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The mountains on the north side run the wrong way. That is why the snow is 210 inches.
The mountains on the north side run the wrong way, and that is the weather
Most ranges in the Rockies run north to south. The Centennial Mountains, which close the north side of Island Park and carry the Continental Divide along their crest, run west to east — one of very few ranges in the Rockies that do. A wall set across the weather instead of along it is a different thing to live under, and the numbers show it.
“The Centennial Mountains are one of only a few ranges within the Rocky Mountains that trend west to east, and the Continental Divide runs along their ridge line.”
— Encyclopedia entry for the Centennial Mountains, describing the range from Monida Pass east along the Divide to the Henrys Fork, with Mount Jefferson as its high point. Retrieved 1 September 2026
The range tops out just over 10,200 feet at Mount Jefferson, and the Divide along its crest is also the Idaho–Montana line. About 100,000 acres of it are roadless. Its south face is the northern rim of the country a cabin gets built in, and the flat below it, where Henrys Lake sits at the foot of the wall, is a different building proposition from the caldera floor, which that page sets out.
What the wall does: 210 inches a year
Weather that reaches Island Park has to get over the Centennials or around them, and what it leaves behind is measured. The Western Regional Climate Center's Island Park station recorded a mean of 210.75 inches of snowfall a season over 71 seasons, 1936–37 through 2012–13. The worst season in that record, 1951–52, dropped 355.5 inches. The climate table for the Island Park Dam station, covering a different period, prints 214.7 inches. Take either; the point is the same.
| Month | Mean snowfall (inches) | What it means on a lot |
|---|---|---|
| November | 25.25 | The ground is closed for excavation by the end of it in most years |
| December | 47.88 | Roof load is accumulating, not melting |
| January | 48.64 | The heaviest month; every access decision is tested now |
| February | 37.52 | Still building |
| March | 29.34 | Pack at its deepest; the melt has not started |
| April | 13.92 | Melt onto frozen ground, which is the drainage test |
Two cautions on those figures, because they are easy to misuse. Snowfall is the depth of snow that fell, not the depth on the ground, which is less because it settles. And neither is the design load. The county's Island Park district carries 170 psf of ground snow load, a separate engineered figure, whose origin and standing that page explains. The 210 inches tells you why 170 is the number; it is not a substitute for it.
Three gaps in the wall, and what a gap does
The Divide is crossed near Island Park at three named passes, and they are the only ways through the north side of the basin. Each is a road, and each is a wind funnel, because a gap in a wall is where the air goes.
- Raynolds Pass, 6,844 feet, on the Divide north-west of Henrys Lake, the lowest of the three.
- Targhee Pass, 7,072 feet, on the state line east of the lake, the route toward West Yellowstone.
- Red Rock Pass, 7,152 feet, on the Divide just north of the eastern Centennials, a back road rather than a highway.
For a lot, the practical reading is orientation. Ground in line with a pass takes the wind that comes through it, and wind moves snow; a lot that faces the gap collects a drift where a lot beside it does not. Where the snow lands is a siting question, and it can be seen in one winter visit and not at all in July.
The line on the crest is also a legal line
The Divide here is the Idaho–Montana border. Everything on the south side of the crest is Fremont County, its Development Code, its snow-load district and its permit path. Everything on the north side is Montana, with different building law, a different county and different rules on almost everything this site writes about. Listings near Henrys Lake occasionally describe ground that is closer to a Montana town than an Idaho one, and it is worth confirming which side of the crest a parcel sits on before assuming any of this applies. Every county figure on this site is an Idaho figure.
Building under the wall
- Visit in January. The snowfall record says that is the month the wall does its work, and drift lines, wind scour and the plowed width of the road are all visible then and invisible in summer.
- Decide who plows before deciding where to build. The record's heaviest three months put roughly 134 inches on the road, and a snowmobile-in lot and a plowed lot are different products at different prices.
- Design to 170 psf from the first sketch — the county requires an engineer above 70, so there is no unengineered path here.
- Put the roof's shed side where the drift is not. The wind through the passes decides that, and it is a free observation in the right month.
- Confirm the state, the county and the jurisdiction before the offer, since the crest is a border and the lake sits at its foot.
The wall is the reason the snow is what it is, the reason the season is what it is, and the reason a cabin here is engineered like a cabin nowhere else in the county. It is also the view out of most north-facing windows. The permit path assumes all of that; the buyer should too.
Common questions
Which mountains are on the north side of Island Park?
The Centennial Mountains, which carry the Continental Divide along their crest and form the Idaho–Montana line. They run west to east, which very few Rocky Mountain ranges do, and top out just over 10,200 feet at Mount Jefferson.
How much snow does Island Park get?
The Western Regional Climate Center's Island Park station averaged 210.75 inches a season over 71 seasons, 1936–37 through 2012–13, with December and January each near 48 inches. The Island Park Dam climate table prints 214.7 inches for a different period.
Is 210 inches of snowfall the same as the snow load?
No. Snowfall is the depth that fell; it settles on the ground, and neither figure is the design load. The county's engineered ground snow load for the Island Park district is 170 psf, and it is that number, not the snowfall record, that a roof is designed to.
What was the snowiest season on record?
1951–52, with 355.5 inches at the Island Park station, according to the Western Regional Climate Center's monthly summary.
Where does the Continental Divide cross near Island Park?
At three named passes: Raynolds Pass at 6,844 feet, Targhee Pass at 7,072 feet, and Red Rock Pass at 7,152 feet. Raynolds and Red Rock are on the Divide itself; Targhee is on the state line in the Henrys Lake Mountains.
Why does a mountain pass matter for a cabin lot?
Because a gap in a mountain wall is where the wind goes, and wind moves snow. A lot in line with a pass collects drifts where a neighbouring lot does not, which is a siting question best answered by a winter visit.
Is the Continental Divide the state line?
Along the Centennials, yes. The south side of the crest is Fremont County, Idaho, with its Development Code and snow-load district; the north side is Montana, with different building law. Every county figure on this site applies to the Idaho side.
Which month is the heaviest for snow?
January, at a mean of 48.64 inches, with December close behind at 47.88. Those two months plus February put roughly 134 inches on the ground, which is the period every access decision is tested in.
How much of the Centennials is roadless?
About 100,000 acres, according to the encyclopedia description of the range. Most of the Idaho side is national forest, so the wall above Island Park is a permanent one.
When should I visit a lot under the Centennials?
In January. Drift lines, wind scour and the real plowed width of the road are visible then and not in summer, and the snowfall record says January is when the mountains do their work.