A bright storm eight miles away can be moving away from you while a smaller-looking storm fourteen miles out heads straight toward town. To judge which storm matters, ask three questions: which way, how likely, and how soon? Distance and radar colour alone cannot answer them.
Start with where the storm is going
Most storms move with winds several miles above the ground. Playing a radar loop lets you follow that movement instead of treating the latest image as a destination. The useful question is how close the storm will get if its recent motion continues.
That is the closest point of approach. In our illustrative example, the nearby storm sits eight miles northwest and moves northeast, away from town. The other sits fourteen miles west-southwest and moves toward town. The farther storm has the more direct track.
A projected miss is not an all-clear: lightning can strike outward ten miles from a thunderstorm. Strong storms can also veer to the right of the mean wind in the Northern Hemisphere. Keep checking the track as new scans arrive.
Ask what the severe probability belongs to
NOAA estimates that about one in ten US thunderstorms reaches severe levels: hail at least an inch across, wind gusts around 58 mph, or a tornado. A bright radar echo can come from heavy rain or hail; its colour does not by itself give the severe-weather risk.
NOAA and University of Wisconsin scientists developed ProbSevere to estimate whether a storm will produce severe weather in the next 60 minutes. It combines radar, satellite, lightning and environmental information and updates every two minutes.
The probability belongs to the storm. A 65% value means the model estimates a 65% chance of severe weather from that storm within the hour. It does not mean a 65% chance that your house will be hit. Direction still matters.
Treat arrival as an estimate
For a storm moving directly toward you, distance divided by speed gives a first arrival estimate. Fourteen miles at thirty miles an hour is about 28 minutes. Those are example numbers, not a current forecast.
The estimate depends on the storm continuing along that path at that speed. It is not a safe waiting period. Use official warnings to guide action, and do not let a countdown delay getting to shelter.
What to take away
Use these checks next time:
- Follow the track, not just the nearest bright patch.
- Separate the storm’s severe probability from its chance of passing over you.
- Treat arrival times as estimates; official warnings come first.
