Last night, a severe thunderstorm rumbled through northern Shelby and Fayette Counties between 4-5am. Fortunately, I was able to grab some screenshots of cross-sectional radar data to go along with a picture sent to me by a Twitter follower in Gallaway, TN (extreme northwest Fayette County, near I-40) that will allow me to demonstrate some of the principles of hail formation and what meteorologists look for when predicting hail. (Note that a Severe Thunderstorm Warning had been issued for the affected areas before the hail storm arrived.)
First off, in traditional situations, there must be a thunderstorm and it's rising currents of air present for hail to be produced. The dynamics within a thunderstorm, including the upward motion of air (or lift) is a necessary ingredient. Hail begins small in storms and grows through a process of ascending and descending in the storm through sub-freezing air. Rising air in a storm, sometimes at a very fast rate, throws the hail higher into the storm. It subsequently gets heavier due to accumulating more ice around the core of the hailstone and descends. Stronger gusts of rising air can push it back up which cause it to accumulate another layer of ice, and the process it repeated. The more times the process is repeated, the larger the hailstone. A good animation of the process can be found here.
Another important factor is the freezing level, or what height above the ground the air drops below 32F (0C). Liquid droplets must be present above this levels to produce ice, and hailstones must rise above this level to continue to accumulate more ice around the core. In addition, if the freezing level is very high, the hail will melt on it's way to the ground and shrink in size, or melt completely. For the largest hail to reach the surface, optimal conditions are a strong thunderstorm with strong updrafts and a low freezing level (near 10,000 feet or less in the Mid-South).
Let's take a look at the storm from last night. The first image below is a cross-section of the storm at 4:22am from the Millington NEXRAD radar. Left to right is roughly west to east (so we are looking north in this image) and bottom to top in the image is increasing altitude (or going up in the sky). Each horizontal white line is 10,000 feet. The brightest colors (reds and purples) are the heaviest rain, or in this case rain and hail. The freezing level last night was fairly favorable at about 11,000 feet. So, any heavy precipitation you see above that level is likely freezing/frozen.
In the image below, hail resides in the white circled area, and actually are being thrown up to even higher levels in the reds between 20,000-30,000 feet. Gallaway is labelled and the storm is moving from left to right, so the storm has not quite reached there yet. Note that nearest the ground, the color above Gallaway is green, or light rain, but the storm is tilted from west to east by upper-level wind and the strongest precipitation (reflectivity) aloft is almost over Gallaway.
In the next image, taken at 4:31am, the color nearest the surface directly over Gallaway is red and that red extends up to about 25,000 feet. Hail is now falling at Gallaway, though the storm has weakened a little, as evidenced by the pack of purples. The darkest reds are now below 10,000 feet and there is much less red between 10,000-25,000 feet, indicating that the storm is in the process of collapsing, the updrafts are weakening, and the hail suspended aloft is falling to the ground.
Twenty minutes later (4:50am), in the last image below, the process appears to be starting over again as plenty of heavy rain and probably some hail is reaching the ground, while another core of heavier precipitation is forming between 5,000-20,000 feet above the ground. This is the nature of the thunderstorm beast, continuing to mature and dissipate repeatedly until an unfavorable atmosphere is encountered.
So what did this storm produce? The picture below, sent by Cam Fondren of Gallaway, tells the story - hail, some of it qurater-sized (1" diameter), which verifies the Severe Thunderstorm Warning that was is effect for Fayette County when the hail fell. Special thanks to Cam for sending the picture!
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