Bob Blog 17 May



Compiled Sun 17 May 2020

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.


If you are having problems with working out where and when the systems on the (surface-based) weather map are going next, then it is worthwhile to check out the jetstream.

Jetstreams are rivers of wind found in the upper atmosphere where jets fly. They snake eastwards around the earth faster the earth’s normal eastward rotation, and often look like large waves marking passing troughs, and sometimes can have small buckles that play with the local weather below.

On average, there are TWO mains jetstreams affecting NZ weather– the polar jet that is sometimes south of us and sometimes on top of us, especially when we have passing troughs. The other is a subtropical jet, sometimes just to north of us, but sometimes on top of us, especially when anything come to NZ from the tropics.

Jetstreams can tell us about development (invigoration) and decay (weakening) of surface weather.

In the southern hemisphere, expect development at the surface slightly north of a jetstream- entrance and south of jetstream-exit. Expect decay at the surface slightly south of a jetstream-entrance and north of a jetstream-exit.

As for a steering field for surface features, well, there are stagnant areas in the jetstream which indicate where surface weather may be stalling. However, for a better surface-steering indicator, go to windy,com and dial that height/altitude bar to around 850 maybe 800 or 700hpa.

An interesting feature that occasionally occurs with jet streams are what we call Omega Blocks. These is when the jet splits into a pattern that looks like the Greek capital letter “Omega” (an upside-down omega in the southern hemisphere). They are caused by two cut-off lows and a blocked high in between, staying in places for days, sometimes a week or more.


(thanks to for this image – the website also describes other basic jetstream patterns, including the REX BLOCK- an interesting pattern that has, in part, upper easterly winds- rather rare)

An interesting jetstream pattern is being set up for NZ at the weekend I’m writing this piece, it’ll be history when you read this. Here is a graphic from– a great website for watching the jetstream. There is bar to the bottom right for height/altltude– toggle the pointer to around 300-250 maybe 200mb/hPa for the best looking jetstream map)


(from, a prognosis for Monday 18 May)

Colours are upper winds, surface isobars are included as white lines.

It’s not an omega block and the subtropical jet is doing its own thing.

The Polar jet is buckled with a weakish EASTERLY jet over central NZ. (Remember, upper easterly winds are rare — they are less than the earth’s rotation). It creates two stagnant zones where the surface weather feature stalls, one over norther North island and the other over southern South Island.

Of course, all this may change in the mix of pattern and chaos which makes weather-watching so interesting.


The Tropics

The latest cyclone activity report is at and Tropical Cyclone Potential is from


Today’s map shows TC AMPHAN in Indian Ocean heading north towards Bangladesh.

Also, a weakfish tropical depression of eastern USA.


Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)


SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomon island to north of Fiji to norther Tonga for much of this week.

A LOW is predicted to form south of Tonga on Thursday and then go south, accentuating a squash zone of strong winds and swell between Fiji and NZ from Tuesday (between High to south and trough to NE).


Accumulated rainfall for next week from (isobars are for Sunday night).

Subtropical ridge (STR)

HIGH 1030hPa is expected to enter Tasman sea tonight and cross central NZ mid-week then go off east along about 40 to 35 S.

Next HIGH is expected to enter South Tasman Sea this weekend.

Tasman Sea /NZ/Aus

Squash zone is expected between NZ and Fiji from Tuesday, travelling southeast on Thursday and south from Friday.

Low expected to form off Queensland this weekend and bring wind and rain to North island next week.

From Panama:

Not good this week, and maybe for the coming months. The SW winds arrive in Panama this week, and the ITCZ visits at times. I suppose the best way to tackle these SW winds is to go SSE then west


If you would like more detail for your voyage, then check to see what I offer.

Or Facebook at /

Weathergram with graphics is at (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at

Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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  1. Morning Bob, Just wanted to say thank you for your continued reports, they are always interesting. Our plans to sail to Fiji this year are on hold like everyone else, but we have a tiny hope this might be possible in July. Got to stay hopeful.

    Regards, Linda


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