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

05 July 2015

BobBlog

YOTREPS

Issued 5 July 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.

 

The 30day anomaly map for 1 July as seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

helps to explain what has been happening around the planet during June and how it is linked into an El Nino episode.

The SOI (Southern Oscillation Index) measures the (normalized) air pressure difference between Tahiti and Darwin. When Darwin is having higher than normal pressures, Tahiti tends to have troughs. Indeed that has been the case in June 2015,  as seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

 

The subtropical ridge – that zone between the roaring 40s and the trade winds – the zone along which anticyclones migrate eastwards in the southern hemisphere—has been much more intense than normal , feeding cold SW winds onto southern NZ and accentuating the trade winds over Tonga Fiji and Vanuatu. The South Pacific convergence zone is north of its normal position, and Solomon Islands have lower than normal pressures as a consequence.

 

There is a zone of higher than normal pressure extending all the way from Australia to India and to the Philippines and this helps explain a stuttering monsoon.

 

The Indian Monsoon index may be seen at

http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html

 

TROPICAL TOPICS

 

Over the past week a MJO pulse brought a burst of extra convection into the West Pacific Ocean, along with a burst in near-equatorial westerly winds (as mentioned last wee). That has triggered 4 tropical cyclones:

RAQUEL which was over the Solomon Islands mainly last Thursday/Friday.

CHAN-HOM which is now north o0of the Philippines.

LINFA which is further east;

And NANGKA, even further east:

During the coming week a tropical depression is expected to move towards Hawaii by local Friday and Saturday. And another to southeast of Acapulco. Meanwhile the North Atlantic is remaining quiet.

 

The weekly rain maps over the past two weeks show the intense rainfall from these tropical cyclones, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing wind, isobars, Sig wave height green lines, swell direction orange arrows, current in small arrows, SPCZ and STR.

 

SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stretch from east of Solomon Islands to Samoa and from Suwarrow to almost Tahiti. The part over Tahiti is expected to fade.

The clouds that were part of RAQUEL and expected to spread out as a convergence zone that is likely to cross New Caledonia on local Tuesday night/Wednesday morning, and the following night over Minerva and Tonga.

 

STR= Sub-tropical Ridge

STR is breaking up into a band along 25/30S and discrete Highs that are mainly at 40S. The High moving into the Tasman Sea on Monday is a slow-mover and expected to cross NZ during next weekend=the middle-weekend of the school holidays. It should travel further eat to east of NZ next week along about 40S.

 

NZ

Cold front is being directed northwards across NZ on Monday and Tuesday, followed by air that is coming directly off the polar ice shelf. Good for alpine snow.

This front is likely to produce gale force NW winds near 30S on Wednesday. Avoid.

 

Departing from Australia to the tropics this week:

Winds between Brisbane and Noumea are expected to be light on Monday and Tuesday then southerly or SSE on Wednesday to Saturday, so that may allow a good enough voyage (either way). However a front is expected to reach Brisbane late Saturday and deepen in the Tasman Sea bringing strong northerly flow to New Caledonia by Mon/Tue 13/14 July. Avoid.

 

Departing from Northern NZ going north.

AFTER that Tuesday front over northern NZ there should be a good weather pattern for departures on wed/Thu/Friday with moderate to fresh SW or southerly winds for starters.

>>>>>>

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

See my website http://www.metbob.com for information on tailor-made voyage forecasts – Feedback to bob@metbob.com.

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