Thunderstorms and storm warnings on Lake Geneva

Updated 17.10.2026

Read the orange lights, recognise thunderstorm winds and know when to head back.

Storm warning lights

Twenty-three orange lights, spread all around the lake, broadcast the warnings:

Light	Warning	What it means
≈ 40 flashes per minute	Strong-wind warning (caution warning)	Gusts of 25 to 33 knots (about 46 to 61 km/h), with no set time.
≈ 90 flashes per minute	Storm warning	Gusts above 33 knots (about 61 km/h), with no set time.

On the French side, a caution warning bans beach craft and swimming; a storm warning bans all navigation except passenger boats, and everyone heads for the nearest shelter (Lake Geneva special navigation police regulations). The lights on shore are authoritative, ahead of anything a phone shows.

Thunderstorm winds

Summer thunderstorms raise the most dangerous winds on the lake, because they arrive all at once:

the joran drops suddenly off the Jura, often heralded by big dark cumulus clouds over the Jura; strong gusts at first, then it steadies, and the temperature falls;
the vaudaire, on the Haut-Lac, comes out of the Rhône plain from the south-east, sudden and strong;
the bornan, between Rolle and Lutry, and the môlan, on the Petit-Lac, come down from the Savoy mountains from the south, in very violent gusts, especially in hot spells.

Wind by wind: the winds of Lake Geneva.

What to do when it picks up

Watch the sky towards the Jura and Savoy, and the lights on shore.
Head back early: the strong-wind warning has no set time, and the blow can come quickly.
Shorten sail and put lifejackets on before it blows.
At anchor, leave before the sea gets up: a spot sheltered from the current wind no longer is once the wind shifts.
In trouble: 117 in Switzerland, 112 everywhere.

The Beaufort scale

On a lake, waves stay smaller than at sea for the same force, but they are short and steep.

Force	Description	Knots	km/h
3	Gentle breeze	7–10	12–19
4	Moderate breeze	11–16	20–28
5	Fresh breeze	17–21	29–38
6	Strong breeze	22–27	39–49
7	Near gale	28–33	50–61
8	Gale	34–40	62–74
9	Strong gale	41–47	75–88

So the strong-wind warning covers the top of force 6 and force 7; the storm warning starts at force 8.

With the Navleman map

While under way, once “Let's go” is started, the app monitors the radar: it sounds an alert when a storm cell is less than 30 km away and will reach you within three quarters of an hour. The “Thunderstorm risk” map shows the forecast potential, hour by hour; the radar shows the rain actually falling and, extended, that of the coming hour. Station readings give the strong-wind and storm warnings by sector (Wind and weather menu).

Open the Lake Geneva map