Hermine reinvigorates to threaten US eastern seaboard
The former hurricane is expected to threaten the coast from the Carolinas to Connecticut in the coming days.
It was always expected that Hurricane Hermine would gain energy over the Gulf of Mexico, and bring damaging winds and flooding rain to the state of Florida.
Usually, such cyclones weaken as they lose their source of energy, the warm waters of the Gulf of Mexico, and interact with the land.
For a time, this is what happened. But once Hermine crossed the Florida Panhandle and reemerged into the Atlantic, it reaped the benefit of passing over the northern end of the Gulf Stream.
Off the coast of North Carolina and Virginia, sea surface temperatures are currently between 26 and 28C, some two degrees above normal, and high enough to support tropical cyclone development.
Consequently, Post-Tropical Cyclone Hermine, which has sustained winds of 100 kilometres an hour, will intensify. Winds will increase to at least 120kph – a Category 1 hurricane on the five point Saffir-Simpson scale.
It is unclear whether the US National Hurricane Center will reclassify Hermine as a hurricane at a later date. Regardless, the impact on the eastern seaboard will be considerable.
Hermine’s northeastward movement is the result of steering by a trough of low pressure. That is now moving away, leaving Hermine somewhat marooned in the Atlantic for the next few days.
During this period, low pressure and strong winds will result in massive waves of up to eight metres, and a significant enhancement of water levels. These could be up to 1.5 metres above the tidal norm.
The coastal region from the Carolinas, northwards through Virginia, Delaware, New Jersey, New York, Connecticut to Cape Cod will be affected by stormy weather and a significant risk of coastal inundation and erosion.
Fortunately, no features in the upper atmosphere are expected to interact with Hermine in such a way that would cause it to develop any further.
This makes it a very different system from Hurricane Sandy which brought devastation to this coast in October 2012.
Sandy was another storm system which lost hurricane status but was reinvigorated by warm Gulf Stream waters. It, too, was a large system which was driven northwards.
Unlike Hermine, however, Sandy interacted with a frontal system from the northwest and was driven onshore by an area of high pressure across the North Atlantic.
Sandy also coincided with spring high tides, which made the coastal inundation and erosion far worse.
It is expected that Hermine will remain far enough offshore not to have a major impact on the coastline, although there is still some uncertainty in its likely behaviour before the polar jet stream dips southwards later this week and hauls Hermine further out into the Atlantic.
Nevertheless, tropical storm warnings remain in place along the coast and the area is best avoided during this period.