Forecasting Hail Occurrence

Photo of large hailstone

Hail occurrence is relatively rare with routine air-mass thunderstorms, but is more common with multicell and supercell thunderstorms. Several factors come into play when attempting to forecast hail occurrence. Important considerations that can be evaluated on the skew-T diagram include the following:

The higher the EL in a sounding, the greater the chance of hail, which tends to be favored in taller thunderstorms. Nearly two-thirds of all storms with radar echo tops over 50,000 feet report hail at the surface.

An evaluation of the freezing and wet-bulb zero levels is also important. Lower zero levels favor hail because a greater depth of the thunderstorm has sub-freezing temperatures where hail production can occur. As a result, hail is more likely to occur in thunderstorms with freezing levels lower than 12,000 feet.

The higher the CAPE, the higher the updraft velocity and greater potential for more and larger hail.

Soundings exhibiting significant vertical wind shear indicate thunderstorms may become titled, with updraft/downdraft separation that can promote enhanced intensity and longevity of updrafts.

Finally, elevation plays a key role as the highest frequencies of hail are observed to the lee of the Rocky Mountains from Colorado and Wyoming northward to Alberta, Canada. In this region surface elevations are 4000-6000 feet MSL so that the climatological freezing level is closer to the surface.

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