Bob’s Blog



Compiled Sun 10 June 2018

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.

CAPE an index for thunderstorms and lightning

One of the parameters available from the sidebar is the CAPE index—the Convective Available Potential Energy.


GFS model for next Wednesday as seen at showing isobars and CAPE.

Basically, this measures buoyancy of the air, the amount of heat released if there is any convection. On earth we normally have static stability. Sometimes parts of the atmosphere become unstable. The way we meteorologists study instability is called the “parcel method”: we look at a parcel of air starting off near the ground with temperature T(parcel) or Tp and compare this with the vertical temperature profile of the wider environment, called T(environment) or Te. If the parcel of air is lifted it will cool at the standard atmospheric lapse rate. So the “lifted-Tp” is less than starting Tp. However, the wider environment may have some unstable layers. In these layers the “lifted- Tp” gets HIGHER than the surrounding Te, or lifted-Tp>Te, meaning the parcel has gained buoyancy (or become less dense than its surroundings) and will accelerate upwards. It will continue accelerating upwards until it eventually finds a matching Te (neutral buoyancy) and then slow to a stop (or until deposited ice particles stick together, but that’s another story). This is the convective process, and you can see it happening in the rising turrets of active cumulus clouds.


Modern weather models have many vertical dots in their matrix, and thus store a vertical profile of the atmospheric temperature or a Te. With aid of a rather complex equation (which I won’t show here), we can turn this Te profile into a single number that shows how much energy is released (in joules per kilogram) if a parcel of air is triggered to rise from the surface. This is called CAPE and if it is zero then the parcel just drops back again. If CAPE is positive then the parcel will rise, and the higher the CAPE, the faster the rise.

It has been suggested (but not yet measured) that the updraft speed in a thunderstorm needs to be over around 25 km/hr (13 knots) in order to be able to be strong enough to separate the charged particles and produce lightning discharge. This seems to correspond with a CAPE of 2000, so this is the threshold I use for the pink area of CAPE in my weather map for these blogs.

There was a squally front over the Lau group of Fiji and southern Tonga last Thursday night.

It occurred in an area well east of the main convergence zone and its sudden onset, and six hours duration, and ferocious content was downright dangerous and, in some cases, damaging.

Computer models don’t have enough incoming data to be able to get a good enough “grip” of the atmosphere to resolve such squall lines, and all they were showing was a zone of light winds. The satellite imagery showers jetstream cloud, mainly further west, and the weather maps mainly showed a convergence zone to southwest of Fiji. The jetstream cloud could be used as a warning sign, for these squalls are known to occur near the equatorward side of the the entrance region of a jetstream.


Thursday morning satellite imagery courtesy of meteo France in New Caledonia

I have used the archive to zoom in on a surface wind map of Fiji area, with a white overlay of CAPE. The CAPE is high all the way between Fiji and Samoa, so does seem to over some warning of squalls, albeit rather vague and imprecise. I suspect that if we had more incoming data, the CAPE view would also be more precise. In the meantime, all I can offer to those afflicted by that thumping, is that weather is a mix of pattern and chaos. Our following of the pattern is vaguer than the reality of the chaos.


Red region denotes likely boundaries of a squall line last Thursday night. Map shows surface streamlines and high CAPE in white.


Off the western Mexico coat we have tropical storm ALETTA and BUD.

Travelling northeast past Japan we have TC MALIKSI and in the China Sea we have Tropical depression EWINIAR

If we compare the past week’s rain map with the previous week we can see a build-up of activity over India to Indonesia/Vietnam, along the ITCZ across the North Pacific and Panama, and also along the SPCZ especially near Fiji.



– see



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

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from Solomon island to Vanuatu to southern Fiji. There may be an arm of extra convection from Tuvalu to Tokelau and possibly to Tuamotu Islands. It is expected to have another quiet week between Tahiti and Tonga


Accumulated rainfall for next week from

Subtropical ridge (STR)

A High over southern New Zealand on Monday is expected to travel northeast to east of NZ towards 35S 120W by end of the week. There may be enhanced trade winds between 10 and 20S to north of this high from mid-week.

Next HIGH from the west is expected to enter the Tasman Sea around Thu 21 June—a long way away

Around Tasman Sea, NZ to tropics /FP

LOW1 approaching NZ from the north is expected to curve westwards and cross the Auckland area on Tuesday. It has strong winds and rain tied up with its warm front in its dangerous quadrant (southeast of centre) and this is expected to affect the Gisborne area, which is vulnerable and flood-prone due to heavy rain last week.

There is another Low, Low2, near Lord Howe Island tonight, and this is expected to go northeast and fade.

This means that after Low1 gets south of Northland, there is a reasonable window of opportunity for sailing north to the tropics—the voyage will encounter a zone of lights winds near 30S.

Another trough is expected to travel east across the Tasman Sea on Saturday reaching South Island on Sunday and North Island on Monday 18 June. Preceded by strong NW winds—finally a good direction for kayaker Scott Donaldson attempting to Kayak across the Tasman Sea (with me watching the weather) Sadly Scott got caught in an eddy loop last week and it has taken a week to get out of it


Follow Scott’s progress at

Australia to tropics

There is a good opportunity this week, with LOW2 until is around until Tuesday and with the trough approaching from the west until Friday.

Galapagos /Marquesas

From Galapagos area to Marquesas, departure can be any time this week. Best path for avoiding direct down-windsailing is to go to 10S 130W, then go direct.

Tahiti to Tonga

South Pacific convergence zone is well to the north this week, and route is clear of any strong convection. Expect Ese winds over 20 knots between 10 and 18S from 12 to 15June with 2m+ swells as HIGH passes to the south.


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

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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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