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

25 August 2012

BOBGRAM issued 26 Aug 2012

WEATHERGRAM
YOTREPS
Issued 26 August 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 Ocean: Seas surface temperature anomalies in the equatorial mid Pacific have been increasing positive, and over much of the area they are now between 0.5 and 1C above. So it is trending towards an El Nino, but not there yet. Between Galapagos and South America a blob of cooler than normal water has welled up to the surface and is now slowly wandering westwards. Interesting.

The Atmosphere: The Southern Oscillation Index or SOI is steadying after a dip during past month. On 25 Aug its 30-day running mean was -1.01. So the atmosphere is now following the Ocean and trending El Nino, following the Ocean.

Tropical cyclones: ISAAC is at present attacking the tent city of 'quake refugees in Haiti and is likely to visit the Florida Keys later this week. In the NW Pacific we have TEMBIN still skirting around Taiwan, and BOLAVEN is heading to southwest end of Japan.

In the South Pacific, the Convergence Zone SPCZ continues to be fairly quiet. The part of the SPCZ over the Coral Sea seems to have shrunk to a zone over the Solomons. There is still a lot of convection between the Equator and 7S from 160E to 170W--- possibly due to the warmer than normal sea temperatures there. Another branch continues around Tokelau, and this seems to be lingering in place. A weak CZ is moving eastwards across the area around Niue and is likely to cross the Cooks on Monday UTC and fade over French Polynesia on Tuesday /Wednesday. In conjunction with this CZ a LOW is expected to form near 30S 155W/150W… this Low should quickly deepen and then move off to the ESE.
Anyone planning to go west from Tahiti this week should let that SPCZ pass by first.

A small CZ is expected to form over New Caledonia on Monday UTC. In conjunction with this CZ a low is expected to form near 30S 170-180E on Tuesday/Wednesday 28/29 Aug and this Low should then deepen a lot as it rolls off to the east southeast, expanding to dominate the weather in the Pacific next week to east of NZ.
These sub-tropical lows are able to feed off moisture from the SPCZ and energy in the nature of upper divergence from the subtropical Jetstream.

SUBTROPICAL RIDGE STR
The STR is mostly reverting to its Aug/Sep normal latitude of 30/35S this week. The High that crossed NZ late last week and is now just east of NZ is expected to slowly go north to 30S 165W by Wed 29 Aug and then should slowly make its way east. The High that is expected to cross the central Tasman Sea on Tuesday 28 Aug and the South island on Wed 29 Aug is likely to then be constrained to move east along 45S from Thurs 30 Aug to Sat 1 Sep.

NZ/Tasman Sea
A frontal system preceded by NW winds and followed by SW wind sis crossing NZ tonight/Monday. This system is at the intensity and latitude that is typical of spring, indicating that the Roaring 40s are now spreading onto NZ.
The next trough should start approaching NZ on Friday preceded by NW/N winds. This system is likely to form a low and then draw in a southerly change over NZ during Sat/Sun/Mon 1/2/3 Sep.

Traveling towards New Zealand this week:
In a simple roaring 40s scenario there is a rhythm in the weather with which you can arrange a reasonable sail to NZ. However this week there is a Low near 30S 170-180E on Wed/Thu 29/30 Aug and this poses a challenge. If you wish to approach NZ from the north this week, try and time your arrival day to be Sat 1 Sep.
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Now that we are at the end of August we can start forming ideas about the coming cyclone season. The SOI is hovering around -1, and such a low SOI is associated with an El Nino (we are not there yet but the trend is in place). During an El Nino the SPCZ tends to be shifted to north and east of its mean position , and this tends to encourage cyclones to form around or east of the dateline with fewer than normal near Australia.
A study of the number of Australian cyclones is available at http://www.aoml.noaa.gov/hrd/Landsea/oz/index.html. Using Figure 2 from this study and -1 for this year's value of the August SOI
we get an estimated count of 8 cyclones (for Australia) compared with the long term average of 11.

The Bureau advice at this stage on the cyclone season at http://www.bom.gov.au/wa/cyclone/seasonal/ is "The tropical cyclone season runs from 1 November to 30 April. The seasonal outlook for 2012/2013 will be issued on 15 October 2012."

I shall try and do some more research on this topic over the next few weeks?
See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at http://metbob.wordpress.com/
Weathergram text only http://weathergram.blogspot.co
Website http://www.metbob.com
Feedback to bob@metbob.com

19 August 2012

BOBGRAM issued 19 Aug 2012

WEATHERGRAM
YOTREPS
Issued 19 August 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 Ocean: Seas surface temperature anomalies around the Galapagos are now relaxing back towards normal, but warmer-than-normal sea are shifting westwards into the central pacific, indicating we are on the verge of a new El Nino episode, but we are NOT there yet.

The Atmosphere: The Southern Oscillation Index or SOI is now dipping quickly. Its 30-day running mean rose during July from -1.21 to +0.27 and dropped to -0.65 by 12th August and -1.08 by 18 August. So the atmosphere is now tending towards El Nino, following the Ocean.

Tropical cyclones: In the NW Pacific we have TEMBIN heading for Taiwan, but it looks mediocre compared with HAIKUI a few weeks ago.
In the Atlantic we have GORDON with an eye near 35N and hurricane winds over Azores. This system is breaking a few of the rules but it is likely to fade after Tues 21 Aug west of Portugal.


In the South Pacific, the Convergence Zone SPCZ continues to be fairly quiet. There has been a weak CZ over the eastern Coral Sea occasionally affecting Vanuatu and New Caledonia. This looks to be reforming over Solomons this week.

Another convergence zone CZ has been lingering between 3 and 10South from 170E to 160W and then tapering off to the SE well east of French Polynesia FP- unusually north and east of normal for the SPCZ even for El Nino conditions. Some welcome rain for Kiribati. This is making another good week for sailing from Tahiti to Tonga without being bothered by the SPCZ. However the trade winds on route may be fresh to strong around 15S on Monday and Tuesday 20nd 21 Aug, and again around FP on wed/Thu 29/30 Aug. There seems to be a link between El Nino and strong trade winds.


SUBTROPICAL RIDGE STR
The STR is sticking to its northern latitude position along 20 to 30S cross much of the South Pacific.
Even the High centre near 45S 140W tonight is moving north-eastwards towards more tropical latitudes so that isobar patterns are approaching climatology in both western and eastern parts of the South Pacific.

A High is expected to move along 47S to south of Tasmania on Monday 20 Aug and cross the South Tasman on Tie 21 Aug and South Island on Wed 22 Aug and then move along 45S to 160W by Sat 25 August. The accompanying high marking the STR should move along 30S into N Tasman Sea on Mon20 Aug and then crawl towards North Island and fade there by Thursday 23 Aug.

Another High marking the STR should move along 30S into the North Tasman Sea on Sat/Sun 25/26 Aug.
NZ/Tasman Sea
South of the SR we have the roaring 40s plus some more in what has been a series of Tasman Lows all during August. There is one crossing central NZ tonight and going steady eastwards, but leaving a trough in its wake. Its upper trough should cross North Island on Monday evening. This should mark the end of the series: The next Low is likely to fade in the Tasman Sea on Sat and Sun 25 and 26 Aug.

Traveling towards New Zealand this week:
If you can time it, then aim to arrive in NZ on late Thursday, during Friday, or early Saturday (before next weekend's trough arrives).
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Anyone with internet access can see some of my talk as guest speaker last Monday to Cruising and Navigation Association of NZ'S Annual dinner:
1. Barcelona: http://www.youtube.com/watch?v=dErqR-AVzDc
2. America's Cup: http://www.youtube.com/watch?v=NL3lBpHJKdo
3. The winning formula: http://www.youtube.com/watch?v=rCwTj8bdRwk
4. El Nino/La Nina: http://www.youtube.com/watch?v=440FjhIQcyk&feature=plcp
5. Spring Outlook: http://www.youtube.com/watch?v=7gKlpxXflx4
6. Questions: http://www.youtube.com/watch?v=zv9WqsWYvCo

Also here is a link to a letter to NATURE magazine about how climate change is making the SPCZ more erratic:
http://www.csiro.au/en/Portals/Media/Warming-causes-more-extreme-shifts.aspx
Note that Raro Met man Arona Ngari is one of teh co-autours (well done-- pity the yachties can't get to rarotonga this year because of harbour reclaimation).

In the "Weathergram with graphics" this week is a satellite photo of the volcanic eruption around 85 miles SW of Curtis Island in the Kermadecs on July 17/18; this was the origin of teh rafts of pumice between Tonga and NZ during last few weeks. the volcano has since gone quiet.

There is also a photo of the WAKA that departed Auckland on Friday heading for Easter Island, as last leg of Hector Busby's mission (http://tvnz.co.nz/national-news/waka-crew-use-stars-easter-island-voyage-5029651)

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics at http://metbob.wordpress.com/
Weathergram text only http://weathergram.blogspot.co
Website http://www.metbob.com
Feedback to bob@metbob.com

12 August 2012

BOBGRAM issued 12 Aug 2012

WEATHERGRAM
YOTREPS
Issued 12 August 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 Ocean: Seas surface temperature anomalies around the Galapagos continue to sneak above normal indicating we are on the verge of a new El Nino episode, but we are NOT there yet.

The Atmosphere: The Southern Oscillation Index or SOI is now dipping again. Its 30-day running mean rose during July from -1.21 to +0.27 and has dropped to -0.65 on 12th August. So the atmosphere is in neutral territory.

This dichotomy between atmosphere and ocean is leading to some unusual weather patterns.

Monsoon is entering the Western North pacific and continues to be more active than normal ,

Tropical cyclones: Things were tragic last week in the NW Pacific with HAIKUI, and there are some other disturbances starting up in the region. There is also a new tropical Low spawning in the Atlantic and there are two disturbances off west coast of Mexico. Still busy.


In the South Pacific, the Convergence Zone SPCZ continues to be fairly quiet. There has been a weak CZ from Solomons to the southeast and this occasionally extends to Vanuatu or sometimes Fiji depending on passing mid-latitude lows and jetstreams. Water-vapour satellite imagery shows Australia to be a source of dry air that goes eastwards, evaporating a lot of tropical features. The main Convergence Zone CZ seems to be located between 5 and 10S from 175E to 160W. This is abnormal and is leading to some great opportunities to sail west from French Polynesia towards Tonga without being molested by the SPCZ for those seeking an easy passage. There is no guarantee this scenario will be around by the end of the month, but it looks good to go this week.


Map for GFS model for Wed 15 Aug 1200UTC with features given in text.

SUBTROPICAL RIDGE STR
The STR is sticking to its northern latitude position along 20 to 30S and having a slowly intensifying time this week.
The Highs are expected to hug Australia until around Tue 21 Aug and after that one may venture into Tasman Sea.
One small cell should zip across the north Tasman sea on Thu 15/16 Aug and fade east of the dateline on Fri/sat/sun 17/18/19 Aug.

NZ/Tasman Sea
We continue with a procession of lows moving across the Pacific, without much opportunity for a high to fit in between them until mid-next week; this is a typical winter pattern.
The Low crossing NZ on Mon/Tue 13/14 Aug is likely to be followed by a disturbed westerly flow on Wed to Fri (15-16-17 Aug), then a prefrontal northwest flow on Sat 18 August and another low on Sun/Mon 19/20 Aug.

Traveling towards New Zealand this week:
If you can time it, then that Wed to Sat gap is the best time this week to approach NZ.

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics at http://metbob.wordpress.com/
Weathergram text only at http://weathergram.blogspot.co.nz
Website http://www.metbob.com
Feedback to bob@metbob.com

05 August 2012

BOBGRAM issued 5 Aug 2012

WEATHERGRAM
YOTREPS
Issued 05 August 2012
Bob McDavitt's ideas for sailing around the South Pacific.
If you have web access the go to http://metbob.wordpress.com/ to read this blog with images.
(Might not be working this week).
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 Ocean: Seas surface temperature anomalies around the Galapagos have been sneaking progressively above normal for the past month indicating that we are on the verge of a new El Nino episode, but we are NOT anywhere near there yet.

The Atmosphere: The Southern Oscillation Index or SOI is now dipping again. Its 30-day running mean rose during July from -1.21 to +0.27 and has dropped to D TO -0.15 by 5th August. So the atmosphere is in neutral territory.

Monsoon is entering the Western North pacific and is currently more active than normal as measured at http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html

Tropical cyclones: as the saying goes, August, a must. And indeed it will be a busy week in the North Atlantic with ERNESTO in the Caribbean and FLORENCE in the wins. Meanwhile HAIKUI is hovering near south of Japan and there is another tropical low in mid north pacific waiting in the wings.

Map for GFS model for Wed 8 Aug 1200UTC with features decode given in text.

In the South Pacific, the Convergence Zone SPCZ continues to be fairly quiet. There has been a weak CZ from Solomons to the southeast and this is crossing Fiji tonight, but it is also evaporating because a large area of dry air that has been sent eastwards from out of Australia has caught up with it. Another CZ lies from Tuvalu to Tokelau along 10S, with occasional squalls extending east wards to Marquesas.

This week the CZ near and south of Fiji is expected to blossom into a trough on Wed 8 Aug that is likely to0 then fade over Tonga on Thursday. That's about all.

SUBTROPICAL RIDGE STR
The STR is in its northern latitude along 25 to 30S and having a weak time this week.
Over Australia the highs are assisted from aloft (probably by extra sinking air thanks to being downstream from the Monsoon) and one should expand on over eastern Oz on Mon/Tue 6/7 Aug and the next should get diverted across Tasmania on Sun 12 Aug.
Further east: One cell is likely to form near 25S 170W on Tuesday 7 Aug and then move East-southeast. Another should form in the same place around Friday and should take a more easterly path High east of NZ near 170W tonight is expected to reform a cell of high pressure between 20 and 30S at 160W on Monday. This new cell should go east and fade away on Tuesday and Wednesday.


NZ/Tasman Sea
We are getting a procession of lows moving across the Pacific, without much opportunity for a high to fit in between them; this is a typical winter pattern, but will probably only last for a few weeks- the pattern started last week.
Last week's Tasman low is now east of NZ and should get taken southwards across Chathams Islands on Monday. The low which is tonight near south of Tasmania should cross northern NZ on Wed 8 Aug and then rush off the southeast. The trough which is crossing Tasmania on Wed is expected to deepen to around 980 hPa and stall in the Tasman Sea on Thu/Fri/Sat/Sun 9/10/11/12 Aug in the Tasman Sea. Avoid

Traveling towards New Zealand this week:
Only Thursday offers good weather for arrival.

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

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