The National Hurricane Center (NHC) issues advisories, forecasts, and warnings on tropical cyclones - the generic term for hurricanes, tropical storms, and tropical depressions. Sometime prior to making landfall, Hurricane Sandy is expected to lose its characteristics as a tropical cyclone and take on the structure of a wintertime low-pressure area. Because the National Hurricane Center only issues advisories on tropical cyclones, there will be changes in the flow of information coming out of the NWS when this transition occurs.
The primary difference between a tropical cyclone and a wintertime cyclone is the energy source. Tropical cyclones extract heat from the ocean and grow by releasing that heat in the atmosphere near the storm center. Wintertime cyclones (also called extratropical or frontal lows), on the other hand, get most of their energy from temperature contrasts in the atmosphere, and this energy usually gets distributed over larger areas. Because of these differences, tropical cyclones tend to have more compact wind fields, tend to be more symmetric, and have a well-defined inner core of strong winds. Wintertime lows have strong temperature contrasts or fronts attached to them, have a broader wind field, and more complex distributions of rain or snow.
The official NWS term for a tropical cyclone that has evolved into something else is “Post-tropical cyclone”, where the post in post-tropical simply means after. Thus, once Sandy loses its tropical cyclone status it will be known as “Post-tropical Cyclone Sandy” in NWS products. Some aspects of this transition are already occurring, and NWS forecasts of storm impacts are based on this expected evolution. Regardless of when this transition formally occurs, Sandy is expected to bring significant wind, surge, rainfall and inland flooding hazards over an extremely large area, and snowfall to more limited areas.
Because Sandy is expected to make this transition before reaching the coast, the NWS has been using non-tropical wind watches and warnings, issued by local NWS Weather Forecast Offices (WFOs), to communicate the wind threat posed by Sandy in the Mid-Atlantic States and New England. (This is why NHC’s tropical storm warnings extend only into North Carolina.) The NWS plans to continue using non-tropical watches and warnings issued by local offices in the Mid-Atlantic States and northward throughout this event. By using non-tropical warnings in these areas from the start, we avoid or minimize the significant confusion that could occur if the warning suite changed from tropical to non-tropical in the middle of the event.