Bob Blog 29 Sep

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.

Compiled Sunday 229 September 2024

Getting to New Zealand from the tropics during the disturbed
westerlies of Sprong

Over the next month or two the yachts that have been enjoying the
tropics take on the migration to NZ or Australia to avoid the cyclone
season that nominally starts on 1 November.

It so happens that spring is the time of the year when the Antarctic
circle experiences sunlight after six dark months. Just as dawn is the
oldest time of the day, so it is that equinox is the coldest time of
the year for the Antarctic .
t is the temperature difference between tropics and pole which powers
the strength of the disturbed westerlies, and this is strongest at the
equinox, hence ethe expression 'equinoctial gales'.

To give a measure of these gales here are the wind roses at 39S 160E
in mid Tasman Sea from https://www.pitufa.at/oceanwinds/ a useful
resource showing monthly averaged winds around the seas of this planet
as measured by scatterometers on satellites.
The bulges to the west in September turning to Southwest by November.

In comparison , the wind rose for March (when the temperature
difference is at its weakest) shows less wind and no bulges.

I have mentioned before that when approaching the 'roaring 40s' from
the tropics are producing disturbed westerlies , we often have a
series slow moving Highs travelling east along around 30S and a
procession of faster moving troughs , some with lows, travelling
southeast along around 40 to 45S. In this pattern sometimes the best
strategy is to deliberately encounter a passing trough at around 33 to
35S so as to arrange arrival in NZ "between trough"

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The routing advice I give may be considered as being an independent
opinion which maybe uses tools which you are not using in your own
weather studies.

I load my expeditionmarine.com program with weather data from the
ECMEF and GFS global models , the new ECMWF AIFS ( using AI) , and the
unified mode, also with ocean data such as sea, swell and currents
from Mercator. This data is merged .

One tool I use is to ask my software to produce a table , for example
for voyages from

Minerva to Opua calculating one departure per day for the days we have
data

for a yacht with up to 6kt sailing boat speed,

that gives (in UTC) the time each voyage takes (the longer this is the
riskier it is).

that avoids swell over 3m, head winds over 15ktt and tail winds over
25kt (a comfortable enough setting)

Highlighting the next "sailing window"

and clearly showing which stays we may just as well stay put.


This way the estimated trip time is a QUICK PROXY that measures the
risk of that voyage. Yes other paths may be more comfortable and
other paths again may be less risky.

In this table the colours are simply assigned to help differentiate
the paths when drawn on a map and are otherwise meaningless.

Ina few days the tables show a trend and usually the trend is the
thing to watch for timing departure ( rather than an analysis
paralysis each day)..

As to the data . Clearly all model data becomes distorted Ina few days
the tables show a trend and usually the trend is the thing to watch
for timing departure rather than overly analysing each day. The
arithmetic of chaos unravels the signal the further it goes into the
future. Usually the idea shown gets unreliable beyond 7 days unless
there is a trend. The data I have covers the next 14 days , but any
finishing date beyond that is just an artifact of numerology.

Another , more time intensive, tool is to calculate the voyage profile
of individual voyage . Here is one for example:

Voyage profile

min 25%ile med 75%ILE max


wind true 4 10 13 14
20 avg kt

apparent 6 9 11 12
18 avg kt

angle true 40 91 122 131
156 avg deg

apparent 31 64 100 110
144 avg deg

Boat 2.3 4.70 4.90 5.35
6 kt

swell 1.3 1.4 1.5 1.7
2 sig m

Baro 0 1013 1014 1015
1016 hPa

beat beam aft

<50 TWA >150 deg

<10kt 9% 18% 0% 27%

TWS 0% 64% 9% 73%

>20kt 0% 0% 0% 0%

Totals 9% 82% 9%

AWA apparent wind angle


<10kt 18% 27% 0% 45%

AWS 0% 55% 0% 55%

>20kt 0% 0% 0% 0%

Totals 18% 82% 0%

If we have two voyages then these voyage profiles help to
differentiate between them .

Another tool is a simple graph of forecast wind and sweel .



TROPICS
ISAAC and JOYCE are in the North Atlantic and Jebi and Krathon are in
the NW Pacific

Tropical Storm Pulasan brought unprecedented flooding rains to areas
of western Japan still recovering from a devastating quake that killed
at least 236 people on New Year's Day. . Hurricane Helene raked parts
of Florida as it roared ashore in the panhandle. . Hurricane John
killed at least three people in mudslides across southwester

WEATHER ZONES

The South Pacific Convergence zone is sitting over Solomon Islands and
northern Coral Sea and extends to Tuvalu and Tokelau then southeast to
Southern Cooks.

There is a squash zone of strong winds and rough seas between Tahiti
and Tonga mid -week.

HIGHS and LOWS

Low L1 is moving east well south of Tahiti and may form a secondary
near 43S.

HIGH H1 over NZ on Monday the expected to travel off to the east, with
a squash zone of enhanced trade winds on its northern side mid-week.

Low L2 is an east coast low near Brisbane on Monday and expected to
turn into a north-south trough over NZ on Wednesday. Associated trough
may affect New Caledonia and Vanuatu rather weakly on Tuesday. The
trough then moves on to the east, as a secondary low forms in the
Tasman Sea. This low may well bother NZ next week.

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If you would like more details about your voyage, then check
metbob.com to see what I offer.
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com
(subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at
weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
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