Ed DunhamAdministrator
(Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017))
Thu Oct 25 2012 09:21 PM
Re: Hurricane Stuff - STC Sandy?

Hurricane Sandy has the potential to become a significant subtropical cyclone that could threaten the Northeast early next week. First, some definitions from NHC:

Hurricane / Typhoon:
A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/hr) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.

Subtropical Cyclone:
A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.

Subtropical Depression:
A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/hr) or less.

Subtropical Storm:
A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 km/hr) or more.

Okay - so here is the question: "What do you call a Subtropical Storm that has sustained winds in excess of 64 knots?"

Answer: "Its still a Subtropical Storm." Go back to the definition of a Hurricane and note that it starts off with "A tropical cyclone...". Since the definition for a Hurricane requires a full tropical system, a subtropical system with hurricane force winds is still a Subtropical Storm.

The only previous event that I know of was on October 24, 1979, at 1800Z (similar timeframe) when Subtropical Storm 1 had sustained winds of 65 knots. It was never named (they didn't name subtropical storms back then) and still carries the designation of STS1 for 1979.

TWC has proposed an interesting theory this evening that The Perfect Storm of 1991 (Hurricane #8) and the unnamed storm off the Florida east coast (Aug-Sept) in 2011 were a rare hybrid system that consisted of a tropical system at its core surrounded by an extratropical system beyond the core. The suggestion is that Sandy could become another one of these. Who knows - maybe so - at least its some novel thinking outside of the box.

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