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FORECASTING HAIL

METEOROLOGIST JEFF HABY

Hail is both destructive to vegetation and manmade structures. Hail is classified as severe by the National Weather Service if it is equal to or greater than 3/4" in diameter. Strong winds make these darting spheres of ice even more damaging. It is difficult to pin point where exactly a large hail shaft will strike just as it is difficult to predict where tornadoes will exactly occur. However, the general region where hail can be expected is very predictable. Hail occurs in association with thunderstorms, particularly supercell thunderstorms. Below are factors to consider when trying to forecast for the likeliness and size of hail.

1. ELEVATION:

Higher elevation areas are closer to the cold layers of the upper atmosphere. When a hail stone falls, it rapidly begins to melt when the environmental temperature rises above freezing. If the hailstone has to fall through a deep layer of warm air, it will melt from the outside in, turning into non-damaging raindrops or decreasing significantly in size. Mountainous regions and the High Plains of the United States have the highest number of hail days per year. Small hail which would normally melt before reaching the surface in a low elevation area reaches the surface in high elevation area. Storms do not need to be as severe in the lee of the Rockies as in lower elevation areas for hail to reach the surface.


2. FREEZING LEVEL:

The freezing level determines the depth of the atmosphere that is above freezing. If the freezing level is high in the atmosphere, hailstones will have more time to melt than if the freezing level is close to the surface. A high freezing level also decreases the vertical depth in which hailstone formation and growth is possible. The freezing level depends on elevation, the season, and the temperature profile of the atmosphere. High elevation areas will have relatively low freezing levels in all seasons. For low elevation areas a general rule to follow is: If the freezing level is closer to the surface than 650 millibars, strong thunderstorms have a good probability of producing hail that will reach the surface. The freezing level can be found readily by examining the morning or afternoon Skew-T Log-P plot or forecast sounding.


3. WET BULB ZERO LEVEL:

The wet bulb zero level is defined as the freezing level that will result due to evaporative cooling. The freezing level will lower if there is dry air in the mid-levels of the atmosphere. This occurs due to evaporative cooling of environmental air that entrains into a thunderstorm. This same entrainment can also produce strong and gusty surface winds. Dry mid-levels are common in the Great Plains. This is another factor that leads to many hail days in this region of the U.S.


4. CONVECTIVE AVAILABLE POTENTIAL ENERGY (CAPE):

This is the most important factor in determining hail size. CAPES under 1000 J/kg generally produce borderline severe hail (near 3/4" or less) while CAPES over 2000 J/kg can produce very large hailstones. High CAPES lead to high upward vertical velocities within a thunderstorm. High UVV's can suspend hailstones and add layers of ice onto already developed hailstones. The amount of CAPE can be approximated by modifying the morning Skew-T sounding for that day. In many cases this is executed by changing the surface temperature and dewpoint to fit current observations. Forecast model soundings can also be examined for changes in CAPE during the day.


5. SUPERCELL THUNDERSTORM (HIGH WIND SHEAR):

Strong upper level winds allow CAPE to be maximized to its fullest potential. Strong upper level winds tilt the updraft of developing thunderstorms. This allows the updraft and downdraft to be separated from each other. This produces higher UVV's in the updraft.


6. PRECIPITABLE WATER:

The weight of moisture and water will influence the strength of the updraft. High moisture soundings result in water loading. CAPE is reduced with water loading since the force of gravity pushes down on the liquid water drops. Precipitable water values of less than 1.0" will not be nearly as influenced by water loading than if precipitable water values are above 1.5". Lower precipitable water values have the potential to produce large hailstones when significant CAPE is present. Low precipitation supercells are notorious for producing large hail. In the lee of the Rockies, PW is climatologically low, adding to the hail potential.


----- consensus -----

Hailstone size is maximized by high elevation, low freezing levels, low PW, dry mid-level air, high CAPE, and large wind shear. The region of the country that these factors come together the most are in the High/Great Plains of the US.

Hailstone size is minimized by low elevation, high freezing levels, water loading (high PW), moist mid-levels, low CAPE, and weak wind shear.