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

10 April 2016

Bob Blog 10 April

WEATHERGRAM

YOTREPS

Issued 10 April 2016

 

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.

 

 

10th of April is a date marked in the heart of my generation of kiwi as being the anniversary of the ‘WAHINE” disaster.

On that date in 1968,48 years ago, the low that was once Cyclone GISELLE re-intensified near central NZ, resulting is large winds and swells that washed our Inter-Islander ferry ‘WAHINE”, with 734 people on board, on to the rocks near the entrance of Wellington Harbour. The storm peaked during the morning, and WAHINE took on water and dragged its anchors, and was pushed into further rocks. The winds started easing during the afternoon, and the ‘Abandon ship’ order was given at 1.30pm. The ship sunk in 12m of water at 2.30pm (no-one then on board). 51 died in the sea that day, 1 died a few days later, and another died of related wounds in 1990 taking the WAHINE death toll to 53. There were 3 other victims that day due to Cyclone Giselle itself.

See www.nzhistory.net.nz/culture/wahine-disaster for more information.

 

The estimated wind speed around Wellington during WAHINE storm, may be seen at www.gapwind.org/southeast-directions-over-study-area

Cook Strait is a gap with the Southern Alps to the SW and North island ranges to the northeast, and, as the saying goes, the wind gallops thru gaps.

 

EXTRA CONVERGENCE ZONE

That ‘extra convergence zone’ which has been between Galapagos and Marquesas since the equinox is STILL expected to affect travellers along this route for the next week.  Best seen on windyty.com using the rain accumulation of next ten days setting.

This extra convergence zone marks the north end of the SE trade winds, and there are only light winds between it and the Intertropical convergence zone, so that area is worth avoiding. Thus the best path at present from Galapagos to Marquesas is to find a gap and get south of the extra convergence zone.

 

EL NINO REVIEW

The heat content in the eastern equatorial Pacific (sea surface temperature) is now easing steadily and is on track to get back to normal levels by end of May.  This is shown by the NINO3.4 index as seen at

www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly.

This graph shows how this El Nino episode compares with the last grand event in 1997/1998.

Switching to a weekly time scale, the Southern Oscillation Index SOI (taken from the atmosphere) is also rapidly relaxing, as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

 

Over the past month the average winds in the equatorial Pacific have been reasonably close to normal, with no sign of any ‘equatorial westerlies’ and the trade winds also look normal in the northern Pacific and southeastern pacific.  Hopefully this means the ‘extra convergence zone’ will slowly disappear soon.  View average winds at

www.esrl.noaa.gov/psd/map/images/fnl/sfcwnd_30b.fnl.gif

 

TROPICS

AT present there are no cyclones around.  There is a tropical depression in the Indian Ocean which may go west and recurve over Mauritius.

Rain maps for the past week or two, as seen at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif , show that the extra convergence zone has become stronger last week. The path of tropical cyclone ZENA just south of Fiji stands out clearly.  And Australia remains dry.

 

Panama to Galapagos:

The last day in this current period of good winds for departure is expected to be this Monday (local), and the remainder of this week is expected to have winds too light for sailing away from Panama Outlook is for light winds next week too.

Convection can be seen in the windyty.com rain accumulation map to be strong to west of 80W so best to divert form the direct route around this, using way point 5N 80W and so going via Isla del Malpelo.

 

Travelling to Marquesas:

Light winds around Galapagos. The west-going current near the equator now only goes to 110W and into an area of light winds, so is not the way to go. Between Galapagos and Marquesas is the extra convergence zone with tropical squalls—this has wind, but is not the way to go. SO, look for a gap in the squalls and get to south of the convergence zone, maybe 9 or 10 South and then go west. OR wait, as things may change next week or the week later.

 

WEATHER ZONES

 

SPCZ=South Pacific Convergence zone.

SPCZ is expected to spend this week active across the Coral Sea to northern Vanuatu to Fiji /Samoa area to between southern cooks and Tahiti. There are signs this zone may travel south late in the week onto Tonga. And computer models are picking that weak tropical low (central pressure around 1005 hPa) may form to NW of Fiji my mid-week and travel southeast across the Fiji area this weekend--- this low may (perhaps) bring another dose of rain to Fiji, and is likely to bring a relaxing of the fresh to strong trade winds there until Friday.

 

STR= Sub-tropical Ridge

New HIGH is expected to travel east across Tasman Sea on Wednesday and northern NZ from on Thursday to Monday. There is likely to be a zone of enhanced easterly winds on the north side of this HIGH.

 

For NZ and Tasman Sea

A trough is forecast to travel northeast across NZ from Monday to Wednesday, associated with the passage of a southern ocean low eastwards well south of NZ.

This is occurring at the same time that SAM, as measured by AAO, is expected to dive negative for the first time since last December. This means the following southerly winds may being noticeably colder air.

To see the AAO go to  www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html

>>>>>>

 

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–

Feedback to bob@metbob.com. Tell anyone you like that to subscribe they should email me.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

 

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