Bob Blog 8 Sep



Compiled Sun 08 September 2019

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.

Parts of a depression.

A depression is a zone where the isobars are depressed below normal. The average surface air pressure in 1013.25hPa and nearest isobar too that is 1012hPa—The 1012hpa is usually the straightest on the weather map, dividing anticyclones or anticyclonic flow with higher isobars — from cyclones or cyclonic flows with lower isobars.

The same thing is called a Cyclone when referring to its winds rather than it’s isobars. It is also called a Low.

Lows come in many shapes and sizes and their intensity (of wind and rain) may be related to the central pressure. The lower this is. the more the intensity. Other factors are the cloud width and shape, and , for tropical cyclones there is Category ( a wind scale),or Dvorak code (size and cloud extent) or ACE (accumulated cyclone energy) , Cyclone DORRIAN has a central prssure as low as 910hpa.

A low in the Tasman Sea today with a central pressure of 990hpa is a good demonstration showing the parts of a mid-latitude depression (not a tropical Cyclone). It occurs when a stream of cold air from the Sothern Ocean (high density air collides with warmer more humid air in the Tasman Sa (lower density air). At the cold front on the eastern side of the system, higher density air pushes the lower density air up and out to eastwards, creating rising air = cloud and rain

The Cold Pool is formed by the upper air reflection of the surface low and is usually found N or NE on the surface Low (in the southern Hemisphere), It can easily be detected from satellite imagery as a circular area of thundery showers, formed by warm surface air rising quickly due to its extra buoyancy when there is colder than normal air aloft.



Between the front (which arrived like and express train this morning) and the cold pool (which arrived late this afternoon/evening) there is a zone with partial l sunshine and no rain which is called the “Dry slot”. Most fronts are followed by dry slots and they can become a sailor’s friend, so long an you to brace for the cold pool that may follow.

After the Cold pool comes the “Back side” of the Low with its curl of cloud containing another dose of rain.

And on the south side of the low there is the frontal occlusion . This zone may become very slow-moving so that even if it only has moderate rain intensity , its volume of rain may be larger than normal over a small area.


The latest cyclone activity report is at and TCFP tropical Cyclone Formation Potential at

TC DORIAN hovered around the Bahama for more than 2 days, making 3 landfalls, and since then has been skirting east coast of North America. It also marked a surge in Cyclone activity: there are now FIVE cyclones, and several areas of potential activity in North Atlantic and North Pacific. ‘There is also a zone of potential development around the Solomon Islands, but models are quiet there.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with quieter convection hovering around Fiji this week.

Passing trough, with northerly winds swinging to southerly winds, now crossing eastern Fiji on local Monday is expected to go SE and reach Tonga by Wed/Thu local date and then Niue the next day (Wed local date) but fade and not affect Aitutaki or Tahiti, but may affect Rarotonga on local Thursday/Friday..


Accumulated rainfall for next week from

Subtropical ridge (STR)

Yet another week with STR north of NZ.

Hight to NE of NZ is traveling east along 30 to 35S on Mon-Tuesday

Another High is expected to form off east of Sydney on Wednesday and then NE to 30S then east across northern NZ by local Friday.

Tasman Sea /NZ/Aus

Low down to 990 hPa in Tasman Sea is expected to cross northern NZ on Monday/Tuesday local, followed by S Wed/SW Thu//W/Friday. Then another low is expected to deepen in Tasman sea over weekend and cross over northern NZ on Mon 16 Sep.

Looks OK to sail from QLD to New Cal early this week in the winds following Monday’s low but no good from Wednesday when SE winds return, and it is then OK to go t’other way.

If sailing from tropics to N this week, then try and time your voyage to arrive in NZ between fronts – as is likely next week..

Tahiti to Tonga

Looks Ok to depart from Monday this week, may need to go north of the direct path to avoid lights winds. Early notice that next week may have a squash zone of enhanced winds.


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

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Weathergram archive (with translator) is at

Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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