Are hurricanes possible on the Great Lakes?
Here in the Midwest we get some pretty nasty storms. I’m not just talking about individual “mesoscale” storms that cover one or two counties. I’m talking entire storm systems on a “synoptic” scale, systems that cover a few states. Even though these storm systems can bring winds of 50mph and heavy precipitation like a tropical storm, a meteorological “x-ray” of a mid-latitude storm system or cyclone is very different than a tropical storm or hurricane that develops over warm ocean waters. The quickest answer to this blog’s title is “Nope”. But there was one known case about 15 years ago that made meteorologists rethink just how close we could come to seeing a Great Lakes tropical system. This is the story of “Hurricane Huron” born in the middle of September 1996.
Lets first talk about the differences between a storm system sweeping across the land around here, vs. one churning the warm ocean waters of the Atlantic. A typical mid-latitude low pressure system for the Midwest will start with a warm sector and cold sector bounded by warm and cold fronts. As the system ages you might see an occluded front as the cooler air wraps around the center and cuts off the warm air. The storm system may seem strong and windy at the surface but it is actually stronger at higher altitudes. This is called a cold core system.
A tropical depression, tropical storm, or hurricane is very different. It doesn’t have fronts or separate warm and cold sectors. It is strongest at or near the surface. At higher altitudes the pressure gradient becomes smaller. At the top of a tropical system you’ll actually find high pressure. So it does a complete reversal from the surface! This is a warm core system fed by the warm ocean waters. Ideally, the water needs to be 80 degrees or better for the heat flux or heat transfer to feed the system. Remember, heat is a form of energy so transferring the heat is simply transferring energy.
In mid September 1996, a cold core mid-latitude cyclone moved over the Great Lakes bringing it’s fronts along with it. But then something happened. The system stalled while centered over Lake Huron. After a summer of sunshine and warm air, Lake Huron’s surface temperature was peaking in the upper 60s. This is well short of the 80 degrees needed for a hurricane, but this storm system was different. The air it brought over the Great Lakes early on was so cool that a water temp in the 60s could send a strong heat flux into the storm system. The “warm” Great lakes were starting to change things. It lost it’s fronts and began to form a more symmetric circulation. The storm strengthened more at the surface instead of at higher altitudes which is something a tropical system would do. The rain bands became heavier. The winds increased to tropical storm strength (+35 mph) with some of the rain bands spinning around the center. Possibly one of the most tell tale signs that this was no ordinary Midwest storm? An eye developed over Lake Huron prompting many to nickname the storm “Hurricane Huron”.
At it’s peak of development the heat flux from the Great Lakes into the storm reached 700 watts per square meter. This heat flux is equal to a typical flux in a category 1 hurricane! Parts of the Great Lakes and Canada ended up with more than 4″ of rain, a total much greater than your typical fall storm system. But of course this could not last forever. As Hurricane Huron churned the waters of the Great Lakes with waves up to 10ft. cooler water mixed in and dropped the surface temperature to a point where Hurricane Huron could no longer feed, and the system faded.
Despite its name Hurricane Huron was never a hurricane, it wasn’t even a tropical storm. But it certainly wasn’t your standard storm system rolling through the Midwest. Hurricane Huron is considered a hybrid with some characteristics of a mid-latitude storm system, and some of a tropical system (you could make the argument it was closer to being tropical). So far it’s the closest we’ve come to a Great Lakes hurricane.