Florence is also moving at a pretty good speed for a hurricane: west-northwest at 17 mph. Florence is rounding the base of a high pressure over Bermuda, which is partially the reason she is traveling at that current speed. However, while Florence may be moving fast now, she will begin to slow down as she nears land, and this will play a notable impact on her intensity.
As Florence nears land on Thursday, she will begin to slow down as an area of high pressure sits across the Great Lakes Region. This means that Florence will have to change her course slightly, but her forward momentum (and other atmospheric conditions) will want her to move forward instead of simply turning. What this means is that Florence will effectively hit the brakes as she nears land on Thursday, causing her effects to be felt along the coastline well before she actually makes landfall.
When a hurricane sits over waters for a long enough time, it begins to cause problems for itself. By slowing down, a hurricane will expose itself to cooler waters churned up by it's own surf. Hurricanes love warm waters, and part of the reason Florence is as strong as she is right now is because she is sitting over warm waters. As she slows down, she will cause the waters below her to cool a bit, and this will impact her intensity.
Also expected to be against her will be the presence of wind shear (that is, change of wind direction with height and speed) as she nears the coastline. Hurricanes like an atmosphere with little wind shear, and currently she is in an environment with little wind shear. But closer to the shore, Florence is expected to encounter shear from a high pressure over the Great Lakes, as well as a stalled out frontal boundary just a little bit inland.
What do all of these factors mean for Florence? Simple, it means that she may actually weaken before she reaches land, potentially down to a Category 3 Hurricane.
What are the impacts that the North Carolina coastline faces? First off, let us talk about the winds: perhaps the most misunderstood event of a landfalling hurricane is the wind speeds. The strongest of winds are located in the eyewall of a hurricane, which is usually found about 40 miles away from the center of the hurricane. Within the eyewall, that is where you will find the 140 mph winds. Once Florence reaches land, however; she will rapidly weaken (as most hurricanes do). This means that places further away from the shoreline will not be seeing 140 mph winds at all. In fact, most places effected by the hurricane will not be seeing 140 mph winds, and only about a third will actually see hurricane force winds. In addition, the strongest winds will likely be along the shoreline, this is because water offers less friction for air than land (i.e., water does not slow wind down as much as land does).
Due to Florence's expected brake slamming late tomorrow night/Thursday, perhaps the most general threat will be several places along the coastline may experiencing rain fall well in advance of her landfall. In fact, it is entirely possible that over 15 inches of rain could fall in certain locations (with locally higher amounts) by the time that the entire event is over. That means that flooding is going to be a concern.
Storm surges will also be a threat along the immediate shoreline, with expected storm surges of 6-9 feet in some locations. Storm surge is effectively the wind pushing water inland, causing flooding of it's own separate from the rain.
Another potential impact are tornadoes, which is a common hazard with most hurricanes, especially major hurricanes. Lots of low level shear caused by the hurricane's rotation itself can cause thunderstorms in the rain bands to rotate, and sometimes they produce tornadoes. Hurricane Ivan in 2004 produced the most tornadoes from a hurricane numbering 120 confirmed tornadoes, but most hurricanes do not produce that many tornadoes, usually they produce a dozen or so and usually the tornadoes stay below EF3 intensity. Nonetheless, they are still a possibility.
The next update will be between 10:30 PM and 11:30 PM CDT.
-Timothy Albertson
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