Gust front off Brunnen: reading the sky and choosing where to run
14:20 on 3 August. You are in a 6 m open glassfibre boat with a 90 hp outboard, three adults and a child aboard, off Brunnen on Lake Uri, 1.2 km from the shore, heading south toward Flüelen. Air 31 °C, water 21 °C, humidity high. Pressure fell from 1013 hPa at 09:00 to 1007 hPa at 14:00. Over the Glärnisch and the Schächental to the south-east a wall of cumulonimbus has built; the anvil is spreading north-west over Brunnen, the base is flat and grey-green, and a ragged roll cloud hangs beneath it. The cell is moving north-west at about 20 km/h and its core is roughly 8 km away. The lake to the south is still glassy, but a dark line is moving across the water and a few whitecaps appear and vanish. The wind goes from 4 kn south to 12 kn north-west in under two minutes. Your phone shows a red thunderstorm warning valid from 15:00. Brunnen harbour is 1.2 km north; a lee under the Axen cliffs is 2 km south. VHF and four life jackets aboard, no radar.
1. Read the situation. What is happening, how long have you got, and what will the wind do?
Read the worked answer
The cell is already mature and the outflow has arrived. The anvil spreading overhead means the storm is no longer building - it is in its mature phase and the top has been blown downwind. The flat grey-green base and the ragged roll cloud are the signature of a downdraught spreading out along the surface.
The glassy water and the dark line are the gust front, not a lull. A patch of dead calm in front of a dark line on a hot afternoon is the cold pool pushing out ahead of the rain, and the whitecaps that appear and vanish are the leading edge arriving in gusts.
Timing. The core is 8 km away and moving at 20 km/h, so the rain and lightning arrive in roughly 24 minutes. The gust front runs ahead of the core by 5 to 15 minutes. So your usable window is about 10 to 20 minutes - and alpine valleys funnel and accelerate outflow, so plan for the short end.
What the wind will do. It has already veered from south to north-west and increased from 4 to 12 kn. Expect it to build rapidly to 30–50 kn, hold for 10–20 minutes, then ease and veer further as the core passes. The direction change is the dangerous part: any anchorage or mooring that was sheltered from the south is now exposed.
Pressure. Six hectopascals in five hours is a useful trend, but it is not the decisive evidence here. The sky and the water are.
2. Where do you go - Brunnen harbour or the lee under the Axen cliffs - and why?
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Go north to Brunnen. It is 1.2 km, which at 15 kn (28 km/h) is under three minutes, and it puts you in a harbour with mole protection, moorings and a lee shore you can tie to. You will be secured before the core arrives.
Why not the cliffs. They are 2 km to the south, straight toward the approaching cell, so you would be running into the worst of it rather than out of it. A 'lee under the cliffs' is the classic alpine trap:
- Downdraughts come over the top. A north-west outflow accelerates down a steep rock face and arrives as violent, shifting williwaws - the water at the foot of a cliff is often the roughest on the lake.
- No landing. Vertical rock, deep water close in, nowhere to step ashore and no mooring.
- Rockfall and lightning. Both are real hazards under a cliff during a storm.
- No shelter once you are there. You would be committed, with the child aboard, in an open boat, for the 20 minutes of peak wind.
Decision logic. Shelter means protection from the wind you are about to get, plus the option to get ashore. Brunnen offers both; the cliffs offer neither.
While you run north: everyone in life jackets, the child seated low and amidships, loose gear stowed, hatches and locker lids closed, and the boat trimmed so the bow rides up. Do not try to outrun the front by going faster into open water.
3. The gust front hits before you reach the harbour. How do you handle the boat, and what do you do if the outboard stops?
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Handling in the gust. Take the wind on the bow or the quarter, never beam-on - an open 6 m boat with a child aboard can be swamped or rolled by a 45 kn gust on the beam. Hold enough power to keep steerage and keep the bow up, but do not drive hard into the chop and start shipping water over the bow. If you cannot make headway, slow to displacement speed and take the seas at 20–30° off the bow rather than head-on. Keep clear of the lee shore: the wind will push you onto it far faster than you expect.
Reduce windage - canopy down if it can come down quickly, nobody standing, weight low and central.
Lightning. Stay low, keep away from the antenna and any rod holders, and do not shelter under a lone tree or a cliff overhang.
If the outboard stops. Get a drogue or a bucket streamed from the bow to hold the bow to the wind - an open boat without way tends to lie beam-on and roll. Do not attempt repairs with the cover off in driving rain and lightning. Then decide on the radio: with an open boat, a child aboard and 45 kn of wind, a PAN PAN to the lake rescue service giving your position and situation is proportionate, and it escalates to MAYDAY if you start taking water, lose the drogue, or drift toward the shore. A MAYDAY is for grave and imminent danger; a boat being blown onto a lee shore in a storm qualifies once you have lost control of the situation.