The Science Behind the Flint-Beecher Tornado
Introduction | Background |
The upper air observations taken at 10 am on the morning of June 8th show the classic features of the event. At 500 mb (Figure 3), a closed upper low was in position just northwest of Michigan with strong winds in excess of 50 knots poised to move over southern lower Michigan during the subsequent afternoon and evening hours. The 500 mb trough axis had a decided "negative-tilt" yielding a broad area of divergence over the Great Lakes region.
At 700 mb (Figure 4), a large area of dry air in the middle troposphere was advancing over the southern Great Lakes, along with an area of strong winds approaching 50 knots. Also, temperatures at this level were at or above 11C south of the Great Lakes, strongly hinting at the likelihood of a mid-tropospheric capping inversion over the Ohio Valley that would prevent the initiation of thunderstorms significantly south of Michigan.
At 850 mb (Figure 5), a great deal of warm, moist air was moving into southern Michigan behind a lower atmospheric warm front and ahead of a lower atmospheric "dry-line" over the Mississippi Valley region. Temperatures at this level were approaching 19-20C with dewpoint likely reaching 14C. This would result in fairly steep late afternoon lapse rates between 850 and 500 mb (near 30C), and a Total Totals Index near 54, certainly supportive of strong thunderstorm potential.
In fact, the 4 pm radiosonde observation taken at Mt. Clemens, MI, after being modified for expected late afternoon temperatures and dew points (Figure 6) - yielded a convective available potential energy (CAPE) estimate through the troposphere of 4500 Joules per Kilogram.
The surface map at 130 pm that afternoon (Figure 7) showed strong lower pressure for early June over the Minnesota "arrowhead" region, with a warm front south of the Great Lakes, and an occluded front immediately west of Lake Michigan. By late afternoon, the warm front had moved quickly into southern lower Michigan, allowing temperatures to climb to near 80F and dewpoints to in excess of 70F.
Not only were the thermal and moisture profiles in the region conducive to strong, severe thunderstorms, but the wind profiles were suggestive of tornado-producing "supercell" thunderstorms. Winds in excess of 50 knots in the 700-500 mb layer were indicative of strong, deep shear through the lower troposphere (0-3 km Storm Relative Helicities ~300 m2/s2 and 0-6 km cumulative shear ~55 m/s) - and supportive of supercell thunderstorm formation. And, meanwhile, the presence of the warm front in southeast lower Michigan added a "veering" with height aspect to the winds in the lowest 1 to 3 thousand feet of the atmosphere, adding to the tornado potential that evening. By 130 am that night, the surface map (Figure 8) showed a cold front exiting southeast lower Michigan, essentially bringing an end to the severe weather threat.
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