Hurricane Florence has strengthened as expected. There will likely be some more strengthening, followed by some weakening near the coast. Florence will remain a major and deadly hurricane until after landfall, and then, it will be a deadly flood causing storm with significant winds.
The best estimate for timing of events is as follows.
On Wednesday morning, Hurricane Florence will likely be looming just off the Carolina coast, with winds of about 140 mph (making it a Category 4 Hurricane).
Tropical storm strength winds will come ashore Wednesday night. Strong surf will have already developed.
The strongest part of Florence will be pounding the Carolina coast at sunup on Friday, and the storm may sit right on the coast for several hours as it winds down to a tropical storm. Landfall and this rapid weakening may happen at about the same time. It is remotely possible that the eye wall will not pass over the coast, thus causing weather reporters and climate disaster deniers to become stupified.
It is important to understand how storm surge warnings work. There are two kinds of estimates that can be made. One is to assume exactly where the eye will come ashore, exactly when (to include the contribution of tieds), estimate the wind speed and forward speed of the storm, and the angle of movement, then use all these variable together with the topography of the coast to estimate how high flooding will be at any given point.
The second is to look at where the storm may go, and how strong it might be, and how those other variables will pan out, but with uncertainty in location and timing being much larger, and produce a map of coastal regions indicating where there might be high flooding, and how high that might be.
The former is generally not what you are looking at when you see a storm surge map. For example, right now there are estimated storm surges of over 9 feet near Kinston and Greenfille North Carolina, but only one foot in and around Albermarle sound. This is based on a combination of the likely track and behavior of the storm but considering that we don’t know exactly where the storm is going to hit. The difference between Greenville and Albermarle sound is a combination of where we think the storm is going and what the storm might do. The two topographies are greatly different, accounting for most of the difference in possible flood height.
In other words, most of the flood levels indicated in most storm surge maps are not going to happen. Only some of them. But, we don’t know which ones.
Anyway, there are storm surges expected to be greater than 9 feet in some places. Even three foot surges are a lot if it is high tide along very low lying barrier islands. If Florence does plow into North Carolina as expected, it is quite likely that the configuration of the barrier islands along that coast will change. Some lines may become dotted lines. Some inlets may be filled, some new inlets created, many tidal channels in the estuaries will shift. Places like the town of Ocracoke may suffer very sever damage, as they lie below the maximum possible food surge.
Aside from storm surge, flooding happens inland with hurricanes. The special feature of East Coast hurricanes, especially in “Hurricane Alley”, between Northern Georgia and southern Virginia, is that behind the coastal plain there is a mountain range. So, if tropical storm carried water dumps on that mountain range, a two or three inch rainfall over a very large area can cause significant flooding.
Rainfall of over four inches is expected to fall over most of Virginia, and about half of North Carolina. Delaware, DC, Delmarva, Maryland, southern Pennsylvania, and parts of West Virginia and South Carolina will also get lots of rain. The area of expected landfall, and a few areas in land (like in south-Central Virginia) may see rainfall totals of over 15 inches, maybe two feet right along the coast.
A lot of roads and bridges will be wiped out, immortal doods in large pickups will be obviated, and low lying villages will be flooded. Unless the storm does a zany (and very unlikely) turn to the north before coming ashore, this may be the biggest problem, since it is hard to evacuate five or six states.
Washington DC could get up to a foot of rain. Note: The White House lies on much lower terrain than the Capitol. I’m not promising you a Rose Garden flood, but it could happen.
Florence is just like any other storm of the day: Enhanced by global warming. But note this. Florence will weaken from a strong Category 4 storm to a strong Category 3 storm as it gets near the coast. This will happen because the warm waters on the surface will be churned by the storm itself into the colder water at depth, weakening the storm. Over recent years, however, we’ve seen storms fail to weaken, or even strengthen when they would normally have weakened, because the deeper water is still at “hurricane-strength” temperature. Katrina, Haiyan, and some of last year’s Atlantic storms exhibited this behavior.
With increased warming of the sea, this will eventually start to happen in the US East Coast littoral. Florence ten or twenty years from now would build to Category 5 strength and keep that level of intensity, or nearly, before reaching the shore. And, in ten or twenty years from now, sea levels will be higher. And, by then, a few other hurricanes would have smashed up the barrier islands. In 20 years, North Carolina and Virginia are not likely to be very well protected by those sand features. James Hansen was right.