Tropical versus Extratropical

For several days, the National Hurricane Center (NHC) has been talking about Ida becoming “extratropical.”  The definition of extratropical used by NHC is as follows:

“Extratropical:  A term used in advisories and tropical summaries that indicate that a cyclone has lost its “tropical” characteristics.  The term implies both poleward displacement of the cyclone and the conversion of the cyclone’s primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.”

The transition from tropical to extratropical is obviously not instantaneous.  Ida will gradually lose its tropical characteristics and is forecast to become “extratropical” within 36 hours.  But it is important to note that tropical storm force winds will still accompany Ida as the transition occurs.  And it is also important to understand that the strong winds and heavy rains are already spreading onshore along the Gulf coast, well in advance of the center approaching the coast, as the NHC has been saying.  The heavy rains will continue and spread from the central and eastern Gulf coast northward and northeastward into the eastern portions of the Tennessee Valley, southern Appalachians, and the southeastern U.S.

Although Ida has been downgraded from a hurricane to a tropical storm, the threat of strong winds, heavy rains, coastal flooding, and the possibility of isolated tornadoes remain.  And even after Ida transitions to an “extratropical” cyclone, these threats will continue.


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