Translator

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

30 October 2011

BOBGRAM issued 30 Oct 2011

WEATHERGRAM
YOTREPS
Issued 30 October 2011
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.

This new La Nina episode continues in a weak to moderate fashion. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 22nd Oct it was 1.0, and on 30th it was 0.96. Computer modelling suggests this La Nina will continue into the southern hemisphere cyclone season. This tends to reduce the tropical cyclone activity around and east of the dateline, but may increase the risk around the Coral Sea.

Some Insurance policies take the start of the cyclone season to be 1 Nov- and indeed the risk rises rapidly during Nov and December.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna and to Samoa, then … slightly weaker … to Southern Cooks. This is its 'normal position' and it has shown a build up of activity during the last week.

It appears that a tropical Low is forming on the SPCZ around the Wallis/Futuna area during Monday and may move across northern/central Tonga on Tuesday, and then take a path ESE-wards to north of Niue on Wednesday and over Southern Cooks on Thursday, then southwards into the mid-latitudes. Avoid.

SUBTROPICAL RIDGE STR
A BFH (big fat high, central pressure over 1036 hPa) has built well to east of NZ and is hovering around 45S 140W. There is a squash zone of enhanced trade winds on the north side of this high near 25S 150W also enhancing this squash Zone a sit moves slowly south. Avoid.

The next high is expected to form slowly in the mid Tasman Sea around 30S but its formation is expected to be delayed until Sun 6 Nov, and this high may just stay put all next week. During this time there will be the characteristic subtropical ridge link connecting this high to the BFH. The STR link should move north next week to 30S, fading out the squash zone.

TASMAN SEA/NZ AREA
An upper low is forming tonight near 28S 175E and it contains thundery squalls. This feature is expected to be collected by the remains of an old frontal system that is currently draped over southern NZ and a trough that is in the Tasman Sea . The combination is expected to move across NZ on wed 2 Nov and lead in a disturbed W to SW flow that looks likely to cover NZ and the area north of NZ from Thu 3 Nov until at least Fri 11 Nov. Drat.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access you may be able to check the weather patterns at http://bit.ly/7daywx or http://bit.ly/ecoz
Those in Tonga may as well wait for that tropical Low to go away on Tuesday before thinking of departure, just it case it changes track.
The STR will remain as a zone of light winds roughly along 30S from wed 3 Nov until at least Fri 11 Nov, with a disturbed westerly flow over Northland. This isn't the easiest pattern for getting to NZ but we may be able to sort out some zig-zags across the S/SW winds for a few boats with have enough fuel to motor across the STR light-wind-zone.

Note that I will be UNAVAILABLE from 8 to 12 Nov—attending to MetService display at Christchurch show.


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

22 October 2011

BOBGRAM issued 23 Oct 2011

WEATHERGRAM
YOTREPS
Issued 23 October 2011
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.

A La Nina episode is kicking in. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 16th Oct it was 1.0, and on 22nnd Oct it was 1.2. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season.
This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds around New Caledonia and another to NW of Fiji at present, and there is likely to be more of these squash zones this week.

The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna… slightly weak over Tokelau/Samoa the strong again over Northern Cooks. It had a brief welcome visit to Tuvalu mid-last-week, and is expected to drift SOUTH this week.

SUBTROPICAL RIDGE STR
This is a week of Big Fat Highs (BFH)
The one building east of NZ today is expected to make steady progress to the east along around 40 to 45S and grow to over 1030hPa when well south of French Polynesia FP from Wed 26 Oct.

A new BFH is likely to cross Tasmania on Mon 24 Oct and then squeeze around South Island on Wed/Thu 26/27 Oct and then build to maybe over 1035 as moves off to east of NZ from Fri 28 Oct to Mon 31 Oct.

A squash zone of enhanced trade winds is likely to form between this new BFH and the SPCZ=== most likely around 20S to 25S form Tonga to New Caledonia from Friday 28 Oct to Tuesday 1 Nov

TASMAN SEA/NZ AREA
A transitional trough between the two Highs is expected to be windy in the east and west in the west as it crosses South Island on Monday, and then weaken as it stalls over the North Island on Tuesday/Wednesday, 25/26 Oct.

Next trough should reach South Island around Sat 29 Oct --- as a LOW is likely to form off Sydney. This front should stall over central NZ as that Low move SE along the frontal zone. Fronts should then cross Northland around Tue /Wed 1 or 2 Nov, followed by a moderate westerly wind, and Thu 3 Nov followed by a moderate SW wind.

SAILING TO/FROM NORTHERN NZ.
If you have Internet access check the weather patterns at http://bit.ly/7daywx or http://bit.ly/ecoz

The BFHs are in charge this week. One leaves light winds over Northland until Wed 26 Oct then E or NE/NW winds over Northland from Thu 27 Oct to Tue 01 Nov. This provides a good window for arriving in NZ, especially for those leaving Tonga /Fiji/New Cal on Mon/Tue 24/25 Oct. AT this stage the winds over Northland on 2-3 Nov are expected to be mod W /SW, so this may be OK as well.

Because the BFH may be making a squash zone from Tonga to New Cal from Fri 28 Oct to Tue 1 Nov, we may as well treat next weekend as a stay ashore weekend, maybe a good time to complete the voyage planning rather than a target date for departure.

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

16 October 2011

BOBGRAM issued 16 Oct 2011

WEATHERGRAM
YOTREPS
Issued 16 October 2011
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.

Northern Hemisphere cyclone season is easing off this week.

A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds over the seas of French Polynesia at present as a high moves east along 30S and this should last until Tuesday 18 Oct UTC. Another squash zone is expected to form in the Coral Sea on Tuesday 18 Oct UTC and this should spread east reaching New Caledonia /Vanuatu on Thu 20 Oct, Fiji on Fri 21 Oct and Tonga on Sat 22 Oct.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 9th it was 0.91, and on 16th Oct it was 1. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season. At this stage is expected to be a weak La Nina, and NOT as strong as the extreme La Nina we had last summer.

The South Pacific Convergence Zone SPCZ is expected to hover near its normal position this week between 10 and 15S—this means that Tuvalu and Tokelau may see some desperately needed showers for a change.

SUBTROPICAL RIDGE STR
The week starts with a BFH (big fat high) near 40S 150W moving east – this is the cause of the squash zone over French Polynesia. It is expected to be followed by another High of weaker intensity near 35S 160W from Wednesday 19th October. And another High should form in the Tasman Sea on Tue 18 to Thu 19 Oct, crossing northern NZ on Sat/Sun/Mon 22/23/24 Oct (Labour weekend in NZ)


TASMAN SEA/NZ AREA
Of major concern to anyone sailing to NZ at present is a trough now in the Tasman Sea. One low in this trough is forecast to cross the South Island on Monday and another should cross northern NZ on Wednesday--- this brings vigorous SW winds and heavy SW swells to the zone between northern NZ and as far north as 30S, maybe 25S, on Wed 19 and Thu 20 Oct. AVOID.

For this year's COASTAL CLASSIC departing Auckland for Bay of Islands on Friday,
The forecast at this stage is for a ho-hum SW between 10 and 20 knots.


SAILING TO/FROM NORTHERN NZ.
The main thing is to avoid 25S to NZ on Wed 19 and Thu 20 Oct, please

There is a squash zone slowly spreading east from Coral Sea, expected to reach New Caledonia around Thu 20 Oct and onto Tonga by Sat 22 Oct. It may be a good idea to depart before this zone arrives.


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

09 October 2011

BOBGRAM issued 9 Oct 2011

WEATHERGRAM
YOTREPS
Issued 9 October 2011
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.

Cruising sailors in South Pacific may be in for an interesting delight this weekend /next week with the waning moon, especially around the third quarter- This usually triggers a spawning of a coral worm called Palolo (or Balolo in Fijian). Sometimes the overnight spawn may be large enough to colour the sea – stuff can be netted and eaten raw or cooked. Ask the locals.

Northern hemisphere cyclone season is still busy: PHILIPPE still in North Atlantic, staying offshore, and now IRWIN and JOVA in NE Pacific.

Equatorial ocean temperatures in west pacific are near normal, and in east pacific are below normal. A La Nina episode is kicking in. This helps intensify the subtropical ridges STR and knocks them pole-wards (in both hemispheres). The stronger highs in the STR help make stronger trade winds.

From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 2nd Oct its 30day running mean was 1.2, and on 9th it was 0.91. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season. At this stage is expected to be a weak La Nina, and NOT as strong as the extreme La Nina we had last summer.

The South Pacific Convergence Zone SPCZ gets knocked south by a La Nina. This was certainly the case early this year, and indeed SPCZ has not been visiting Tokelau or Tuvalu much this year… this explains their drought which is now getting desperate. The SPCZ over past week is now hovering between Solomons and inbetween NE of Vanuatu and SW of Fiji. Rather weak.

SUBTROPICAL RIDGE STR
The STR is usually around 30S at this time of the year, but at present seems to be more near 35/40S. And some BFH (big fat high) specimens are on show this week, getting over 1035hPa near 35S 100W and near 37S 140W. These BFHs are also blocking highs so that troughs get directed around them rather taking a normal journey southeastwards. They also strengthen the easterly winds in the tropics on their northern side - be aware of the prospects of a squash zone of enhanced easterly winds between French Polynesia and Niue from Tue 11 Oct, with huge surf/swell over the Cooks. Avoid.

For Tasman Sea, next high should move in from west on Thu 13 Oct, but will be a slow-mover…should fade over Northland on Sun/Mon 16/17 Oct. The following High is likely to catch up with this and cross the North Island more sedately next week from 18 to 20 Oct. If this turns out to be the case, it may offer a few reasonable voyages from Tropics to NZ / Australia.


TASMAN SEA/NZ AREA
Broad multi-edged trough is expected to cross New Zealand this week. First trough already has a tight and intense low centre moving south along eastern Tasman Sea, and this should cross southern NZ on Monday 10 Oct, preceded by strong NE flow over NZ. Second trough has a low that should move east over southern NZ on Wed and then the trough line should cross remainder of NZ on Thu 13 Oct, followed by cold SW flow that may be thundery.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see http://www.islandcruising.co.nz/ for this voyage.

If you time your voyage to depart Tonga/Fiji/New Caledonia before the squash zone of an incoming high arrives (New Caledonia already sat 15 Oct, and Fiji /Tonga by Tue 18 Oct. Also time it to arrive in NZ after the SW winds following Thursday trough have faded away (around 16 Oct) – then that has the offerings of a reasonable voyage.

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

02 October 2011

BOBGRAM 2 Oct 2011

WEATHERGRAM
YOTREPS
Issued 2 October 2011
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.

This edition comes from Ohope beach… "Is that a dark fin I see before me?"
I shall be discussing weather with local community groups here on Monday.

I hope you managed to avoid the stronger than normal tidal currents of the past week, all due to a Perigean new moon - currents should be quieter this week, allowing for better diving :)

Northern hemisphere cyclone season is still going strong. NALGAE is second destructive typhoon to visit Philippines in a week, and OPHELIA and PHILIPPE are still in North Atlantic, staying offshore.

The tropical ocean temperatures in west pacific are near normal, however there is a slow intensification of a cool anomaly around Galapagos --- so there is a tendency towards a La Nina, but it is more in the east than in the west.

From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.75 on 21 Sep and 1.2 on 2nd October. First time is has exceeded 1 since April, so yes there a change occurring in the air. Over next few weeks I shall explain what this may do to the coming South Pacific cyclone season.

The South Pacific Convergence Zone SPCZ has been in its normal zone from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks. The eastern side to this zone is weakening at present as a new zone has formed – rather weak - over Coral Sea. This trough is at present linked via upper jetstreams with a mid-latitude feature and should cross New Caledonia on Monday and Fiji on Tue/wed 4/5 Oct and fade over Tonga on wed/Thu 5/6 Oct. Another tropical trough may form between Southern Cooks and French Polynesia this week, speaking up for the 'proper' SPCZ.

SUBTROPICAL RIDGE STR
The STR seems to be south of its norm position of around 25S. That High around 1030hPa near 35 to 40S to east of NZ should continue to migrate along that latitude this week, out to way southeast of French Polynesia--- with a slight enhancement of the trade winds on its northern side.

The next High is likely to take a path along slightly more southern latitudes---around 45S to 47S—crossing Tasmania on Tue 4 Oct and skirting southern South Island on Thu 6 Oct, then spreading north be cover North Island by Sun 9 Oct.


TASMAN SEA/NZ AREA
The low that is currently in the Tasman Sea is under a slow-moving long wave trough and so is likely to take its time crossing the NZ area this week. Its main centre should move onto central NZ by Tuesday 4 Oct, and then form a secondary 'back' in the Tasman Sea on Wednesday 5 Oct. This complex should finally get to east of the North Island by Sat 8 Oct—mostly being pushed off its perch by the rising pressures of the High, rather than moving via its own pressure falling techniques. Some spats of heavy rain with the low's frontal bands are likely here and there Mon and Tuesday, and the cold southerlies arriving on Tuesday may bring a dash of snow to the Kaikoura ranges.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then you may be interested in following the ICA All Points Rally, see http://www.islandcruising.co.nz/ for this voyage.

The slow-moving Low crossing NZ is actually GOOD NEWS, as it provides a reasonably steady westerly flow over seas north of NZ from Tues 4 to Friday 7 Oct. OK there may be a mediocre front and southerly change near 30-35S on Saturday 8 Oct, but then the forecast is for light winds over Northland on Sun 9 Oct and steady northerly flow there on Mon/Tue 10/11 Oct. So if you can arrange to depart New cal/Fiji/Tonga at the right time (considering the incoming trough) then a relatively straight forward voyage is in the offering this week. Not 100% but not bad either.

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

Blog Archive