Translator

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

24 June 2012

BOBGRAM issued 24 June 2012

WEATHERGRAM
YOTREPS
Issued 24 June 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 been steadily dropping into the negative during June. It has fallen from -0.14 on 27 May to -0.96 on 24 June.

Indian Monsoon is about a week behind its normal position at present, and seems to have reached its peak intensity which is near normal. It is taking a while to penetrate NW India.

In the South Pacific, the Convergence Zone SPCZ is slowly rebuilding in the north after a few weak weeks. It is active across Solomons to Tuvalu and Tokelau then weak over the Cooks. Several lines of convergence are lingering around French Polynesia FP. There is a slab of very dry air over New Caledonia to Samoa with its heart over Fiji. The SPCZ is expected to start heading south this week so that the dry slab should slowly fade.

There is a Low developing south of the Cooks near 25S 160W by Monday and this is feeding off some of those convergence lines over French Polynesia, and slackening the breeze over the Cooks. It should wander off to the south-southeast on Wednesday and Thursday allowing a southerly wind to clear conditions over FP and SE trades to return to the Cooks.

As the SPCZ drifts south into the Coral Sea, a Low may form there on Wednesday and Thursday, mature on Friday and then move south of Saturday and southeast on Sunday when it may weaken over New Caledonia. Avoid this low when it is mature.

SUBTROPICAL RIDGE STR
Large High between 35 and 40S to south of FP is the one that left NZ last week and it continues to move east this week. It has expanded so that its central pressure in now over 1036, and it is expected to hold this intensity until Wednesday and then start to fade.

Next high is likely to move from central Australia into the Tasman Sea at around 35S on Tuesday/Wednesday and this should travel east across northern NZ on Friday and then continue further east.

NZ /Tasman Sea troughs
Between those two Highs a series of fronts are crossing the NZ area. One of these did so over the last two days, greasing the way for others to follow. The front that is expected to cross NZ on Tuesday should usher some polar chilled into the Tasman Sea, producing a rapidly deepening Low that is expected to drop to below 960hPa just east of Campbell Island, making severe gales over Southern NZ. The SW winds that follow the accompanying front are likely to bring heavy SW to Southerly swells all the way to 20S between New Caledonia and Tonga on Thursday and Friday. This will be long period swell, so slope may not be much of problem expect around the Lord Howe Island area where there may well be a squash zone of enhanced winds between the Tasman High and Coral Sea Low. Avoid that.
Another trough is expected to cross Tasman Sea on Sat 30 June and then NZ on Sunday 1 July; this front may be thundery and is likely to be followed by strong showery SW winds on Monday 2 July.

Traveling between NZ and the tropics:
Best days to depart from northern NZ this week are Monday and Thursday.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

17 June 2012

BOBGRAM issued 17 June 2012

WEATHERGRAM
YOTREPS
Issued 17 June 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 becoming more negative. Its 30-day running mean was near zero at the start of June and was -0.68 on 17 June.
Indian Monsoon is about a week behind its normal position at present, but seems to be of normal intensity. It is taking a while to penetrate the Indian subcontinent interior. This may have ramifications downstream later this year.
In the South Pacific, the Convergence Zone SPCZ is active from north of the Solomons to Fiji to south of Tonga and south of Southern Cooks, with another band occasionally across the Vanuatu/New Caledonia area. A mid-latitude low dumped rain along 30S across the Tasman Sea. Conditions have been drier than normal elsewhere east of 180.
This week part of the SPCZ may move south over Fiji on Thursday 21 June, and the remainder of the SPCZ is likely to stay quasi-stationary. Early next week, the SPCZ may shift south across Coral Sea along with a drop in pressure.
SUBTROPICAL RIDGE STR
There is a narrow ridge over NZ tonight and this should expand and grow as it moves off to the east this week. A corresponding enhancement in the trade winds north of this high-that is , a squash zone- is likely over Samoa Tonga Niue on Monday to Wednesday and the Cooks then French Polynesia on Wednesday to Saturday. The squash zone may make diving conditions hazardous.
High over the interior of Australia is likely to bud off a ridge across the northern Tasman Sea along 30S from Thursday, fading to north of New Zealand by Sat/Sunday. This High is tracking north of the normal latitude-and that's an El Nino characteristic.

NZ /Tasman Sea troughs.
Welcome to the week with the least daylight - the winter solstice.
A Low and its associated front is set to cross NZ on Monday/Tuesday. A secondary Low is expected to cross NZ on Wednesday and Thursday after a brief break. There should be another break in the weather- good for departing off to the north – on late Friday and Saturday. Then another front is likely, with NW winds preceding it late Saturday and a good SW flow following it on Sunday.
Departing from NZ for the tropics:
It is probably a good idea to let the Lows go first and wait for a good SW. There is a brief window of suitable SW winds late Friday, and then another better looking one on Sunday 24 June.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback or requests for voyage forecasts to bob@metbob.com

10 June 2012

BOBGRAM issued 10 June 2012

WEATHERGRAM
YOTREPS
Issued 10 June 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 becoming more negative. It was near zero at -0.14 on 27 May and was -0.56 on 10 June.
Indian Monsoon seems to be weakening at present. It is on time for India's west coast but behind time along their East Coast.
In the South Pacific, the Convergence Zone SPCZ is active across Solomons to Coral Sea and northern Vanuatu, and a second branch is sitting from Northern Tonga to Southern and central French Polynesia.
A new LOW is rapidly deepening off the Brisbane coast this evening and has already dropped below 1000hPa, feeding off a jet stream and the East Australian Current EAC. This Low is expected to drop to 987hpa by late Tues UTC (early Wednesday local time) between Lord Howe and Noumea. Avoid. It should then gradually weaken and slowly pick up speed as it travels east then southeast passing by Kermadecs at around 990 hPa on Friday.
SUBTROPICAL RIDGE STR
A weak high cell is likely to form at around 27S between NZ and Fiji on Monday and this should then wander east for remainder of the week.
The High over the Tasmania area tonight is expected to hover over southern Tasman Sea this week and cross the South Island on Sat 16 June.
NZ /Tasman Sea troughs
The High stuck in the south Tasman Sea is working in concert with a large area of low pressure east and southeast of NZ to shovel polar chilled air onto the eastern coast of NZ, in a series of fronts, between Monday and Wednesday, with each colder than the previous. Swell from the south should peak around Monday.
Departing from NZ for the tropics:
It is probably a good idea to let that LOW go first. So, at this stage, Friday or Saturday 15/16 June may be Ok days for departure.
The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

03 June 2012

BOBGRAM issued 3 June 2012

WEATHERGRAM
YOTREPS
Issued 3 June 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 relaxed to near zero. It fell to -0.14 on 27 May and was -0.1 on 3 June.

Indian Monsoon seems to be running on schedule and is lapping onto southern India this week.

In the South Pacific, the Convergence Zone SPCZ is active across Solomons to near Tuvalu, and a second branch is sitting from East of Samoa to around the Southern Cooks. On around Tue 5 June a Low is expected to form on the end of the convergence zone to southeast of the Southern Cooks. This low should then move southeast to the south of French Polynesia. This should drag the SPCZ across French Polynesia and turn the winds there from NE to light-and-squally and then to moderate SE. Anyone doing the Marquesas-to-Tuamotu trip should take this into account.
The western branch of the SPCZ is likely to move south across Vanuatu during Sat/ Sun 9 / 10 June and onto Fiji on 11 June.


SUBTROPICAL RIDGE STR
High that is east of the North Island tonight should wander east and northeast and end the week south of the cooks at around 30S.
Next high is likely to move for Aussie Bight onto New South Wales on Sat 9 June.

NZ /Tasman Sea troughs
Between those two Highs there is room for a trough to open outwards across the Tasman Sea and for a series of lows in the Southern Ocean to extend polar-chilled winds and bursts of swell onto the Tasman/NZ area.

Front is expected to cross the Northland area on Tuesday and a secondary Low that forms on it should cross central NZ.
A stronger front, marking the apex of the upper trough, should cross the area north of New Zealand on Wednesday.
One burst of polar-chilled SW wind and heavy swell from the Southern Ocean is likely to reach southern NZ on Tuesday and another on Thursday, hanging around until Saturday. Avoid.
The primary Low, now deepening off Sydney, is expected to go NE – it is caught on the west side of the upper trough and so has SW winds as its steering field. It should go all the way NE to New Caledonia by the weekend, but weaken into a slack zone after Thursday.

Departing from NZ for the tropics:
Probably a good idea to let those fronts go first. Wind and weather should settle during Thursday and so Friday or later may be the best departure days this week.

The terms used are more fully explained in the METSERVICE Yacht Pack.
More info at http://weathergram.blogspot.com and http://www.metbob.com
Feedback to bob@metbob.com

Blog Archive