This article is from the Storms FAQ, by Chris Landsea firstname.lastname@example.org with numerous contributions by others.
A sub-tropical cyclone is a low-pressure system existing in the
tropical or subtropical latitudes (anywhere from the equator to about
50N) that has characteristics of both tropical cyclones and mid-latitude
(or extratropical) cyclones. Therefore, many of these cyclones exist in
a weak to moderate horizontal temperature gradient region (like mid-latitude
cyclones), but also receive much of their energy from convective clouds (like
tropical cyclones). Often, these storms have a radius of maximum winds which
is farther out (on the order of 60-125 miles [100-200 km] from the center)
than what is observed for purely "tropical" systems. Additionally, the
maximum sustained winds for sub-tropical cyclones have not been observed to
be stronger than about 64 kt (33 m/s).
Many times these subtropical storms transform into true tropical
cyclones. A recent example is the Atlantic basin's Hurricane Florence in
November 1994 which began as a subtropical cyclone before becoming fully
tropical. Note there has been at least one occurrence of tropical cyclones
transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973).
Subtropical cyclones in the Atlantic basin are classified by the maximum
sustained surface winds: less than 34 kt (18 m/s) - "subtropical depression",
greater than or equal to 34 kt (18 m/s) - "subtropical storm". Note that
while these are not given names, they are warned on and forecasted for by
the National Hurricane Center similar to the treatment received by tropical
cyclones in the region.