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

27 July 2014

BobBlog issued 27 July 2014

WEATHERGRAM
YOTREPS
Issued 27 July 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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

ENSO
SOI. The Southern Oscillation Index SOI sums up the weather pattern over the
South Pacific as one number. It is based on the standardized difference in
the barometer readings between Tahiti and Darwin.
The SOI has dived negatively in the past month but needs to remain in the
pink area of the graph (shown in illustrated version of this blog) for more
than a month for this to be called an El Nino episode. Even though we are
not there yet, there have been some signs of El Nino like weather in the
South pacific recently: the westerly winds of the roaring 40s have extended
further north than normal over the New Caledonia area.
Looking at the 30 day averaged pressure anomaly map shows this clearly, and
also shows near neutral conditions around French Polynesia. For an El Nino
to kick in, Darwin needs to have a sustained above normal barometer, and
this is happening. Also Tahiti needs to have a sustained below normal
barometer and that is not happening yet.

TROPICAL TOPICS
Action this week seems to be shifting to the NE Pacific GENEVIVE approaching
the south side of Hawaii HI And HERNAN is lurking off Mexico These features
are expected to fade over the next few days. There are also a few features
that may develop into cyclones in the NW Pacific.
The weekly rain maps for the past fortnight show a burst of tropical
activity spreading eastward along the ITCZ International Tropical
Convergence Zone. The South Pacific Zone SPCZ has also shown signs of
redeveloping after a few quiet weeks.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is intensifying and is expected to drift south this week so that it
visits Samoa on Tuesday UTC and the Southern Cooks on Wed/Thu UTC.

STR= Sub-tropical Ridge
The High moving east off NZ on Monday is expected to travel along 30 to
40 S. There should be a zone of enhanced easterly winds along 15 to 20S on
its north side from Wed to Sunday.

Departing westwards from Tahiti:
OK to depart before Tuesday local but any later in the week and there are
likely to be some strong SE winds encountered on the voyage, except maybe
for a trip to Suwarrow.

Departing from NZ to the tropics
A broad trough is likely to take all week to cross the Tasman Sea and NZ.
Its first feature should be preceded by northerly winds and followed by NW
winds and cross the South Island on Monday and the North Island on Tuesday/
Tuesday night. The second feature is likely to be more intense, preceded by
NW winds on Thursday and Friday, accompanied by a front on Saturday and
followed by SW winds that may be strong on Sunday 3 August.
SO it is not a good week to depart from NZ.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

20 July 2014

BobBlog issued 20 July

WEATHERGRAM
YOTREPS
Issued 20 July 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 of weather maps, so please fine-tune to your place. Dates
are in UTC unless otherwise stated.

Negative storm surge
In this blog last week we looked at the storm surge at Marsden Point (off
Whangarei) in the storm that bothered Northland earlier this month. With
vigorous on-shore easterlies in the past few days a similar storm surge of
around .5m has been measured at Whitianga (off Coromandel).
One of my blog followers pointed out a corresponding drop in sea level at
the south end of NZ at Green Island (off Dunedin). During last week this
was as much as 0.6m and now it is still .4 of a metre. Imagine the
embarrassment of boats taking shortcuts across a mudflat when the real tide
is .6m below the computed tide!

Can anyone offer an explanation for these strong sea sucks  I dont think
the winds at Dunedin have been strong off-shore for all of last week, so
whats the cause? It seems to mirror the shape and trend of the storm surge
on the NE coast- is this a clue to its cause, or a coincidence?

TROPICAL TOPICS
Typhoon MATMO is winding up and set to cross Taiwan on Wed 23 July 2014.
The weekly rain maps for the past fortnight show heaps of convection over SE
Asia weather and along the ITCZ. Also it shows to very wet weeks over
Northland in NZ.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
A weak convergence zone is expected to stay put over Solomons towards the
Tuvalu/Tokelau/ Samoa area.

STR= Sub-tropical Ridge
The STR is behaving normally over Australia and in the eastern South
Pacific, and is weak and slowly returning to normal across the Tasman Sea
and NZ area. The High departing eastern Australia on Wednesday 23 July is
expected to travel east along 30 to 35S across the Tasman Sea on Thursday
and Friday reaching northern NZ on Saturday 26 July.

Departing westwards from Tahiti:
North end of a trough in the middle-latitudes is expected to bring squally
showers and variable winds to Tonga by Tuesday 22 July UTC and Southern
cooks by Thursday/Friday 24/25 UTC. A voyage from Papeete to Suwarrow may
work OK but trips to places further south should take this trough into
account.

Departing from NZ to the tropics
A compact and intense Low has been travelling east past northern NZ over the
weekend causing damage with heavy rain and strong onshore- easterly winds
for a while. This low has now gone off to the east of NZ and has opened the
way for a trough to form along 170W to east of the whole of NZ.
Lows from the southern ocean are likely to be drawn into this trough and
one should deepen there on Wednesday, and another on Friday..

It looks to me that the coldest week of the year for NZ is very likely to be
this week. I say this because the coldest time of the year is usually late
July/early August, and during the coming week air from the southern ocean is
likely to be shovelled onto New Zealand thanks to the southerly flow that is
being directed by the High to the west and the Lows to the east what is
sometimes called an eggbeater southerly.

See http://blog.metservice.com/2009/05/an-eggbeater-southerly/

The polar vortex parameter I use, the AAO, is expected to be negative this
week, HOWEVER the weather forecast is NOT for a full polar outbreak over NZ
this week, as the polar vortex is expected to remain intact (in our part of
the world anyway).

So, when to depart? The SW flow over Northland is expected to be too strong
for comfort on Monday, but weather conditions should be OK for going north
on Tuesday to Thursday. After Thursday there may be problems with the voyage
as the High then in the Tasman is expected to travel east across northern NZ
by the weekend, bringing light winds, and may then be followed by NE winds
and another Low.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

13 July 2014

BobBlog issued 13 July

WEATHERGRAM
YOTREPS
Issued 13 July 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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Scott Donaldson (http://doubleditch.co.nz/Tracker) considered conditions to
be getting too unsafe on Friday, after facing gale winds from E then NE then
N since last Monday, with 3 rolls and spending most of time on his side
early Friday and with forecast for another boost of wind to come with a
passing front on Friday evening and with communication battery
failing. SO he abandoned Double ditch after about 85 days at sea.

The Low that got stuck off northern NZ early last week had its normal exit
to the southeast blocked by a belligerent high. This blocking pattern
happens when the steering winds aloft weaken away over a high, as happened
in this case with the High to southeast of the South island.

There is a blocking index which measures this and its latest readings can
be found at http://www.cpc.ncep.noaa.
gov/products/precip/CWlink/blocking/real_time_sh/real_time_index_nrm.shtml
This shows there has marked blocking just east of NZ since the beginning of
last week.

All the wind and the lower than normal pressures in that system that stayed
near Northland for a few days generated a storm surge of around up to 0.4m,
but this happened at a time of neap tides, and that was a mitigating effect.
There is however now a perigean moon (the full moon and the perigee of the
moon are within a day of each other tonight) so if this event occurred in
the king tides over the coming few days rather than last week then its
impact would have been more severe.


TROPICAL TOPICS
Typhoon RAMMASUN is winding up and set to make landfall over Philippines on
local Wednesday 16 July. Also, the weekly rain maps for the past fortnight
show heaps of convection over SE Asia weather and along the ITCZ, and not
much at all in the South Pacific.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
A weak convergence zone is expected to stay put over Solomons towards
Tuvalu.

STR= Sub-tropical Ridge
The STR is behaving normally over Australia and in the eastern South
pacific, but is weak to-non-existent across the Tasman Sea and South
Pacific area. In this area the highs centres are found to be south of 45S
and slow-moving. One is expected to travel from SE Australia across the
south Tasman Sea on Tuesday and Wednesday and then sneak around the south
coast of NZ.

Departing from NZ to the tropics
Low is expected to form in the cold air belonging to a cold front in the
Tasman Sea tonight and then move northeast across northern NZ and deepen on
Monday (a sign that it has SW winds aloft, so is to west of the upper-
trough-apex.
This Low is expected to take a clockwise path thru 30S 180 and then to the
ESE. Anyone departing Northland on Monday should be able to keep well enough
away from is centre and take a good enough ride northwards with the
southerly winds in the wake of this low for a few days.
On Thursday and Friday another dose of cold air and SW winds is expected to
flood into the Tasman Sea from the southern ocean. A similar process should
recur, allowing a low to form in this cold air, this time it is likely to
form over Saturday and Sunday 19/20 July around the North Island. Westerly
winds are likely to be found as far north at 20S, and there are likely to be
strong to gale west to SW winds as far north at 25S along with large swells.
Avoid.

Departing westwards from Tahiti:
This should be a good week for such a voyageno troughs are expected.
However it will be a good idea to deviate to north of the direct path ,
because the contrary westerly winds may stretch as far as 20S at times and
the trade winds may retreat to north of 15S.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

06 July 2014

BobBlog issued 6 July

WEATHERGRAM
YOTREPS
Issued 6 July 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 of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

Scott Donaldson (http://doubleditch.co.nz/Tracker) got within 36m/65km WNW
of New Plymouth in calm conditions on local Sunday morning, and then an
easterly wind developed so that he has now stopped paddling and will try and
stay-put and hunker down in a NE gale on Tuesday and Wednesday.
He may be able to start paddling again on Friday, and by then may have
drifted 30 miles to the SW of his current position. The whims of the weather
are tossing him a new adventure.

John Furnell of Helicopter Services, based in Taupo, took advantage of
todays rare calm conditions to do another airdrop to top up Scotts
supplies for the week ahead. Scott seems to still be determined to continue
kayaking, it is his adventure and he doing this adventure to support Asthma
NZ. MetBob is helping by watching the weather for him. To cheer him up drop
in a few dollars via http://DoubleDitch.co.nz.

During June the subtropical ridge STR in the Southern hemisphere was
stronger than normal and pressures in the southern ocean lower than normal,
thereby increasing the disturbed westerlies in-between  and thats an El
Nino trait. At present there is a zone of abnormally high pressure close to
Darwin and another close to Tahiti. In a typical El Nino the pressure at
Tahiti drops below normal, and this isn't happening yet. The most remarkable
information from the June anomaly pressure map is the zone of lower than
normal pressure across the North Pacific tropics. This is leading to extra
convection and tropical activity, especially off the Mexican west coast;
latest feature there is tropical depression DOUGLAS.

The Southern Oscillation Index SOI measures the impact of an El Nino or La
Nina episode on the atmosphere. An El Nino episode indicates more heat than
normal within or emanating from the Eastern equatorial Pacific Ocean EEP. We
have known for a few months now that extra heat is being stored in the EEP
mainly beneath the surface. During the past week the SOI has slipped into
negative territory, indicating that there may be a trend towards an El Nino
episode starting in the atmosphere.

TROPICAL TOPICS
Hurricane ARTHUR formed in the past week and managed to briefly sideswipe
North Carolina of the USA and cancel 4 July celebrations for many.

The weekly rain maps for the past fortnight show weather drying out over
Brazil for the World Cup, a burst of tropical convection just to east of
Philippines (Typhoon NEOGURI is likely to travel from there to Japan for
9/10/11 July) and Monsoon rain for Bangladesh. There has also been a strange
tropical depression in the South Indian Ocean (near 9S 83E).

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
A weak convergence zone is expected to travel from Vanuatu on Monday to Fiji
on Wednesday and is likely to weaken. Otherwise the tropics in the South
pacific is likely to be quiet this week.

STR= Sub-tropical Ridge
The STR is weak and north of its normal position at present. There are some
large highs on the weather map but these seem to be related to jet streams
near 60S

Departing from NZ to the tropics
The subtropical jet stream emanating from Australia is north of normal, and
this is allowing a large upper trough to encircle the northern Tasman Sea,
allowing a surface low to form near Norfolk island on Monday and then deepen
a lot as it moves South until it finally weakens west of Taranaki on
Thursday. This is the low that has caught and is delaying Scott Donaldson,
and this is a typical winter weather pattern for NZ.
Associated front with heavy rain should cross Northland on Tuesday and
Wednesday, preceded by strong NE winds and followed by showery NW winds.
SO wait until Friday or Saturday before departure.

Departing westwards from Tahiti:
A trough and accompanying low is expected to travel for midway between Tonga
and NZ on Wednesday onto the Southern Cooks by around Friday UTC/Thursday
local. Avoid.
Also note that is not really a tropical feature, rather it is a feature in
the mid-latitudes, south of the STR, so is likely to be followed by SW winds
and swells.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

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