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

16 August 2015

Bob Blog 16 August

Issued 16 August 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.
Rivers of wind, and puddles of calm:
The wind flow across an island gets distorted by the island itself as shown
in this image from Vitu Levu taken from last Wednesday, as shown
in my illustrated blog at

SOI (the atmosphere)
The SOI (Southern Oscillation Index) measures the (normalized) air pressure
difference between Tahiti and Darwin. With Darwin having higher than
normal pressures, and the preponderance of Tahitian troughs, it is no
surprise that SOI has been so negative lately.
SOI is seen at

GONI and ATSANI are in the western North Pacific and TD 11 is on west coast
of North America
The weekly rain maps over the past two weeks show the intense rainfall from
these tropical cyclones. There has been an easing in the monsoonal rain
over Thailand in the past week, and the rain in the South Pacific has eased
for a spell.
Weekly rain signature is seen at

SPCZ=South Pacific Convergence zone.
SPCZ is expected to weaken over South pacific this week, and remain from
east of
Solomon Islands to Tuvalu/Tokelau, with a branch that moves onto New
Caledonia on Tuesday and Wednesday.
The LOW that is south of French Polynesia mid-week has come from NZ, not
form the tropics. There is likely to be a weak trough on its northern side
and this is expected to reach Tahiti around Thursday UTC (Wednesday local).
Apart from these places mentioned above, this week looks OK for hopping from
Island to Island.

STR= Sub-tropical Ridge
The STR is the zone that divides the trade winds of the tropics from the
disturbed westerly winds of the southern ocean. This week it is mostly
north of its normal position and weaker than normal. However, a HIGH that
is expected to enter the Tasman Sea on Thursday should travel east along
30/35S across NZ on Fri/Sat and then to east of NZ early next week.

Trough is expected to cross NZ on Tuesday followed by a SW flow on
Wednesday, and then light winds from the next high for the remainder of the
week, turning northerly as the next trough approaches , due over North
island on Wednesday 26 August.
See my yotpak at for terms used.
See my website for information on tailor-made voyage
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