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There is no positive way to look at a thunderstorm in the distance and tell if it will produce hail reaching the ground.
Meteorologists use weather radar to "look" inside a thunderstorm. Since hail reflects more energy back to the radar than raindrops it often shows up in red shades.
The WSR-88D Doppler Radar can also estimate the size of the hail based on the amount of energy reflected back.
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The presence of large hail indicates very strong updrafts and downdrafts within the thunderstorm. These are also possible indicators of tornadic activity.
Often large hail is observed immediately north of a tornado track. The presence of hail doesn’t always mean a tornado. And, the absence of hail doesn’t always mean there isn’t a risk of tornadoes.
Fortunately, most hail is small - usually less than 2 inches in diameter
The largest hailstone ever recorded fell in Coffeyville, Kansas on September 3, 1970. It measured about 17.5 inches in circumference (over 5.6 inches in diameter) and weighed more than 26 ounces (almost 2 pounds)!
Hailstones can begin to melt and then re-freeze together - forming large and very irregularly shaped hail
It’s often difficult to get an accurate measurement of hail diameter - especially when it’s falling. When in doubt - play it safe and wait until the thunderstorm has moved away before going outside to measure the size of hail