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

11 October 2015

BOB BLOG 11 Oct

WEATHERGRAM

YOTREPS

Issued 11 October 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.

 

A comparison of the Sea surface temperature anomaly map at start of October of this year with 1997 – as shown by NOAA may be  seen in the LA

Times:  LA Times link:

http://www.latimes.com/local/lanow/la-me-ln-massive-el-nino-is-now-too-

big-to-fail-scientist-says-20151009-story.html

Notice how this year there is an extension of cooler than normal sea from the Papua New Guinea to the Fiji Tonga area—whereas back in 1997 this

zone was warmer than normal.   It is reasonable to postulate (but hard to

prove in advance) that this finger of coolness may help reduce the risk of cyclone formation or cyclone support. A Mercator projection view of this cool blue finger-zone of reduced risk may be seen at  http://www.

ospo.noaa.gov/data/sst/anomaly/2015/anomp.10.8.2015.gif

 

Now that 1997/1998  South Pacific Cyclone season was the most active and longest on record with 16 named cyclones in the South Pacific basin between early October and early May. 

8 of these 16 affected French Polynesia Cook Islands

1982/83 was also an extreme El Nino year with 14 named cyclones:

10 of these 14 were in the French Polynesia/ Cook Islands area.

To compare El Nino seasons a good parameter to use is the NINO3.4, (based the sea surface temperature anomalies in the target area) and a long term graph of this may be found at https://besfeg.wordpress.

com/2015/05/27/will-a-big-el-nino-spell-doom-for-tropical-forests/

Data (up to Sep 2015) is from http://www.cpc.ncep.noaa.

gov/data/indices/sstoi.indices

 

The September 2015 value of the NINO3.4 anomaly was 2.28 compared with 1.

39 in September 198 and 2.21 in September 1997, so at first glance this is a match –and we can postulate (even if it may be difficult to prove in

advance) that the French Polynesia/Cook Islands area are at a higher risk than normal of being affected by tropical cyclones during this season.

However this is just one parameter and, as we can see for the opening image, there is more happening even in the SST anomaly map than just the

NINO3.4

 

Some web sites such as Wikipedia think that this cyclone season has already started, for they start their count from 1 July and RAQUEL formed

30 June, left the basin area and then re-entered on 4 July and faded the

next day.   However, I think that the vernal equinox, when the overhead

suns shifts into the southern Hemisphere, is a more logical date to start the count rather than from 1 July.

 

TROPICAL TOPICS

There is just one named storm in the tropics at present – NORA near Hawaii (again).

The Weekly rain maps  from http://trmm.gsfc.nasa.

gov/trmm_rain/Events/big_global_accumlation.gif show that convective activity has jumped in the past week from the Intertropical Convergence zone to the South Pacific Convergence zone.

This increased activity in the SPCZ coincided with a period of near equatorial westerly winds.

The tropical low on the north side of the equator near 10N 150E is expected to deepen further and may form a Tropical Cyclone.  The Low east of Solomon Islands is expected to weaken and move off to the SE.  SO the “twinning” mechanism seems to have missed working this time.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

There is a lot of activity between Eq. and 15S from 160E to 180 at present and this moisture is expected to feed along the SPCZ that extends

from eastern Solomon Islands to the Fiji area.   SO squally wind and rain

is likely to visit parts of Fiji around mid-week. The GFS model has a tropical low with this on Wednesday UTC but the more reliable EC model at www.tinyurl.com/ecaus has the low further west and from Sat 17 Oct—so the models are flip-flopping and the area is worth watching with care.

Strong SE winds on the south side of the SPCZ are expected to affect Fiji area until Wednesday, and then this zone of strong winds should shift south to around 25S and stay there for a few days.  The strong SE winds that have been affecting Vanuatu are expected to ease from Wednesday.

 

Further east a convergence zone of the north side of a passing front is expected to pass across French Polynesia over the next few days changing the wind direction over the south half of the area.

 

STR= Sub-tropical Ridge

High in the Northern Tasman sea on Sunday (HIGH1) is expected to stay there and fade away on Tuesday/Wednesday as a cold front travels north into it and across North Island and also fades. 

A new High, HIGH2,  following that cold front,  is expected to dominate the South Tasman Sea  by Wednesday and travel across Northern NZ on Friday to Saturday.

This pattern is expected to repeat with a front fading over northern NZ on Sunday 18 Oct and another High, HIGH3 form Monday 19 to Friday 23Oct.

HIGH2 is intense  enough to have a squash zone of strong SE winds on its northern side so that over New Caledonia, SE winds have eased over the weekend but are now expected to increase again from late Tuesday until Friday. And over Tonga, strong SE winds are expected from Wednesday to Saturday, and this squash zone should spread east to affect Niue/Southern Cooks/Society Islands.

 

Travelling Tahiti to Tonga or from Fiji to anywhere:

A difficult week, affected by SPCZ and a squash zone, so maybe stay put.

 

Between Tropics and NZ:

HIGHS 1, 2 and 3 dominate northern NZ over next two weeks, so winds are not strong, but the E/SE winds on the north side of the STR mean that trips from Tonga and New Caledonia have to start setting off with a leg to the SW and then turn to NZ around about 30S.

The weak “replacement” fronts between the highs on 13/14 Oct and 17/18 Oct and have NW winds ahead of them that offer good sailing breeze to northern NZ, but are followed by SW winds. 

As for southern NZ --- well, here be dragons:  vigorous disturbed westerly winds dominate proceedings.

>>>>>> 

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.

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