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

29 May 2022

Bob Blog 29 May

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 29 May 2022

At 8am local time on Wednesday 25 May, Lisa Blair sailed her yacht "Climate
Action Now" into Albany, knocking 10 days off the Around Antarctica record.

27 year old Australian solo sailor Lisa Blair now holds the World Sailing
Speed Record for the fastest solo unassisted circumnavigation of Antarctica,
beating Russian Fedor Konyukhov's 102-day record from 2008 and her own
attempt in 2017 when she was catastrophically de-masted in a horrific
Southern Ocean storm. Lisa is only the third person to ever succeed with
this perilous journey which stays below 45 South encountering the storms of
the Southern Ocean. goes close to Point NEMO (the most remote oceanic place
on the planet) and traverses Iceberg Alley east of Cape Horn..

During her mission Lisa also used "Climate Action Now" to carry out numerous
ocean science studies making worthwhile contributions to our understanding
of how the Southern Ocean is changing. Well done, Lisa.

A photo, taken by her family and put on her blog site at
https://lisablairsailstheworld.com/ shows a double rainbow shining as she
approaches the finish line.


It has been my pleasure to act as weather guide for Lisa during this
mission, discussing with her the path of minimum risk ahead with daily
updates via email and text.

TROPICS
AGATHA is about to make landfall over western Mexico tomorrow and is the
first cyclone of 2022 for the Eastern Pacific. There are potential hotspots
east of Philippines and to NW of Australia.

WEATHER ZONES
SPCZ continues to recover after losing a dose of moisture to the south
recently and stretches from Solomons to Vanuatu to Samoa.
A convergence zone between Niue and Southern Cooks is expected to reach
Tahiti area mid-week.

HIGHS and LOWS
Low L1 from the Southern Ocean is expected to travel northeast to 45S 135W
mid-week

High H1 has been blocked east of NZ for a while and this week is expected to
finally move off to the east along 40S following L1.

L2 is tonight in central Tasman Sea and associated front is traveling south
over NZ on Monday

After L2, L3 rapidly travels eastwards: its front is expected to cross NZ on
Tuesday/Wednesday then L3 may cross central NZ on Friday.

After a brief ridge over eastern Australia on Thu/Fri, L4 is expected to
cross northern Tasman Sea this weekend.

With low pressures over the Tasman Sea and NZ this week, it is a good time
to get east or north from Australia or NZ.

The convergence zone over Tahiti area mid-week might delay departure from
there going west.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more detail for 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 txt 64277762212
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

22 May 2022

Bob Blog 22 May

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 22 May 2022

LISA BLAIR is on target to complete her attempt at the Around Antarctic this
Tuesday see https://youtu.be/TpYWIllg59o

BEAUFORT SCALE
On a wet winter's Saturday afternoon when I was a schoolboy growing up in
Wellington, with nothing better to do, I started reading an encyclopaedia
which our Mum had bought for us. Volume one covered A to C and when I came
across the Beaufort Scale for measuring wind, I became intrigued in
all-things weather. This probably triggered my career choice even when I was
only 11. However, what intrigued me so much was the descriptive definition
given to Force 10 or storm on the Beaufort Scale. There it was in writing
from a reliable source that

Storm is "seldom experienced inland. Trees uprooted; considerable structural
damage occurs."

I was immediately quizzical. The wind in Wellington that day was storm
force, and we seemed to be having a gale every week and a storm every month
in Wellington that winter. In spite of that there was only an odd tree being
blown over and nothing seemed unusual in our storms.

It wasn't until I visited other places that I realised that Wellington's
proximity to Cook Strait made it a wind funnel and funnelled winds have
their own characteristics.

The history of the Beaufort scale is an interesting illustration of how
science works.understanding the pattern of things by observing and
analysing. Analysis requires numbers. So it was in 1805 that the British
navy navigator Francis Beaufort (later Rear Admiral, and knighted in 1846)
developed a code to help navigators to write in the log what the wind force
looks like using a scale of observable effects. Before his scale the ship's
log was filled with indeterminate terms and one mate's "stiff breeze" is
another's "soft breeze". Initially Beaufort used descriptive terms that
applied to a British Man-of-war frigate ship sails, such as "just sufficient
to give steerage". These ships deploy up to 12 sails and the code could
indicate how many sails the ship should unfurl, hence the Force numbers are
from 0 to 12. The important thing to note about the Beaufort scale is that
the levels on the scale are based on increments of observable effect. In
the 1830s the scale became the navy standard. In the first International
Meteorological Conference in Brussels in 1853 it was adopted as a standard
code for all shipping and turned into numbers for ease of reporting.

In 1916, to accommodate steamships, the descriptive terms were shifted from
effects of the wind on sails to effects of the wind on the sea, and a range
of knots for each force was established. In 1923 the director of the UK
Meteorological office added the land descriptors. These are the ones I read
so quizzically early in the 1960s.

The UK Met office shipping forecast still uses the Beaufort scale, but most
marine forecasts are now given with the wind speed in knots. Nowadays the
accuracy of a marine forecast can be around plus or minus 5 knots, and
that's finer than the Beaufort scale where for example the speed range for
each force is around 8 knots. However the definitions used for warning
thresholds (Strong, Gale, Storm and Hurricane) are all still based of the
Beaufort scale.

The Beauford Scale speed in knots do NOT rise in a linear way.
It is as if each step up in "observable effect" gives more grunt.

We know that the force of the wind goes up with the square of the speed.so
a 20kt wind has FOUR TIMES the pressure force of a 10 knot wind. This
explains why sailing in a 20knot wind is fun and a 30 knot wind, just a
small jump in speed, isn't fun anymore.

TROPICS

After a few quiet weeks we had an out-of-season cyclone over Vanuatu on
Friday and Saturday, GINA.
And for the coming week the focus for potential development is shifting to
the China Sea.

WEATHER ZONES
SPCZ stretches from Solomons to Vanuatu to Samoa, recovering after GINA.
A passing trough is expected to move east from New Caledonia to Fiji towards
the end of the week.clip_image010

HIGHS and LOWS
Low L1 is the remains of GINA combined with a frontal zone that extends
southeastwards.
High H1 is moving east along 25S in tandem with L1.
Low L2 is expected to form mid-week between Queensland and New Caledonia and
deepen as it travels south into the south Tasman Sea
High H2 in over Tasmania tonight and expected to travel east along 40S
reaching NZ by Friday.
In the wake of L2 there should be an opportunity to sail from Queensland to
New Caledonia/maybe Fiji
Looks ok for sailing wet from Tahiti this week.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more detail for 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 txt 64277762212
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

15 May 2022

Bob Blog 15 May

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 15 May 2022

CIRCULAR SHAPED CITIES HAVE HEAVIER DOWNPOURS

AN interesting study has been pointed out to me at

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022EF002654

which investigates the shape od a city (either 1) edged like a triangle in a
valley or 2) rectangular around a river or 3) edgeless and sprawling like a
circle) and its impact on the intensity of its rainfall,

Comparing edged-shaped cities with circular-shaped cities, clouds find it
easier to rise over circular -shaped cities and this helps intensify
downpours. For inland cities the maximum rain is usually in the morning and
for coastal cities it is usually in the afternoon. It would be interesting
to extend this study to cities like Auckland which are on an isthmus and get
converging sea breezes.


The South Pacific Convergence Zone explained

Now that many yachts are about to travel west from Tahiti to Fiji and are
thus about to sail thru or around the SPCZ, this obstacle has become a
talking point.it is something like a rite of passage for crossing the South
Pacific, and some have asked what is it, why is it there, how does it drift
and what makes it tick. Stand by for lots of jargon,

Weather is a mix of pattern and chaos, and meteorological teaches
concentrate on the pattern. In tropical meteorology the first idea given is
the Hadley cell.

Because the sun is most directly overhead at the equator, that's where the
warmest seas are, and this causes rising air. Once the rising air reaches
high enough it spreads outwards and sideways to the north or south, where it
sinks at dries out. The sinking air reaches the surface again around 30N or
30S (subtropical ridge) and then recirculates back to the equator as surface
winds know as trade winds (so reliable they can be used to determine trade
routes). The trade winds from each hemisphere converge together in a zone,
and this convergence narrows the zone of rising air into a feature called
the Intertropical Convergence Zone or ITCZ

But in the Southern Hemisphere, the Andes of South America cause a split in
the trade winds. They block a HIGH near 30S around 90 to 110W, or near
Easter Island. It is quasi stationary, just like the High between California
and Hawaii, and also has a gyre that is collecting a rubbish heap just as
badly (Henderson island).

1. There are easterly winds on the north side of this "Andes" High: they are
dry due to continental outflow from off South America. These easterly winds
travel well to west of the dateline along around 10 to 15S.

2. And there are migratory Highs that travel east along the subtropical
ridge from Australia to east of NZ, with a zone of south to southeast winds
on their northern side. These South/SE winds come and go according to the
migratory high and are usually found around 15 to 25S.

3. The convergence zone between these easterly and Southeasterly winds is
called the South pacific Convergence Zone, or SPCZ.

It is typically located from the Solomon Islands southeastwards to the
Southern Cooks, but sometimes may have large gaps or be very quiet.

Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone


It is affected by many things: 1) the PDO which takes many years to switch,
2) the El Nino/La Nina which last a year or so, 3) the strong annual cycle
which makes the seasons, and 4) the MJO which comes for a week or so every
six weeks or so.

So far this year we have been having a LA NINA, and this helps shift the
SPCZ south and slightly west of its normal position and intensifies the
strength of the southeast trade winds.

However it is normal for troughs to travel eastwards along the SPCZ,
connected to travelling lows in the Southern Ocean, and sometimes these
passing troughs induce the formation of deepening lows near 30S , usually
south of the Southern Cooks.

I have found that the easiest way to determine the position and severity of
the SPCZ is to use satellite imagery, and the easiest way to decide what it
may do over next few days is to use the 5day rain accumulation parameter on
windy.com.

TROPICS
All quiet at this time of the year,

WEATHER ZONES
SPCZ=South Pacific ConvL1ergence zone.

The SPCZ stretches from Solomons to Vanuatu to Samoa A passing trough is
expected to move east across Tahiti around mid-week.


HIGHS and LOWS

Low L1 is forming in a trough over central NZ tonight and expected to track
to the northeast this week all the way into the tropics. It's on the
backside of its upper trough.

High H1 east of L1 is also expected to travel NE this week in tandem.

Low L2 is expected to form mid-week over Vanuatu, then fade by end of the
week

High H2 in the North Tasman Sea is expected to slowly travel east along 30S.


Low L3 is expected to travel from 50S and deepen over southern NZ late this
week followed by strong SW winds that may bring 4m swells as far as 30south.

H2 and L3 make trans-Tasman travel difficult this week.

For departing westwards from Tahiti may be an idea to wait until the passing
trough has gone.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more detail for 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 txt 64277762212
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

08 May 2022

Bob Blog 8 May

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 8 May 2022
BLOCKING
Anyone watching weather maps in this part of the world is familiar with high
pressure systems moving from west to east around about 40 degrees south.
This motion is due to the stronger westerly winds aloft. These rivers of
wind aloft meander north and south as they travel eastwards in a roller
coaster fashion and push the surface features around.
To understand how anticyclones form and affect our seasons, we need to
quickly recap on the basics. Our global weather engine is powered by
evaporation from the equatorial oceans. This causes a zone of rising air
around the equator. When this air has risen to the stratosphere it spread
outwards and pole-wards, and finally sinks back to the surface at around 30
to 40 degrees latitude. This zone is called the subtropical ridge and is
the main latitude belt where anticyclones form. Near the Americas these
anticyclones do not migrate but are usually locked in much the same position
by mountain and given names such as the Bermuda High and the Pacific/Marine
High. In the South Pacific anticyclones and normally mobile.
Sometimes a split develops in this steering field, with some air going north
and the rest south, leaving a calm zone in-between. Then the surface high
that is underneath this calm zone stays put and is referred to as a blocking
high. This "blocking pattern" can last for weeks and affect the weather on
a seasonal scale. It is the antithesis of mobility in the roaring 40s.

In this image from windy.com the jetstreams at the 250hPa height are shown
in white with a split in the flow and a calm zone in-between above the
surface High centre east of NZ. Further east the jetstream has caught up
with the surface high.
In the days after ANZAC day a large region of high pressure (anticyclone)
moved over southern New Zealand and since then the same system has been
lingering over the South Pacific Ocean east of New Zealand. Trade winds on
the north side of a large high were enhanced so much that Fiji
Meteorological Service issued heavy rain, flash flooding and coastal
inundation warnings last weekend.
This has been a classical example of a blocking high., and by occurring at
the end of April and start of May it has held back the normal autumn
chilling in New Zealand.


Time section for the 170E to 150W longitude band from 15S to 50S showing
isobars from 21 Mach to 4 May. The normal pattern during April turns into a
big block over past two weeks.


This block serves as a reminder of the ten reasons there are to hate a high
Ten reasons to hate a High:
. Near the centre are "dead" winds and usually an area of low cloud called
"anticyclonic gloom" or dirty air causing fog.
. Highs intensify the trade winds. It may take about a week for a high to
travel eastwards past New Zealand, and during this time the stronger trade
winds tend to give night time rain to the eastern side of the larger Fiji
Islands.... this is called BOGI WALU (eight nights).
. An intensifying high tends to "squeeze" isobars together on its periphery.
If the central pressure is over 1030 there is usually a gale somewhere on
the outside of a high.
. As air flows around a High it spins out, and has a speed as much as 20%
MORE than that indicated by the isobar spacing.
. Stronger fronts: When an incoming front encounters a strengthening high,
its wind and rain may be strengthened, and its frontal movement may stall.
. If a range of mountains blocks the air flowing around a high, the air
tends to accelerate around the mountains and through gaps rather than over
them. This causes a river of wind at the downwind end of a mountain chain.
. In summer, a high may help trigger inland thunderstorms and hail
. In winter, a long night with the clear skies and light winds of a high may
bring frost.
. The bigger they are, the slower they move, blocking the fronts that are
trying to follow them and feeding those fronts with warmth and moisture.
. When a new high forms in the Southern Ocean it shovels a polar-chilled
southerly flow into the mid-latitudes.

TROPICS
The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical
Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

Tropical Cyclone TWO in the north Indian Ocean is going north and tropical
cyclone KARIM (out o season) in the South Indian Ocean is going south.

WEATHER ZONES
Weather Zones Mid-week GFS model showing isobars, winds, waves (green to red
+arrows), Rain (Blue), STR (Subtropical Ridge), SPCZ (South Pacific
Convergence Zone) and CAPE (pink)

CAPE mid-week as seen by ECMWF and GFS from Predictwind.com
Different models seem to produce different values.

SPCZ=South Pacific ConvL1ergence zone.
The SPCZ stretches from Coral Sea to Vanuatu to Samoa A convergence zone
is expected between Tahiti and Tonga this week.

Rain Accumulation next five days from windy.com
HIGHS and LOWS
Low L1 formed at the SE end of the SPCZ last week and is now near 30S to
south of the Coos and travelling off to the southeast
High H1 has ben a blocking high east of NZ since late April and is now
expected to move off to the east.
Low L2 is crossing the Tasman Sea and should reach central NZ on Monday and
then move off to the northeast, following H1.
High H2 is expected to move into the South Tasman Sea by mid-week and then
northeast onto central NZ by end of the week. OK for sailing westwards
across the Tasman, but not Eastwards.
There is likely to be a squash zone of enhanced trade winds between NZ and
Fiji from mid-week.
A tropical low is expected to from off the north Queensland coast by
mid-week.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more detail for 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 txt 64277762212
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

01 May 2022

Bob Blog 1 May

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 01 May 2022

REVIEW OF THE LAST MONTH (April 2022)
Sea surface temperature anomolies: the cool eddies along the eastern
equatorial Pacific that signify LA Nina are still there. The surrounding C
shaped ring of warm water across north Pacific Indonesia and South Pacific,
and warm zones across the south Tasman Sea and off South Africa are more
intense than last month. And the Gulf stream is lit up like the stars and
stripes.

The subtropical ridge in the northern Hemisphere has strengthened since
last month.
The anomaly map shows more lows than normal over Australia and a weakening
and shifting to the north of the anomalous Highs over southern NZ. The North
Atlantic has swapped from High to Low.
Maps show that the subtropical ridge has sifted north and weakened, as shown
by the 1015 over Australia/NZ, and the fading of the 1025 off South America.
The Antarctic High has increased to over 1040 .

Tasman Sea Time-latitude pressure map from PSL for April shows Cyclone Fili,
6 Highs and 5 southern lows in 4 weeks

TROPICS
JASMINE was a late season cyclone that faded over Madagascar last week.
There are no cyclones around at present and the Southern Hemisphere cyclone
seasonal is now nominally ended.

WEATHER ZONES
SPCZ=South Pacific ConvL1ergence zone.
The SPCZ stretches from PNG to Vanuatu to Fiji/Tonga.
Another convergence zone is lingering south of Tahiti.
The best route this week from Tahiti to Tonga seems to be the northern
route.
HIGHS and LOWS
Low L1 is forming between Fiji and southern Tonga and expected to travel
southeast this week.
High H1 is a blocking High east of NZ travelling slowly east along 42S
holding a ridge over central NZ
This should help maintain easterly winds between NZ and Fiji.
Towards the end of this week the next low is expected to travel into the
south Tasman Sea form the west and another may form in the Coral Sea. These
may combine to help anyone planning to sail eat from Australia
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
If you would like more detail for 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 txt 64277762212
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

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