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

02 September 2012

BOBGRAM issued 2 Sep 2012

Issued 2 Sep 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 Ocean: Seas surface temperature anomalies in the equatorial mid Pacific have stalled recently or even relaxed a little and are all around the plus 0.5C mark. So the recent trend towards El Nino is now on hold.

The Atmosphere: The Southern Oscillation Index or SOI is relaxing after a dip during past month. On 25 Aug its 30-day running mean was -1.01, and on 2 Sep it was -0.34. So the atmosphere is in neutral, and that's different from last week's indicators.

Tropical cyclones: It's a busy time around North America with KIRK and LESLIE and an area under Investigation in the Atlantic, and ILENA + another rare under investigation in the Pacific. ISAAC made landfall near New Orleans around 7 years after KATRINA, but was a very low category hurricane and didn't pose much of a problem—rather, its rain has already been lapped up thank you.

The Indian Monsoon failed to deliver its normal dose of rain to some parts of India, but is now very wet over Myanmar/Burma, but dry over Indonesia/Malaysia, so it proving to be a mixed bag--- hard to use as an indicator.

In the South Pacific, the Convergence Zone SPCZ continues to be subdued. There is an active part over Papua New Guinea and Solomons. The zone of welcome rain that has been over Tuvalu/Kiribati /Tokelau is now weakening. Weak Convergence zones CZ are wandering from west to east across the South Pacific Islands, linking in with the transient troughs of the mid-latitudes further south. These systems move east because they are propelled by upper winds, but the surface winds around them are easterly.

This week there are two CZs in the pattern. CZ1 is moving away from French Polynesia FP tonight. CZ2 is bigger; it was over Vanuatu on Sunday and should cross Fiji on Tuesday, and Tonga on Wednesday. This CZ is expected to deepen into a LOW near 25S and to south of Niue on Thursday. Avoid this. The low should then go off to the SE and the CZ is likely to fade as it crosses the Cooks on Friday 7 Sep.

The STR is in its normal position along 25 to 30S. The High cell near 170W is expected to wander eastward along 30S. This helps to squeeze the isobars on its northern side closer together, and thus accentuates the trade winds between Northern Cooks and Tahiti from Tue to Thursday 4 to 6 Sep.
Another high is expected to move into the north Tasman Sea on Tue and then wander east along 30S go that it gets east of the dateline by end of Friday 7 Sep UTC.

NZ/Tasman Sea
The strength of the polar vortex is measured by a parameter called SAM (Southern annular mode) and a proxy for this is the AAO (Antarctic Anomaly) at . Its looking positive for the next few weeks, and that helps support the idea of disturbed westerly conditions for the Tasman Sea/NZ area.

One of these transient troughs in the disturbed westerlies is crossing NZ on Monday and the next is expected to do so on Saturday, followed by a day or so of vigorous SW winds with the Tasman Sea filling up with large but long-period southerly swells from the Southern Ocean

Traveling towards New Zealand this week:
Approaching NZ from the north between Tue 4 and Thursday 6 Sep is the go.
Now is a good time for those of you in planning mode for sailing across the Pacific Ocean to check that you have access to good weather information. Sailmail offers good access to GRIB data, and to view these files you may use a grib viewer such as Viewfax at http:// . Also you should download the Nadi Fleet code by sending an email to, no message, with text "send nadi-fleet", and view this in PhysPlot from

Updates are also available from ZKLF Radio fax on 3247.4, 5807, 9459, 13550.5 or 16340.1 kHz Or High Seas on ZLM 6224 kHz and 12356 kHz at 0303Z, 0903Z, 1503Z and 2103Z and on 8297 kHz and 16531 kHz 30 minutes later or, for warnings, send email to, No subject, saying SEND

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