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

04 October 2015

Bob Blog

WEATHERGRAM

YOTREPS

Issued 04 October 2015

Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

 

Tropics to NZ: - Strategy

  This is the time of year that yachts are staging themselves in Tonga (or Fiji or New Caledonia) and waiting for the right weather pattern for sailing to NZ or Australia.

 

In deciding upon a departure date, the first factor to consider is the local weather:  the South Pacific Convergence Zone SPCZ sometimes brings squalls, but is expected to stay north from Solomons to Tuvalu to Tokelau /Samoa to Northern Cooks this week.  It might visit Wallis and Futuna during middle of this week.  

 

Some like to use Minerva reef as an extra staging post since it is 1.5 days sail south of Tonga, and just out of the tropics (so marine insurance that is void in the tropics may work in Minerva).  But it only shelters you from the waves (so long as they don’t topple over the reef) and not from the wind. 

 

The second factor to watch are the HIGHS that travel along the Subtopic ridge STR, that zone between the trade winds of the tropics and the disturbed westerlies of the roaring 40s.  There is a HIGH in the Tasman Sea at present and it is expected to intensify on Tuesday, producing a SQUASH ZONE of enhanced trade winds on its northern side stretching from Coral Sea to Tonga.  During Wednesday this High is expected to weaken and the squash zone should retreat to the Coral Sea area by Thursday. Another HIGH is expected to travel from 45S over South Tasman Sea on Thursday to 30S over Northern Tasman Sea on Tue 13 Oct – this High is not expected to be intense enough to have a squash zone, but the strong SE winds are expected to remain in the Coral Sea. 

 

The third factor is to avoid strong southerly winds and heavy swells during the trip.  Good news is there isn’t much to report this week. There is a front moving off to east of NZ tonight followed by a SW/S flow, but it isn’t strong.  It may bring some SW swells from Southern Ocean to the area north of NZ but these have a lengthening period are reasonably flat. 

 

And you should try and time your trip so that it does not reach NZ with an active weather feature.  Since these voyages may take 8 to 10 days, the weather outlook that far out is not reliable on any weather models and things will change in the real world during the voyage.  However with resources such as windyty.com now available we can at least get an idea of the winds around NZ a week ahead.  Today’s data shows that a FRONT is likely to cross Northland around wed/Thu 14/15 Oct followed by a burst of southerly winds extending north of NZ to at least 30S.  SO avoid arriving in NZ then, and that means don’t depart Tonga late this week, unless you can beat that front or don’t mind encountering it.

  

This is the season that we sometimes get a procession of fronts travelling across Northern New Zealand, maybe 4 or 5 days apart. Not this week. When this pattern sets in, it may be necessary to deliberately time you departure to encounter one of these fronts near 30S (where they are usually weak) (and then dash for NZ in the SW winds that follow.  This strategy may require you to travel as far west as 170E before encountering the front, and that adds days to the journey if it is starting form Tonga.

 

A useful resource is www.yit.co.nz.  Patricia and David from Gulf Harbour Radio provide a method where by your position can be plotted on the web, and also provide daily weather updates via shortwave radio. You need to first register on this web site with your boat and crew details and then you simply email (send@yit.co.nz) or radio (call in to the sked) in your position and conditions in preferably each day.  These reports help me (and your loved ones) keep track of your progress.

David checks these reports and does a weather sked, Monday thru Saturday (NZ date), on 8752 at 1915UTC to make sure the models reflect your actual conditions.  He does a round-up of the weather in each island group, including passage weather, at 1930UTC. That’s 7.30am depending on your time zone.

 

The High seas forecast SUBTROPICS  is the official forecast for this area issued by the marine team at MetService is broadcast by Taupo Maritime Radio ZLM at 0903UTC on 6224 and 12356kHz , and 1003UTC on 9297 and 16531kHZ, then again on 2103UTC on 6224 and 12356kHz , and at 2203UTC on 9297 and 16531kHz.  If you have access to email on board then the text of this bulletin is available by sending an email to query@saildocs.com, no subject needed, with message SEND http://weather.noaa.gov/pub/data/raw/fq/fqps43.nzkl..txt

Or remember to email or TXT your position report to me at bob@metbob.com 64277762212

 

TROPICAL TOPICS From http://tropic.ssec.wisc.edu/

Tropical storm OHO is near Hawaii, Hurricane JOAQUIN is in the Atlantic, Typhoon MUJIGAE in in the China Sea, and Tropical Storm CHOI-WAN is in the NW Pacific 

The weekly rain maps as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif show that rainfall in the South Pacific is confined to the SPCZ 

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See my yotpak at boatbooks.co.nz/weather.html for terms used.

See my website www.metbob.com for information on tailor-made voyage forecasts

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