Bob Blog 19 Feb 2017



Compiled Sun 19 February 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.

A new weather viewer has been brought to my attention and I’d like to share it with you all at It has good animation graphics, but at present is limited to the weaker models GEM and GFS.


The Atmosphere: 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 standardized difference in the barometer readings between Tahiti and Darwin. A La Nina event occurs when the SOI is more than plus one (standard deviation from its mean) for more than a month. Recently the SOI has been relaxing to near zero.


SOI as seen

The Ocean: The eastern equatorial Pacific Ocean can sometimes act as a storage area for extra heat. Those periods are called El Nino and we had an extreme example of one of these last year, as measured by the NINO3.4 index. In the past couple of months the NINO3.4 was a mild negative, hinting to the possibility of a weak La Nina. However it is now relaxing to near zero.


Ocean Heat index NINO3.4 is showing a relaxing trend as seen at

The Tropics

The MJO that visited the South Pacific Ocean last week has triggered THREE tropical depressions. None of these managed to find the correct upper air conditions to deepen into a tropical cyclone (not yet anyway, too windy aloft) and that is perhaps a blessing.

Tropical depressions: TD90 (near French Polynesia) , 91 (in Gulf of Carpentaria) and 92 (South of Fiji) as seen at


However, Tropical cyclones are the current “safety valves” that allow our atmosphere to “let off steam” when it has stored to much energy. If they fail, then it begs the question, “What’s next?” For a media investigation into this read with help from Neville Koop of Ndraki weather

Activity in the tropics is strongest around TD 90. 91 and 92. And the ITCZ has almost disappeared across the North Pacific! Also, note the interesting convergence zone that has formed just south of Marquesas. That is something of a concern, and we hope it disappears before the panama to Marquesas puddle jumpers get going over next few weeks.



Rain for the past fortnight from



Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, swell black arrows / Sig wave height purple lines, SPCZ and STR.

SPCZ=South Pacific Convergence zone.

SPCZ is active at present with three tropical depressions, but the MJO has moved on and things are expected to weaken gradually during this week. TD 90, 91 and 92 may deepen as they move off but not for long. As mentioned before, that convergence zone between Galapagos and Marquesas is abnormal and of concern.


Tropical accumulated rainfall for next week from

Subtropical ridge (STR)

Wow, a Big Fat High (BFH), over 1038hPa has become stationary near 50S 170W today. This week it should slowly weaken and only finally go NE on Thursday and be replaced by a 1024 high near 40S 170W by end of the week.

Per one of my sayings: “If the High is over 1030 then it’s going to get dirty”. In this case, we can expect a squash zone of enhanced easterly winds between the High and that tropical depression to the north, and this zone of enhanced winds should bring a period of moderate easterly swell on to east coast of Northern NZ this week. So, avoid travel east of NZ now to Thursday.

The good news is that swell fades away by Sat 25 Feb for CANANZ SUMMER CRUISE to Mahurangi, see

Replacement High is expected to travel into central Tasman sea on Wednesday and across central NZ on weekend 25/26 Feb.

Tasman Sea/NZ Area troughs.

There’s a big low in South Tasman Sea tonight, and it’s going off to the south-south-east.  Associated tough—a gap between Highs—is expected to cross central NZ around Thursday 23 Feb. Nothing much to avoid near northern NZ except for an easterly swell until Wednesday.


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