Mount Index mountain forecast

Summit

5,991 ft (1,826 m)

Forecast grid ground

5,272 ft (1,607 m)

Summit temperature

41°F

Summit wind

ESE 4 mph

Freezing level

8,691 ft

High at the summit, by lapse rate

49°F

The last tile carries the Weather Service’s high from its grid’s 5,272 ft up 719 ft to the summit at the standard atmosphere’s 6.5 °C per kilometer (about 3.6 °F per 1,000 ft). Real lapse rates vary: an inversion can make the summit warmer than the valley.

NOAA’s GFS model, run Sat, Sep 26, 5:00 PM PDT, interpolated to 5,991 ft between its pressure levels: the free air over the summit, which on a ridge is close to what a climber feels and in a sheltered spot is more than it. Wind chill is the National Weather Service’s formula, given where the temperature is 50°F or less and the wind above 3 mph.

The Weather Service’s forecast asked at the summit’s coordinates (47.7743, -121.5804), by its office in Seattle/Tacoma, WA. Its grid cell is 2.5 km across and sits at 5,272 ft (1,607 m), 719 ft below the summit, so its temperatures and winds are for that height.

An estimate: the height a rising parcel of the grid’s air would reach saturation, about 125 m for every degree Celsius between temperature and dew point (Espy’s rule), added to the grid’s ground. It fits cumulus and fog made on the spot, not a deck of stratus carried in from elsewhere.

No snow in the National Weather Service’s forecast for the summit’s grid cell over the next several days.

For a sea-level horizon at the summit’s position, local time. From a summit the horizon sits lower, so the sun is seen a few minutes before sunrise and after sunset; a ridge to the east or west does the opposite.

Mount Index is in the West Slopes Central zone of the Northwest Avalanche Center. Status: off season: a summary, no rating.

Why a summit forecast differs

The air cools with height, by about 3.6°F per 1,000 feet on average, and the wind above a ridge blows at the speed of the free atmosphere rather than slowed by the valley floor. A town’s forecast says little about the top of the mountain above it.

Freezing level

The height where the air is at 32°F (0°C). Above it precipitation falls as snow and wet rock freezes; the snow line usually sits a few hundred to a thousand feet below it.

Wind chill

How cold the air feels on exposed skin, from the temperature and the wind, by the National Weather Service’s 2001 formula.

Cloud base

Where cloud begins. When it is below the summit, the top is in cloud: no view, and in cold air, rime ice and poor visibility.

Prominence

How far a summit rises above the lowest contour line that encircles it and no higher summit: the measure of a mountain’s own height, not its neighbors’.

Summits

• Baring Mountain6,125 ft · 7 mi
• Sloan Peak7,835 ft · 22 mi
• Chimney Rock7,726 ft · 23 mi
• Mount Pugh7,201 ft · 27 mi
• White Chuck Mountain6,988 ft · 31 mi
• Glacier Peak10,541 ft · 32 mi
• Mount Stuart9,413 ft · 38 mi
• Mount Fernow9,249 ft · 45 mi
• Bonanza Peak9,514 ft · 46 mi

Ski areas

• Summit West26 mi
• Three Summits27 mi

Forecasts from the National Weather Service. Summit-height temperature, wind and freezing level from NOAA’s Global Forecast System on NOAA Open Data. Summits from Wikidata (CC0): prominence >= 600 m, or elevation >= 4267 m, or >= 12 Wikipedia editions; English Wikipedia article required; read 2026-09-25. Mountain weather changes fast; check the forecast again before you go.