14 September 2018

Florence makes landfall but life-threatening storm surge and rainfall lingers

Radar image from the time of landfall.
Florence made landfall around Wrightsville Beach NC at 8am on Friday as a Category 1 hurricane. As expected and feared, the flooding from both storm surge and rainfall has been terrible and will continue throughout the weekend. Widespread catastrophic wind damage is the only aspect of the storm that was avoided because of its pre-landfall weakening.

Satellite image from the time of landfall.
PRELIMINARY TOTALS: From Thursday morning through Friday morning, Morehead City up near Cape Lookout has received 30" of rain, and down by Cape Fear, Wilmington is at 17" so far and Wrightsville Beach is at 18.5".  Unfortunately, there's lots more to come. Additionally, the storm surge up in the New Bern area has been reported at 10-15 feet, just to give an idea of the severity of this.

24-hour rainfall totals, spanning Thursday morning through Friday morning (https://nc.water.usgs.gov/realtime/rainfall.php)
The storm surge and rainfall will continue today and into tomorrow.  Coastal areas will experience multiple peaks in the storm tide (that's the total observed water level which is a combination of the astronomical tide and the storm surge) because the strong onshore winds will persist through several high tides. Florence is just drifting to the west at 6 mph -- at that rate, it will move less than 100 miles by tonight.

Storm surge warnings still cover a lot of the NC coast and the northern SC coast. Inland flooding due to excessive rainfall is likely from today through the weekend.
After a slow, drawn-out landfall from southern NC to northern NC, Florence will eventually start pulling out of the Carolinas by Monday morning.  The storm surge will have ended by then, but not the flash flooding. Beyond that, it should turn toward the northeast and dissipate... but still bring gusty winds and heavy rain to the northeast on Monday and Tuesday.

A key phrase and message that needs to be repeated over and and over is "there's more to the story than the category!".  The category rating of a hurricane only refers to the peak sustained winds in the eyewall, so it does not account for the storm's size or the tremendous impacts from storm surge and rainfall.  There are many examples of Category 1 and 2 hurricanes being far more destructive, deadly, and costly than the rarer, stronger "major hurricanes" (Category 3+).

These non-wind-related risks have been emphasized for many days... pulling from some of my posts over the past 8 days:

Sept 6: "an inland track would bring multiple hazards, including torrential rain and damaging winds over coastal and some inland areas, and a substantial rise in water above normally dry land in coastal zones. A storm hugging the coast offshore would result in heavy rain and strong winds near the coast, dangerous surf, beach erosion and coastal flooding" 

Sept 7: "coastal areas will deal with a damaging wind threat, flooding rain and a substantial storm surge — which is a rise in ocean water above normally dry land. Some areas further inland could also contend with damaging winds and flooding rain."

Sept 8: "impacts such as rainfall and storm surge will blanket a large portion of the coastline regardless of if and where it makes a direct landfall."

Sept 9: "threat in the Carolinas and Virginia: a life-threatening storm surge at the coast [and] life-threatening freshwater flooding from a prolonged and exceptionally heavy rainfall event” from the coast to interior sections."

Sept 10: "storm surge and coastal flooding will almost certainly be a big problem from North Carolina up into the Long Island area. ... models suggest ... a stalled tropical cyclone producing incredible and dangerous amounts of rain over the mid-Atlantic region."

Sept 11: "This could be an unprecedented disaster for North Carolina ... enormous rainfall totals could be in store for coastal areas as well as inland ... Storm surge is a very big concern ... large swells, elevated water levels, and coastal flooding will affect Florida through Massachusetts over the coming days"

Sept 12: "Florence is still expected to reach the coast early Friday morning as a major hurricane, but could actually drift southward along the coast, delivering a destructive storm surge to hundreds of miles of coastline, as well as feet of rainfall at the coast and inland."

Sept 13: "The storm has grown in size over time as they typically do, so although the peak winds are weaker than they were, the storm surge threat has actually increased for more of the coastline. Rainfall has been and continues to be a significant danger.  A tropical cyclone of any intensity can dump feet of rain if it moves slowly, and Florence is no different. 20-40 inches of rain could fall over parts of eastern North Carolina which would easily induce tremendous flash flooding. Even further west into the mountains, 6-10 inches of rain could fall. The rain will persist for a few days in this region."

The public must get beyond the "it's only a Category 1" mentality.  Wind is one of the three primary hazards associated with tropical cyclones, and in the US, it's by far the least deadly and costly.

The rest of the tropics can be summarized in a single map... Helene is quickly losing its tropical characteristics over the cold eastern Atlantic but could bring stormy conditions to the Azores, Isaac has entered the eastern Caribbean and weakened to a tropical depression, and Joyce is a tropical storm that will orbit around and get obliterated by Helene in the near future.  This surge in activity will come to an abrupt end this weekend.

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