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

13 August 2017

Bob Blog 13 Aug 2017

Compiled Sun 13 Aug 2017

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.

At present tropical cyclone JOVA is travelling ESE (a strange direction) off the west Mexican coast and a tropical depression is expected to take a clockwise path around the Atlantic Ocean.
Meanwhile TC BANYAN is in the Northwest Pacific and is expected to go north and then northeast - its remains may reach Alaska, perhaps.
We can see these tracks at

Last week's rain map, compared with the week before, shows continuing high rainfall around the NW Pacific, and the equatorial Indian Ocean, and the equatorial Atlantic Ocean. It has been very wet around Bangladesh, and there is a build-up of activity along the South Pacific Convergence zone.

Rain for the past week is from

SPCZ=South Pacific Convergence zone.
The SPCZ is expected to shrink and shift to the north this week. The build-up of shower activity that occurred last week is expected to be gathered by a tropical trough and conducted south-southeastwards across Tonga /Niue area on Wednesday 16 Aug UTC.

Subtropical ridge (STR)
High to northeast of NZ is expected to travel east along 30 to 35s this week and build to a peak of over 1035 on Thursday 17 Aug UTC. This is expected to produce a squash zone of enhanced trade winds on the north side of this High on 17 to 19 Aug UTC-this should mainly affect French Polynesia.
New High is expected to move off eastern Australia into North Tasman Sea on Monday, and to travel along 20 to 30S past northern NZ on wed/thu UTC. This high is taking the typical path of a winter anticyclone.

Between Australia /NZ and the tropics:
Light winds in the subtropics between 20 and 30s with the incoming high.
South of 30s the pattern is a typical winter of disturbed westerlies, made stronger and squally by a deep Southern Ocean low travelling east along 45 to 55S, 960hpa, in Aussie Bight by Tuesday and south of the Taman Sea by Friday. SW swells up to 5+ metres with a period over 10 seconds in Tasman sea by this weekend.

French Polynesia to the west:
The trough that is expected to travel south-southeast over the Tonga/Niue area on Wed 16 Aug is likely to dominate the landscape this week for any vessels hoping to travel westwards towards Tonga.
It is likely to be an active trough because it is taking away (southwards) the build-up of extra energy from the SPCZ. Also, as this extra warmth comes south it is likely to encounter the colder denser airmass which crossed North Island tonight showing itself as an upper trough with lots of thunder and lightning.
I'll assume that it may be better to wait for this trough to make its way south-southeast, and so delay that trip toward Tonga until later this week or early next week.
The trough should bring a brief period of strong NE winds to Niue on Wednesday morning local or Rarotonga around Friday morning local.
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