HELENA, Mont. >> The first major snowstorm of the season dropped up to a foot (30 centimeters) of snow in the Helena, Montana, area by Wednesday, sending an army of snowplows and sand trucks out onto the roads.

Residents woke up to swirling snow and the sound of shovels on sidewalks just days after temperatures rose into the lower 80s. Trees decked out in fall colors and some Halloween decorations were weighed down with snow. Helena Public Schools canceled six school bus routes Wednesday morning, but no schools were closed.

The National Weather Service warned of hazardous travel on snowy mountain passes and ice on some highways when snow initially melts and then freezes as road temperatures drop.

Plows scraped snow off the highways, streets and parking lots and sand was scattered on roads to increase traction as the Montana Highway Patrol responded to dozens of crashes and slide-offs, including jackknifed semi-tractor trailers, according to the patrol’s incident website.

The first snowfall of the season “is always the most dangerous because people just aren’t used to it yet” after driving for months on mostly dry pavement, said Matt Ludwig, a meteorologist with the National Weather Service in Great Falls. Drivers aren’t used to dealing with less traction, slower speeds and longer stopping distances, he said.

Cold air moving down from northwestern Canada has combined with a moist Pacific weather system, leading to freezing temperatures and expected snowfall amounts up 14 inches (36 centimeters) in Washington’s northern Cascade Mountains and 18 inches (46 centimeters) in the mountains of Montana, the National Weather Service forecasts. Some higher elevations in the northern Rockies could see snow totals of 2 feet (61 centimeters) or more.

After the first wave of snow, it was Helena that saw the most accumulation, with a couple spots reporting 13 inches (33 centimeters) of snow. However, the official site at the airport had half that amount, said Cody Molvan, chief meteorologist for the National Weather Service in Great Falls said Wednesday.

Some towns in central Montana reported 10 inches (25 centimeters) of snow, while other areas along the Rocky Mountain Front had 6 to 8 inches (16 to 20 centimeters) of snow as the storm moved east. There was black ice on roads as the storm moved into the Billings area in southeastern Montana, the state’s Department of Transportation said.

The first wave of snow in western Montana ended at midday Wednesday and a second round — less widespread and with much less accumulation — was forecast from Wednesday night into Thursday morning, the weather service said.

The storm brought a sharp change in weather.

Helena tied record temperatures in the lower 80s (high 20s Celsius) late last week, which is about 25 degrees above average for this time of year, Ludwig said. Great Falls also had a day in the low 80s late last week, before being covered in snow on Wednesday.

Temperatures could fall into the low single digits with wind chill values below zero Wednesday night into Thursday morning in Great Falls, the forecast says.

“If that’s not a shock to your system, I don’t know what is,” Ludwig said.

Helena’s Walmart store still had a display of kayaks outside on Wednesday, their prices nearly covered in snow.

The snow had also moved across northwestern and north-central North Dakota by early Wednesday, prompting the North Dakota Department of Transportation to advise residents not to travel in five counties, including the Williston area, because of icy roads and areas of near zero visibility. Oversized loads are not allowed to travel through the area.

State law restricts travel for oversized loads when inclement weather causes the vehicle or attachment to swerve, whip, sway or fail to follow the path of towing vehicle, the agency said.

North Dakota state Rep. Jeremy Olson was headed home from the Legislature’s adjourned special session in Bismarck to his farm near Arnegard, in the area where the heaviest snowfall amounts are expected. He said he bought 400 pounds (181 kilograms) of rock salt to put over his pickup truck’s back axle for extra weight and greater traction on snow and ice.

In the days before the storm, his wife and daughters went shopping for the next week, got shovels ready and prepared a generator in case their power goes out.

“Things we’ve learned over the past years, a few lessons learned over past experiences gave us a good chance to get prepared for this thing,” Olson told The Associated Press.

The area of Williston, Watford City and Minot, in North Dakota’s oil field, could receive the heaviest snowfall, potentially 8 inches to a foot (20 to 30 centimeters), said Nathan Heinert, a meteorologist with the National Weather Service in Bismarck. Bismarck could see 4 to 6 inches (10 to 15 centimeters) of snow late Thursday after rain on Wednesday, he said.

The snow closed U.S. Highway 14-16-20 outside the east entrance to Yellowstone National Park, according to the Wyoming Department of Transportation. There was no word on when the gate 50 miles (80 kilometers) west of Cody, Wyoming, would reopen. The park’s east, south and west entrances and nearly all roads in the park are scheduled to close to car and truck traffic for the season next Wednesday.

Associated Press reporters Jack Dura in Bismarck, North Dakota and Mead Gruver in Cheyenne, Wyoming, contributed to this report.