Mountain weather for campers: reading forecasts, storms, fog and flash floods

Mountain weather is not the same as weather in the valley, and a forecast for the nearest town can be wrong for a pass 1,500 metres higher. Temperatures fall with altitude, wind accelerates over ridges, and clouds build quickly when warm air rises along slopes. For campers the consequences are practical: a pitch that was dry at lunchtime may be exposed to a storm by evening, and a pass that was open in the morning can be in cloud by afternoon.

None of this needs to stop a trip. It requires a habit of checking the right sources, planning around the hours when weather is most likely to turn, and knowing a few simple rules for the three situations campers meet most often: storms, fog and floods.

Where to read the forecast

The most reliable source is the national weather service of the country being visited. In France, Météo-France publishes a four-colour vigilance map of green, yellow, orange and red covering nine phenomena, including thunderstorms, rain and flooding, floods, snow and ice, avalanches and heat. It is refreshed at least twice a day, at 6 am and 4 pm, and there are separate mountain zones for avalanche warnings. In Switzerland, MeteoSwiss monitors the hazard situation on behalf of the federal government and issues warnings of dangerous weather events. In Austria the service is GeoSphere Austria, and in the United States the National Weather Service. Apps that aggregate several models are useful for planning, but a warning from the national service takes priority.

Read the forecast for the altitude and valley where the vehicle will be, not for the regional capital, and compare two sources. If a warning is in force for the area, the sensible option is to descend, not to wait to see what happens.

Afternoon thunderstorms

In summer, mountain storms follow a pattern. Mornings are often clear, cumulus clouds build around midday, and storms break in the afternoon or early evening. A day plan that puts the exposed activity, such as a high pass, a ridge walk or a cable car, in the morning and the sheltered time in the afternoon follows that rhythm. Towering clouds with dark bases, a sudden wind change and distant thunder are signs that the storm is close.

Lightning and the vehicle

The National Weather Service gives a simple rule: when thunder is audible, get inside a substantial building or a hard-topped metal vehicle, with the windows closed. It also advises staying inside for at least 30 minutes after the last thunder, because lightning can strike well away from the rain, up to about 10 miles (16 km) from it. Tents, sheds, picnic shelters and covered porches provide no protection. A camper with a metal body and closed windows is a reasonable shelter; a pop-up roof, a tent awning or the open air is not. While in the vehicle, avoid touching metal parts and plugged-in equipment. If there is a choice, do not park on an exposed summit, under an isolated tree or on a ridge when storms are forecast.

Dry riverbeds, creeks and low ground

A dry creek bed or gravel river bank can be an attractive and flat place to park, but it is also where water goes when it rains in the mountains above. A storm several kilometres away, out of sight and hearing, can send a flood down a valley within minutes. The NWS notes that 6 inches (15 cm) of fast-moving water can knock over an adult, 12 inches (30 cm) can carry away most cars, and 2 feet (60 cm) can carry away SUVs and trucks, and that more than half of flood drownings involve a vehicle driven into floodwater. A motorhome is not exempt. Park above the flood line, away from the bed and the outer bend of a river, and move early when rain is forecast upstream.

On the road, the same applies: never drive through a flooded section, even when it looks shallow, because the surface underneath may have been washed away. Turn around and find another route.

Fog, snow and mountain passes

Fog on a pass requires a slow speed, dipped headlights and, on narrow roads, the courage to stop at a wide point instead of continuing blind. Do not rely on the sat-nav alone: the road may be closed, or the pass may be in cloud. Passes also have a season. In May 2026, for example, Swiss authorities and media reported the Furka, Grimsel and Nufenen passes reopening on 29 May, while Susten and the Great St Bernard were still closed, and typical season dates for high passes are late May or June to October or early November. Snow can fall in the high Alps even in summer, so winter tyres or snow chains may be needed outside the usual months, and a pass closure should be checked on the day with the relevant road authority.

A short checklist

Before leaving the valley: check the warning map and the forecast for the destination altitude, and set the time you will turn back. Fuel up and fill the fresh water. In the vehicle: keep a torch, a warm layer and a charged phone, and tell someone where you are going. During the day: start early, be down from exposed places by early afternoon, and choose the night pitch with drainage and escape routes in mind. If a storm arrives: stay in the vehicle, windows closed, wait 30 minutes after the last thunder, and avoid walking out to see the damage until the weather has passed.

Sources

• Lightning Safety brochure (National Weather Service)
• Turn Around Don’t Drown (National Weather Service)
• Un dispositif d’avertissement sur les dangers météorologiques (Météo-France, Vigilance)
• Hazards (MeteoSwiss)
• More Swiss Alpine passes open for summer (SWI swissinfo.ch)