When to hike the JMT — season, snow and weather

The John Muir Trail has a short, weather-defined season. In broad terms it is walkable from late June to early October, and comfortably walkable from mid-July to mid-September. The exact edges move by weeks depending on one number: the previous winter’s snowpack.

Snowpack sets the start

The passes are what close the trail, not the valleys. Forester Pass at 13,200 feet, Muir Pass at 11,955 feet and Mather Pass at about 12,100 feet all hold snow on their north-facing approaches long after the trail below is dry. In a low-snow year the route can be effectively clear by late June. After a winter at 150 percent of average, firm snow on the passes and dangerous creek crossings can persist into late July.

Check the April 1 snow-water-equivalent figures for the southern Sierra before you commit to a June or early-July start. If they are well above average, plan for an ice axe, traction, real snow-travel skill, and fords that are genuinely hazardous rather than merely cold.

Month by month

June. Long days and empty trail, but the highest risk. Creek crossings are at peak flow — Bear Creek, Mono Creek, Evolution Creek and the Whitney-side drainages have all injured or killed people at high water. Snow travel on the passes is likely. Mosquitoes are ramping up.

July. The mosquitoes peak in the first half of the month around lakes and meadows, and they are not a minor annoyance; head nets earn their weight. Fords drop steadily. From roughly mid-July the North American monsoon starts pushing moisture north, so afternoon thunderstorms become a daily planning constraint. Days are warm, nights are usually just above freezing.

August. For most people the best compromise. Snow is gone from all but the shadiest aspects, fords are manageable, and mosquitoes fade noticeably after the middle of the month. Thunderstorms are still common in the afternoon. This is also when the trail is busiest.

September. Arguably the best trail conditions of the year: stable air, no bugs, clear light, low water. The trade is cold. Nights at 10,000 to 11,000 feet regularly drop into the 20s Fahrenheit and can reach the teens late in the month. Daylight shrinks fast. The first real Pacific storm can arrive any time from mid-September, and it can drop snow on the passes overnight.

October. Occasionally glorious, frequently over. Resupply points close, days are short and cold, and a single storm can end the trip.

Daily weather rhythm

Between mid-July and late August the pattern is reliable enough to plan around: clear mornings, building cumulus by late morning, thunderstorms between roughly 1pm and 5pm, clearing in the evening. The practical rule is to be over your pass and off exposed ground before early afternoon. That means starting at first light on pass days, which also gets you firmer snow in early season and softer light for the walk.

Temperatures to plan for

Most nights are spent between 9,000 and 11,500 feet, so the numbers that matter are alpine numbers, not valley ones. Expect daytime highs of roughly 60 to 75°F and overnight lows of 25 to 40°F through the core season, colder in September. A 20°F sleeping bag is the common choice; a 30°F bag works in August for people who sleep warm.

Smoke is now part of the calculation

Late summer and autumn are peak wildfire season in California. Smoke from fires hundreds of miles away can sit in the Sierra for days, and closures can appear with very little notice. Check air quality and active incidents in the days before you start, and again at every resupply where you get signal.