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

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.
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