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

24 September 2011

BOBGRAM issued 25 Sep 2011

WEATHERGRAM

YOTREPS

Issued 25 September 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 is still at peak cyclone season, with HILARY west of Mexico, OPHELIA and PHILIPPE in Atlantic and NESAT about to cross northern Philippines.

 

The tropical ocean temperatures in mid pacific continue to be near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos---so much that it has exceeded USA's Climate prediction centre CPC's threshold and they have called it a new La Nina.  It is sort of neutral with bursts of La Nina at times.

 

From the atmosphere, the Southern Oscillation Index SOI is positive and increasing: its 30day running mean was 0.44 in 17 Sep and 0.75 on 21 Sep.  Some lingering La Nina weather patterns persist in the atmosphere, and the Subtropical ridge STR in the Australia/Tasman Sea area is now being taken south of its normal position.

 

The South Pacific Convergence Zone SPCZ is persisting from Solomons to Fiji then southeast across Tonga and to south of Southern Cooks.  The section across northern Vanuatu/Fiji/Tonga is weak at present but is likely to activate from Thursday 29 Sep to Sunday 2 Oct.

 

SUBTROPICAL RIDGE STR

The STR seems to be south of its norm position of around 25S.  That High near 40S in the South Tasman Sea tonight is expected to travel northeastwards across NZ this week and may be located near 30S 170W by wed 28 Sep and then wander southeast to 38S 130W in early October.  Its intensity should stay below 1030hpa so that the trade winds on its northern side are likely to be enhanced a little but stay less than strong.

A large area of heavy SW swell associated with a low from the southern ocean is expected to reach NZ south coast by Wed 28 Sep and then move into the eastern flank of the High in early Oct – so avoid the centre of this high as it will have the combination of light winds and heavy swells – no good even for a power boat.

 

TASMAN SEA/NZ AREA

There is a developing Low between Sydney and Lord Howe Island tonight feeding off the jetstream aloft and the east Australian current from beneath. It should  move east along 30S until wed 28 Sep and then when it gets knocked a little to north of Norfolk Island,  but may redevelop near and to east of Kermadecs on Fri/Sat 30 Sep/1 Oct.   This low has strong to gale easterly winds on its south side, worth avoiding.

After that high crosses central an northern New Zealand on Tuesday 27 Sep, a North to NW flow is expected over the country for the remainder of the week : down-slope winds may be a problem for eastern coastlines, with temperatures foehning up into the 20s.

 

Looking further ahead a Low from the southern ocean is likely to reach Southland by Sunday 2 Oct, turning the flow over NZ to a squally westerly, then from Mon 3 to Wed 5 Oct next week, this low may deepen east of NZ,  bringing a polar chilled southerly outbreak to NZ eastern coastlines. Avoid.

 

 

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.

 

On Mon 3 Oct to Wed 5 Oct a polar chilled flow hits Northland with W then SW then S winds, so don't plan on departing this week unless you are quick enough to reach Northland by end of Sun 2 Oct – and if you come south from New Caledonia, then also watch the progress of that low wandering along 30S.   

 

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

18 September 2011

BOBGRAM issued 18 Sep 2011

WEATHERGRAM

YOTREPS

Issued 18 September 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.

 

North Atlantic, still boisterous with cyclones ROKE and SONCA – they seem to be staying offshore.

 

The tropical ocean temperatures in mid-pacific are near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos--- so much that it has exceeded USA's Climate prediction centre CPC's threshold and they have called it a new La Nina.  Over the entire Pacific, it is sort of neutral with bursts of La Nina at times.

 

From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.49 on 10 Sep and 0.44 in 17 Sep.   There has been some lingering La Nina weather patterns in the atmosphere last month, but the sub tropical ridge STR is now near its normal position. 

 

In fact: the South Pacific this week seems to be a continuation of last week- with trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S—something like the seasonal norm, and this week's disturbed westerlies are looking to be quieter than last week's.

 

SUBTROPICAL RIDGE STR

A steady STR staying along about 25S.   STR in this position should maintain fresh to strong SE winds in the Coral Sea. 

One weak high cell should cross the North Tasman Sea on Monday 19 Sep and North Island on Tuesday 20 Sep then weaken away.  Another high cell is likely to weaken away in north Tasman Sea on Thursday and Friday 22/23 Sep.  Next high should advance across Tasmania on Sat 24 Sep, Tasman Sea on Sun 25 Sep and North Island on Mon 26 Sep.  These systems are not penetrating past the dateline at this stage.

 

TASMAN SEA/NZ AREA

Disturbed westerly flow in Tasman Sea latitudes is a good sign--- it is caused by the warmth of the extra sunlight in the Southern Hemisphere getting further south.  The stronger temperature difference between tropics and polar regions is what feeds these westerlies.  The equinox is around Friday 23 Sep, so this is THE week of the year when we have the strongest temperature difference.

 

Last week the disturbances were severe at times… hail in Wellington, a tornado in Auckland and, today,  more hail in Auckland and Bay of Plenty.  It seems that the coming week should be slightly quieter. The main disturbances should cross NZ on Mon 19 Sep (followed by cold air over southern districts), Thu/Fri 22/23 Sep and Sat/Sun 24/25 Sep (with more cold air in the south).

 

 The timing of these sequences may still change and anyone following the Rugby games in NZ can catch an update on the weather forecasts for the games from

http://www.metservice.com/towns-cities/rugby-world-cup-2011

 

SAILING TO/FROM NORTHERN NZ.

If you are planning to sail from Tonga/Fiji./Vanuatu/ New Caledonia to NZ, then the next few months is the optimum time.  You can join the ICA All Points Rally, see http://www.islandcruising.co.nz/, for this voyage.  My suggestion is that you try and meet a front at 30S – that way you will avoid meeting a front upon arrival in Northland. -

 

I'm just back this evening from a great Boat Show in Auckland.

Great new site-- thanks to organizers, and to Auckland City for helping sorting the parking!

 

The terms used are as 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

10 September 2011

BOBGRAM issued 11 Sep 2011

WEATHERGRAM
YOTREPS
Issued 11 September 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.

Three cyclones (KATIA, MARIA and NATE) in the North Atlantic and some areas of interest to wets of Mexico…so Northern Hemisphere Cyclone season is still raging.

The South Pacific this week offers trade winds in the tropics, disturbed westerly winds to south of 30S and the subtropical ridge STR sitting mainly along 25S. The daily maps look reasonably close to seasonal norm, but just for this week.

The tropical ocean temperatures in mid pacific are near normal, however there are signs of a growing pool of cooler-than-normal sea near Galapagos. This has prompted USA's Climate prediction centre to call this a new La Nina, but Australian and NZ climatologists are happy enough to call it neutral - let's compromise with neutral + bursts of La Nina at times.

From the atmosphere, the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.46 on 2 Sep and .49 on 10 Sep. There have been some lingering La Nina weather patterns in the atmosphere--- the STR was further south than normal earlier this month, but is near its normal latitude this week.


SUBTROPICAL RIDGE STR
No large highs this week in our area, just a weak STR along about 25S. Next high should be moving off Australia and along 30S into northern Tasman Sea from Thu 15 Sep to crossing NZ on 22/23 Sep. This provides a good window for sailing westwards across the north Tasman or eastwards across the mid Tasman Sea.

Rising pressures this week over eastern Australia are helping to enhance strong SE winds in the Coral Sea.

TASMAN SEA/NZ AREA
The Low that crossed southern NZ today in followed by a period of squally SW wind and heavy swells in the Tasman/NZ area until at least Wednesday 14 Sep, and these then spread eastwards of NZ as a disturbed westerly flow. Avoid.

Another trough is likely to deepen in the central Tasman Sea on Friday 16/ Sat 17 Sep and cross North island on Sat 17/ Sun 18 Sep then move off to east of NZ - but this feature is still somewhat uncertain, so if it concerns you then get updates on it .

SAILING TO/FROM NORTHERN NZ.
The Island Cruising Association ICA fleet are starting to think about their All Points Rally from Tonga/Fiji/Vanuatu/New Caledonia to Opua. Well, my idea about the best time to leave from any of these places is so that you encounter a front at 30S – one that is followed by a mediocre ridge. The reasoning is that this voyage takes around a week and fronts in this part of the world have a frequency of at least once a week, sometime more frequent. The disturbed westerlies over Northland this week do not offer much opportunity for this pattern. So let's just enjoy another week of tropical bliss this week, and check in again next Sunday/Monday.

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

04 September 2011

BOBGRAM issued 4 Sep 2011

WEATHERGRAM
YOTREPS
Issued 4 September 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.

We are now moving into the month where daylight increases fastest in the Southern hemisphere ---It breaks out onto the Antarctic circle at around the equinox – that's the Antarctic dawn, so its their coldest time of the year. Since the westerly winds of the roaring 40s get their energy from the temperature difference between the tropics and the polar regions, these winds are normally at their strongest and furthest north at this time of the year--- some call these equinoctial gales—I like to think of them as the gales of the Antarctic dawn.

This year things are not quite normal. The tropical ocean temperatures are near normal so the ENSO is in neutral and not having much impact at this stage. However the Southern Oscillation Index SOI is hovering slightly above zero: its 30day running mean was 0.38 on 28 Aug and 0.46 on 2 Sep. This has been producing some lingering La Nina weather patterns in the atmosphere--- and one which has been happening in the South Pacific has taken the current high of the subtropical ridge south of "normal", weakening the roaring 40s.
HOWEVER, it now looks as though the South West Pacific may be in for a period of roaring 40s weather next week and possibly the following week as well.

4 Cyclones are raging in the Northern hemisphere… In NW Pacific TALAS is weakening over Japan and a new one (currently called TC SIXTEEN) is waiting in the winds. Around USA, Southern Louisiana seems to be coping OK with LEE whilst KATIA awaits offshore (KATIA is the replacement name for the now retired name KATRINA).

In our tropics, the South Pacific Convergence Zone got weak over the past week. There are clusters of convection north of Solomons, just east of Samoa and around Northern cooks but northing organised. The main convection that went from southeast of New Caledonia to southeast of Fiji (where it is tonight) is fed by a jet stream rather than convergence--- and will bring squally rain to southern Tonga until late Monday UTC (avoid). This system should continue heading east southeast and bring variable winds of a trough over Southern Cooks on Tue 6 Sep UTC.

SUBTROPICAL RIDGE STR
The current HIGH making its way eastwards across NZ is somewhat south of the normal STR September position, so kiwis should enjoy its light winds whilst they can. Meanwhile it is likely to produce strong trade winds near 20S all the way from about 160W to 180 until Wed 7Sep. Fiji Met Service have a gale warning is place on this squash zone.

Next high is expected to move east off Australia next week , around Tue 13 Sep , and along 25 to 30S into the North Tasman Sea This is the more normal latitude for the STR in September and more indicative of spring and more encouraging to the roaring 4os pattern.

TASMAN SEA/NZ AREA
Between every two highs there is a trough, and in fact there will be a run of troughs across NZ after the High moves off on Thu 8Sep. Firstly a small low may form between New Caledonia and Northland on Friday 9 Sep and then move southeast brushing past NZ's northeastern areas on Sat 10 Sep. This just marks where a trailing upper trough gets moistened by an upper jet - not expected to bring much.

The next trough should be more substantial and should build into a well-wound up low off Sydney by late Friday 89 Sep, and followed by gale force south to southwest winds in the Tasman Sea for Sat/Sun /Mon 10/11/12 Sep, along with strong wet mild nortehrlies over NZ on Sun 11 Sep. Avoid.

Considering the location of the next high being to north of most of NZ, it seems that roaring 40s or disturbed westerlies are likely over NZ during the week Mon-Fri 12 to 16th Sep--- This may change , so read in again next week.

SAILING TO/FROM NORTHERN NZ.
The passing high makes this an OK week to depart or arrive, but NE winds ahead of approaching trough rules out a good departure from Thu to Fri 8 0r 9 Sep.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com
Feedback to bobmcd@xtra.co.nz

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