Bob Blog



Issued 24 May 2015

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.

Northern NZ Saturday LOW

A low started to form near Lord Howe Island on Thursday 21 May. Here it is as seen at the surface level of the web site.


Wind speed


Cloud and rain

It was near and slightly southeast of the apex of a JetStream as shown at the FL340 level map


At this position air may be sucked upwards and outwards from the low in a process similar to how we can use a hose to spray out garden chemicals. This causes the central air pressure (as measured in hectoPascals) of the low to drop quickly, creating more and more isobars near the centre, making for stronger winds around the system.

Many meteorologists monitor the 500 hPa (FL 160) map as it is in the middle-atmosphere and combines the features of the upper air (jet streams) and the surface . Last Thursday the FL160 map looked like this:


The flow at this level is a good approximation of the steering flow for surface features, and this map shows the strong NW winds above the surface low, so that is was no great surprise that this surface low travelled so quickly across northern New Zealand on Saturday.

The low has already been blown to the far SE of NZ and thus dislodged a chunk of air out of the Southern ocean onto New Zealand as shown in this plot of the current snow expected for the next 24 hours over NZ


The Atmosphere:

The Southern Oscillation Index SOI is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean) and sums up the weather pattern over the South Pacific as one number. It has dived very negative in the past few weeks, possibly due to troughs near Tahiti. We haven’t seen it this negative since Jan 2010.


SOI as shown at

If this index remains lower than -10 for four weeks in a row then we have a fully- blown El Nino (both ocean and atmosphere). .

This is NOT a typical El Nino. When compared to the past it seems to be mimicking the 1986-87 event rather than the biggest ever 1997-1998 event:



The Ocean

Here is an interesting theory:


Theory is that heat is being taken from atmosphere over the Pacific Ocean and then driven by wind into the Indian Ocean. SO the Pacific Ocean temperature indicators may take on the values of an El Nino, but might not be delivering an atmospheric El Nino.


TC DOLPHIN has moved off to the NE of Japan, and at present we are having a quiet period between cyclones. Next one is likely to form near 10N 105W by Thursday, off the Southwest coast of Mexico.


In an El Nino year the Indian monsoon is usually late, slow, and not-so-wet. Well, this El Nino hasn’t really got into the atmosphere yet, and the Indian Monsoon is starting up already and is maybe a day or so EARLY.


Latest Indian Monsoon index – it is a few days early, and so far is wetter than normal

—NOT a typical El Nino-Monsoon at all.

The rain map for the last two weeks shows how quickly the South Pacific has dropped in rainfall since that early May low over the Solomons. It also shows maximums of rainfall in Southeast Asia as the Monsoon kicks in. And troughs over southern French Polynesia.



Weekly rain signatures for past two weeks, as seen at


Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing wind, isobars, Sig wave height green lines, swell direction orange arrows, current red arrows, SPCZ and STR.


Galapagos to Marquesas:

Better winds for departing are on local Monday/Tuesday this week.

There is still signs of a good west-going current along around 5 to 6S from 95W to 130W, so use this if you can. There may be some convergence shower activity around the Marquesas this week.

SPCZ=South Pacific Convergence zone.

SPCZ is expected to be weak this week and stretch from Tuvalu to Suwarrow, with a relaxation of the convergence zones that have been bothering French Polynesia over last few week.

Late in the week a trough is expected to form between eastern Fiji and south of Tonga (Minerva area). This trough is likely to have a squash zone of strong easterly winds on its southern side.

STR= Sub-tropical Ridge

Next HIGH is expected to move east into central Tasman sea along 35S by Wednesday and then across the North island on Thursday and off to the east by Friday, and hug 35to 40S as it travels off to east of NZ next week.

Departing from Australia to the tropics his week:

Once that HIGH starts travelling east from Wednesday this will make for strong SE winds in northern Tasman Sea, and there is a zone of light winds within the HIGH for anyone motoring.

Big trough is expected to move into southern Tasman Sea from Thursday, bringing strong SW winds and heavy swells as far north as Lord Howe Island by Sun 31 May. Avoid.

Departing from Northern NZ going north.

Southerly blast is still strong on Monday and should ease on Tuesday. Then easterly winds and light winds over northern NZ on Wednesday and Thursday, and then the NE winds ahead of the next front are expected from Friday. SO best day to depart is Tuesday 26 May.

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