Mar–May
High-elevation Idaho stays snowbound. Sawtooth NRA campgrounds (Redfish, Stanley Lake) sit at 6,500-plus feet and typically open Memorial Day weekend. The Sawtooth Wilderness holds snow into July. Highway 21 from Lowman to Stanley closes seasonally for avalanche danger and reopens mid-April through May. Craters of the Moon is at 5,900 feet on the open Snake River Plain — accessible year-round, though wind and wide temperature swings make April and early May variable. Hells Canyon at 1,300 feet is at its best now: green hillsides, rattlesnake-pre-warmth, and rafting season starting to build. City of Rocks shoulder season runs through May with cool-rock conditions for traditional climbing. The Idaho Panhandle is wet and gray, and Priest Lake holds ice into April.
Jun–Aug
Everything is open. Sawtooth NRA campgrounds bind tight on six-month reservation windows — Redfish Lake's Mount Heyburn and Glacier View fill within minutes for July and August weekends. Stanley sits at 6,260 feet and can drop below freezing in any month — a 30°F night in mid-July is unremarkable. The Salmon River's Main and Middle Fork peak permit lottery runs through Recreation.gov in January for the following summer; private permits are notoriously hard to draw. Hells Canyon water temperatures climb and rattlesnake activity peaks. Wildfire smoke is the seasonal risk from mid-July onward — central Idaho carries the highest fire load of any state in the West most years.
Sep–Nov
The peak season for sockeye salmon viewing at Redfish Lake — late August through mid-September, when the fish that swam 900 miles from the Pacific up the Snake and Salmon to spawn are visible from the boardwalks. Larch turn gold in the northern panhandle and the Bitterroots in late September through mid-October. Highway 21 over Banner Summit can close in late October with the first heavy snow; Sawtooth NRA campgrounds shut down by late September. Hells Canyon stays in the 70s well into October — the best rafting weather of the year. Craters of the Moon shoulder season is excellent through October. Hunting seasons (elk, deer, antelope) open in September across most units; expect heavy backcountry use on USFS land.
Dec–Feb
Most of the state's developed campgrounds are closed and behind locked gates by Thanksgiving. Sun Valley and the Sawtooth crest hold deep snow — typically 10 to 15 feet at Galena Summit. Craters of the Moon stays open year-round, and Pittsburg Landing in Hells Canyon is accessible most winters, though the gravel road from White Bird can ice. City of Rocks stays open through mid-November before winter closure. Cross-country and skate ski networks run at Galena Lodge, Bogus Basin, and across the Boulder-White Cloud foothills. Cold-air-pool inversions trap fog and minus-20 mornings in the Stanley Basin — among the coldest reliable readings in the Lower 48.