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WEATHERGRAM

Bob McDavitt's ideas for sailing weather around the South pacific

18 February 2018

When does a cyclone lose its name?

Incoming email this morning:

 

Bob hi

 

I hope you can help me out with this.

 

Marine insurers have, in the past, imposed extra conditions for damage or loss due to “named tropical storms”. This is the phrase they use rather than “tropical cyclone” or such and presumably covers the early stages of the storm before the eye develops and the later stages when they are often referred to as “ex tropical cyclone …”.

 

Recently there has been a move to exclude all damage or loss from named tropical storms. This is one of underwriters responses to the big losses they recently suffered in the Caribbean etc. This is a significant reduction in cover and I would like to understand it better.

 

The naming of tropical storms appears to be quite systematic worldwide so their birth is reasonably clear cut. However I can find nothing about their un-naming, about that point in time when they are no longer a “named tropical storm”. In an official metrological sense when do “named tropical storms” cease to be?

 

I hope you can shed some light on this for me.

 

Regards

 

+++++++++++++++++++++++

 

My reply

 

Hi there Bruce.

 

In summary, when the system is in the tropics and is expected to be able to produce gale winds it can hold a name as a tropical cyclone. .

 

As for the insurance term “named tropical storm”, the Insurance company is clearly reducing/limiting its coverage.  I suppose such a term only applies to marine insurance covering, in part, tropical areas. . It may be easier for you to dump that policy and get a “storm” policy which offers reasonable coverage for damage done by wind /rain/waves.

 

When a tropical cyclone leaves the tropics (latitude 23.5, as defined by the axial  tilt of the earth)   it is nominally  no longer a tropical cyclone.

 

Usually from that point it is called ex-tropical cyclone , or former tropical cyclone NAME  some also call it a cyclone/depression/low of tropical origin, or extra-tropical cyclone ( extra here has the meaning “out of”, as in extraordinary or extra-terrestrial) .

 

Usually the cyclone will be weakening by that stage and losing its tropical characteristics.  The main feature that distinguishes a tropical cyclone is the “eye wall” = a ring of intense wind and rain around the centre , with winds reaching at least gale force , averaging 33 knots  (gusting around 50 knots).   Another feature is to have spiral shaped convective bands . 

 

As the cyclone loses these characteristics , the TCWC (tropical cyclone warning centre) that monitors it reduces its Category , after dropping below Cat 1, it is deemed to be a depression or a low.   When less than Cat 1 is no longer has winds of gale force in its eyewall, and thus is unlikely to produce much damage.   Since this decision may require good evidence from a visible satellite imagery, it is usually made during daylight rather than overnight.

 

  Some lows of tropical origin take a while to unravel , and some manage to have a second life as a mid-latitude low. If these lows are still carrying damaging wind and rain then it helps the media to drop the word “tropical” and simply refer to the system as CYCLONE NAME.  I started doing this when on duty during Cyclone BOLA back in 1988.

 

Now, mid-latitude lows  do not have an eyewall  but may indeed have some areas with  gales. Most mid-latitude lows  are not given names.  (There has been some attempts by UK and USA to have a list of “winter named storms” recently , but that’s to help media coverage, not something that insurance companies have had a hand in).

 

There are two interesting cyclones in the  Australia/ NZ region as far as the relevance of the term  “named tropical storm” is concerned.

1.KELVIN has damaging winds and rain and was CAT2 when in made landfall in NW Australia last week .  It is now inland and has lost its tropical characteristics , but is still in a tropical latitude and bringing damaging rain.  The Bureau of Meteorology are keeping it as a named storm this morning , Cat 1 , and anticipating to drop its name (and treat it as an L) by this afternoon, as it leaves tropical latitude. The systems will still have a coherent Low centre and a zone of potentially destructive wind even when t loses it name.  It may well be called ex-KELVIN for a while (or maybe as a Low/depression of tropical origin).

see www.bom.gov.au/products/IDW60281.shtml , in AWST time

 

2.GITA is already south of the tropical border but still has a name and is described this morning to be Cat2.

 

Satellite imagery shows it has already lost its eyewall, and only has one spiral band of convection left at this stage.  Also it is beginning to encounter stronger NW winds aloft (part of a jetstream ahead of an approaching upper trough).  These winds are ripping off the upper clouds of GITA , leaving behind a rotating low. 

 

Satellite image source: www.meteo.nc/nouvelle-caledonie/observations/images-satellite

 

In New Caledonia time = UTC+11

 

MetService is the TCWC (tropical Cyclone warning Centre) for GITA and this morning have it as a “named tropical cyclone” of Cat 2  even tho’ it is near 30S, well out of the tropics.

 

The idea shown in the image above is that the system may continue to be a Cat2  this afternoon, but from then on will be a mid-latitude LOW, and no longer a “named tropical cyclone”.   The system has an estimated central pressure this morning of around  980 hPa and may deepen to 977hPa on Tuesday , thanks to the injection of cooler air on its southern side.  It will take a few days for this system to weaken, for its isobars to “unravel” and for the winds it brings to ease.

 

In a mid-latitude low the zones of strongest winds is usually alongside the “fronts” (leading edges of an invading airmass). As a mid-latitude low approaches New Zealand the zones of strongest winds become distorted by terrain effects due to the mountains, producing “rivers of wind “ and “puddles of calm” .  In the image above no attempt has been made to show  the likely boundary of gale winds for the likely future positions of this mid-latitude  low.    Instead, these are detailed at the Severe weather pages

 

www.metservice.com/warnings/severe-weather-warnings

www.metservice.com/warnings/severe-weather-watch

and www.metservice.com/warnings/severe-weather-outlook

 

 

So “Cyclone GITA”  has the potential to damage in New Zealand  over the next few days but will by then not be a “named tropical cyclone”.  There will be plenty of insurance claims , and they will be covered by “storm” damage --- but maybe not by the policy you seem to be referring to.  It may or may not help your case if the media continue to call this system “Cyclone GITA” while it has damaging wind and rain.

 

Bob McDavitt

 

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