENA — The jet stream descends down from Canada into the northern United States at this time of year as Montana descends into Fall.
MTN’s Curtis Grevenitz explains how the shift creates a rare cloud formation here in Montana -- that occurs in few places around the world.
A Chinook is the strong downsloping wind that develops off of the Rocky Mountain Front, but typically there is a signature cloud formation that forms over the mountains which coincides with the meteorological setup that creates the wind.
The Chinook Arch is a type of mountain wave cloud that is unique to our area because of geography and storm tracks. This formation occurs on the east slopes of the Rocky Mountains particularly in the Montana-Alberta area where mountains run roughly north-south.
The Chinook Arch cloud has a sharp western (upwards) edge. The arch refers to the shape of the western edge as seen by someone looking westward from the ground on the plains to the east. The arch is especially noticeable when the blue sky to the west of it is completely clear of other cloud layers.
Upper level moisture hits the Continental Divide in a perpendicular direction forcing the invisible water vapor up. As the air rises, it cools and reaches the condensation point at which a cloud forms. As long as there is a source of water vapor hitting the divide from the west, the cloud will continue to develop.
The Chinook Arch cloud often proceeds a Chinook Wind as the upper level changes arrive before strong winds develop on the ground. Because of this, the Chinook Arch cloud has an important place in local weather lore as a predictor.