|LOCAL FREAKISH WX SERIES:|
METEOROLOGIST JEFF HABY
This series looks at freakish weather events that can occur locally. The weather event is freakish in that it display an extreme weather
characteristic. It is local is that it can occur in one location and a mile away there can be nothing or a much reduced intensity of the
weather. This writing looks at extreme hail.
The hail size that falls from a supercell storm is highly variable. The hail sizes experienced will vary dramatically over the region
the storm moves over. One person may report marble size hail, another may report quarter size hail while even another could report
tennis ball size hail. Why is the hail produced from a supercell not uniform in size?
The location that receives the largest hail will have the best combination of the storm being at its most intense and where the updraft
and growth factors are maximized. A storm is a process that varies in intensity during its lifecycle. A storm will tend to increase
in intensity, then reach maximum intensity and then decrease in intensity. This is only a generalization though since storms are continuously
varying in intensity as they evolve and move. Locations directly under the hail core will often get larger and more hail than
locations on the periphery.
The wind interactions with the hail stones will help determine where they fall after the stones grow in the updraft region of the storm. A
stronger updraft will typically develop larger hailstones. The updraft will typically be stronger in the core of the updraft than at
the outer edges. Stones in the core of the updraft can be suspended longer and thus have more opportunity to cycle through the updraft
and continue to grow. The size of this core region is localized within the storm, thus when this hail falls out it will influence a
local region below it. Since a storm will typically have a forward motion as the hail falls out, the hail will fall in a more or less
linear but narrow path over the ground surface. The intense and turbulent winds aloft can influence where the hail stones move and how
some are recycled back into or away from the updraft. This can cause some hail to fall farther from the storm core than expected.
Huge hail stones can devastate local regions along with linear path while just half of mile away from the path there may be no
hail at all. Neighbors just a few houses away from each other can report different hail sizes.
The question to ask is this, what is the largest hail stone that landed on the ground from the storm? Often this largest stone is
not found if it lands for example in an empty field. If this largest hail stone and stones that are similar in size to it land on
a car, in a front yard or on a roof then it is much more likely it will be observed. Depending on the strength of the storm and
the location with respect to the storm, the largest hail stone may be golf ball size or it could be grapefruit size for example.
It is difficult to know the largest hail stones produced until pictures and reports come in.