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

26 April 2009

BOBGRAM7 issued 26 April 2009

WEATHERGRAM
YOTREPS
Issued 26 APR 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

TROPICS
South Pacific Convergence Zone SPCZ has been weak in the past week,
strongest in central Coral Sea (320 mm/week) and between New Caledonia
and Fiji (250mm/week).
This branch of the SPCZ should be slow-moving this week. It may become a
bit more active than last week but the cyclone season seems to have
wound down now.

There is another branch of the SPCZ near Niue this weekend and this is
expected to move east across Southern Cooks this week.

That zone of convection mentioned last week mainly from 5 and 8S between
Marquesas and Galapagos is still there, but is weakening.

The trade winds easterlies are robust between New Caledonia and
Queensland this week, (they were weak there last week).

SUBTROPICAL RIDGE STR
The Big Fat high BFH that spent this weekend over Chatham Islands is now
moving slowly east along 40S and there is a squash zone between it and a
trough from Kermadecs and Niue, mainly along 30S. By the end of this
weak the BFH is likely to be east of 120W, and that trough is likely to
have grown to a large LOW and travelled south to 40S along 160W. This
may become useful for sailing from NZ to Tahiti, but only if you wait
for the current NE flow over northern NZ to turn northerly (that should
happen on Tuesday).

NZ AREA
It was a good-old southern-ocean southerly that brought rain to Adelaide
for ANZAC day, and the associated trough/low is expected to cross NZ on
Wednesday. This will be preceded by several fronts and a NW flow,
possibly dumping 400 to 500 mm of rain over the Southern Alps in 3 days,
with relatively warm days in eastern areas.

On Thursday, what follows the trough/low is a cold southerly over NZ,
but this may not bring much rain to Nelson/ Hawke's Bay Gisborne, our
driest areas.

On Friday the next HIGH is expected to cross Campbell Island, allowing
a southeast change to bring some wind and rain to eastern North Island
on the weekend of 2/3 May - double check this nearer the time as it may
delay that "perfect departure from Northland to the tropics".

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

18 April 2009

BOBGRAM7 issued 19 April 2009

WEATHERGRAM
YOTREPS
Issued 19 APR 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

TROPICS
It is the planetary scale features that best explain what is going on at
present-a deviation from the normal weather pattern with a large scale
upper low in the north Tasman Sea 30S 160E and a twin large scale upper
low near 30S 160W . These two features are a magic 40degrees longitude
apart, which means that the upper high in-between them , near 40S 180,
is blocked and that surface feature rotate around these blocked upper
features for a few days.

To see this , if you have access to full fast Internet go to
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/real_time_s
h/real_time_index_nrm.shtml
and scroll to the 500 hPa map.

The South pacific Convergence Zone SPCZ links the subtropical fronts of
those two upper lows..., so it snakes from near ne Caledonia/Vanuatu to
Tuvalu/Samoa (this section is weak this weekend but may some back) and
then southeastwards across the zone between Southern Cooks and French
Polynesia.
Over the past week, there has been some noticeable convection along
mainly 5 to 8 South between Marquesas and Galapagos but this is
weakening.

The trade winds easterlies are robust in most parts of the South
Pacific, but are lacking in the zone from Solomons to Fiji/Tonga. A
zone of weak wind is forecast to move along 20S from Queensland to New
Caledonia/Fiji this week, not good for sailing but OK for motoring.

SUBTROPICAL RIDGE STR
This has been displaced well south of normal at present, sort of a La
Nina trait and indeed, the 30day running mean of the SOI has bounced up
in the past two weeks after a falling trend. SO the Big Fat High BFH
that moved to east of NZ during the past week is now moving along 40S.
That LOW which popped up south of Niue in the past few days deepened
more than originally expected but is weakening away now. However, there
may be a few another small low developing near 30S 150W on 19-20 April
UTC and then moving off to the SE-all happening on the "western
shoulder" of the BFH.

The next BFH is crossing Tasmania at present and forecast to cross the
South Tasman Sea along 45S on 21-22 April and NZ on 23 April, and then
move off to the east of NZ along 40S.

NZ AREA
The Low near Lord Howe Island today has a weaker companion near Norfolk
Island. These two features are both caught in the circulation of an
upper low and thus are likely to rotate clockwise around each other,
with the eastern one fading away, its fronts moving onto northern NZ
tonight and Monday. Then the western Low should move south and weaken
on the western flank of the BFH, finally crossing the South Island on or
around ANZAC day Sat 25 April, preceded by its fronts and a NE flow on
Friday and followed by a NW flow over the South Island on the Sunday.
SO, looks to me that there are no-good-days for departing from northern
NZ for overseas this week.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

12 April 2009

BOBGRAM7 issued 12 April 2009

WEATHERGRAM

YOTREPS

Issued 12 APR 2009  

Bob McDavitt's ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas come from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

 

LAST WEEK

Finally a relatively quiet week in the South Pacific tropics, after the drama of LIN last weekend.

 

TROPICS

The South pacific Convergence Zone SPCZ is not as active as it was a week ago.  There is one branch of activity from the Coral Sea to New Caledonia ad another, coming and going, from Tokelau/ Samoa to Northern Cooks to Marquesas… This branch used to extend to a section between Marquesas and Galapagos, but that section is weakening now.

 

During the coming week not much change is expected to happen in the tropics… There will be some developments in the subtropics with one Low  likely to briefly form south of Niue on Wednesday UTC then head off to the southwest, and another low to form near Lord Howe Island in the Northern Tasman Sea next weekend. Both these lows will “steal wind”, broadening apart the isobars in the tropics immediately to their north. Avoid these lows.

 

SUBTROPICAL RIDGE

The Subtropical ridge line is still at its equinoctial latitude of around 40S.  There is a big fat high BFH mainly over the North island of NZ at present and stretching out along 40S to the east, with a squash zone of enhanced trades on its northern side mainly along 20S from around Kermadecs all the way to about 160E from Monday to Wednesday.  Avoid.

 

 

NZ

The forecast is for the BFH over NZ to be kicked off east by a front arriving over the South Island on Monday and fading over central NZ on Tuesday. Then a new BFH is expected to grow to 1033 near Chathams on Wednesday and to move slowly east over the next few days, as another front moves onto the South Island on Thursday and fades over the central NZ on Friday.

 

This week the fronts will likely be preceded by strong northerlies and followed by westerly winds.

 

 

The terms used are more fully explained in the METSERVICE Yacht Pack.

           More info at http://weathergram.blogspot.com

             Feedback to bob.mcdavitt@metservice.com

 

05 April 2009

BOBGRAM7 issued 5 April 2009

WEATHERGRAM
YOTREPS
Issued 5 APR 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

LAST WEEK
The "low from Rotuma" that was mentioned in last week's Weathergram
turned out to be LIN which blossomed when it moved south-southeastwards
across southern Tonga earlier today giving Easterly then NW gusts to
over 50 knots for around 5 hours. It is expected to continue to peel
off to the southeast.

That strong wind and heavy rain which hit Coffs Harbour early last week
was associated with a small trough in tango with the strong squash zone
on the north side of big fat High.. the High was over 1040 hPa in the
south Tasman Sea, and when its over 1040 it's going to get naughty.

TROPICS
The South pacific Convergence Zone SPCZ is still very active in the
Tuvalu/Tokelau/Samoa/Tongs/Niue but it looks as tho' LIN will take much
of this activity with it off and away to the mid-latitudes.
Indications are that the tropics are about to go thru a period of more
settled weather , starting in the Coral sea and spreading east, over the
next week or two . This settled wetaher has already arrived in the
Coral Sea.

There is a zone of convection between 9S 120W and EQ 105W about 300
miles wide. This is the ITCZ that normally lives at 3 to 5N and has
been knocked across the equator by the equinox. Well it is a few weeks
after the Equinox now so the overhead sun in the Northern Hemisphere and
this zone of convection should soon fade. Meantime, anyone who is going
from Galapagos to Marquesas should head south of 9S in those longitudes
to avoid the squalls.

There is a branch of the SPCZ between northern Cooks and Marquesas.


SUBTROPICAL RIDGE
The HIGH that crossed NZ over the past week or so hit over 1040 in the
south Tasman sea last Tuesday and tossed dry cold southern ocean air
onto NZ, producing as much as 6 degrees of frost to harass the grape
growers at the start of harvest. This High is expected to continue its
migration east between 40 and 45 South from 160 to 120W this week. The
Low that belongs to LIN is well positioned to travel south "down the
western shoulder" of this High. Beware the squash zone between High and
Low.

Next HIGH is already moving into the Australian Bight and is also taking
the near 40S latitude that the subtropical ridge prefers near the
equinox. It should cross Tasmania on Thu 9 Apr and NZ on Good Friday 10
April. It is not likely to get as much vertical support as its
predecessor, so will likely fade to a slither along 35S as it crosses
NZ.

Use these Highs to make your travel plans; they have more cohesion than
the lows and fronts. Also remember the squash zone of enhanced trade
winds that exists on the northern side of these Highs.

NZ
NZ will be "between Highs" this week, and thus will have a series of
fronts.
The first front is already in the Tasman Sea and Fiordland. On Monday,
its rain clouds may need to stall over the SW of the South Island, as
another front approaches. On Tuesday, both these fronts should reach
the North Island and, on Wednesday, a secondary may low form on these
fronts over the South Island, as some cold air arrives aloft. Avoid
sailing these days.

On Thursday a SW flow should settle over NZ leading in the Friday High.
So it looks like a period of settled weather in NZ for the main part of
Easter, except for a disturbed westerly flow over the South Island.
Enjoy.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bob.mcdavitt@metservice.com

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