Winter Weather

A Record-Breaking Winter Storm for Montana

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Early autumn is usually pretty tranquil for the Pacific Northwest. The semi-permanent ridge of high pressure in the Eastern Pacific that keeps us so dry and mild through the summer is usually still robust enough to keep the storm track to our north, and any storms that do impact our area are generally pretty wimpy, and sometimes downright laughable.

However, early autumn also brings the potential for some of the strongest and highest-impact storms of the year. Sea-surface temperatures over the tropics and subtropics are just past their yearly peak, creating an extremely warm, humid airmass for any storms that do develop to tap into, particularly when the remnants of tropical cyclones are entrained into the westerlies. Meanwhile, the northern latitudes are cooling dramatically as the days rapidly shorten and positive feedbacks from increasing snow and ice cover help reflect the little solar radiation they do receive back out into space. When you combine this increasing north/south temperature gradient with abundant moisture, we can get some truly breathtaking storms. The Columbus Day Storm of 1962 is the most extreme example for the Pacific Northwest, but there have been others, like the floods of October 2003 and the windstorm of October 1934. And now, you can add the snowstorm of September 2019 to that list.

Over the past two days, an incredible amount of snow has fallen and continues to fall over the Northern Rockies, particularly Northern Montana. The graphic below shows the snow forecast, not actual amounts, but thus far these amounts have verified and then some.

Montana snow forecast
Credit: NWS Great Falls

Glacier County in NW Montana has lived up to its name, with 40 inches at Browning and 38 inches at St. Mary since Friday with another 6-10 inches forecast through 6 pm Monday. For reference, Browning averages 59 inches of snowfall in an entire year!

And it’s windy too. Look at Cut Bank early yesterday morning. Not exactly shorts weather.

Some brave souls at Flathead Lake took advantage of the strong winds to do some pleasant September surfing on Flathead Lake in NW Montana. At least it wasn’t snowing at the time!

This record-breaking snowstorm began its life as a vigourous shortwave trough that slid down the BC coast into the Pacific Northwest on Friday. This trough subsequently stalled over the Columbia Basin and deepened into a broad upper-level low, entraining moisture from the Eastern Pacific and creating a strong horizontal temperature/moisture gradient (front) along its southeastern edge. This front then evolved into a strong mid-latitude cyclone as it interacted with large-scale upward motion associated with this upper-level low and underwent cyclogenesis.

In plain English, the combination of cold northeasterlies bringing cold air from Canada, warm/moist southwesterlies bringing warm, moist air from the Eastern Pacific, and a favorable upper-level environment created a strong, cold, and moist storm. This, combined with orographic enhancement from terrain, resulted in a terrific snowstorm that absolutely crushed snowfall records for many portions of Northern Montana.

The following images show the upper-level and surface development of this system from 5 pm Friday to 5 pm Sunday

5pm Friday


5am Saturday


5pm Saturday


5am Sunday


5pm Sunday

Washington and Oregon

While Montana definitely took the brunt of this storm, it still brought heavy snow to many locations above 4,000 feet in Washington and Oregon – and even some spots below 4,000 feet. Government Camp, which is often rainy in the heart of winter, saw 2 inches this weekend, and Timberline got a foot. And because we had easterly flow over our area due to being northwest of the upper-level low, the normally-drier eastern slopes of the Cascades saw heavy snow. Looks like Mission Ridge has picked up nearly a foot so far this weekend – not bad at all!

Nearly a foot at ~5,000 feet at Midway Lodge, Mission Ridge
Credit: Mission Ridge Ski and Board Resort

Fortunately for snow-lovers, extended forecasts keep us cooler and wetter-than-normal for the next two weeks, albeit significantly warmer than the past several days. It’s going to be tough to get much snow over the SW Washington/NW Oregon Cascades below 6,000 feet, but locations above 5,000 feet in the North Cascades by Mt. Baker should start to build a nice base.

Thanks for reading and enjoy your week!
Charlie

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