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

29 April 2012

BOBGRAM issued 29 April

WEATHERGRAM
YOTREPS
Issued 29 Apr 2012
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.

Production a little later than normal this evening because I was driving from Wellington to Auckland this afternoon. All OK here.

The Southern Oscillation Index or SOI is now holding at around -0.6… That was its 30day running mean value on 8 April and 22 to 29 April – there was a slight dip to -0.8 around 15 April. Since it is holding at -0.6 we are on the cusp of describing it as a weak El Nino or still in neutral territory. Either way, this means that the tropics are having only a minor impact on South Pacific weather patterns at present—so OK you can use those Routing text books which pick early May as the end of the cyclone season as far as Yacht Insurance is concerned and so an OK time for departing NZ and sailing to the Islands.

South Pacific Convergence Zone SPCZ has been weakening during the past week and now there is only scattered convection across the Coral Sea.
There has been a line of convection between northern Vanuatu and Fiji, but this has mostly weakened away. There is a steady line of convection from Tuvalu to Samoa to Southern Cooks and over French Polynesia. A small tropical Low is likely to form on the SPCZ over Southern Cooks around 5 May and this should go SE to southern parts of French Polynesia on Sun 6 May.

Galapagos to Marquesas: Much the same as last week. There are squally patches between 115 and 118W otherwise OK. Depart quickly SSW from Galapagos until 3S then WSW to 5S then west to 125W then direct.

SUBTROPICAL RIDGE STR
The weak ridge to NE of NZ today is expected to go east and fade away on Monday. Next ridge is likely to be held in place in Tasman on Monday and Tuesday to allow the passage of the northern semicircle of a huge Southern ocean Low. This STR Hugh should cross NZ from Wed to Friday 2 to 4 May and leave a ridge over South Island on Sat/Sun 5/6 May.
There will be an enhancement in the trade winds on the north side of this High near 23S on Friday but it isn't expected to be much of a squash zone.

NZ /Tasman Sea troughs
There is a Low between Brisbane and Lord Howe tonight and its clouds do look menacing on sat pictures… but models are picking all this to die 'in situ' during Monday as the next High sets up in south Tasman Sea. Basically it i8is a good looking seed of a trough, but lacks a good jet stream and so hasn't as much upper support as does the South Tasman High.

That Southern ocean LOW that has been moving along 60S from 150E to 180 over the weekend has a northern semicircle of wind and swell that already extends as far north as 40S = southern North Island. As the Low makes its way further east on Monday is may throw northwards a secondary low as far as 45S 170W by wed 2 May—Along with SW swell reaching 9 occasionally 13 metres east of NZ . Avoid.

Next Tasman low is likely to deepen off Sydney on Friday 4 May, as the High over NZ moves off. This Low should deepen and stay in the Tasman on weekend of 5/6 May, allowing a strong N to NE flow to form from New Caledonia to western NZ. Avoid.

NZ to Tropics: There is a nice window-of-departure for a good voyage based on the incoming High between 1 and 3 May. It's a balance between the squash zone on north side of this high (which peaks at 25S on Thursday/Friday 3/4 May) and encountering the NE winds of the following trough (which makes departures from Thurs 3 May onwards no good)… so a lot depends on vessels individual circumstances of speed and destination.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

22 April 2012

BOBGRAM issued 22 April 2012

WEATHERGRAM
YOTREPS
Issued 22 Apr 2012
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.

The Southern Oscillation Index or SOI is now hovering in negative territory. On 8 April its 30 day running mean was -0.6 on 15 April it was -0.8, and on 22 April it was -0.6 again.

South Pacific Convergence Zone SPCZ has been moulded around a BIG FAT HIGH BFH that stayed near New Zealand last week. So there has been extreme convection along and just off Australia's East Coast. This convergence was also intense over New Caledonia and Fiji and then average convection over Tonga and from there to the southeast. Another convergence zone lingered along 10S between Tokelau and Northern Cooks, and another was active at times between 5S and the equator from 130 to 150W.

A small tropical low may form on the SPCZ on wed 25 April around 30S 175W … this system is likely to get pushed to the southeast by an upper Jetstream and should steal the winds off Tonga on Thursday.

Galapagos to Marquesas: There are squally patches between 110 and 115W otherwise OK. Depart quickly SSW from Galapagos until 3S then WSW to 6S then west to 125W then direct.

SUBTROPICAL RIDGE STR
The BIG FAT HIGH BFH that lingered over NZ for much of the past week is now slowly going east along 37/40S. Its western lobe should weaken over North Island on ANZAC Day Wed 25 April, allowing the remains of the now-intense trough that is currently in the west Tasman Sea to cross the NZ on Fri 27/Sat 28 April preceded by some NW winds and followed by W/SW winds. A passing Low in the southern ocean should bring a cold southerly outbreak to southern NZ on Sun 29 Apr.

Next HIGH is expected over Aussie Bight on Sat 28 April/New South Wales on Sun 29 Apr, and NZ on Tue 1 May.

NZ to Tropics: It looks to me that the cyclone season is over and this week is OK for sailing north. We may as well wait for that trough to cross North Island late on Friday and depart on Sat 28 to take advantage of the post frontal tail winds.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

15 April 2012

BOBGRAM issued 15 April 2012

WEATHERGRAM
YOTREPS
Issued 15 Apr 2012
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.

The Southern Oscillation Index or SOI continues its dive into negative territory. On 8 April its 30 day running mean was -0.6 and on 15 April it was -0.8; still way too early to call this a swing , but if it remains negative for another fortnight then we may be witnessing a swing into El Nino. And we may see trade winds easing, and the subtropical ridge may drift north.

South Pacific Convergence Zone SPCZ has been settling into more normal activity over the past week, compared to its extreme ran with an MJO event just before Easter. There's an extension of the zone in the Coral Sea, and the main SPCZ stretches from eastern Solomons or northern Vanuatu to Fiji. There has been a weak rather scattered zone from Tokelau to North Polynesia, and another weak branch just south of French Polynesia linking with a mid-latitude low that is tonight near 30S 140W.

Another mid-latitude low is likely to form at the SE end of the SPCZ to south of Niue on Tue 17 Apr. It should peel off to the SE by Fri 20 April

.
Galapagos to Marquesas: There are squally patches near 3-4S 93-98W and near eq-2S 98-102W. Depart quickly SSW from Galapagos until you find the easterly winds they are near 5S, and then take the direct route to Marquesas.

SUBTROPICAL RIDGE STR
In recent weeks the STR has been taking on Indian-summer latitudes. The term 'Indian summer' has been applied in USA when autumnal anticyclones sit over the prairies and allow settled weather (after the first frost) to help Indians do some Buffalo hunting before winter sets in. Of course USA is now having a tempestuous spring. But NZ is having a large autumnal anticyclone this week, and some may well call its weather an 'Indian Summer'. It is forecast by some models to have a central pressure over 1038 in the Tasman Sea by Wed 18 April and to move slowly east over central NZ, not weakening until it gets east of NZ after Sun 22 April.

There is a zone of enhanced trade winds on the north side of this high,
But if you don't mind enhanced trade winds then this is an OK week for sailing from NZ for the tropical Islands.


The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

08 April 2012

BOBGRAM issued 8 April 2012

WEATHERGRAM
YOTREPS
Issued 8 Apr 2012
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.

The Southern Oscillation Index or SOI has had an interesting week starting off around plus 0.5 (30-day running average) and ending up on 8 Apr at minus 0.6. We are at the time of year when overhead sun in moving into the northern hemisphere, and any change in SOI at this time tends to set the trend for the next year, so maybe we are in for an El Nino (consistent negative SOI). Still too early to say fully, but if so then trade winds may start easing, making tropical travel over next few months….interesting.

South Pacific Convergence Zone SPCZ has been active from north of Solomons to Fiji. There has been another branch mainly along 10S across Tuvalu and Northern Cooks/French Polynesia – this is the northern extent of the SPCZ, and a favoured position in an El Nino event, so maybe another pointer in that direction.

This recent MJO that brought extremely heavy rain to Fiji has now done its dash, and we are likely to go through a period of less than normal activity in the tropics over the next few weeks so this is an indication that we are that's the end of the southern hemisphere cyclone season. Voyaging sailors can now start thinking about sailing to the tropics.

Galapagos to Marquesas: Showers and convergence zones have eased away so now so it's time to go. Indications are OK this week for ignoring the equatorial current and taking the southern route—get quickly to 5S to catch the easterly winds to go west. There are stronger Easterlies further south with bigger seas so not so comfortable. 5S is a good compromise.

SUBTROPICAL RIDGE STR
High east of NZ has helped to keep the Low that has been NE of NZ away from the most of the mainland over Easter. High cell over Tasman Sea should spread over northern NZ on Monday and Tuesday and the main high in the east should migrate east along 40S, reaching 1032.

A big Fat high BFH is moving across the Aussie Bight. It is a slow-mover and should peak with central pressure around 1038 near 40S on Tue 10 April. It should spread across Tasman Sea/NZ along 37S from Fri 13 to Mon 16 April.

There are zones of enhanced trade winds on the north side of these highs, especially the BFH- avoid Coral Sea from Wed 11 to Sun 15 April.

TASMAN SEA/NZ AREA
Trough between the highs is expected to cross NZ on Tuesday 10 and Thursday 11 April. If you want to take an 'early' departure from NZ to the tropics, weather is looking Ok from Thursday 12 April – a good SW for departure. Avoid Fri 13 if you are tridentophobic.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

01 April 2012

BOBGRAM issued 1 April 2012

WEATHERGRAM
YOTREPS
Issued 1 Apr 2012
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.

The Southern Oscillation Index or SOI had a brief burst form the recent MJO or pulse of enhanced convection moving from North Australia to Coral Sea/South West Pacific. it has been hovering around 0.5 to 0.8 over past two weeks, but is likely to weaken over next two weeks. Still enough strength in it to be called a weak La Nina.

This MJO's arrival into the Southwest Pacific has brought a zone of near-gale near-equatorial NW winds from the Solomons to Fiji during the past week, enhancing the South pacific convergence zone SPCZ and triggering two tropical lows.

The first of these, Low 1, hovered near Fiji since Thursday and has produced extreme rainfall over western Viti Levu. It is expected to drift S and SW during the coming week hovering near Northland on Tue to Thu then drifting west to Norfolk/Lord Howe area , being directed by upper winds around an upper Low.

The second is currently over Vanuatu and expected to go SE to southwest of Fiji on Monday and northeast of Kermadecs on Tuesday then turn to the south and fade away.

The near-equatorial near-gale NW belt on the north side of the SPCZ should turn into a zone of squally westerly quarter winds all the way from Coral Sea to Tonga/Southern cooks by the end of this week - Islanders in this region need to be aware this is the opposite of normal , and several safe anchorages will become unsafe.

Kiribati and Tuvalu should miss these westerlies and just get NE/N/NW winds.

The SPCZ stretches all the way from Solomons to Fiji to Southern Cooks and should move northwards this week to be from Solomons to Tuvalu/Samoa to Southern Cooks by Saturday 7 April with squally NW winds on its northern side in places.

Galapagos to Marquesas: Still some tropical showers in the way between Galapagos and 5S as far west as 100W , along with weak west to NW winds. These are slowly fading, however this week it may be better to avoid the direct or 'more southern' route and motor up to about 1 degree North and catch the equatorial current west wards to around 125 W then going direct. Those who have already set off will need to get south to 6 South to find some useful easterly wind for sailing.

SUBTROPICAL RIDGE STR
Large High east of Chathams tonight should wander northeastwards this week to 35S south of French Polynesia by Sat 7 April.
A new High, High2, should stretch across Bass Strait on Wed and move over southern Tasman Sea on Thu and southern NZ on Good Friday to be over Chatham Islands on Sat 7 April. The track of this high will help mould the track of Low1 into the Tasman Sea.

TASMAN SEA/NZ AREA
Weather is dominated by Low1 and High2 during this week and into Easter.
Easterlies for Easter for the North Island.

The easterly winds and frontal rain around Low 1 should have its greatest impact over the North island from Tue to Wed. The E to NE winds in the squash zone of High2 are likely to build on Good Friday and last until Easter Sunday, when a weak trough moving onto the South Island helps the isobars to spread out again.

Things may work out that Thursday is the day between wind blasts -- so it may be an OK day to start the annual Auckland to Tauranga Yacht Race. But Friday's likely increasing E/NE wind in Bay of Plenty may also be upsetting near the RENA reef, so race participants should arrange checks for latest updates.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback: bobmcd@xtra.co.nz, or bobmcdavitt@hotmail.com

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