At a pressure of 919 millibars, Michael ranks as the third strongest storm (according to pressure) to make landfall in the continental United States. The lower the pressure, the stronger the storm.
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The strongest storm to ever make landfall was in 1935 dubbed the "Labor Day" storm with a pressure of 892 millibars.
This storm is unprecedented as a Category 4 storm has never made landfall in the Florida Panhandle before Michael.
Why Hurricane Michael grew so quickly:
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The main factor for Michael's quick intensification was the warm waters in the Gulf of Mexico. Hurricanes thrive off of ocean waters that are warmer than 80 degrees Fahrenheit and the Gulf is currently above that.
Also contributing to Michael's strength were the winds. Storms fall apart in environments with high wind shear, where the winds change direction and speed as you go up in altitude.
Michael was in an area with very low shear, favoring its rapid development.