Bob Blog 21 Oct

WEATHERGRAM

YOTREPS

Compiled Sun 21 Oct 2018

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.

 

There has been a Big Fat High (BFH) parked over northern NZ since 13 Oct as seen on the Auckland Airport barograph (using windy.com)

We can see how the intensifying high squeezes to isobars on its periphery closer together.  This makes a squash zone of enhanced winds, in this case from Tonga to Fiji and almost to Vanuatu.

Windy.com can also plot the past 20 days of reports.  The bottom trace is a barograph and shows that this BFH relaxed between 17 and 19 October—This also caused a temporary easing of the squash zone, and several yachts picked this window of opportunity to depart Fiji/Tonga for NZ.  They still encountered a (weakened) squash zone, and it wasn’t comfortable, but could have been worse, and they got the rare opportunity of sailing from Fiji or Tonga to New Zealand without encountering a passing trough.  Sweet, and no longer on the menu.

The squash zone is expected to relax again from Tuesday.

 

The illustrated edition shows bumps twice a day in the Auckland barograph--- these are the impact of the semi-diurnal atmospheric tide.  It is always best to log your barometer reading around 10am or 10pm (local) when it’s at the top of this cycle.

 

A quick look at the mid-atmosphere 500 hPa map around the southern hemisphere shows how the blocked high around NZ is linked to two other highs around the hemisphere. This is called the wave 3 pattern.

So, when the winds aloft decrease to a crawl because of gaps in the jetstreams, that is when the surface Highs just stay where they are and are called “blocked”. 

 

 

THE TROPICS

Latest cyclone activity is seen at tropic.ssec.wisc.edu

WILLA is aiming to make landfall near SW tip of Mexico, VICENTE is expected to weaken as it travels northwest along the coast.

 Looking at the weekly rain maps, last week’s shows a build-up of convection around Mexico and just north of Fiji, but decreasing convection elsewhere, especially around the Indian Ocean. 

 See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is sitting over northern Coral sea and northern parts of Vanuatu across northern Fiji and northern Tonga towards Niue and may spread towards Southern Cooks by the end of this week.

A tropical low may forms on the SPCZ to south of Niue late this week and then travel off to the southeast. Watch our for this if travelling from Tahiti to Tonga. 

 

Subtropical ridge (STR)

The BFH which has been blocked over northern NZ is expected to start relaxing by Tuesday and then to move off to east of NZ.

Next HIGH is expected to form in South Tasman Sea from Thursday and to move onto NZ early to mid-next-week.

 

Tropics to New Zealand

The squash zone between Fiji/Tonga and NZ is expected weaken by Tuesday 

A Low is expected to form between New Caledonia and New Zealand late this week and then travel east, providing the challenge of a period of southerly winds to yachts sailing south.  Yachts from Tonga can anchor in Minerva reef and wait this out.  Yachts from Fiji will need some waypoints to handle those southerly winds.

 

Between Tropics and Australia.

For those headed to Queensland or Coffs with the Down Under Go West Rally

An active trough may reach Bundaberg area from around Sat/Sun 27/28 Oct,

so that a departure from New Caledonia by local Tue 23 may encounter that trough near the Bundaberg coast where it may be active /squally. And a departure on local Wednesday may encounter that trough further offshore where it may be weaker. This timing can change.

For more info about the Go West Rally go to www.downunderrally.com/about-go-west

 

From Tahiti to Tonga

Squash zone in this area is now fading.  An active trough is expected to reach Tonga around 27-28 Oct UTC followed by a period of NW winds, avoid.

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

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

Contact is bob@metbob.com or txt 6427 7762212

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