Tonga to NZ

Finally on Thursday morning the GFS model and ECMWF models (http://bit.ly/ecoz)  have come together (almost) after deviating for  a few days.

They still produce different scenarios after 8 November .  The GFS is the one that most sailors are using for their GRIB data and I suppose many were having nightmares on Wednesday when it went all over the place with the situation near New Zealand next week.

So we have a cold front crossing NZ on Friday and Saturday- this introduction of cool air into a medium wave trough area has really got the GFS excited (oh Ok, lets say it got the numbers gelling) so that it stalled that front and turned it into a low lingering in the area for a few days .  its good that we update these models with observations from the real world twice a day—today’s run has gone off that idea and let the Friday /Saturday front proceed .

 

The next feature is a HIGH in the subtropical ridge.  This is taking a track more to the south than the previous high. It is expected to cross Southern NZ on Monday and straddle NZ from Tuesday to Thursday. This sets up an easterly flow onto Northland.

 

Then comes the baddy – as far as the sailors are concerned.  in yesterday’s run the GFS the scenario was for a new Low to form around northern NZ with a squash zone between it and the New Zealand high producing strong winds that turned to a gale early next week . Enough  of a chance to deter those planning to sail to NZ.  But now the GFS  scenario has ‘caught up’  with the ECMWF and both are introducing the new low between New Caled0nia and Fiji on Thursday 8 Nov — so that leaves plenty of time for someone departing Tongatapu on Friday or Minerva by Saturday to have  a reasonable voyage to NZ.  After that, this window closes.

During Thurs 8 to Sat 10Nov that baddy Low is expected to go southeast  and deepen a lot.  There will THEN be  a significant squash zone of enhanced wind between it and the high over New Zealand. GFS scenario hasn’t  caught up with this yet but ECMWF is on to it.  It should pass by close to south of Minerva on Thursday 8  to Friday 9 November – so please get away from Minerva before then  .

Here is spot weather data at North Minerva using this morning’s GFS data- and I suspect that the winds will be stronger than this on 8 Nov.

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Here is what a voyage from Tongatapu to Opua for  a vessel doing up to 6.5 knots departing noon Friday looks like using today’s data – red path is the optimized  path and red barbs are the winds likely to be encountered , each barb represents 10 knots – click to enlarge

 

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There are some light winds – so some motoring is required.  And we do encounter  a brief period of strong easterlies near  33 South ,  not too bad.  Any departures later than this  are likely to get more strong winds there  and if you depart  on 4-5-6 November you’ll likely encounter a gale around the incoming Low.   Next window may open  around 10 November.

 

If you would like me to email me  a voyage forecast for your trip to NZ please let me know at bob@metbob.com