The short answer
Fastpacking weather planning means checking each day of a multi-day run as its own forecast: heat while you move, cold at camp, rain on kit you can't dry, wind on exposed ground and rivers after rain. Forecasts are dependable about five days out, so firm up each day as it comes into range.
Key takeaways
- Plan each day on its own; start time, camp and river crossings follow its forecast.
- In our worked example, starting at 06:00, not 10:00, cut the heat's cost from 21 minutes to 7.
- NOAA says five-day forecasts are right about 90% of the time; ten-day ones, about half.
- Mountain storms build on hot afternoons, so cross high passes in the morning.
On this page
- What is fastpacking weather planning?
- Which weather matters on each day of a fastpack?
- How much does your start time change a hot day?
- How far ahead can you trust the forecast for a multi-day run?
- When is the season on the best-known multi-day routes?
- How do you plan each day around the weather?
- What can't a forecast tool do on a fastpack?
- Questions runners ask
- Sources and method
Words this guide uses
- Fastpacking
- Running and fast hiking over several days with your sleeping kit, food and spare clothes carried on your back, kept as light as possible.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll be there, from your start time and pace.
- WBGT
- Wet-bulb globe temperature, a heat-stress number combining air temperature, humidity, sun and wind.
What is fastpacking weather planning?
Fastpacking weather planning is reading the forecast for every day of a multi-day run, at the hours you'll move and the place you'll sleep. It matters more than on a day run, because your shelter and dry clothes are on your back.
Gear Patrol describes fastpacking as carrying less "so that you can move faster through the wilderness", with nights out. Our ultra and multi-day running weather hub covers the wider picture; this guide plans the days.
You need a pace-timed forecast for each day: the weather for each stretch of your route at the time you'll be there.
Which weather matters on each day of a fastpack?
Six things, and each one changes from day to day.
| What to check | The number to read, with units | Why it matters |
|---|---|---|
| Heat while you move | WBGT, °C (°F): Moderate from 18.3 (65), High from 22.8 (73) | You can't sit out the afternoon and still reach camp |
| Humidity | Dew point, °C (°F): sticky from 15 (59), hard from 18 (64) | Sweat, and sweaty kit, dry slowly |
| Cold at camp | Overnight low where you'll sleep, °C (°F) | You stop making heat; wet layers can't be swapped |
| Rain | Chance, %, and amount, mm (in) | Wet kit stays wet, and rivers rise |
| Wind | Speed and gusts, km/h (mph) | Exposed ridges and passes chill and slow you |
| Storms | The hour thunderstorms are forecast | Mountain storms build on hot afternoons |
Rivers need their own check: read yesterday's rain as well as today's.
How much does your start time change a hot day?
A lot, at fastpacking pace. On an illustrative clear July day in the Alps, starting at 06:00 instead of 10:00 cut the time the heat cost from about 21 minutes to under 7.
The day: 24 km (14.9 miles) at 12:00 per km (19:19 per mile), at 1,500 m (4,900 ft), with the air rising from 13 °C (55 °F) at 06:00 to 27 °C (81 °F) by mid-afternoon. It is not a forecast; the numbers come from RunFast's engine.
| Start | Finish | Peak WBGT, °C (°F) and level | Hottest feels like, °C (°F) | Weather cost |
|---|---|---|---|---|
| 06:00 | 10:48 | 21.7 (71), Moderate from km 15 | 29.6 (85) | +6 min 40 s |
| 08:00 | 12:48 | 23.8 (75), High from km 19 | 34.7 (94) | +14 min 50 s |
| 10:00 | 14:48 | 24.6 (76), High from km 9 | 36.3 (97) | +20 min 55 s |
Slow movers lose more time to heat than fast ones (Ely et al., 2007), and at 5 km/h (3.1 mph) you make little cooling breeze. Dawn has its own catch in humid places, as our guide to the best time to run in hot weather explains.
How far ahead can you trust the forecast for a multi-day run?
About five days for planning, and the last day or two for decisions. NOAA says a five-day forecast is right about 90% of the time, a seven-day one about 80%, and a ten-day or longer one only about half the time.
So on a five-day trip, plan the last day from the season, then recheck each evening if you have signal. Our guide to when to trust a forecast goes deeper. RunFast's hourly forecasts reach 10 days ahead.
When is the season on the best-known multi-day routes?
When the trail's managers say, then day by day from the forecast. Here is what the official pages say about three routes on three continents.
| Route | Distance, km (miles) | Days, per the official page | Season, per the official pages | Weather risks the official pages name |
|---|---|---|---|---|
| Tour du Mont Blanc, France, Italy and Switzerland | 170 (106), with 10,000 m (32,800 ft) of climbing | About 10 (60 hours of walking) | Huts open from June and close in September | Storms on hot summer days; snowfields above 2,200–2,300 m (7,200–7,500 ft) in mid-June 2021; fog; sudden cold |
| John Muir Trail, California, US | 340 (211) | Not stated | Yosemite's ground above 3,050 m (10,000 ft) usually snowy until mid-June or later | Afternoon thunderstorms; snowmelt rivers; nights around freezing |
| Otter Trail, South Africa | 45 (28) | 5 | Not stated | Frequent rain at the Lottering and Bloukrans crossings; Bloukrans best crossed near low tide |
How do you plan each day around the weather?
Move in the cool, cross high ground early, and choose camp for the night's weather.
- Start early in heat. In our example, four hours earlier saved about 14 minutes.
- Know where you'll be at midday. Make it shade or water, not an exposed climb.
- Cross passes early. The official Tour du Mont Blanc site says mountain storms are common on hot summer days. Sequoia and Kings Canyon National Parks say to seek shelter when you see distant thunderheads.
- Camp sheltered on cold, wet nights. The park service warns that hypothermia can set in with air as mild as 16 °C (60 °F) if you're exposed or wet for long.
The American College of Sports Medicine (2007) sets out the signs of heat illness. Park rules, official warnings and medical advice override this article, which is not medical advice.
What can't a forecast tool do on a fastpack?
It can't come with you: RunFast doesn't work offline, and there's no live tracking yet. It also maps one day at a time. Our guide to the weather along your running route shows how to get each day's route file.
And no forecast sees what rangers see. Check trail-condition pages and hut wardens for snow, closed paths and swollen rivers.
Questions runners ask
Can I get one forecast for a whole multi-day route?
Not as one route in RunFast yet. One route is one run, and there is no multi-day tour, so load each day's GPX, FIT or TCX file on its own, with its own date, start time and pace. Hourly forecasts reach 10 days ahead, so you can map every day inside that window.
What pace should I enter for a fastpacking day?
The moving pace you'll really hold with a loaded pack; our worked example uses 12:00 per km (19:19 per mile). Map your run takes paces up to 25:00 per km (40:14 per mile) and slows you on climbs by itself. Set it a little slower to allow for stops.
How cold does it get at camp on a summer fastpack?
Colder than the daytime forecast suggests. At Tuolumne Meadows in Yosemite, 2,620 m (8,600 ft) up, the National Park Service says mid-summer days usually reach the low 20s °C (70s °F) and nights fall to around freezing. Check the overnight low where you'll sleep, and keep one dry layer for camp.
Should I cross a river after heavy rain?
Not if it looks high or fast. SANParks warns that frequent rain can cause problems at the Otter Trail's Lottering and Bloukrans crossings, and says to cross the Bloukrans near low tide. In the Sierra Nevada, the park service calls summer snowmelt crossings extremely dangerous. Wait, or take the official escape route.
Does RunFast work offline on the trail?
No. Map your run and the free account need a connection, and there is no live tracking yet. Plan each day before you go and save them in a free account, so each day's forecast reaches your email the evening before. The phone apps, with live tracking, are coming.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- Gear Patrol (2022). Everything you need to know about the crazy world of fastpacking (Tanner Bowden)
- NOAA National Environmental Satellite, Data, and Information Service (2026). How reliable are weather forecasts?
- Autour du Mont-Blanc (official Tour du Mont Blanc site) (2026). Tour du Mont-Blanc (distance, climbing and days)
- Autour du Mont-Blanc (official Tour du Mont Blanc site) (2026). Mountain hazards and equipment
- Autour du Mont-Blanc (official Tour du Mont Blanc site) (2026). Trail conditions
- Autour du Mont-Blanc (official Tour du Mont Blanc site) (2026). Refuge des Mottets (opening season)
- US National Park Service, Yosemite National Park (2026). John Muir Trail
- US National Park Service, Yosemite National Park (2026). Seasons
- US National Park Service, Sequoia and Kings Canyon National Parks (2026). Trail safety
- South African National Parks (SANParks) (2026). Otter Trail
- Medicine and Science in Sports and Exercise (2022). Effects of weather parameters on endurance running performance (Mantzios et al.)
- Medicine and Science in Sports and Exercise (2007). Impact of weather on marathon-running performance (Ely et al.)
- Journal of Occupational and Environmental Hygiene (2008). Modeling the wet bulb globe temperature using standard meteorological measurements (Liljegren et al.)
- American College of Sports Medicine (2007). Position stand: exertional heat illness during training and competition (Armstrong et al.)
The worked example runs RunFast's forecast engine (engine-core.js, version 56f40ec887bf) on an illustrative day, not a forecast or a measurement: a clear 15 July at 45.9° N, 6.9° E and 1,500 m (4,900 ft), with the air rising from 13 °C (55 °F) at 06:00 to 27 °C (81 °F) at 15:00, relative humidity falling from 76% to 37%, a 10 km/h (6 mph) wind at 10 m and 10% cloud. A 24 km (14.9 mile) day on flat ground at an even 12:00 per km (19:19 per mile), no stops, from three start times. For each kilometre, at the minute its midpoint is reached, the engine reads sunshine from the sun's height (Haurwitz, dimmed for cloud by Kasten and Czeplak), WBGT (Liljegren et al., 2008), feels like at walking airflow (Steadman's apparent temperature with sun) and the weather cost: 0.4% of time per °C of WBGT above 15 °C (59 °F), from Mantzios et al. (2022), scaled up to 2.2 times for slower movers after Ely et al. (2007), summed over the day. Heat-stress levels: Moderate from 18.3 °C (65 °F), High from 22.8 °C (73 °F). The cost model was fitted to running races, so at hiking pace treat it as a guide. Route facts are quoted from the official pages listed as written on 14 October 2026; the Otter Trail distance is the sum of SANParks' day-by-day distances (the same page also gives 42 km). Imperial values are conversions.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.