Bob Blog 26 April 2015



Issued 26 April 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

Three Chilean Volcanoes have recently thrown ash into the atmosphere; Calbuco, Caliaqui and Cordon Caulie.

Here is a copy of the current forecast for the VA (Volcanic Ash) as it drifts into the South Atlantic:


Volcanic ash advisory from

The Atmosphere:

The Southern Oscillation Index SOI sums up the weather pattern over the South Pacific as one number and is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean). It relaxed to zero in early March and then went negative, to be -10 units for a few weeks in late March/early April, but is now relaxing again.


SOI as shown at

If this index remains lower than -10 for four weeks in a row then we have a fully- blown El Nino, but since it is now relaxing the trend is away from an El Nino at present.


The tropics are quiet at present. The outlook is for a tropical LOW to form over Marshall Islands from 4 May.

The rain map for the last two weeks show an increase in activity along the ITCZ in the Northern Pacific and generally around the South Pacific, and a decrease in activity in that convergence zone west of Galapagos which has been a “mirror image” of the ITCZ and appeared soon after the equinox. Also notice the intense rain around Sydney. Yuck.



Weekly rain signatures for past two weeks, as seen at



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 red arrows, SPCZ and STR.

Galapagos to Marquesas:

Good winds for departing on local Monday then fading winds around the Galapagos.

First of all head for 5S at around 97 to 100W, even though this involves motoring into light SSE winds. Then the middle leg is westwards along 5 to 6S as far as 125W enjoying a west-ward going current. And then the third leg is to go direct in SE winds

SPCZ=South Pacific Convergence zone.

SPCZ is expected to be reasonably intense this week across Solomons/ Tuvalu and Samoa and may visit French Polynesia for a few days. The part of the SPCZ over Tuvalu is expected to drift southwards towards Vanuatu/Fiji by the end of the week, 2/3 May.

STR= Sub-tropical Ridge

STR is reasonable north of NZ this week,

Next significant HIGH is expected to ridge across Tasmania on Wed 29 April and then across the central Tasman Sea on Thu/Fri and across NZ on Sat/Sun/Mon 2/3/4 May.

Departing from Australian East coast for the tropics this week:

Not a good week for going to the tropics, as that HIGH moving east along the STR from Wednesday is likely to have a squash zone of enhanced easterly wind son its northern side. Also a LOW is expected to form off Brisbane on Thursday/Friday and then move south along the western flank of the then departing HIGH, the LOW may bring another gale to the Sydney area on Sat and then weaken and go south.

Departing from Northern NZ for the tropics.

Northern NZ is experiencing a passing active FRONT tonight and on Monday, then West to SW winds fresh to strong at times on Tuesday and Wednesday, weakening away on Thursday, followed by light winds in that passing HIGH on Sat/Sun/Mon 2/3/4 May. As that HIGH travels east, it bunches the isobars on its northern side into a squash zone of strong SE winds (25 gust 35 knots and 3 metre+ swells) over, and on the way to, Fiji/Tonga from 1 to 6 May. Minerva Reef is likely to have strong trade winds from 3 to late on 5 May so please time your trip to avoid these if you can— maybe delay departure from NZ until Thursday, or, if you do depart on Tuesday/Wednesday then go slow so as to avoid getting north of 25S until after 5 May.

See my yotpak at for terms used.

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