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

22 September 2013

BOBGRAM issued 22 Sep 2013

Issued 22 Sep 2013

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.

Background influences
EQUINOX: It is logical to divide the seasons on earth use the four 'corners' of our orbit around the sun as a guide. At the equinox the earth's equatorial plane lines up with the sun's, or the sun's sub-solar point (overhead sun) crosses the earth's equator. This happens at 22:20:44 UTC, bringing spring into the southern hemisphere.
Note that day length as measured by sunrise and sunset tables is slightly MORE than 12 hours on the day of the equinox--- Because sunrise is times when the top limb of the sun clears the horizon, rather than the mid-point of the sun, same for sunset,-- and that adds 7 to 10 minutes to the day.

SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.
It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has dropped to just below zero in early September and bounced back to 0.52 on 22 September.

Mexico was hit by the combo of Cyclones MANUEL in the eastern Pacific and INGRID in the Atlantic, last week, but America is free of cyclones at present. In the NW Pacific TC USAGI is about to make landfall just north of Hong Kong and the large catherine-wheel like structure of TC PABUK is posed south of Japan and expected to go then northeast skirting the east of Japan.

Looking at the anomaly map of last week's average wind we can see the monsoon has got across Philippines into NW Pacific. Another feature that shows up is the extreme anticyclone in the South Pacific - between NZ and South America intensified to over 1040 at one stage , creating squash zones of enhanced wind on its northern and southern fringes, explaining the strong winds over Tahiti.

South Pacific Convergence Zone SPCZ
The SPCZ has broken into several small branches and these are likely to remain separate during the coming week. One is positioned from Solomons to near Tuvalu and another weak one occasionally extends west east from around Tokelau to Northern Cooks. A more intense Convergence Zone CZ over Vanuatu/Fiji is expected to go south during Monday and form a rapidly deepening Low that is likely to affect New Zealand's North Island on Tuesday and Wednesday. Avoid.
Another CZ is expected to linger over the Tahiti area this week, bring some squally showers and variable winds that come and go.

Sub-tropical Ridge STR
The STR has a weakness in the New Zealand area at present and this is expected to allow a low to deepen in the area between NZ and Fiji on Monday. This low, of tropical origin, is expected to completely disrupt the STR—and go south during Tuesday and Wednesday affecting North Island, and then peel off to the southeast.
The old STR should get pushed to the south by this low and weaken, A new STR is expected to start forming over New Caledonia by Tuesday and the slowly expand south and east into the Tasman Sea North island area by the weekend of 28/29 September.

Roaring 40s and New Zealand
A disturbed westerly flow, typical of spring, is expected to spread from the Aussie Bight across the South Tasman Sea and onto the South Island from Wednesday.

Route Briefings
Tahiti to Tonga:
There are likely to be some squally showers near Tahiti this week, but the weather pattern is looking OK on the way to Tonga.

Between NZ and Fiji/Tonga
Will need to wait for the Low to clear away, then may be OK to travel either from N to S or from S to N--- voyage will encounter some light winds from the new STR. At this stage don't have enough data for the entire voyage and so need to get another update.

See my yotpak at for terms used.
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