Two of Michigan’s lakes saw out-of-season cold air moving in over the waters. Unstable weather conditions developed across the eastern parts of the state.
National Weather Service forecasters in Detroit reported a contrast in temperatures between the surfaces of the lakes and the air sitting above. The result was showers and thunderstorms.
When the storms crossed Lake Huron and Lake St. Clair, low-level updrafts could have developed into waterspouts, which are quite rare.
How thermal instability over warm lakes creates turbulent conditions
Late summer at Lake St. Clair and Lake Huron (one of the Great Lakes) saw temperatures reaching 70 degrees Fahrenheit, and they remained there for some days.
But then, a trough in the upper level of the atmosphere saw a mass of cool air being pushed from Canada to the lake region. Temperatures dropped at the 5,000-foot altitude level to close to 40 degrees.
The result of the difference in temperature was a thermal gap that crossed the 23-degree Fahrenheit threshold. The air at the lake surface, which was warmer and more moist, was drawn into the cooler part of the atmosphere above, creating storms.
On land, heat from the sun during the day means thunderstorms form in the afternoon. It is not the same on the lakes, where the stormy conditions happen at night.
The atmospheric dynamics behind vortex formation
Waterspouts need particular conditions to form. There needs to be high moisture near the water surface, low wind shear, and rotation of air.
When the convective shower cells started getting organized above Lake Huron, shifts in the wind created zones of spinning air near the water. Light wind movements in the background meant that the vortices endured rather than getting dispersed by wind shear in the upper atmosphere.
When an updraft with thunderstorm conditions passed over one of the spinning air zones, the rising column of air was dispersed vertically.
This action accelerated the rotation speed. The funnel column was narrowed, and cloud bases were pulled downwards.
At the same time, spray is drawn from the lake, creating a waterspout.
According to Doppler radar tracking, cloud roofs grew to above 15,000 feet. This is more than enough convective updraft wind power to sustain non-supercell waterspouts.
Specialized forecasting tools like the Szilagyi Waterspout Index were used by meteorologists tracking the atmospheric profiles. This was to determine real-time weather risks above shipping channels.
Seasonal risks and marine safety management
The National Weather Service monitored developing hazards from its radar networks in Detroit and Alpena.
However, small waterspouts often form below the height of the radar beams. This means forecasters have to supplement their data or rely entirely on high-resolution satellite images, offshore weather monitoring buoys, and visual reports coming in from commercial shipping vessels.
The conditions combined to be conducive to vortices. Warnings went out from Port Huron to Saginaw Bay.
Traffic on the water was warned about wind gusts reaching above 40 miles per hour. Boaters and commercial vessels also had to prepare for heavy downpours, bringing visibility drops. It was even possible that waterspouts may make landfall on some beaches.
The cold air that moved from Canada to the lakes saw moisture profiles moving away from the kinds of convective cells that create waterspouts.
Safety lead time is essential
The potential for waterspouts to form remained high on two of Michigan’s lakes until the thermal gap between the warm water surface and upper air narrowed.
It is important to track thermodynamic interactions. This is needed to give early safety lead time for those on the waters of the lakes during weather transitions.
Local waterspout indices were validated. This ensures regional numerical weather prediction models accurately capture thermal dynamics. This aims to prevent sudden marine hazards without issuing unnecessary coastal disruption warnings.
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