Upper Level Jet and Large Hail in Summer Jonathan D. Finch

Low to mid level shear (0-6 km), lapse rates and instability parameters like CAPE are often used to forecast large hail. Two parameters that are often overlooked as predictive tools of large hail are high level winds and high level storm relative flow. Several case studies are shown here to illustrate this.

Case 1 July event map 700mb chart 500mb chart 250mb chart Dodge City 00z sounding A storm moved very slowly SSE or SE near Dodge City producing quarter size hail. Golf ball size hail occurred near Garden City. The high level storm relative flow was 40 to 45 kts.

Case 2 August event map radar image(.5 slice) radar image(8.7 small) radar image(8.7 large) 700mb chart 500mb chart 200mb chart Dodge City 00z sounding A storm was nearly stationary with 55 to 65 kts storm relative flow at high levels ( mb). Two-inch hail was reported with this storm—with hail falling for 45 minutes.

Case 3 September event map Radar images 700mb chart 500mb chart 200mb chart Dodge City 00z sounding Storms were nearly stationary or moved slowly northwest at 10 mph with 55 to 65 kts storm relative flow at high levels ( mb). Nickel-sized hail fell with the first storm. The second storm was in open country and was even stronger than the first based on radar. A funnel was reported for 6 minutes that came about 1/3 of the way to the ground.

Conclusion When the mid level flow is weak (hence slow-moving storms) the high level jet can help evacuate the updraft leading to large hail. This can make the difference between storms that go up and collapse quickly (pulse storm environment) and longer lived storms with hail. Large hail can occur despite very weak mid level winds or very weak mid level storm relative flow.