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Thunderstorm Forecast
What forecasters look at before they say “chance of thunderstorms”, and how to read it for your own afternoon.
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Three ingredients, all required
A thunderstorm needs instability, moisture and something to lift the air. Miss one and you get nothing — which is why hot, humid days sometimes pass without a rumble.
Instability is measured as CAPE, convective available potential energy, in joules per kilogram. Below about 500 J/kg storms struggle; 1,000–2,500 supports ordinary summer storms; above 3,000 is the territory of severe weather. It is the single number most worth watching, and it is what the storm-day counts on this site are built from.
Lift comes from a front, a sea breeze, terrain, or plain daytime heating. This is why storms so often fire in mid-afternoon: the sun has spent all day building the instability and the ground finally lets it go.
What “40% chance of thunderstorms” means
It means that at any given point in the forecast area, there is a 40% chance of storms — not that 40% of the area gets one, and definitely not that the storm will be over your house. Summer storms are patchy by nature: two neighbourhoods 5 km apart routinely get 20 mm and nothing at all on the same afternoon.
This is also why a forecast can be “wrong” in your garden and right for the region. Judge it over a season, not over one barbecue.
How far ahead it is worth trusting
For storms specifically: the day itself is good, tomorrow is decent, three days out tells you whether a pattern is coming but not where cells will fire. Beyond about five days, a thunderstorm symbol is a statement about the season, not about your plans.
The practical approach is to check the pattern a few days ahead, then check instability again on the morning itself.
Storm risk by city
Live conditions and storm-day history for the cities that get the most storms.
Common questions
- What is a good CAPE value for thunderstorms?
- Roughly: under 500 J/kg storms are unlikely, 1,000–2,500 supports ordinary storms, and over 3,000 J/kg is severe-weather territory when there is lift to trigger it.
- Why do thunderstorms happen in the afternoon?
- Because daytime heating builds the instability that storms feed on. It peaks a few hours after the sun does, which is why mid-to-late afternoon is the classic firing time.