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

30 June 2019

Bob Blog 30 June

WEATHERGRAM

YOTREPS

 

Compiled Sun 30 June 2019

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.

 

JUNE REVIEW

Sea Surface temperature anomalies as at end of April may be seen at www.weatherzone.com.au/climate/indicator_sst.jsp?lt=global&lc=global&c=ssta

 

The main pattern changes are a return to near normal across the central and south Pacific, but still slightly warmer than normal around NZ and Tasman Sea. Similar easing of warm anomalies are occurring in Indian and South Atlantic Oceans. A zone of cool is now established west of Australia (part of a Positive Indian dipole, depriving Australia of rain from the west). The med is hot. And both the Southern Ocean and the Arctic have lots of cool melt water.

 

To see how the annual weather cycle and the seasons are working out, we can check the average isobar maps for past 30 days and their anomaly from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

 

The subtropical ridge is stronger than normal, especially in the Southern Hemisphere,  due mainly to an anomalously intense and slow-moving High (1042) in late June, now east of NZ.

 The ITCZ trough has deepened and shifted north especially over Asia. This year’s Indian monsoon was late this year, and, to Jun 26th, its rainfall was 24% below average. It should reach Delhi from 3 July.

 

Zooming into the NZ area, and comparing monthly anomalies  from end of last month with now, shows that the subtropical ridge STR has strengthened, especially over Australia, and its southern parts have drifted south (except over NZ). Troughs are deepening east of NZ, increasing the SW winds.

 

 

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THE TROPICS

 

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

TC ALVIN briefly bloomed during the week near west of Mexico. TC FOUR is near the Philippines.

There are areas of potential development in the China Sea, near Bangladesh, and to west of Mexico.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stay over Solomons and another branch stretch from Samoa to Suwarrow to French Polynesia north of normal. There is a trough (1011) lingering around east of New Caledonia for Monday and Tuesday and then that should go south and cross NZ on Wed and Thu.

 

Subtropical ridge (STR)

Here’s a snapshot of that Big fat High when it was crossing NZ yesterday

This Big fat High is expected to travel east to east of NZ along around 38S and possibly reaches its peak of 1042 hPa on Wed UTC. Long lasting squash zone on its northern side means “stay-out and hunker down” for yachts between French Polynesia and Tonga this week.

Next HIGH is expected to move from Tasmania to Tasman Sea on Friday at 1030+, and then go NE across the Tasman Sea and easing to 1020 near Northern NZ mid-next-week.

 

Tasman Sea /NZ/Aus

Between the Highs, a trough is expected to move from South Tasman by Tuesday and the merge with a trough from the north over the North Island on Wed/Thu, followed by a southerly flow on Friday/Saturday.

Departures from NZ to tropics should wait until the Southerly arrives, and maybe wait another day for northeast swell to ease.

The trough east of New Caledonia is turning winds between New Caledonia and Australia to a light south-southeasterly for Monday, briefly helping voyages between Noumea and Australia and between Australia and Noumea.

However, by Wednesday to Friday, a squash zone is expected to form SE of Bundaberg due to an incoming High, worth avoiding.

 

Tahiti to Tonga

Squash zone this week on northside of BFH. Stay put.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>> 

23 June 2019

Bobgram

WEATHERGRAM

YOTREPS

Compiled Sun 23 June 2019

 

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.

 

Solstice was Friday 21 June 1554UTC, but for the next six weeks, as the days’ get longer in NZ the cold gets stronger.

 

Tahiti to Tonga

Now that many yachts are about to travel west from Tahiti to Tonga, it is time to review the weather, in general terms, for this route.

Getting from the Tahiti area to Tonga/Fiji means crossing the South Pacific Convergence zone, SPCZ

 

1. This zone may be weak or may contain squally showers. In the models it is portrayed as a zone of light winds. To see a forecast for this zone, use windy.com and rain accumulation 5 days.

This zone may linger in the north (it does this in an El Nino, and we are nearly in an El Nino), or, it usually hovers between Samoa and Southern cooks. To avoid it choose a different latitude or aim for gaps in the zone.

2. Another thing to avoid is a SQUASH ZONE, when a large High travels east along 30S.

When the central pressure in the High is 1030hpa or above, it gets dirty (above ten thirty is dirty) and squeezes the isobars north of the high in the trade wind zone closer together making a "squash zone" of enhanced winds and rough seas. These usually occur around 20 to 25 South, and may last for several days. They are reasonably easy to forecast, and are worth avoiding. If caught in one, change your latitude rather than your longitude.

3. The third thing to avoid is the passing trough or passing low.

The systems tend to form on the SPCZ and travel southeast along it, propelled by upper NW winds as a steering field. They also tend to have their own mini-squash zones. Isobars give a clue about the intensity of a passing trough: those above 1010hP are usually weak and those below 1007 are worth avoiding.

There are three routes between Tahihi and Tonga, northern, central and southern:

The northern route goes via Suwarrow to Samoa and can be used when the SPCZ is further south

The middle-route offers stopovers such a Palmerston Island and Niue and is a good alternative when the SPCZ is further north than normal.

The southern route, going via Rarotonga and maybe Beveridge reef, is not often used but may help avoid a squash zone or sometimes a passing trough.

Basically, avoid passing troughs and squash zones, and go thru the SPCZ when it is weak. This may mean taking short hops rather than getting direct from Tahiti to Tonga in one go.

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THE TROPICS

TC VAYU brought moderate damage to NW India last week when it made landfall. It was the strongest cyclone in this area for 21 years. ( 8 fatalities and 12 injury reports).

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are areas of potential development near the Philippines and to north of PNG, also off the west coast of central America.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomons to near New Caledonia to south of Tonga this week. A weak passing trough (down to 1016hPa) is expected to travel southeast along this zone to east of New Caledonia and south of Tonga on Mon/Tues, and then another stronger passing trough is likely in the New Caledonian /Fiji this weekend (down to 1004, worth avoiding).

 

Subtropical ridge (STR)

HIGH 1028hpa is expected to travel along 30S to south of Tahiti next few days—with a small squash zone near Southern Cooks.

Large HIGH is expected to slowly travel along 40S this week, reaching 1040 hPa over South Tasman Sea on Thursday, and crossing central NZ this weekend. There are likely to be squash zones between NZ and New Caledonia/Fiji with this High.

 

Tasman Sea /NZ/Aus

Weak trough is expected to linger around North of NZ on Monday/Tuesday with light winds and scattered showers.

After that the winds north of NZ are easterly, increasing to strong in squash zones. Avoid. Should be Ok for getting to Australia , not the other way.

HIGH is expected to be followed by a trough in the Tasman Sea next week.

 

Tahiti to Tonga

Trough now moving off to east of Society Islands. Small squash zone west of Tahiti on local Monday. Weak passing trough over Tonga on Tuesday 25 traveling east to be south of Southern Cooks local Thursday.

Next passing trough expected to reach Tonga this weekend, worth avoiding.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>

16 June 2019

Bob Blog 16 June

WEATHERGRAM

YOTREPS

 

Compiled Sun 16 June 2019

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.

 

The South Pacific Convergence Zone explained

Now that many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, this obstacle has become a talking point…as if it guards the eastern entrance to the South Pacific like a protective dragon, and some have asked what is it, why is it there, how does it differ from the ITCZ and what makes it tick.

 

Weather is a mix of pattern and chaos, and meteorological teaches concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell.

 

Because the sun is most directly overhead at the equator, that’s where the warmest seas are, and this causes rising air. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds. The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ

 

But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W, or near Easter Island. It is quasi stationary, just like the High between California and Hawaii, and also has a gyre that is collecting a rubbish heap just as badly (see Henderson island). blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-in-paradise/

 

1. There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. These easterly winds travel well to west of the dateline along around 10 to 15S.

 

2. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. These South/SE winds come and go according to the migratory high and are usually found around 15 to 25S.

 

3. The convergence zone between these easterly and Southeasterly winds is called the South Pacific Convergence Zone, or SPCZ.

 

It is typically located from the Solomon Islands southeastwards to the Southern Cooks, but sometimes may have large gaps or be very quiet.

 

It is affected by many things: the PDO which takes many years to switch, by the El Nino/La Nina which last a year or so, by the strong annual cycle which makes the seasons, and by the MJO which comes for a week or so every six weeks or so.

See the diagram at  www-gte.larc.nasa.gov/pem/pemt_flt.htm

 

I have found that the easiest way to determine the position and severity of the SPCZ is to use satellite imagery, and the easiest way to decide what it may do over next few days is to use the 5day rain accumulation parameter on windy.com.

 

Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone

 

THE TROPICS

TC VAYU is in the Indian ocean and weakening and about to make landfall over NW of India.

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are areas of potential development in China sea and North of PNG, also off the west coast of central America. This week.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomons to Tuvalu and Samoa this week.

A trough is expected to travel across Southern Cooks and Tahiti late in the week.

 

Subtropical ridge (STR)

HIGH over South Tasman Sea is expected to cross New Zealand on frosty Tuesday and then travel to east of NZ.

Next High is expected to spread into Tasman Sea next week.

 

Tasman Sea

Low is expected to deepen east of Coffs on Monday and then travel east and weaken, with associated front crossing NZ on Thursday followed by a westerly flow on Friday.

This Tasman trough is likely to replace the trade winds over northern Tasman Sea with light winds or SE winds, good for getting from Australia to Noumea, but not the other way.

That Tasman trough is expected to cross NZ on Thursday so the last good day to depart NZ for the tropics is Tuesday, and even that will need waypoints to zigzag across a period of northerly winds on Wednesday and Thursday.

 

Tahiti to Tonga

A passing trough is expected to affect Southern Cooks on local Tuesday and maybe the Tahiti area on local Thursday. Departures from Tahiti until Wednesday should be able to avoid that trough and have trade winds for a while. Passing trough expected to reach Tonga by local Tuesday 25 June.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>

 

 

09 June 2019

Bob Blog 9 June 19

WEATHERGRAM

YOTREPS

 

Compiled Sun 09 June 2019

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.

 

Where’s the El Nino?

The state of the ENSO =almost an El Nino at times

 

The Atmosphere:

 

El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather: the La Nina, caused by cooler than normal seas along the equatorial eastern pacific. shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

 

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number of isobars between them on the weather map. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus-one we call it an EL NINO event.

 

Since February the SOI has been negative, and for a while in March was touching a weak El Nino. Since then it has weakened, but in the last week it is touching into El Nino again. If it continues as negative as this then the South Pacific may be in for weaker trade winds and more SW wind events than normal

Near El Nino conditions are seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

(Note that in this graph on the vertical axis 10= 1 standard deviation)

 

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2016. Since then there has been several cool periods. Sea has warmed since May 2018 and touched El Nino territory late last year and in Feb/March 2019 and since then has been relaxing, Data missing for May and June. The ocean has not been in tandem with the atmosphere.

Near El Nino at times are seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

 

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas should stay much the same for the next few months but may warm a little more between the coming October to January period.

CPC/IRI predictions  are at iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

 

The latest SST anomaly map from www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html shows lots of warm anomalies across the entire tropics, except for NW Australia and the Humboldt current off South America.

 

THE TROPICS

There are no cyclones at present, but there is potential off the central American west coast and about the south on India. The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomons to Fiji and Tonga this week. A trough is expected to travel across Fiji and Tonga on Monday to Wednesday and then to travel off to the southeast.

 

Subtropical ridge (STR)

HIGH at 30S to north of NZ on Monday is expected to travel slowly east.

Next HIGH is expected to enter from Australia into Tasman Sea on Tuesday and slowly cross the Tasman Sea to ne north of NZ on Saturday.

 

Australia to Noumea

With the near southerly winds ahead of each of these Highs there is a brief opportunity for sailing from Australia to Noumea, departing Monday or Friday/Saturday.

A passing trough crossing the Tasman Sea on Tuesday and Wednesday may  impede a voyage from Noumea to Australia.

 

NZ to tropics:

Monday departure should be OK to go around the back side of the trough that is crossing Fiji and Tonga Monday to Wednesday, and trips to Tonga are able to avoid the larger swells from the Tasman Sea. Trough is expected to cross Northland on Tuesday night /early Wednesday, Stay put for that, but should be OK to depart in the SW flow that follows

 

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>

 

02 June 2019

Bob blog 2 June

WEATHERGRAM

YOTREPS

Compiled Sun 02 June 2019

 

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.

 

REVIEW of MAY

Sea Surface temperature anomalies as at end of April may be seen at

www.weatherzone.com.au/climate/indicator_sst.jsp?lt=global&lc=global&c=ssta

 

The main pattern changes are a slight warming around New Caledonia and slight reduction in the warmth of the Tasman Sea, a warming in the India Ocean, warming in the Humboldt current off western South America, and a reduction in the warmth of the North Atlantic. The Caspian Sea and Black Sea are outstandingly warmer than normal.

Another month that has been mostly warm, with the main cool areas being melt water around the Arctic or Antarctic.

 

To see how the annual weather cycle and the seasons are working out, we csn check the average isobar maps for past 30 days and their anomaly from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

The subtropical ridge has started its northern shift in both hemispheres. This has allowed troughs to develop in the South Tasman Sea. The near equatorial trough has also drifted north, especially over India. But the Indian Monsoon is only now starting and is about a week late, as seen at

www.imd.gov.in/pages/monsoon_main.php. A late monsoon encourages a heat wave over northern India, and sure enough today’s high is 45C

 

Zooming into the NZ area, and comparing monthly anomalies for end of May with end of April, shows that the subtropical ridge STR has moved north as shown by the northern 1015 isobar over Queensland. Also, the STR has weakened in the South Tasman Sea with the 1015 isobar shifting from 50S to 40S. The westerly winds over NZ are increasing.

 

THE TROPICS

There are no cyclones at present, but there is a tropical depression off the central American east coast. The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is rather weak and mainly from Solomons to the Tokelau area this week, north of Samoa.

There is also an active convergence zone lurking to southeast of Tahiti around the Gambier Islands.

 

Subtropical ridge (STR)

HIGH at 30S to north of NZ on Monday is expected to travel quickly off to east.

Next HIGH is expected to enter from the west into the south Tasman Sea on Friday and travel across central NZ on Sun/Mon 9/10 June.

 

Australia to Noumea

Travelling Tasman Trough expected to break the trade winds and reach Noumea around Wednesday, followed by southwest/South winds and big swells  on Thursday, then the trade winds return.

 

NZ to tropics:

Stay put until Friday or Saturday.

A Low is expected to develop off Sydney on Monday and deepen into a large system with a Low of 986hPa on Tuesday, then cross the North Island on Wednesday, followed by a cold and vigorous SW flow from the Antarctic over NZ on Thursday, slowly easing on Friday.

Departures to the north from NZ may be possible on Saturday (perhaps for some from late Friday).

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>> 

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