Severe Weather 101

Hail Forecasting

To forecast hail, deep moist convection is required, in addition to these three basic ingredients:

  • Adequate updraft to keep the hailstone aloft for an appropriate amount of time,
  • Sufficient supercooled water near the hailstone to enable growth as it travels through an updraft, and
  • A piece of ice, snow or dust for it to grow upon.

There is no clear distinction between storms that do and do not produce hailstones. Nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground.

Multi-cell thunderstorms produce many hailstones, but not usually the largest hailstones. In the life cycle of the multi-cell thunderstorm, the mature stage is relatively short so there is not much time for growth of the hailstone.

Supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud. In general, hail 2 inches (5 cm) or larger in diameter is associated with supercells.  Non-supercell storms are capable of producing golf ball size hail.

In all cases, the hail falls when the thunderstorm's updraft can no longer support the weight of the ice. The stronger the updraft the larger the hailstone can grow.

What we do: NSSL is developing techniques to use dual-polarized radar data in short-term computer forecast models to improve forecasts of hail and large hail.