You are all set for the UTMB® race, and the only uncertainty left is the weather in Chamonix in August. It is impossible to make precise forecasts several months in advance, but reliable trends can be identified from the region's historical data and temperature differences by altitude can be anticipated.
The period concerned
HOKA UTMB Mont-White® week takes place every year during the last week of August, with the flagship race starting late on Friday afternoon. This is therefore the weather window to analyse.
Average temperatures
On the valley floor, Chamonix sees an average maximum temperature of around 26°C during the last week of August, with a daily average of around 21°C. Average lows are around 10°C at night, with some years dropping close to 6°C.
These figures apply to the valley. Expect significantly lower temperatures at altitude on the course, with a drop of around 6.5°C for every 1,000 metres. At the start of the 2025 edition, forecasts predicted around 12 to 19°C on the valley floor, 4 to 9°C at around 2,000 m and only 1 to 6°C at around 2,500 m: the difference between the start in Chamonix and the high points of the course is considerable.
Sky conditions and risk of rain
On average, the last week of August in Chamonix remains fairly sunny with some cloudy spells, and historically it rains on only one day during the week, rarely continuously.
These statistics should be treated with caution: in the high mountains, every scenario remains possible. It is best to prepare your mandatory equipment while always factoring in the possibility of deteriorating weather, even if the averages are favourable.
Two consecutive editions disrupted by the weather
The last two editions show better than any statistic why you should not rely on averages.
In 2025, the race took place in near-continuous rain for several days, with feels-like temperatures dropping to -7°C at altitude. The organisers had to reroute part of the course after Courmayeur.
In 2026, a storm forecast for the 5:45 pm start window led the organisers to delay the start by two hours and alter the route at the last minute. The TDS®, held on Monday, had already seen a night of cold rain, with temperatures close to 0°C on the passes and nearly 45% of runners withdrawing.
For two consecutive years, the weather scenario has therefore altered the race itself. That is the strongest argument for kit designed for the worst conditions, not the average.
Where can you check the real-time weather forecast before the race?
Historical trends can help you prepare your kit in advance, but they do not replace a short-term forecast. In the days before your start, check the Météo-France forecast for the Chamonix valley, reviewed each morning by a local forecaster and broken down by altitude band, as well as the official UTMB® forecasts, published in the days leading up to the races.
Useful guide: forecasts only become genuinely reliable 72 hours before the event. Beyond that, Météo-France's confidence index drops quickly.
Frequently asked questions
What weather can you generally expect during the UTMB®?
The last week of August in Chamonix is generally mild, with days around 21°C and little rain. But differences from one edition to the next are significant: the last two were disrupted enough to alter the race.
What temperatures can you expect at altitude on the course?
Allow for around 6.5°C less for every 1,000 metres of altitude compared with the valley. At the high points, temperatures can drop to around 0°C, with feels-like temperatures well below that as soon as there is wind, as in 2025.
Can the race be altered or delayed because of the weather?
Yes, and it happens regularly. The course was rerouted in 2025, and in 2026 the start was delayed by two hours with a last-minute route change. These decisions are announced by the organisers in the hours before the start.
Should you expect rain or snow?
Rain remains possible at any time, and snow cannot be ruled out on the highest sections in low temperatures. This is why the mandatory kit required by the organisers includes waterproof and warm equipment, whatever weather is forecast at the start.
