When our 4Warn Weather team forecasts chances for severe weather, that isn’t your run-of-the-mill heavy rain or lightning. Instead, three specific hazards determine whether a storm meets the criteria: tornadoes, damaging winds, and hail.
Each of these elements signals the potential for significant impact -- and sometimes danger -- on the ground.
The Three Key Ingredients of a Severe Storm
1. Tornado Potential
If a storm shows rotation on radar, it immediately raises concern. Rotating storms -- often called supercells -- can produce tornadoes, even if one hasn’t formed yet.
Because of this, the National Weather Service may issue a tornado warning based on the potential for development, not just confirmed sightings. Rotation alone is enough to classify a storm as severe.
2. Damaging Winds
Strong winds are another defining factor. When gusts reach or exceed 58 mph (93 km/h), they are considered capable of causing damage -- downing trees, power lines, and even causing structural harm.
At that threshold, a severe thunderstorm warning is typically issued.
3. Hail: The Most Complex Factor
Hail is where things get more nuanced. Not all hail is dangerous per se, and not all hail-producing storms are considered severe. The size of the hail is what matters most.
- Pea to nickel-sized hail: Generally, it is not severe and causes minimal damage
- Quarter-sized hail (1 inch / 2.5 cm) or larger: Meets severe criteria
- Larger hailstones: Can cause serious damage to vehicles, roofs, and crops
Small hail alone won’t usually trigger a warning unless it’s combined with strong winds or tornado potential. But once hail reaches that one-inch threshold, it can prompt a warning on its own.
Where Do the Worst Hailstorms Happen?
Hailstorms don’t occur randomly -- they thrive in specific atmospheric conditions.
Across North America, some of the most damaging hailstorms form in a corridor stretching from Texas through the Midwest and into the Great Lakes. This region becomes a battleground of air masses:
- Warm, humid air travels north from the Gulf of Mexico
- Cool, dry air moves south from Canada
When these collide, the atmosphere becomes unstable -- perfect for powerful thunderstorms and large hail development.
The Science Behind Hail Formation
Hail forms inside strong thunderstorms with intense vertical motion. Here’s how it works:
- Thunderstorms develop strong updrafts and downdrafts
- Raindrops are carried upward into colder parts of the atmosphere
- These droplets freeze into small ice pellets
- The pellets cycle up and down within the storm
- As they collide with supercooled water droplets, they grow larger
- Eventually, they become too heavy -- or the updraft weakens -- and they fall to the ground as hail.
- The stronger the updraft, the larger the hail can grow before falling.
Why It Matters
Understanding what makes a storm “severe” isn’t just about definitions -- it’s about awareness. A storm with rotation, high winds, or large hail has the potential to cause real damage.
Recognizing these threats early helps you and others be prepared ahead of the storm.