Florence Passing South of Bermuda


Florence_Satellite
GOES-EAST geocolor satellite imagery of Hurricane Florence just prior to the latest NHC advisory. Source: Slider – CIRA/RAMMB

Hurricane Florence underwent a remarkably well-forecast period of rapid intensification into a major hurricane yesterday evening. As of this morning’s 11am advisory from the National Hurricane Center, Florence is still a major hurricane with maximum sustained winds near 130 mph. Florence is moving west-northwestward, and is expected to continue that motion around deep-layered high pressure centered between Bermuda and Nova Scotia over the next day or so. Florence is expected to remain a very dangerous major hurricane during this time as it tracks through an environment conducive for hurricane growth.

Bermuda

Florence should make its closest point of approach to Bermuda mid-afternoon as it passes about 300 nm south-southwest of the island. Impacts on Bermuda should be fairly limited. Expect elevated and hazardous surf and rip currents along the South Shore from southeasterly swell generated by the major hurricane. Isolated showers are possible and winds could briefly increase to strong (20-33 kts) in the southern marine area. A Small Craft Warning is in effect for elevated surf and strong winds.

Follow the Bermuda Weather Service for the latest official forecast for Bermuda.

East Coast/Mid-Atlantic

Florence is expected to continue west-northwestward today before turning northwestward on Wednesday through its final approach to the US East Coast. Florence is forecast to track over open ocean with sea surface temperatures near 29°C. Furthermore, environmental vertical wind shear is low and humidity sufficiently high. These ingredients suggest that Florence will continue as a major hurricane through its approach to the coastline on Thursday, and expected landfall Friday morning. Changes in the hurricane’s intensity are likely to be related to internal structural changes known as eye-wall replacement cycles. One such cycle is nearing completion this afternoon.

During such a cycle, the eye-wall (region of the hurricane with strongest winds and heaviest rains) is encircled by outer bands that form a secondary eye-wall.  This has the effect of increasing the overall size of the hurricane. If the cycle is uninterrupted, the secondary eye-wall begins to intensify at the expense of the original, resulting in an overall weakening of the hurricane. Eventually, the original eye-wall dissipates and the secondary eye-wall assumes the role of the primary eye-wall, and the larger hurricane resumes strengthening.

Coastal and inland watches have been issued by the National Weather Service for parts of South and North Carolina in response to the wind and storm surge threat.

Florence_Steering
Map of the steering flow appropriate for hurricanes of Florence’s strength. White streamlines indicate the direction of the steering flow, while the filled colors indicate the strength of the flow. Image valid at 1200 UTC 11 Sep 2018, Source: CIMSS

Increasing surf and dangerous rip-currents are expected to commence tonight or tomorrow along the US East Coast.

On Thursday, as Florence nears the coast of North and South Carolina, the hurricane is expected to enter a region of fairly weak steering. Florence should be near the southwestern edge of high pressure centered over the North Atlantic, and near the northeastern edge of high pressure centered over the Northwest Caribbean Sea imposing steering flow in opposite directions across the hurricane resulting in a weak steering regime. Both the track and intensity, and therefore the severity and duration of impacts from the hurricane depend strongly on the structure of the steering flow as Florence approaches the coast, and the timing with which the hurricane approaches this change in steering. While confidence is high that there will be severe impacts somewhere in the Mid-Atlantic region, exactly who gets what is still to be determined.

Residents should prepare for life-threatening storm surge inundation at the coast, freshwater flooding inland, and damaging winds – particularly at the coast, but extending some distance inland. See the National Hurricane Center’s Key Messages for more.

Stay up-to-date on the latest official forecast products from the National Hurricane Center and warning products from your local National Weather Service office.

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