Why thunderstorms can form in the afternoon in the mountains

A sunny morning in the mountains can be followed by a thunderstorm. When warm, moist air can rise through unstable air, a storm can develop rapidly. Plan around the forecast and decide in advance how to get off the ridge.

How daytime heating can help build a storm

Thunderstorms need moisture, unstable air and a way to lift air. In suitable conditions, convection can build clouds that produce showers or thunderstorms.

Sunshine warms the ground and the air near it. That heating can help air rise. Winds blowing moist air up a mountainside or along a front can also provide lift. How much the air rises depends on the day’s conditions.

As air rises, it expands and cools. Water vapour can condense into droplets once the air is cool enough. If the updraft continues to grow in sufficiently moist, unstable air, a tall cumulonimbus cloud can develop.

On some warm-season days, daytime heating favours storms later in the day. The timing varies with moisture, stability, wind and weather systems. Thunderstorms can form earlier or later, so check the forecast for the actual day and route.

What a clear morning tells you

Sunshine shows the current conditions near you. Check whether the forecast mentions thunderstorms or instability later, and how long it would take to leave exposed terrain if the sky changes.

Planning to avoid thunderstorms

Compare expected storm timing with your route, especially summits, ridges, long open slopes and the descent. Make a plan that lets the group leave exposed ground before a storm reaches it.

Start early when that keeps you ahead of the weather

An early start can help on days when storms are forecast later. Plan backwards from the earliest likely deterioration, leave time for delays, and choose an alternative if the group cannot get down in time.

• Check hourly conditions for the full route, including the descent.
• Agree where and when to turn back before entering exposed terrain.
• Identify a substantial enclosed building or hard-topped vehicle that you could reach safely if needed.

Adjust the plan when timing is uncertain. If storms may reach the route before the group can leave exposed ground, choose a shorter, sheltered route with suitable access or postpone the outing.

Watch the sky while you walk

Look for changes, especially as you approach a ridge or leave a place where you can turn back. A developing storm can appear between forecast updates.

• Clouds growing rapidly upward or darkening nearby.
• A sudden cool gust as a storm approaches.
• Thunder or visible lightning, even if rain has yet to reach you.

If you hear thunder, move promptly off summits and ridges and seek a substantial enclosed building or a closed hard-topped vehicle if safely reachable. A tent, an open shelter or standing under a tree offers no reliable lightning protection. Avoid isolated trees and bodies of water. If you cannot reach a building or vehicle, leaving exposed high ground along a known route can reduce exposure, but the outdoors remains hazardous.

Read the forecast and nowcast together

Check official thunder advisories and the forecast for the elevations and hours you will reach. A general precipitation probability is only one part of the picture. Look for thunderstorms, strong gusts, heavy rain and poor visibility.

If the forecast describes unstable air or thunderstorms near the route, assess the earliest timing and how quickly the group can descend. Allow for a storm arriving earlier or forming closer than expected.

The Japan Meteorological Agency’s lightning nowcast shows analysed and predicted activity. Check the latest display where you have coverage. A storm can form where no activity is shown, so respond to thunder or visible lightning immediately.

Before setting out

Check the latest forecast and advisories, and share the turnaround point and shelter options with your group. Where a safe exit is uncertain, choose a different route or another day.

While hiking, watch the sky and leave exposed terrain early if conditions deteriorate. If you reach suitable shelter, stay there until the storm has passed. The US National Weather Service advises at least 30 minutes after the last thunder.