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

27 September 2009

BOBGRAM7 issued 25 Sep 2009

WEATHERGRAM
YOTREPS
Issued 27 September 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.

Well, the days are now longer than the nights and soon the annual
migration from tropics to NZ/OZ will be upon us, analysis paralysis will
be returning to the Minerva virtual yacht club. At this stage you've
probably heard that El Nino is in the equatorial Pacific Ocean, well
that's true but it isn't yet manifest in the atmospheric weather
patterns. This is expected to gradually become noticeable over the next
few months, as an equator-wards shift of our favourite weather zone: The
SPCZ, the subtropical ridge and the roaring 40s.
Since this is the season when the subtropical ridge normally shifts
southwards, maybe the resultant kerfuffle will appear to be just a
continuation of the current pattern.

Anyway those of you heading for NZ or Aus in yachts that average around
150 mpd or less are reminded that fronts and troughs cross the Tasman
Sea /NZ area around once every 4 days or so, thus your voyage is likely
to encounter a front. Normally the bets latitude to do this is around
30S--- sort of a mid-way rite of passage, and at that latitude fronts
are normally at their weakest, plus, if you time in rightly, it will
allow you to make landfall "between fronts" and that's a good idea.

TROPICS/SUBTROPICS
Cyclonic conditions are raging in Philippines and NW Pacific at present
and this extra convection extends across the equator to just north of
Solomons to Tuvalu, the weakens a little in a South Pacific Convergence
zone this stretches to between Southern Cooks and French Polynesia FP .
SPCZ is expected to take a rare excursion northwards across FP this week
- allowing, after its passage, a nice voyage westwards from Tahiti.

The northern end of a elderly mid-latitude front dumped hail into Suva
market last Tuesday-just in passing-just some cold air getting from
mid-latitudes into the tropics.

SUBTROPICAL RIDGE ZONE
Not much action expected here this week.
Today's HIGH in North Tasman Sea/New Caledonia area is expected to
weaken but remain as a zone of light winds that will migrate east along
around 20 to 30 S reaching Niue around 03 Oct UTC.

Another HIGH is expected to form in North Tasman Sea between Lord Howe
Island and 25S on 30 Oct and reach to 180 line around 4 Oct UTC. There
should be a squash zone of enhanced easterly winds along 10 to 20S to
north of this HIGH.

TASMAN SEA/ NZ AREA
Basically it is expected to be a week of two LOWS/

Low 1 is already deepening off Sydney, with strong SW winds over New
South Wales. Frontal zones associated with this low should cross NZ on
Monday, and the LOW itself, with attendant intense showers in a cols
pool, is expected to cross NZ on Wed, followed by strong westerly winds
on Thu. Avoid. Should be OK to make landfall in Northland on
Thursday-Friday-Saturday, but please check.

Second LOW is expected to form off southern New South Welsh coast on
Friday and turn into a trough as it crosses South Island on Saturday 2
Oct and North Island on Sun 4 October. Avoid, and this timing may
change so update.

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

20 September 2009

BOBGRAM7issued 20 Sep 2009

WEATHERGRAM
YOTREPS
Issued 20 September 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/SUBTROPICS
Last week's Low between NZ and Fiji was blocked form its normal exit
route to the southeast by a retrogression of a long wave ridge around NZ
--- This meant NZ had settled weather (after a strong NW last Monday)
and Fiji to Niue got a concentrated burst of rain from that blocked
trough. Yep, that's what flooded that Nadi bus station.

We have the equinox on Tuesday UTC, and finally the overhead sun will be
getting into the southern hemisphere. Around this time of year we often
get a mirror image of the ITCZ along 10S for a few weeks, and this does
seem to be forming now-at least from Solomons to Tokelau. In some years
the equinoctial convergence zone has tropical cyclone twins - one
forming on ITCZ triggering another in southern hemisphere and "out of
season". Not much is known about what triggers this pattern, but it is
something to add onto to list of possibilities to watch out for over
next few weeks


SUBTROPICAL RIDGE ZONE
HIGH cells migrating eastwards along 25/30S this week will keep the
South Pacific convergence zone off to its northern most position,
merging with that equinoctial convergence zone. One High will be on the
date line around Wednesday UTC and the next Sun/Mon 27/28 UTC. North of
these highs will be an area of enhanced trade winds, mainly at 10 to
15S. Because these periods of enhanced winds are no goof for fishing or
diving, and because all you need to time them is to watch the passing
Highs , I recommend that's what you do using the Fiji Map website at
http://www.met.gov.fj/aifs_prods/0641.html or our 7-day site at
http://www.metservice.com/public/maps/rain-forecast-7-day.html.

There's a trough between those two highs and it will be crossing Fiji
and Tonga around Friday/Sat 25/26 Sep. This brings fluky winds and
tropical showers so not the smoothest for sailing.

TASMAN SEA/ NZ AREA
It's too complicated and changeable for this bulletin. In this area
there is no blocking at present, so it's a ridge/trough/ridge/trough
pattern, but some of the troughs are building into deep LOWS in the
Tasman Sea, lapping onto NZ but not quite doing their worst there (yet).

For settled periods use the ridges--- one over NZ Tuesday UTC and next
is Friday UTC. There's a mid Tasman LOW between these systems on Thurs
UTC that models are deepening briefly to 975 hPa to west of Fiordland
travelling southeast. Avoid.

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

13 September 2009

BOBGRAM7 issued 13 Sep 2009

WEATHERGRAM
YOTREPS
Issued 13 September 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/SUBTROPICS
This week a MJO, which encourages extra convection near the equator, is
expected to weaken around the Solomons area. There was an interesting
tropical low that briefly formed in the Coral Sea to NW of New Caledonia
mid last week, and some of the energy of this is now wrapped up in the
LOW today between NZ and Fiji. This MJO is also in the India Ocean and
one computer model has come up today with the scenario of a Tropical Low
deepening near the Diego Garcia (near 5S 70E) in the Indian Ocean this
week. However the MJO is expected to be fading in this area by then, so
this will maybe just be a small low.

We are approaching the equinox, and that's when the sun gets overhead at
the equator. Around this time of the year a line of convergence
sometimes forms across the South Pacific along 10S-something like a
mirror image of the ITCZ... sort of an equinoctial convergence zone. We
might see this starting to form during the next week or two.

The South Pacific Convergence Zone SPCZ has been mainly located from
Solomons to Wallis/Futuna in the past week, and I expect its eastern
edge to wander north towards Samoa this week, and then stretch southeast
to be active in-between the Cooks and French Polynesia.

The LOW that is today between Fiji and NZ formed as an upper wave ridge
was arriving over NZ, masking NZ a place of rising rather than falling
pressure-this has blocked that low, causing it to do a loop and knocked
it a little North. Soon it will wander east (accompanying tough is over
Fiji today and should cross Tonga on Mon/Tue 14/15 Sep) and fade with a
little of it moving south well east of New Zealand.

A new trough is expected to form over Fiji and Tonga on Thu/Fri 17/18
Sep and then, on sat 19, deepen into a low near 30S 170W, to south of
Niue, and then wander off to southeast. People in Tonga looking to
sail early to NZ should wait for that trough to move off on Sat 19,
however there is a day between troughs-wed 16 Sep-which may be an OK day
to depart.

SUBTROPICAL RIDGE/ TASMAN SEA/ NZ AREA
HIGH crossing Bass Strait on 14 Sep is expected to slowdown as it
approaches NZ, should be over North island on Thu /Fri 17/18 Sep, and
linger to north of North Island Sat/Mon 19-21.

There is a disturbed westerly flow south of this ridge, with one front
crossing the South Island on 14 Sep, next on 16 Sep, next on 18/19 Sep,
next on 21/22 Sep. Each front should be preceded by a burst of NW winds
and followed by west/southwest winds.

HIGH near 30-40S 165W today is expected to wander slowly east along 35S
all week, and the squash zone of enhanced easterly winds on its northern
side should cross French Polynesia around Thu 17 Sep UTC. Good for
surfing but no good for diving or fishing.

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

06 September 2009

BOBGRAM7 issued 6 Sep 2009

WEATHERGRAM

YOTREPS

Issued 6 September 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.

 

Special thanks from me to the Tokoroa Weather Watchers who treated me to some fine hospitality last night – that was fun, watching a dry slot move onto the North Island and chatting into the night about barometers, weather maps and severe thunderstorms.

 

TROPICS/SUBTROPICS

TC DUJUAN is wandering northeast off the Philippines this week,  and the Solomons/Micronesia area is likely to have a boost to its convection activity as a Madden Julian pulse of enhanced activity rolls in .

 

As for the South pacific – well, just as last week, the main part of the South Pacific convergence Zone SPCZ is in its northern-most position and is likely to stay put this week as well from Solomons to Tuvalu to Tokelau/Samoa, to Southern Cooks. A burst of convection is likely to visit Samoa on Monday UTC

 

Next weekend, around 12/13 Sep, tropical lows are likely to form to south of New Caledonia and Southern Cooks.  These Lows are likely to deepen into a compact centre that will then move off to the south east. Time any voyages to avoid these lows.

 

SUBTROPICAL RIDGE

A HIGH that is today over New Zealand is expected to become a blocking HIGH and stay put until Wednesdayn9 Sep, and then to move slowly along 30/35S as far as 170W by Sun 13 Sep, with an squash zone of enhanced easterly winds on its northern side in the tropics.  

Another HIGH is expected to form in the Tasman Sea on or around Sat 12 Sep, crossing NZ on 14 /15 Sep.

These Highs are steady enough to help in the timing of your voyages.

 

TASMAN/NZ AREA

A Low over Tasmania is expected to wander eastwards then off southwards over next few days.

Between the HIGHS there is a trough crossing NZ on 11/12 Sep, preceded by strong northerlies. Avoid.

 

 

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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