Appalachian Trail Weather Planner

Several sections of the AT present weather conditions significantly more extreme or variable than the regional averages shown in the planner. Hikers in these areas should consult point forecasts (Mountain-Forecast or ATWeather, linked below) and allow extra margin in their plans.

• Great Smoky Mountains (TN/NC, miles 165–230): The Smokies are among the cloudiest and wettest places in the eastern US, receiving over 85 inches of rainfall annually at higher elevations. Fog and low visibility are common even in summer, and temperatures at Clingmans Dome (6,643 ft) run 10–15 °F cooler than nearby valleys. Snow is possible from October through May, and afternoon thunderstorms are frequent from June through August.
• Roan Highlands (TN/NC, miles 370–390): The Roan Massif carries the trail across some of the longest stretches of open, exposed ridgeline on the entire AT. Lightning is a serious hazard here in summer; hikers are advised to be off the balds by early afternoon during thunderstorm season. Winds are sustained and strong year-round, making apparent temperatures considerably colder than nearby sheltered valleys in winter and spring.
• Presidential Range and White Mountains (NH, miles 1,840–1,930): This section includes Mount Washington, which holds the record for the highest wind speed ever directly measured on Earth (231 mph). Weather on the Presidentials is notoriously fast-changing and severe in any season; above-treeline conditions can be life-threatening even in July. The AMC summit observatory issues mountain-specific forecasts that are essential reading before crossing this section.
• Mahoosuc Range (ME, miles 1,930–1,980): The Mahoosucs mark the transition into Maine's demanding terrain. Cold fog and surprise freezes extend well into June at elevation, and Mahoosuc Notch (often called the hardest mile on the AT) can hold ice and snow in crevices through late spring. Nights turn cold again in mid-August, and early September snowfall at the higher summits is not unheard of.
• 100-Mile Wilderness (ME, miles 2,090–2,190): The final major section before Katahdin is also the most remote stretch of the AT, with no paved road crossings and limited bail-out options. Cold snaps can arrive by mid-September, and Baxter State Park (which manages Katahdin) closes the summit trail to through-hikers after mid-October due to snow and ice. Hikers should carry adequate cold-weather gear even in late summer and monitor the park closure date when planning their finish.

Routes

Typical northbound and southbound route options are included as hiking direction options, as are various "Flip-Flop" options that re-arrange the usual hiking order. Actual flip-floppers can do this at any point along the trail, though for this tool we use the standard half-way point of Harper's Ferry, West Virginia as that is most popular pivot point accommodating multiple types of flip-flop routes. For the purpose of hike duration calculations I've added a standard 2-day travel period to each flip-flop. More on these options can be found at the Appalachian Trail Conservancy's flip-flop page.

Temperature Adjustments

This site uses average daily maximum and minimum temperature data from the last seven years, along with apparent temperature data that addresses heat index and wind chill issues. This approach provides stability in planning values while also reflecting more recent shifts in climate conditions. We use the standard Steadman approach to these, and feel free to read more about them in the notes of the Florida Trail (for Heat Index) and New England Trail (for Wind Chill) sites where I first incorporated each into the planning tool.

All that being said, given that elevations do not change nearly as much on the Appalachian Trail as they do on other trails (like the Pacific Crest Trail or Continental Divide Trail), and they correspondingly do not cause as much of a weather shift, I have not created an AT-specific algorithm to estimate changes based solely on elevation or for specific AT peaks. But those are in development for the Arizona Trail and (if successful) may be for the AT in future builds.

Comfort Index and BestStart! Date

The BestStart! date tool uses a thermal comfort scoring system modeled off of the Universal Thermal Climate Index (UTCI), a scientifically grounded measure of the human body's physiological response to outdoor thermal conditions. Unlike a simple temperature reading, UTCI-based approaches account for the combined effects of air temperature, humidity, wind, and solar radiation on perceived comfort. All this makes them far more meaningful for planning extended outdoor activity than raw thermometer values alone.

Because our historical weather data does not include the solar radiation measurements needed for a true UTCI calculation, we use the apparent temperature (the Steadman heat index and wind chill already incorporated throughout this site) as a practical proxy. Research has shown that apparent temperature correlates well with UTCI across many conditions and thus works well with the TrailTemps data sets. For more on the UTCI methodology, see the foundational paper by Bröde, et al. (2010) and the comparative analysis by Bröde, et al. (2012).

For scoring purposes, each day of your hike is assigned a thermal comfort tier based on the apparent high and apparent low temperatures at that location and date. The five tiers are:

• Comfort Zone: Apparent high ≤ 79 °F and apparent low ≥ 48 °F. Ideal hiking conditions. Scores 10 points.
• Moderate Stress: Apparent high 80–90 °F or apparent low 32–47 °F. Manageable with preparation. Scores 8 points.
• Strong Stress: Apparent high 91–100 °F or apparent low 9–31 °F. Requires meaningful mitigation (extra water, insulation, adjusted schedule). Scores 5 points.
• Very Strong Stres: Apparent high 101–115 °F or apparent low −17–8 °F. Hazardous conditions for unprepared hikers. Scores 2 points.
• Extreme Stress: Apparent high above 115 °F or apparent low below −17 °F. Any start date producing even a single day at this level is automatically excluded from consideration.

Each day receives a combined score (the average of its heat and cold tier scores) and those daily scores are summed across the entire hike. The start date with the highest total score is recommended as the BestStart! date. When a day falls outside the Comfort Zone, the tier displayed in the Thermal Comfort Profile reflects whichever of the two stresses (heat or cold) is more extreme relative to its zone.

The scoring treats heat stress and cold stress symmetrically; a day scoring 5 points for heat counts the same as a day scoring 5 points for cold. This is intentional. One might expect cold stress to deserve less weight for hikers, since the exertion of hiking generates significant body heat that partially offsets ambient cold. That logic has merit, but hikers vary widely in pace, clothing, and planned schedule. Also, some hike early mornings or evenings to beat the heat, some layer up aggressively, and others choose a lighter pack at the cost of warmth. Any asymmetric weighting would embed assumptions about hiker behavior that are too subjective and variable to be defensible as a general recommendation. The symmetric approach lets you interpret the result and adjust for your own style.

Note that this tool uses historical planning averages, not a forecast. It identifies the statistically most comfortable time of year to hike based on the past seven years of weather data; actual conditions will always vary.

• AT Weather Hazards: Learn more about weather hazards along the AT from the Appalachian Trail Conservancy, including how to prepare for and respond to them.
• ATWeather: Provides 7-day weather forecasts at specific sites (e.g., shelters and campgrounds) along the Appalachain and Pacific Crest Trails. Includes sunrise and sunset hours for timing daylight hikes.
• Mountain-Forecast: Obtain precise weather forecasts at high elevation and other highly variable weather areas.
• Wildlife of the AT: Comprehensive guide to the diversity of animals you may encounter along the trail, a plentiful number given the diverse range of ecosystems the PCT traverses. Lists 74 species of amphibians, 233 species of birds, 83 species of mammals and 45 species of reptiles, as well as the approximate area hikers may encounter them.