A significant snowstorm is set to unfold over the northern Rockies. Three to four feet — with locally higher amounts — (makes 1 tot 1.4 meter) is possible in the higher elevations in the Northern and Canadian Rockies. Regardless of specific totals, the impacts may be severe. With leaves still on the trees and the potential for this to be a heavy, wet snow in lower elevarions, we’re looking at lots of tree damage, fairly widespread power outages, and travel concerns.
The story behind this storm consits of several pieces that are coming together at the same time. First of all there is e deep and strong upper level low, secondly the prolonged upslope winds, loads of moisture and very cold polar airmasses coming in from the North.
Below you can see the cold Polar airmasses infiltrating the Northwest this weekend. Blue is the cold air moving in. To make sense of just how cold this air is for this time of year on the map to the right you can see that the air that is infiltrating is about 15 degrees celcius colder than what is normal (purple/pink is way colder than normal and orange is warmer than normal.
The hige differences in tempearture feed a strong upper low. The strength of this feature in the upper atmosphere contributes both to the strong winds, and the vorticity (large scale lift), which is critical in a storm producing precipitation. Last but not least there is a lot of moisture moving in from the west. The combination of cold air, strong vortcity and loads of moisture will make this an exceptional storm for september.
In December or January a storm like this won't turn heads, but in september with all the leafs still on the effect may be severe.