Thunderstorm building over the ridge on a day trip
You are on a 6 m fibreglass runabout with four people aboard, cruising a large lake at 25 knots on a hot, humid afternoon. The forecast called for a 30 percent chance of thunderstorms late in the day. It is 15:30. To the west, a tall cumulonimbus is building over the ridge, with a flat dark base and visible rain shaft. You can hear distant thunder about every 20 seconds. The nearest safe harbour is 9 km east; your launch ramp is 14 km southeast. The wind has shifted from southwest to gusty west and the water has gone from calm to 0.5 m chop in ten minutes. You have a VHF radio, lifejackets for all, and half a tank of fuel.
1. What does the cloud, the thunder interval, and the wind shift tell you, and what is your decision about route?
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The signals point to a mature, electrically active storm that is close and moving toward you. A tall cumulonimbus with a flat dark base and a visible rain shaft means heavy rain, strong outflow gusts, and possible hail. Thunder every 20 seconds means lightning is frequent and near; the rough rule is that the flash-to-bang interval in seconds divided by five gives distance in kilometres, so 20 seconds is roughly 4 km. The sudden shift to gusty west wind and the rapid chop are the storm's outflow arriving ahead of the rain.
Decision: do not try to outrun it to the ramp 14 km southeast, and do not head west. Turn east toward the nearest safe harbour 9 km away, increase speed only as sea state allows, and get everyone into lifejackets. If the harbour cannot be reached before the gust front, the fallback is to reduce speed, keep the bow into the wind and seas, and ride it out rather than run for a distant ramp. The exam is testing whether you read the sky, act on the outflow, and choose the nearest shelter rather than the familiar destination.
2. The gust front hits before you reach harbour. Describe how you handle the boat and the people, and what you do about lightning.
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Handling:
- Reduce speed before the gust arrives, put everyone in lifejackets, and have them sit low and centred.
- Take the seas at an angle, not straight on, easing throttle to avoid burying the bow or launching off a wave.
- Keep the bow up and steer a steady course; avoid turning broadside to the chop.
- Secure loose gear and close hatches; keep the bilge pump ready.
Lightning:
- Get low and stay away from the radio antenna, metal rails, and the console if possible.
- Do not hold a fishing rod or any long metal object.
- Keep away from the water; do not swim or dangle limbs over the side.
- If you can reach harbour, do so; a enclosed cabin or a building ashore is better than open water.
People:
- Watch for cold, fear, and seasickness; keep them informed and calm.
- Keep the VHF on and monitor; if you are in trouble, call early with position and number aboard.
The half tank of fuel is a constraint: do not burn it fighting the storm at high speed. Slow, controlled, and pointed at shelter is the exam answer.