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

26 July 2015

Bob Blog issued 26 July

WEATHERGRAM

YOTREPS

Issued 26 July 2015

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.

 

Prof Valentina Zharkova from the UK’s Northumbria University pointed out recently at a symposium in Wales that the recent low sunspot activity was pointing to a downward trend consistent with a longer-term beyond the 11-year cycle. “The model predicts that the magnetic wave pairs will become increasing offset during Solar Cycle 25, which peaks in 2022. Then during Cycle 26 (~2033) the two waves will become exactly out of synch, canceling each other out. This will cause a significant reduction in solar activity…. We predict that this will lead to the properties of a ‘Maunder minimum’ ”.

The ‘Maunder Minimum’ was a period of almost no sunspots between 1645 and 1715, It wasn’t long for the mainstream media to pick this up and come out with sensational headlines such as “Predictions from the model suggest that solar activity will fall by 60 per cent during the 2030s to conditions last seen during the ‘mini ice age’ ” (Royal Astronomical Society).

That 60% drop is referring to the fractional difference between the sun’s maximum and minimum output. In fact our sun, in spite of these ups and downs, has only varied by 0.1 per cent in its output during the past 2000 years. Some people are associated the ‘Maunder Minimum’ 1645-1715 with the mini ice age 1300-1870, but the mini-ice age started earlier, finished later, and is generally attributed to a period of volcanism on planet earth.

As Prof. James Renwick of Victoria University in Wellington explains “This effect is unlikely to bring on the type of cold conditions seen from 1645 to 1715. The big difference now is that there is a lot more greenhouse gas in the [Earth's] atmosphere than there was in the 1700s."

For a full review of the media miss-hype check out http://www.radionz.co.nz/national/programmes/mediawatch/audio/201763733/winter-is-coming-churnalism-freezes-out-the-facts

 

Perhaps we should instead be devoting all this time and effort to something of more immediate importance, such as a study of the intensity of anticyclones – are they getting higher?

The anticyclone in the Tasman Sea mid last-week was large and had a central pressure of 1036 hPa.

This anticyclone is now east of NZ and the computer models are having a hard time dealing with it; they want to re-intensify it to over 1040 when it is south of French Polynesia by Wed 5 Aug, as seen at  windyty.com

 

TROPICAL TOPICS

Out west, HALOLA is now in its third week and a tropical depression near southwest of Japan.

There are a few tropical depressions around but the activity of the past two weeks now seems to be subsiding this week.

 

The year is turning: This week marks the start of the ‘dog days’ of the northern summer, when the northern continents normally get the hottest day of the year as the dog star Sirius rises just before dawn. Because it takes long clear nights to make a frosty mornings, we in New Zealand may already have had our coldest morning of the year (but our coldest day-time temperature also usually occur in late July/early August).

 

The weekly rain maps over the past two weeks show an increase in intensity in the convection across Tuvalu and Tokelau, also along the ITCZ, and a burst of activity in the Asian Monsoon.

 

Weekly rain signatures for past two weeks, are seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ

SPCZ this week is expected to stretch from south of Solomon Islands to Tuvalu to Tokelau to Northern Cooks. It is active at present (squally) and looks likely to cross the Bora Bora and Papeete region on local Tuesday (Wednesday UTC).  Avoid.

There is a possibility that a tropical depression may form near the Solomon Islands area late this week, with heavy rain and squalls. Sad to report.

 

STR= Sub-tropical Ridge

The High that was in the Tasman Sea mid last-week is now east of NZ travelling slowly east along 40 to 45S.

A new High is expected to move from inland Australia into Tasman Sea along 30S starting mid-week and cross northern NZ on Thursday and Friday. There is a squash zone of enhanced SE wind on the north side of this high and in the Coral Sea

 

NZ

A passing front on Monday and a trough of Tuesday then a decreasing SW flow on Wednesday. Light wind sin the passing High on Thursday and Friday. Then another front in the north on Saturday/Sunday.

 

Travelling Westward from French Polynesia:

Wait form the SPCZ to come and go, hopefully all on Tuesday local, and then there appears to be an OK voyage to the west for a week or so.

 

Departing from Australia to the tropics this week:

The squash zone on north side of that Tasman High makes Brisbane to Noumea very difficult for first part of the coming week. A trough moving into the Tasman early next week may offer the correct weather pattern for a while.

 

Departing from Northern NZ going north.

A departure on Thursday seems to have the best pattern for a trip northwards from northern NZ this week.

 

>>>>>>  >>>>>>>>>>>>>>>>>>   

See my yotpak at boatbooks.co.nz/weather.html for terms used.

See my website www.metbob.com for information on tailor-made voyage forecasts  Feedback to bob@metbob.com.

Tell anyone you like that to subscribe they should email me.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

To unsubscribe,  send a reply email saying LEAVE.

 

 

 

19 July 2015

Bob Blog

WEATHERGRAM

YOTREPS

Issued 19 July 2015

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.

 

Late autumn in New Zealand saw rain return to west coast of South Island after a very dry period.  This can be seen in the daily storage of the NZ hydro dams at http://www.electricityinfo.co.nz/comitFta/ftaPage.hydrology

 

In The Atmosphere:

The Southern Oscillation Index SOI sums up the weather pattern over the South Pacific in one number and is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean). The weekly update was lower than -10 for four weeks in May, enough to start a moderate El Nino episode. The value relaxed during June had dived rapidly during July, reaching -20 last week, as seen at  http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly  .

 

El Nino’s impact: During an El Nino episode, weather patterns tend to be drawn closer to the equator. The South Pacific Convergence zone tends to be tugged north and east of its normal position and there tend to be more troughs over Tahiti. The subtropical ridge in the southern hemisphere tends to be north of its normal position and this weakens the trade winds overall, but doesn't stop the squash zones that can be found on the north side of the migratory HIGHS.

During El Nino years, tropical cyclones in the South pacific enter middle latitudes anywhere between the Australian coast and about 150°W and typically weaken quickly as they move away to the south-east, due to strong winds aloft.

I am NOT aware of any association between SOI and an early onset to the southern hemisphere cyclone season.  And cyclone RAQUEL over Solomon Islands earlier this month was a “one-off” so there is no reason that I know of for sailors to speed up their visiting plans across the South Pacific,

 

 

TROPICAL TOPICS

Out west, HALOLA is now a tropical depression and heading for Japan .

East of Hawaii the remains of ENRIQUE are fading Further east, near the Mexican west coast, DOLORES is travelling north The weekly rain maps over the past two weeks show the intense rainfall from tropical cyclones.  There also seems to have been an increase in intensity in the convection across Tuvalu and Tokelau.

 Weekly rain signatures are at  http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

 

SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stretch from south of Solomon Islands to Tuvalu to Tokelau to Northern Cooks.  A small convergence zone (on the north side of a Southern Ocean Low) is expected to travel eastwards (upper westerly winds) and cross the Palmerston Island on Tuesday/Wednesday UTC (local Monday/Tuesday).

 

STR= Sub-tropical Ridge

STR is along 40S in Tasman Sea and 30S in the SE Pacific, but is having a disjointed week in-between.

A cold HIGH is moving slowly across the Tasman Sea and expected to cross NZ on Thu Fri UTC.

There should be a squash zone of enhanced SE winds on the north side of this Tasman sea HIGH, affecting Minerva to Coral sea around mid-week.

 

NZ

The weekend low is now east of NZ and moving off to the eats,  but on Monday one more cold front is expected to skirt the east coast bringing a southerly gale (and alpine snow) until Wednesday morning (mainly Wellington to Gisborne).

For the remainder of the week the incoming High brings light winds and frosty nights.

This is the same low /cold High combo which brought snow to parts of Queensland late last week.

 

Travelling Westward from French Polynesia:

A good week for going west, but be aware of the strong SE winds expected over Tonga from Wed 22 to Sun 26 local.

 

Departing from Australia to the tropics this week:

The squash zone on north side of that Tasman High makes Brisbane to Noumea very difficult for first part of the coming week.  A trough is expected to travel east across the eastern seaboard on Thursday and then local Friday may be Ok for a departure.

 

Departing from Northern NZ going north.

A departure on Monday is likely to encounter strong tail winds at first.  A departure after Wednesday may encounter NE head winds by Friday.

 

>>>>>> 

See my yotpak at boatbooks.co.nz/weather.html for terms used.

See my website www.metbob.com for information on tailor-made voyage forecasts– Feedback to bob@metbob.com.

Tell anyone you like that to subscribe they should email me.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

To unsubscribe  send a reply email saying LEAVE.

 

 

12 July 2015

Bob Blog 12 July 2015

WEATHERGRAM

YOTREPS

Issued 12 July 2015

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 30day anomaly wind map for 11 July at http://www.esrl.noaa.gov/psd/map/images/fnl/sfcwnd_30a.fnl.html

shows a mediocre monsoon.  Also extra S to SW winds feeding onto NZ (much colder than last winter).  South Pacific trade winds have been stronger than normal in the Coral Sea and south of Tahiti, but about normal in-between.  The burst of near-equatorial westerly winds just north of Solomons Islands also stands out.

 

WESTWARD HO

We are coming to the time of the year when cruising yachts travel westwards across the South Pacific from French Polynesia to Tonga/Fiji.  There are three popular routes

1.      The northern route: via Suwarrow and Samoa

2.      The central route via Palmerston/Aitutaki

3.      The southern route via Rarotonga/ Niue

Since we are having an El Nino, the South Pacific convergence zone SPCZ is spending more time than normal over the northern route, so that may be the squally way to go unless you can pick a period when the SPCZ is weaker than normal.

There are occasional squash zones of enhanced trade winds over the central route on the north side of the large HIGHS that travel east along the sub-tropical ridge about 30S.  And the southern route is most at risk from the big swells that roll out of the southern ocean.  This year I think that often the central route may provide the least risk for adverse weather--- but there will be times when the central route is not good.

 

To help visualize the position of the SPCZ over the next week or so use the rain accumulation overlay option on windyty.com (found on the right) and select Next 10 days.  The day-to-day variations of the SPCZ may be difficult to follow,  but the 10-day accumulation shows its trend well.

 

Sailing from Tahiti to Tonga usually involves an encounter with a convergence zone CZ.  First the trade winds start to falter and swing to the left, so that the winds are from the Northeast. This is a reliable herald that within 24 hours the sky becomes overcast and the wind becomes light and variable.  This is when you should go on squall watch:  reduce sail, close the hatches, and be prepared to motor away from any squall that gets in the way.  The squalls usually produce 30 knots for 30 minutes, but can be stronger and last longer.  There is usually only a few days between convergence zones so your voyage is likely to encounter at least one – something like a rite of passage.  Try and arrange your departure so that the convergence zone that you encounter is a weak one.

 Since the convergence zone normally stretches from NW to SE, the best way out is to go to the SW. 

 

TROPICAL TOPICS

CHAN-HOM is still going and was busy last week, briefly making landfall near Shanghai and now heading for Korea.

NANGKA is also still going and is now going to the north, heading for Japan.

 Four new cyclones have been named across the North Pacific during the past week , and there are at least three tropical depressions that are being watched closely.

The weekly rain maps, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif , over the past two weeks show the intense rainfall from these tropical cyclones along the ITCZ. 

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stretch from east of Solomon Islands to Tuvalu to south of Tahiti this week.  A LOW is expected to form to southeast of Rarotonga on Monday UTC / Sunday local and then move southeast and deepen so that Southern Cooks is likely to get a period of fresh to strong SW winds and some big swells by mid-week.  Avoid.

STR= Sub-tropical Ridge

STR is along 30S in the Tasman Se and in the SE Pacific, but is having a disjointed week in-between.

The cold HIGH that has been over NZ this weekend is expected to travel east along 45S and release its grip over NZ by Wednesday.

NZ

The front which moved across eastern Australian coast on Saturday has deepened into a Low off New South Wales today.  This front is expected to reach NZ on Wednesday, preceded by a northerly blast on Tuesday, and followed by disturbed SW flow on Thursday.  There is expected to be only a brief gap for a weak ridge on Friday and then another LOW is expected to cross NZ this weekend 18/19 July.

 

Departing from Australia to the tropics this week:

A departure on Tuesday may offer a suitable weather pattern as the LOW in the Tasman Sea moves off towards NZ. A departure on or after Wednesday may encounter a front on Friday.

 

Departing from Northern NZ going north.

AFTER the Wednesday front, SW winds should ease so that a THURSDAY departure may be the best on offer this week.  That voyage is likely to encounter northerly headwinds ahead of the next front on Sat/Sun and so will need a few waypoints.

>>>>>> 

See my yotpak at boatbooks.co.nz/weather.html for terms used.

See my website www.metbob.com for information on tailor-made voyage forecasts– Feedback to bob@metbob.com.

Tell anyone you like that to subscribe they should email me.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

To unsubscribe send a reply email saying LEAVE.

 

 

05 July 2015

BobBlog

YOTREPS

Issued 5 July 2015

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 30day anomaly map for 1 July as seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

helps to explain what has been happening around the planet during June and how it is linked into an El Nino episode.

The SOI (Southern Oscillation Index) measures the (normalized) air pressure difference between Tahiti and Darwin. When Darwin is having higher than normal pressures, Tahiti tends to have troughs. Indeed that has been the case in June 2015,  as seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

 

The subtropical ridge – that zone between the roaring 40s and the trade winds – the zone along which anticyclones migrate eastwards in the southern hemisphere—has been much more intense than normal , feeding cold SW winds onto southern NZ and accentuating the trade winds over Tonga Fiji and Vanuatu. The South Pacific convergence zone is north of its normal position, and Solomon Islands have lower than normal pressures as a consequence.

 

There is a zone of higher than normal pressure extending all the way from Australia to India and to the Philippines and this helps explain a stuttering monsoon.

 

The Indian Monsoon index may be seen at

http://apdrc.soest.hawaii.edu/projects/monsoon/realtime-monidx.html

 

TROPICAL TOPICS

 

Over the past week a MJO pulse brought a burst of extra convection into the West Pacific Ocean, along with a burst in near-equatorial westerly winds (as mentioned last wee). That has triggered 4 tropical cyclones:

RAQUEL which was over the Solomon Islands mainly last Thursday/Friday.

CHAN-HOM which is now north o0of the Philippines.

LINFA which is further east;

And NANGKA, even further east:

During the coming week a tropical depression is expected to move towards Hawaii by local Friday and Saturday. And another to southeast of Acapulco. Meanwhile the North Atlantic is remaining quiet.

 

The weekly rain maps over the past two weeks show the intense rainfall from these tropical cyclones, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing wind, isobars, Sig wave height green lines, swell direction orange arrows, current in small arrows, SPCZ and STR.

 

SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stretch from east of Solomon Islands to Samoa and from Suwarrow to almost Tahiti. The part over Tahiti is expected to fade.

The clouds that were part of RAQUEL and expected to spread out as a convergence zone that is likely to cross New Caledonia on local Tuesday night/Wednesday morning, and the following night over Minerva and Tonga.

 

STR= Sub-tropical Ridge

STR is breaking up into a band along 25/30S and discrete Highs that are mainly at 40S. The High moving into the Tasman Sea on Monday is a slow-mover and expected to cross NZ during next weekend=the middle-weekend of the school holidays. It should travel further eat to east of NZ next week along about 40S.

 

NZ

Cold front is being directed northwards across NZ on Monday and Tuesday, followed by air that is coming directly off the polar ice shelf. Good for alpine snow.

This front is likely to produce gale force NW winds near 30S on Wednesday. Avoid.

 

Departing from Australia to the tropics this week:

Winds between Brisbane and Noumea are expected to be light on Monday and Tuesday then southerly or SSE on Wednesday to Saturday, so that may allow a good enough voyage (either way). However a front is expected to reach Brisbane late Saturday and deepen in the Tasman Sea bringing strong northerly flow to New Caledonia by Mon/Tue 13/14 July. Avoid.

 

Departing from Northern NZ going north.

AFTER that Tuesday front over northern NZ there should be a good weather pattern for departures on wed/Thu/Friday with moderate to fresh SW or southerly winds for starters.

>>>>>>

See my yotpak at boatbooks.co.nz/weather.html for terms used.

See my website http://www.metbob.com for information on tailor-made voyage forecasts – Feedback to bob@metbob.com.

Tell anyone you like that to subscribe they should email me.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

To unsubscribe  send a reply email saying LEAVE.

==

 

 

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