Hail in Asotin County, Washington

A severe thunderstorm developed in the northeast Blue Mountains and drifted north-northeastward. The thunderstorm produced hail up to the size of quarters before it weakened as it moved off the northern foothills of the Blue Mountains.

A deep upper level trough of lower pressure was situated just off the west coast of the United States in the eastern Pacific. Good dynamics at the base of this trough and a southerly, divergent flow pattern at upper levels generated a baroclinic zone that stretched from eastern Oregon to eastern Washington. Low level moisture streaming northward east of this zone resulted in an unstable atmosphere across the warm sector that included extreme eastern Washington. A strong veering shear profile in this same area provided the ideal ingredients for supercell development.