How Severe Storm Detection Works: Satellite, Lightning, CAPE

Contingencias alerts you to a storm when there is lightning nearby. Lightning is recorded by the GLM, the sensor on the GOES satellite, in the Americas, and by the Lightning Imager on the MTG satellite (EUMETSAT) in Europe, Africa and the Middle East. Two other signals adjust what the alert says: convective energy (CAPE), calculated by a forecast model, and, only in the Americas, cloud-top temperature in GOES satellite imagery. It is a risk signal, not a confirmation that hail will fall at an exact spot.

What each signal contributes

• Lightning (GLM in the Americas, MTG in Europe, Africa and the Middle East). It records electrical discharges from space. It is the condition for an alert: with no lightning, nothing is sent. It does not say how severe the storm is or where hail is falling.
• Convective energy (CAPE). The energy available in the atmosphere to sustain a storm, calculated by a model. It grades severity: below 1,000 J/kg the alert says “thunderstorm”; at 1,000 or more, “severe storm.” It says whether convection is possible, not whether it is happening.
• Cloud-top temperature (GOES satellite, the Americas only). A very cold top means a very tall cloud, typical of storms with deep vertical growth. On its own it cannot say whether there is convection: in midwinter, a deck of high clouds can have a top as cold as a summer thunderstorm. It never decides whether an alert is sent: it can only change its text. To see how these images are read, look at How to Interpret Clouds and Systems in Satellite Images.

More about the satellites behind the imagery in The GOES-R Series and Its Weather Imagery.

How they are combined: when an alert is sent and what it says

An alert is sent when there is lightning nearby. If the lightning is within 25 km (about 15 miles), that is enough; if it is farther away, enough convective energy is also required where it struck. Cloud-top temperature plays no part in sending. Once the alert is decided, there are three possible texts:

• “Thunderstorm”: CAPE is below 1,000 J/kg. It does not mention hail.
• “Severe storm, with possible hail cores”: CAPE is 1,000 J/kg or more. It is the usual text for a severe storm.
• “Severe storm, with high hail risk”: CAPE is 1,000 J/kg or more and, in addition, the coldest cloud top reaches −60 °C (−76 °F) or lower. That temperature is measured only with GOES, so this text can only appear in the Americas.

None of the three says hail is there: “possible hail cores” and “high risk” describe conditions, not a detection.

An illustrative example

Imagine a summer afternoon: the sensor records lightning 20 km (about 12 miles) from your field and the model shows a CAPE of 1,800 J/kg. Vigía sends “Severe storm 20 km away, with possible hail cores.” If GOES imagery also shows a top at −62 °C, the text changes to “with high hail risk.” With the same lightning and a CAPE of 400 J/kg, the alert would read “Thunderstorm 20 km away.” And if there were only very cold clouds with no lightning, there would be no alert. It is an example to understand the logic, not the record of a real event.

How often the data is updated

Lightning data and satellite images are refreshed every few minutes; the model that calculates CAPE is updated several times a day. So an alert reflects what was happening a few minutes ago, not what is happening this second.

What it cannot do

• It does not detect hail. No satellite signal confirms hail at an exact spot; that is why we say possible hail cores.
• It gives no lead time. The alert arrives when the storm is already under way near your location.
• Its resolution is limited. Satellite images have a resolution of a few kilometers per pixel and refresh every few minutes, so they miss very small storms and very fast changes.
• It can be wrong in both directions: alerting you to a storm that produces no hail, or missing one that does.
• It does not use Doppler radar. The alerts cannot see rotation inside a storm, so they are not a way to detect tornadoes.
• The data depends on the region. In the Americas, lightning comes from the GLM (GOES satellite); in Europe, Africa and the Middle East, from the Lightning Imager on the MTG satellite (EUMETSAT). We measure cloud-top temperature only with GOES (the Americas).

How to turn it on

The alerts are part of Vigía, the alerts feature of Contingencias Premium: choose your city or field and turn on “Severe storm.” By default the alert is triggered when the storm is within 25 km (about 15 miles) of your location, and the radius can be adjusted; the “Lightning nearby” alert is separate and by default warns of a strike within 10 km (about 6 miles) in the last 30 minutes. Learn about Vigía, watch storms on the radar, and read an overview of the app in Weather App With Severe Storm Alerts.

Frequently asked questions

What does “possible hail cores” mean?

That the storm has characteristics with which hail can form. It does not mean hail was detected or that it will fall on your house or field.

Why use lightning and not just the satellite?

Because cloud-top temperature alone cannot tell a storm from a deck of high clouds. Lightning confirms that a storm is active, which is why it is the condition for an alert.

Can you know minutes ahead that hail is coming?

Not with these signals. Contingencias does not promise lead time; it tells you a storm is nearby and how far away it is.

Does it work in Europe?

Yes, with MTG lightning data. The difference is that without cloud-top temperature (GOES only, the Americas) the alert cannot reach “high hail risk.”