National Weather Service United States Department of Commerce


Weather Review - June 28, 2009


Above is an animation of a storm that formed over southwest Coosa County on June 28, 2009. At the beginning of the loop, an outflow boundary from an earlier storm (the light blue line of reflectivities in a NW-SE orientation) is moving to the southwest. Once the outflow boundary meets with the already forming storm just to the east of Mitchell Dam, the storm intensifies warranting the issuance of a Severe Thunderstorm Warning.

Below is a still image of the above storm at its most intense. Depicted here is a radar phenomenon known as Storm Top Divergence. Storm Top Divergence is an indicator of how strong a storm has become. Once the updraft in a thunderstorm reaches the tropopause, it can no longer go any higher in elevation. In turn, it spreads out at the top of the thunderstorm into what is called the anvil. The intensity of this spreading is measurable on radar using velocity data. Measured by adding the intensity of the inbound velocities and the outbound velocities, most studies have concluded that 75 knots of Storm Top Divergence is enough to sustain hail growth large enough to meet severe criteria (3/4 inch).

This example of Storm Top Divergence illustrates 124 knots of divergence at 54,000 feet. Though no hail or wind damage was reported with this storm (very rural area), this storm was by far the strongest of the day in central Alabama.