Hurricane Florence decreased in intensity early Thursday, but is still slated to be strong hurricane as it nears the Carolina coast late tonight. While the storm has had a steady, forward momentum around 15-20 m.p.h., it is not expected to continue once it interacts with land. This morning's forecast from the National Hurricane Center shows a slowing left-hand turn. That's mainly because of the friction of the land.
As a hurricane moves on the ocean, there is little to no obstruction and no friction. That means that storms are able to develop from tiny waves coming off the coast of Africa. Warm ocean currents feed developing low pressure systems, turning some of them into hurricanes. But they almost immediately weaken upon impact to large land masses.
The simple way of thinking of this is rolling a ball across a smooth floor versus thick shag carpeting. A low-friction surface will keep the ball moving faster and farther versus the same ball moving at the same force onto a carpeted floor.
Once Florence comes ashore, a rise in land elevation, foliage, buildings, and other protrusions cause friction on the bottom of the hurricane. This will cause a turn to the left and a slowing of its forward progression.
This is bad news when it comes to storm surge (the wall of water that will come ashore). Storm surge will be maximized as the storm decreases in forward speed. In addition, epic amounts of rain are expected. For comparison, Hurricane Hugo from 1989 came in and didn't decrease in forward speed. This one could stall out, bringing two to three feet of rain to the Carolinas into the weekend.
-Meteorologist Eric Sorensen