Exceptionally Strong Cyclone Brings Wild Mix of Summer, Winter Weather to Northern U.S.

Nick Wiltgen
Published: July 28,2015

In the dead of summer, a time when attention-getting storms are typically tropical and the atmosphere is otherwise stagnant and steamy, a cyclone straight out of the spring and fall playbook is winding up north of the U.S.-Canada border. It whipped up a wild combination of snow showers and straight-line thunderstorm winds of 90-100 mph within the same state on Monday. The storm then produced extremely strong non-thunderstorm winds on Tuesday in the northern Plains, including gusts in excess of 70 mph in parts of two states.
The National Weather Service in Billings, Montana, summed the situation up bluntly in a tweet Tuesday: "This pattern should not happen in July."
Overall, impacts from the storm will lessen by Wednesday as it pulls away, however brisk winds will continue from the Dakotas to the western Great Lakes.
We'll explain the science behind this rare display later – but first, let's get to the show itself.

National Weather Service storm reports Monday included accumulating snow in Idaho and western Wyoming while severe thunderstorms broke out in north-central Wyoming. See the bottom of this article for photos of the snow.

Monday: Fire and Ice – and Tornadoes

Even as wildfires raged in parts of Idaho and Montana, snow showers broke out in mountains of both states as well as in neighboring Wyoming on Monday.
More than an inch of snow accumulated in the mountains of central Idaho and the Tetons of western Wyoming.
(MORE: Rare July Snow Falls on Rockies Ski Resorts)
Meanwhile, less than 200 miles away within the borders of Wyoming, straight-line winds from a severe thunderstorm unleashed wind gusts of 90-100 mph in north-central Wyoming. In Big Horn County, at least four camper trailers were damaged or destroyed by those straight-line winds, according to National Weather Service.
The wild weather wasn't limited to the American side of the border. In Canada, a tornado was confirmed by storm chasers as it tore a path through rural southwest Manitoba, staying on the ground without lifting for up to three hours, according to Environment Canada.
Though it damaged a bridge, power poles and possibly a farm, amazingly it missed every town in its path. Environment Canada meteorologist Mike Macdonald told the CBC that a tornado of such long duration is considered "phenomenal ... and to miss everything is basically a miracle."

Piles of hail linger in the yard of a home near Custer, South Dakota, on Tuesday morning some 13 hours after a hailstorm struck the area on Monday, July 27, 2015. (Credit: Nature Tech Family/Twitter)

In the States, the ground turned white in South Dakota's Black Hills late Monday not because of snow, but from hail. Hail up to 3.5 inches in diameter – larger than baseballs – was reported near Hill City, not far from Mount Rushmore National Monument.
In Custer, South Dakota, yards were still partially covered in hail Tuesday morning as temperatures plummeted into the 40s.

Tuesday's Top Wind Gusts

 

 

 

 

 

 

 

 

Tuesday: Winds Top 70 mph

Monday's wild weather was associated with a developing low-pressure system – a cyclone – which continued to strengthen Tuesday as it moved east over the southern part of the Canadian Prairies, just north of Montana.
This cyclone is unusually strong for the middle of summer, and as a result it's kicked up a large area of strong winds more typical of the fall, winter and spring months.
A few wind gusts of 70 mph were reported in northwest North Dakota, including a 76 mph gust near Crosby early Tuesday evening. At the time of that wind gust, sustained winds were an incredible 56 mph. A separate gust to 76 mph was measured in Garrison later in the evening. Gusts in excess of 60 mph were clocked in Dickinson (68 mph), Tioga (67 mph) and Bismarck (67 mph).
(INTERACTIVE MAP: Current Wind Speeds)
In eastern Montana, numerous locations have reported wind gusts of 50 to just over 75 mph. One location in northeast Montana reported sustained winds of 60 mph with a gust to 76 mph on Tuesday evening. Other top gusts as of Tuesday evening are 71 mph near Navajo and 73 mph near Comertown.
High winds have also gusted over 60 mph in western South Dakota, including top gusts of 67 mph near Ludlow and Lemmon.
The gusty winds were accompanied by cool temperatures for late-July standards, with some areas only in the 50s and 60s Tuesday afternoon.

The Science Behind the Cyclone

A strong punch of wind energy, well anticipated by computer forecast models many days in advance, has been zipping east across the northwestern U.S. and southwest Canada in the upper levels of the atmosphere. As is usually the case, low pressure has spun up at ground level thanks to the interaction between that wind corridor and the sharp elevation drop from the Rockies to the Plains.
This combination regularly gives rise to what meteorologists call "lee cyclogenesis" – the development of low pressure downwind (i.e., east) of the Front Range of the Rockies. These low-pressure systems can bring very strong winds to the Front Range and adjacent High Plains, and can also trigger major severe weather outbreaks farther east as they encounter moist air drawn north from the Gulf of Mexico.
The thing is, strong lee cyclogenesis doesn't usually happen in the middle of summer. That's because winds in the upper atmosphere – even in that wavy corridor of west-to-east wind we call the "jet stream" – are usually near their weakest point this time of year and are often well north of the international border.
As you probably know from experience, July is not known as a typically windy month over most of the country, except during severe thunderstorms and landfalling tropical cyclones.
This storm is an exception to that norm. Large-scale winds are driven by pressure differences in the atmosphere, and this unseasonably strong low pressure is bringing strong winds over an unusually large area.
The National Weather Service in Billings said in a tweet Tuesday morning that Tuesday could end up being the "windiest July day ever" in Billings, presumably based on the day's average wind speed.
MORE: Snow in Montana and Wyoming, July 27, 2015