A “potentially historic” fall snowstorm is set to wallop a large stretch of the country — evolving into an “all-out blizzard” as it passes over the Dakotas and sending an arctic blast into much of the northern US, forecasters predict.

Denver was downright balmy on Wednesday with a high in the lower 80s — but the temperature had already plummeted into the upper 20s by 1 a.m. as snow began to fall, according to Accuweather. From 1 to 3 inches of snow is expected to fall in the Mile High City on Thursday.

A similar drastic change happened in Rapid City, South Dakota, which reached a high of 80 degrees Tuesday — but the mercury dipped 40 degrees Wednesday, the weather service reported. As much as 1 foot of snow could fall in the city, according to predictions.

An “all-out blizzard” is expected to slam the region spanning central and northeastern South Dakota into central and eastern North Dakota, AccuWeather chief broadcast meteorologist Bernie Rayno said.

Most of the Dakotas will see 6 to 12 inches of snow, but 12 to 24 is likely — with 30 inches possible from north-central South Dakota to central and northeastern North Dakota, Accuweather predicted.

“A significant, and potentially historic, October winter storm is expected to impact the northern Rockies into the central and northern Plains through Saturday,” the National Weather Service said in a tweet. “Significant travel disruptions, power outages, and impacts to agriculture and livestock are all anticipated.”

Winds from the northwest will average around 15 to 30 mph with frequent gusts above 40 mph over the northern and central Plains, according to forecasters. The Dakotas could be battered with gusts up to 60 mph.

Gusty winds will also send a chill way out to the south and east, over the middle Mississippi Valley, Great Lakes and Ohio Valley this weekend, according to Accuweather.

Temperatures in Minneapolis will drop from the low 70s to around 40, and from the 70s to about 50 in Chicago over the weekend, forecasters say.