Snow At The End Of Summer Dazzles The Northwest.

How is it that a storm can produce a four foot snowfall before the first week of the new season is over!?  That is exactly what happened last weekend as portions of Montana got absolutely buried.  Parts of Idaho, Washington, California, Oregon, Wyoming, and Colorado also saw snow, which undoubtedly got skiers in those states really fired up, as well.

How does a storm of that magnitude happen in September?  Well, first, you need a dynamic jet stream pattern, with plenty of amplitude in the troughs and ridges strung out across the hemisphere.  Here is the jet stream map from Saturday morning.

Jet Stream Last Saturday

Notice the strong ridge in the Gulf of Alaska.  That feature is consistent with the warm pool of water discussed in last week’s article.  The ridge’s clockwise circulation helps to tap colder air from the high latitudes of the northwest corner of the continent, and that cold air flows directly into the trough as it strengthens over the Northwest on the map. 

The trough is where rising air is found, and the stronger the trough, the more effective it is at drawing cold air southward.  Rising air cools, and in this fashion, strong troughs can actually “manufacture” their own cold air, which helps fight off the more seasonable air found away from the center of the circulation.  Everything has to be just about perfect to create the cold for a snow producer like last weekend’s storm, and it pretty much was a perfect setup.  Here is a look at the anomalies of air found in the mid-levels at the same time stamp. 

Temperature Anomaly: Deviations From Normal

You can see that the deviations from normal were on the order of 8 to as much as 18 degrees below normal (Centigrade) when the trough was at its strongest, plenty cold enough to support early season snow, even in the relatively lower elevations.   Not surprisingly, if you follow the bulge of the coldest air southward from the core that is located in southern Canada, it points right at Browning and Cut Bank, Montana, two spots that saw some of the deepest snow. In order to produce four feet of snow, you need plenty of moisture, and the broad fetch off the Pacific delivered the goods. 

While the storm was a record breaker in many locations, you might be surprised to know that autumn snowfall in North America has shown an increasing trend in recent years, as this chart clearly shows.

Autumn Snowfalls Since 1997

Early season snow across Canada and the northern U.S. helps refrigerate autumn air masses before they spread out and move further south, so I interpret this snowfall as a positive sign for the early stages of the upcoming season.  If you are looking for the next early season dump, keep your eyes open for the next trough to dig into the Northwest/northern Rockies.  

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