In article , Les
Crossan writes
I always thought hurricane-force winds could come from extratropical
cyclones or mid-latitude depressions too?? Has the thinking changed on this
recently.... please correct me if I'm wrong..

the usw glossary has this: Hurricane Force: This term (in UK Met Office use)
is only used in shipping bulletins and associated Gale/Storm warnings. It is
strictly defined as a mean (10 minute) wind of 64 knots or more. (Gusts not
defined) (See also comments at Severe Gale).

The USW FAQ has a distinction in 2B.1 too between tropical cyclones and deep
depressions. I'm confused now. HELP!

Hurricane force winds (Beaufort Force 12) can be produced by tropical
cyclones or mid-latitude depressions or even by katabatic effects in the
Antarctic. Hurricane force refers purely to the strength of the wind and
is not dependent on the nature of the weather system producing the wind.

Tropical cyclones (including regional names such as hurricanes and
typhoons) are warm-cored low pressure systems which are fuelled by large
quantities of water vapour evaporated from the sea surface. They develop
and persist in regions of low vertical wind shear. Increasing vertical
shear generally results in weakening.

Mid-latitude lows form in highly baroclinic areas. In maturity they are
cold-cored. During the developmental phase there is almost always
considerable vertical wind shear above the developing surface low.

When a tropical cyclone is said to be in transition to an extratropical
low this means that it is losing its warm core and the strong convection
immediately surrounding it. Typically, this happens when the centre
moves over colder water. A major hurricane/typhoon can take a
considerable time to completely lose its tropical characteristics.

Hope this helps!

Norman.
--
Norman Lynagh Weather Consultancy
Chalfont St. Giles
Buckinghamshire