Sun Valley, Dec 15–21

• Each week is the same dates every year: the 1st–7th, 8th–14th, 15th–21st, and the 22nd to month end. Thanksgiving, MLK Day and Presidents' Day always land in the weeks marked.
• Comparing weeks: the last week of each month runs 7 to 10 days, so weeks are compared by the share of their days with fresh snow (a percentage of the days), never by a count that grows with the week's length. "In 13 of 20 winters" describes one week and is true for it, but isn't used to rank weeks. The same goes for the best shot at a Bluebird day: Bluebird days per lift day.
• Fresh snow: the federal SNOTEL station near the mountain gained about 4" or more in 48 hours (3" when it stayed below freezing, or it caught heavy, cold snow that packed down), the same test Snowcall uses every morning. A big day is 8" or more.
• Lifts running: open at least 3 days of the week, from resort announcements and news, so approximate. A week counts at a mountain when that happened in 14 of the 20 winters, or in 4 of the last 5: snowmaking has moved some openings earlier, so recent winters say more about opening weeks than older ones. For some winters, mostly before 2016, no opening or closing day could be found; those use that mountain's 2016-26 average.
• Percentages are shares of days, rounded to whole numbers. Twenty winters of one week is about 140 days, so two mountains within a few points of each other are a near-tie (marked "close" when within 5). Counts of winters stay counts ("13 of 20"), since the 20 shows how small the sample is.
• Clear skies: days with under 30% cloud from 9 AM to 4 PM, the same test the calls use, over all of the week's days in every winter. Past skies come from the ERA5 weather record, which sees the whole sky, so a day when the summit sat above valley fog can count as cloudy. Most weeks sit within a few days of each other; mid-January stands out as the clearest.
• New snow is how much deeper the station got that week; snow settles, so it runs a little under what fell. On the ground is the station's depth at the start of the week, not the resort's base.
• What the days were like: each lift day's call, the same five words as the home page, over every winter. Bluebird is rare everywhere, so it's also counted in winters: did that week have at least one Bluebird day while the lifts ran? Past days use a reconstruction of the sky (the ERA5 weather record), not the forecast the app had that morning.
• Day type squares: with Day type on, each winter's square shows its best day that week, in call order (Bluebird, Powder, Storm, Bluebird Groomer, Quiet).
• Twenty winters is still a small sample: a pattern, not a forecast. Any week can be dry.

Snow data: USDA NRCS SNOTEL, winters 2006-07 to 2025-26. Rule v3.3. Sky history: contains modified Copernicus Climate Change Service information 2026. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.