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Don Paul: On thunderstorms and a cold Lake Erie's dampening effect

Many people know the chilly waters of Lake Erie in the spring and early summer can seriously weaken thunderstorms passing over the lake.

For example, a chilly breeze from the lake is what saved Western New York on May 31, 1985, from the full impact of a major tornado outbreak that killed 88 people in northeast Ohio, northwest Pennsylvania, southern Ontario and eastern New York. Violent tornadoes crossed over into Chautauqua County from Pennsylvania late that day and were intersected by a comparatively cold lake wind, quickly weakening those storms, which had killed numerous people in northwest Pennsylvania.

Surface-based thunderstorms depend on “parcels” of air becoming buoyant and unstable, rising into a colder and often drier environment aloft. This usually occurs with increased surface heating and moisture, producing convection linked to this lifting. Here is a classic, simple NOAA diagram of the stages of thunderstorm development:

(Diagram from NOAA National Weather Service training materials showing the formation and dissipation of a thunderstorm)

The diagram depicts what we call surface-based convection, in which the lifting occurs right above the surface. Some otherwise healthy thunderstorms moving northeast from a hot and humid air mass over northern Ohio would find the colder surface of Lake Erie and cool air just above it at this time of year to be an unhealthy environment. Typically, some weakening occurs as the storm moves over eastern Lake Erie.

Typically does not mean always, however.

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There is a phenomenon we call elevated convection, which causes a disconnect  between those thunderstorms crossing over the lake and the lake itself. Over the years, quite a few astute viewers have contacted me asking why some storms did not weaken passing over a still-cold Lake Erie on a given occasion. Elevated convection can occur when a warm, buoyant air mass exists aloft, above a still stable, cool surface layer. This diagram applies.

Although that diagram shows an elevated thunderstorm above a cold layer over land, the same principles apply over cold water. Most elevated convection occurs as a warm front, the leading edge of a warmer air mass, pushes into a cooler air mass at the surface. The unstable layer is aloft in this case and is not surface-based. The lighter, less-dense warm air is forced to rise over the heavier, stable, colder air at the surface. The warm layer associated with an approaching warm front can become quite “bubbly” or unstable. This buoyant layer is where thunderstorms will form, not depending on surface heating.

In effect, the elevated convection above the cold layer at the lake is disconnected with the lake and feeds off the instability aloft. So, the lake has no weakening effect on such storms, and they can survive a trip over cold Lake Erie in good health.

Elevated convection is less often associated with tornadoes than surface-based convection. But it can certainly produce severe weather, including larger-scale flash flooding. A long-term study by the University of Missouri shows elevated storms tend to bring more rainfall than surface-based storms and produce more of the most dangerous lightning strikes, cloud-to-ground. Complexes of elevated convective cells can also cause enormous rainfall amounts over a larger area because the elevated unstable layer often covers more geographical area than the instability feeding surface-based storms, in a smaller area. And, with a shallow cold layer at the surface, freezing rain with thunder from the elevated storms can develop as well.

Now, the next time someone ponders why the lake didn’t kill some storms barreling into Buffalo on a spring day, you’ll be all set to bedazzle that someone with more than they wanted to know. This can be very handy if you prefer to be alone. I have frequently found this to be the case in newsrooms and the gym.

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