Bob McDavitt's ideas for sailing weather around the South pacific

21 September 2014

BobBlog issued 21 Sept

Issued 21 September 2014

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

1. Resolution
The global computer models take weather observations and transfer them onto
a grid of dots, thereby averaging the weather out. Features that are
occurring between the reports of the observation network are missed
altogether, but some data such as temperature is observed in a continuous
fashion by remote sensing from satellites and is covered well. The model has
to balance this matrix of dots to ensure that certain rules are observed,
for example all the computed upward motions need to be balanced by down ward
motions so that the total atmosphere remains intact. Once a captured and
balanced pattern is obtained, it is then pushed into the future using
dynamic equations in small time-steps. Then another balancing computation is
done and so forth. SO the GRIB data of a weather forecast is just a
mathematical idea based on the extrapolation of a captured pattern. In the
real world chaos is continuously jiggling the weather pattern in to a
different direction, so that the weather forecast deviates from the real

You may already be aware of the limitations of GRIB data. Sometimes someone
will offer you GRIB data with a better resolution, and the question arises
if it will have better accuracy. The answer is somewhat mixed, as I hope to
show you today:

If you can, check my Weathergram graphical edition on the web at/metbob. to see the comparison of a map showing normal GRIB data from
the GFS model with another from the 8km resolution Predictwind version of
the GFS model (PWG) as available (under registration) from

The PWG model captures some terrain effectslowering the air pressure in the
interior of the main islands of Fiji in the heat of the day, and there by
forming a westerly sea-breeze around Nadi/Lautoka and causing an
acceleration of the wind on the SW end of Fiji near Navula Passage. All of
these terrain effects truly happen, even if they are not resolved by the
normal GRIB data.

These models with higher resolution and closer dots (and smaller time-
steps) require heaps more computing than the normal global models and give a
pattern that is somewhat closer to the real world. However there is no
increase in the basic weather observation resolution that goes into these
models, so they still have limitations. Their output is still just an
averaged idea.

2 Equinox

Welcome to the vernal equinox today. That was when the overhead sun shifted
from the northern hemisphere to the southern hemisphere. It is only on the
equinox that the sun rises due east and sets due west.
Astronomers, logically, time the seasons using the solstices and equinoxes
of earths orbit around the sun. By this reckoning, spring starts with
vernal equinox. Note that this makes our winter last 93 days and summer 89
days because the earths orbit is slightly elliptical and the earth speeds
up a bit during our summer.
For practical purposes climatologists measure climate data averaged over
each calendar month, and by this reckoning the months of September to
November are taken as spring. Note that by this reckoning summer lasts
90 days (91 days in a leap year). In New Zealand this method of marking the
seasons is more popular than using the equinox and solstice. Fair enough, it
adds 1 or 2 days to summer J You may have noted that the time between
sunrise and sunset is NOT 12 hours today, but around 7 minutes LONGER. This
is because sunrise is defined at the time the top limb of the sun is just
visible on the horizon, and similarly sunset is defined with the top limb of
the sun disappears below the horizon. Due to refraction of the atmosphere
the sun appears and disappears when is slightly below the horizon. This
explains the extra day light. The name used to describe the date when time
between sunrise and sunset crosses twelve hours is the equilux as described
at Over the next few weeks, as we get to notice
the longer days there will be extra warmth reaching the southern ocean. Sun
starts to shine reach Antarctica after six dark months. Just as the coldest
part of night for us is just after dawn, so it is that Antarctica gets is
coldest temperatures of the year around the equinox. It is the temperature
difference across the Southern Ocean that energies the westerly winds found
there, and so these at their most energetic, and expand to their furthest
north, around the equinox. This is the origin of the term equinoctial
gales, however I prefer the phrase gales of the Antarctic dawn.
Note that New Zealand will be putting their clock forward an hour next
Sunday (28 Sep) and changing to NZDT. New South Wales changes on 5 October.

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one number. It is
based on the standardised difference in the barometer readings between
Tahiti and Darwin. It has been negative since July and dived below -10
(Australian units) at the start of September where it has hovered ever
since.  if it holds below -10 for another few weeks then we can call this a
full-blown El Nino.

In the east pacific we have POLO and in the west Pacific we have FUNG- WONG.
The weekly rain maps show a shift of the heaviest rain from India/Pakistan
to the Taiwan area with TC FUNG-WONG.

SPCZ= South Pacific Convergence Zone
The SPCZ is fragmenting into several branches this week, and slowly
intensifying. One branch is expected to form in the northern Coral Sea
between Monday and Thursday, with a squash zone of enhanced SE winds on the
south side of this zoneavoid.
One of these convergence zones is expected to affect Fiji on
Friday/Saturday, and another to affect Vanuatu/ Loyalty Group on Sunday.

STR= Sub-tropical Ridge
The STR is at its normal latitude for this time of the year, along 20 to 30S
The HIGH over eastern Australia is expected travel across the Tasman Sea
from Tuesday to Thursday and northern NZ on Friday followed by a trough on
Saturday night and disturbed SW over NZ by Sunday.

Departing westwards from Tahiti:
A trough is travelling west along 20S followed by southerly winds and big
SW swells. It is expected to reach Niue by local Sunday, southern cooks by
local Monday/Tuesday, and fade over Australs by local Wednesday.
It is expected to be followed by a period of trade winds that look good for
going west , but might be a squash zone forming along 10 to 15S this weekend

Between NZ to the tropics
Vigorous SW/S flow over NZ on Monday as a Low from the southern ocean
sideswipes its eastern coasts. Then settled weather until the next trough
approaches with NW winds on Friday night/Saturday, then the trough itself on
Saturday night, and then a SW flow on Sunday 28th.

See my yotpak at for terms used.
Weathergram text only (and translator) is at
Weathergram with graphics is at, click FOLLOW at bottom
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