Forecast Parameters for U.S. Hail Occurrence and Size

Forecasting the occurrence of different types and sizes of hail can be challenging on operational time scales. Despite posing the risk of damage to property and vehicles, high insured losses and producing a higher frequency of loss swathes than tornadoes, comparatively little attention has been given to our understanding of forecasting these events. This is mainly driven by a poor understanding of the relationship between environmental characteristics and hail occurrence or size. Thermodynamic sources of energy are essential to promote the strong updrafts that support hail. However, enhancement of updrafts via vertical wind shear and helicity relative to storm motion is also an important contribution to large hail formation. Interactions on the microphysical scale, moisture loading and structure of the vertical temperature profile can also influence the potential for relatively large hail, meaning that indices that are calibrated for other severe phenomena may not be representative of the appropriate set of ingredients.

Observations of hail can pose problems, as they suffer from a number of non-meteorological temporal and spatial inhomogeneties and thus make it difficult to identify physical relationships. The availability of a recently compiled dataset of near-hail Rapid Refresh model proximity soundings and storm modes from the Storm Prediction Center (SPC) presents an opportunity to improve our understanding of the parameters for hail forecasting. For the purposes of this presentation, classes considered include larger accumulations of smaller hail, very large hail (>4 inches), and long-lived and significant right and left moving supercells. New results characterizing forecast parameters and profile characteristics for different hail classes and storm characteristics will be presented.