East Idaho is swinging from summer to winter and back again — a temperature whiplash dramatic enough to put a rare winter storm warning on the books and put some local cities within striking distance of decades-old cold records.

Just days after daytime highs across the region sat in the low 90s, the National Weather Service has issued a winter storm warning — for late June — covering the backcountry of Lemhi County above 6,500 feet. The warning, in effect Saturday evening through Monday morning, calls for 4 to 8 inches of heavy wet snow in Lemhi County with up to a foot possible across the highest peaks, driven by west winds gusting up to 30 mph.

It's the kind of warning forecasters don't issue lightly. The weather service pointed out that while mountain snow in late June in East Idaho is not all that unusual, a winter storm warning is a very much rare occurrence.

The cold isn't confined to one county, either. The weather service said snow is likely above 6,500 feet across all of East Idaho into Monday morning, while the lower elevations swap their summer sun for clouds, possible rain and a chance of thunderstorms Sunday and Monday.

But the headline isn't the snow — it's the plunge. Temperatures are forecast to fall into the high 30s and low 40s both Saturday night into Sunday and again Sunday night into Monday throughout East Idaho, with daytime highs struggling to climb out of the high 50s to low 60s. That's a swing of 30 degrees or more from the low 90s heat the region was sweating through just days earlier.

The cold is forecast to be sharp enough to threaten record books that have stood for decades. The weather service said there's a good chance Pocatello breaks its June 28 record low of 37 degrees set in 1965 and June 29 record low of 35 degrees set in 1969. Rexburg could easily break its own June 28 record of 39 degrees set in 1998 as well as its June 29 record of 41 degrees set in 2014. Other East Idaho cities including Idaho Falls are expected to flirt with their own records, falling just 2 to 3 degrees short.

For anyone heading into the East Idaho backcountry through Monday morning, forecasters say to plan for true winter conditions. Cold temperatures, wind and snow raise the risk of hypothermia, and the weight of wet snow can be enough to snap trees and block access to backcountry roads.

The National Weather Service is urging people to delay travel through the backcountry and higher elevations if at all possible. Anyone who must travel should carry a winter storm kit — tire chains, booster cables, a flashlight, a shovel, blankets and extra clothing — along with water, a first aid kit and anything else that could help if they become stranded.

As abruptly as it arrived, the cold is expected to leave. Forecasters say warmer conditions will return regionwide starting Tuesday — closing the books on one of the stranger stretches of East Idaho weather in years.