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

08 July 2012

BOBGRAM issued 7 July 2012

Issued 8 July 2012
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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

The Southern Oscillation Index or SOI continues on its downward trend. Its 30-day running mean has fallen from -0.14 in late May to -1.21 on 7 July. If this remains below -1 for the remainder of July then we are in El Nino territory.

Indian Monsoon has re-surged so that it is now showing above normal activity as it bursts into NW India. However an El Nino episode is usually associated with a weak monsoon, so in the next few weeks its activity over SE and central Asia will be worth watching

In the South Pacific, the Convergence Zone SPCZ continues to rebuild in the in the Coral Sea region. Activity is also slowly stretching out along 10 to 15S from Tuvalu to Tokelau – and so by Friday 13 a trough is expected to deepen between Samoa and Southern cooks. This area has only had dry air for the past few weeks so this trough may bring rain that could be a shock to their systems. A low should form south of the Cooks on Sat 14, compacting isobars around its centre and opening the gradient out over the Cooks. This Low is then expected to deepen further as it wanders off to the SW and then the south. Avoid.

This is expected to dominate proceedings in the South West Pacific this week: The High which is crossing central NZ on Monday to Wednesday is expected to continue migrating eastwards along around 40S from Thursday to Sunday, and then it should get pushed to the SE by the low developing south of the Cooks. For a while it should be a banana-cushion south of that low.
North of this high there is likely to be a squash zone of enhanced trade winds mainly over the Coral Sea, but also stretching from Vanuatu to Fiji on Tuesday and Wednesday and from Tonga to Southern Cooks on Thursday and Friday. This squash zone should then be caught between the Low and its high cushion.

The next STR high is expected to move across central Australia on Friday and Saturday and then weaken in the Tasman Sea on Sunday and Monday.

NZ/Tasman Sea troughs
A large Low is expected between those two highs. It should move across New South Wales on Thursday, deepen rapidly east of Tasmania on Fri 13 July and then start weakening as it travels on a SE path to south of NZ on Sat.
A secondary low is likely over central NZ on Sunday and then stall over northern NZ.

Traveling between NZ and the tropics:
Best days to depart from northern NZ this week are Monday to Wednesday.
A Thursday departure is likely to get caught by the secondary low over the weekend.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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