Translator

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

26 May 2013

BOBGRAM issued 26 May 2013

WEATHERGRAM
YOTREPS
Issued 26 May 2013
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.

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardised difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number.
SOI rose to over plus 1 briefly in early April and dropped to minus 0.39 in early May and bounced back into the positive in May, was plus 0.53 on 26 May. So the signal coming from the Atmosphere continues to be erratic.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.
Over the past few months conditions have been near average, and in the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is possibly a sign of a trend towards La Nina conditions.

ITCZ and Monsoon
ITCZ is strong over eastern Pacific and weakening over western Pacific. Not much sign of monsoon over southern India but active convection for Burma/Myanmar .

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is expected to move south this week, reaching Fiji by Wednesday and maybe developing a low over Tonga by Sat 1 June. Avoid.
There is a left over convergence zone CZ along 10S between Samoa and half-way-between-Marquesas-and Tahiti. Rather inconvenient.

Sub-tropical Ridge STR
The STR straddles 30S this week. The High east of NZ is expected to travel along 30S and the High west of NZ should travel along 33/35S, crossing NZ on Thursday to Sat 30May to 1 June. There should be a squash zone of enhanced SE winds over New Caledonia and Fiji mid-week between the SPCZ and STR. Avoid.

Roaring 40s and New Zealand
Southerly outbreak AND large swells from southern ocean are expected to travel NE across NE on Monday and reach as far north as Tonga by Tuesday. Avoid.
Next front is expected to reach Tasmania on Fri 31 may and then NZ by Sun/Mon 2/3 June. Avoid.

Route weather picks
Galapagos to Marquesas: Good to go this week. Light to moderate South to southeast winds over Galapagos and moderate trade winds trade winds from 2S onwards, but light easterlies and a nearby CZ around Marquesas.

NZ to the Tropics.
Wait for the polar blast to get across NZ on Monday and past Tonga by Tuesday.
Wednesday looks to be best day to depart.
Light winds on Thursday and Friday and a strong to gale northerly flow on Saturday and Sunday close the window of good weather quickly.

See my yotpak at http://lnk.ie/FVHJ/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is http://lnk.ie/FVHK/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/FVHL/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/FVHM/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

19 May 2013

BOBGRAM issued 19 May 2013

WEATHERGRAM

YOTREPS

Issued 19 May 2013

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.

Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardised difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number.

SOI rose to over plus 1 briefly in early April and dropped to minus 0.39 in early May and bounced to plus 0.6 by 18 May. So the signal coming from the Atmosphere continues to be erratic.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.

Over the past few months conditions have been near average, and in the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is possibly a sign of a trend towards La Nina conditions.

ITCZ

The 5-day rain map ending 13 May shows some new life in the Intertropical Convergence Zone ITCZ. However, the ITCZ is staying slightly south of the Marshall Islands and so their long term drought continues.


WEATHER ZONES

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ is weak and spread out at present, but there is an active branch between Tuvalu and French Polynesia. It is expected to drift south later this week and reach Fiji from Tuesday 28 May.

A small convergence zone CZ lies between Fiji and New Zealand tonight and is expected to cross Minerva on Tuesday and then ease away as it travels east/southeast.

Sub-tropical Ridge STR

This is rather weak at present. There is a High centre east of 180 in the normal winter latitude near 30S. This holds a weak and tenuous link to another high which is moving east along 45S from the Australian bight towards NZ. This High is preceded by a southerly flow that is shovelling cold air northwards. The southerly should reach southern NZ by Wednesday and the high is expected to cross the South Island on Thursday and the North Island on Friday.

Roaring 40s and New Zealand

Low1 in the Tasman Sea is expected to cross northern NZ on Wednesday. Low 2 is expected to move off Australia east coast on Thursday or Friday and then over northern NZ on Sunday 26th May followed by a strong SW flow.

Route weather picks

Panama to Galapagos: Not a good week for this voyage. S to SW winds to south of 8N are getting persistent and may be up to 20 kt. Intertropical Convergence Zone ITCZ quite active between 6 and 2N and acting like a sort of barrier.

Galapagos to Marquesas: Good to go this week. Light southerly winds over Galapagos and moderate trade winds trade winds from 2S onwards. To get the most from wind and surface current head off for 5S110W and follow current to 5S124W and then go direct to Marquesas. There may be some tropical showers between 5S and 8S.

NZ to the Tropics.

Departure on Monday is likely to be in light winds then fresh NW winds for a few days.
Tuesday and Wednesday departures are likely to get strong NW winds. No good.
On Thursday, strong southerlies are expected. No good.
Friday Sat Sun Monday departures are likely to be affected by winds from L2. No good.

So the next day with possible good enough weather for departure is Tuesday 28 May. Perhaps. More next week.

See my yotpak at http://lnk.ie/FO2L/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is http://lnk.ie/FO2M/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com
Weathergram text only and translator is http://lnk.ie/FO2N/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz
Website http://lnk.ie/FO2O/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com
Feedback to bob@metbob.com

12 May 2013

BOBGRAM issued 12 May 2013

YOTREPS

Issued 12 May 2013

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.

Some sailors have asked me recently how good are the Grib files they are downloading. Well, most of you are looking at data gleaned from the GFS model, and it's good, but not the best. A study done in 2012 shows ECMWF is usually the best. Grib data for EC is not easily available but when you have Internet you can check its latest for the next 10 days for http://lnk.ie/FGBJ/e=bobmcd1.bobgram@blogger.com/http://bit.ly.ecoz

Second best is the UK model, again its Grib data is not easily available, but MetService pick the model of the day to feed their regional model , and it is usually EC or UK that they pick, and then it is further processed to give maps at http://lnk.ie/FGBK/e=bobmcd1.bobgram@blogger.com/http://bit.ly.7daywx

So check these web sites when planning your trip (and add http://lnk.ie/FGBL/e=bobmcd1.bobgram@blogger.com/http://www.swellmap.com to look at the likely swells). And if your Grib data agrees with these then all is good, but if not then you need to consider multiple scenarios.


Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardized difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number.

SOI rose to over plus 1 briefly in early April but has since dropped to minus 0.39. So the signal coming from the Atmosphere has been erratic.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.

Over the past few months conditions have been near average, except for some cooling near South America. So the signal coming from the Ocean is mostly neutral.

The Monsoon : The annual arrival of the monsoon over India usually sets in late in May and can set up weather which has downstream ramification going west (Sahel drought/ easterly waves for TC in the Atlantic) and east (MJO pulses from Indian Ocean to Pacific ocean triggering TC action in NW Pacific). At this time of the year mountaineers are watching jet streams and Monsoon indices to calculate the :"Everest climbing window"- it only lasts around 10 to 20 days so getting it right counts a lot.

This year there is talk of a summit climbing window around May 18 to 21. This is a few days early , and the monsoon index at http://lnk.ie/FGBM/e=bobmcd1.bobgram@blogger.com/http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html indicates things are happening earlier than normal , but monsoon hasn't reached south India yet (usually arrives first over Sri Lanka around 25 May). Interesting to see an early tropical cyclone, MAHASEN, in the Bay of Bengal.

WEATHER ZONES
South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ has eased in activity a lot since last week. There is some activity between New Caledonia and Fiji tonight and this sis expected to result in the formation of a Low to SW of Fiji on Monday, L1, and this is then expected to move off to the SE.

Left behind are some discrete convergence areas like beads on a necklace mainly along 10 to 15S from Papua New Guinea to Marquesas.

Sub-tropical Ridge STR

This is generally in its winter position along 30S except for Tasman Sea.NZ latitudes where a high that is now over eastern Tasman Sea is expected to move only slowly over central NZ by Wednesday and then fade east of NZ on Thursday.

Roaring 40s and New Zealand

Main event is another Low L2. It is expected to move east across Tasmania on Monday and Tuesday, the Tasman Sea on Wed and Thu, then central and southern NZ on Friday and Saturday. It is likely to have gales near its rather complex centre and generate large swells in the Tasman Sea on Friday and Saturday. Avoid.

Route weather picks

Panama to Galapagos: A reasonable N or NW breeze to depart with from now until Thursday 16 UTC. Not a good week for the voyage overall— S to SW winds to south of 5N are getting persistent and may be over 15kt. Intertropical Convergence Zone ITCZ quite active between 2 and 5N and acting like a sort of barrier.

Galapagos to Marquesas: Good to go with light southerly winds over Galapagos and moderate trade winds trade winds from 2S onwards. To get the most from wind and surface current head off for 5S110W and follow current to 5S124W and then go direct to Marquesas.

NZ to the Tropics.

Departure son Monday and Tuesday should start with light winds and get some wind assistance around the west side of L1 but then encounter light winds north of 30S and finally get to trade winds around 20S. Wednesday departures may start in light winds but encounter fresh to strong NW winds on the way as L2 approaches NZ. Thursday Friday and Saturday, and maybe also Sunday are likely to be adversely affected by L2, with strong and squally W/NW winds ahead of a front that may not clear Northland until early next week, followed by a day or so of strong SW winds.

See my yotpak at http://lnk.ie/FGBN/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is http://lnk.ie/FGBO/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com

Weathergram text only and translator is http://lnk.ie/FGBP/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz

Website http://lnk.ie/FGBQ/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com

Feedback to bob@metbob.com

05 May 2013

BOBGRAM issued 5 May 2013

WEATHERGRAM

YOTREPS

Issued 5 May 2013

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 Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the standardized difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI rose to over plus 1 briefly in early April but has since dropped to minus 0.24 on 5 May. So the signal coming from the Atmosphere has been erratic.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.

Over the past few months conditions have been near average, except for some cooling near South America. So the signal coming from the Ocean is mostly neutral.

Weather on Route:
Panama to Galapagos: Locally around Panama the best day this week to depart is Wednesday 8 May –along with some northerly winds. These will not last long but should be good enough to get away OK. The International Convergence Zone ITCZ is weaker than last week and mainly located between 8N and 2N and mainly to west of 80W.

Galapagos to Marquesas: Good trade winds over Galapagos between now and 11 May, so this is a good time to go. To get the most from wind and surface current head off for 6S110W and follow current to 7S124W and then go direct to Marquesas.

WEATHER ZONES
South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ was very active and broad this time last week, but much of its activity has been taken away by a tropical low that deepened over Tonga last Monday and then moved off first south then southeast into the mid-latitudes. The path taken by this low has upset several yachts heading east from NZ to French Polynesia last week. Some have been able to use the westerly winds on the north side of this Low to help them east. There is still some convection of this branch of the SPCZ over French Polynesia.

New SPCZ is now near its weakest and well north mainly between 10 and 12.5S from Solomons to 175E. It is expected to gradually shift south and may spread onto Fiji from Thursday 9 May and may form a tropical Low south of Fiji from Monday 13 May.

Sub-tropical Ridge STR
The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is well north of its normal position in the eastern Pacific (south of French Polynesia) and this is helping to enhance strong trade winds over the Austral group.

However the STR is at its more normal latitude over the Australia/NZ area. A new high is expected to bloom in the Tasman Sea after Wednesday 8 May and may spread over NZ during the 11/12 May weekend. The STR seems to be doing a good job in keeping apart the polar chilled southern ocean air from the tropically warmed air, but for a few small excursions.

Roaring 40s and New Zealand
An active trough and its Low L1 is crossing the North Island on Monday and moving off to the E/SE on Tuesday, but weather is complicated by the likely formation of a secondary L2, which may cross northern NZ on Wednesday.

NZ to the Tropics.

It's likely to be squally with L1 over northern NZ on Monday and start of Tuesday. Winds should then briefly ease but then things get complicated by the likely passage of L2.

L2 may clear off NZ on Thursday or Friday and that could then make a good weather pattern with a settling southerly flow for anyone looking to sail away from northern NZ. Too early to tell at this stage what may happen during the remainder of such voyages.

See my yotpak at http://lnk.ie/F97V/e=bobmcd1.bobgram@blogger.com/http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is http://lnk.ie/F97W/e=bobmcd1.bobgram@blogger.com/http://metbob.wordpress.com

Weathergram text only and translator is http://lnk.ie/F97X/e=bobmcd1.bobgram@blogger.com/http://weathergram.blogspot.co.nz

Website http://lnk.ie/F97Y/e=bobmcd1.bobgram@blogger.com/http://www.metbob.com

Feedback to bob@metbob.com

Blog Archive