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

07 April 2019

Bob Blog 7 Apr 2019

WEATHERGRAM

YOTREPS

Compiled Sun 07 April 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.

 

For those of you who are now planning to depart Australia or NZ for the tropical Islands (after the cyclone season) and would like as assisted passage: here is some contacts.

 

Island Cruising New Zealand are organising a rally from Opua to Tonga. Sadly the registration for this is now full, but if you’d like to join the waiting list then see www.islandcruising.co.nz/ or visit their facebook site at www.facebook.com/islandcruising.nz/ for newsy tidbits

 

And the Down Under crowd are in full swing arranging the GO EAST Cruisers rally from the Gold Coast to New Caledonia/Vanuatu in May 2019 (6 Maty or soon after).

 

For more info see www.downunderrally.com/go-east-rally-2019

 

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So, when will this cyclone season finish?

 

Nominally, the cyclone season ends at the end of April. At present TC WALLACE is travelling southwest well offshore of NW Australia, and there is a tropical depression travelling westwards near Darwin, so this cyclone season is still with us.

 

One parameter we watch is the MJO or Madden Julian Oscillation. It takes 10 to 20 days for the active part of an MJO event to travel across the Pacific (a burst of extra convection that can trigger cyclones) and we get such a passage around once every 4 to 6 weeks. There have been active MJO events over the Australia/Pacific region during much of December/early Jan, then in late Jan/early Feb, and now in late March/early April. These can be seen as the blue (for bubbly) OLR or Outgoing Radiation zones in a time-longitude diagram from www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/forca.shtml

 

The stage is set for the non-active or settled phase of the MJO to travel across the Pacific over the next few weeks, but there may be another active MJO late in April, so even if the next few weeks are clear of cyclones we can’t say the season is “over” yet.

 

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A touch of weather chaos.

I often use the expression that weather is a mix of pattern and chaos, and I use it as a way of explaining away the weather features that the models just do not resolve. I noted an example from the real world near Wellington yesterday, as seen at windy.com

The smoothed-out pattern of the global model isobars did NOT capture a chaotic little whirl of winds that formed to southeast of Cook Strait. This little feature caused a gale of southerly winds through Cook Strait, as seen in the Wind observations. The computed wind forecasts were constantly underestimating the real world.

 

Another thing to note is that the barometer was reasonably steady throughout this gale. This is often the case near a “squash zone” and that is little comfort to a sailor looking for signs.

 

MetService was on the ball, watching weather radar and analysing the reported observations with isobars down to 1hPa apart, they could track this chunk of chaos and incorporate it into marine, aviation and general media forecasts and warnings. The MetService analysis was tweeted at twitter.com/MetService

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THE TROPICS

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

 

TC WALLACE is staying offshore (fingers crossed) off NW Australia. There is also a low travelling west across the Darwin area, but it is not expected to deepen much further.

 

There are some spots of possible activity this week around Solomon Islands and Micronesia, otherwise all looks quite quiet.

 

Rain in the past week from trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif was strongest near WALLACE and around Micronesia and weaker than last week elsewhere around the Pacific.

 

The “mirror CZ” continues just south of the equator in the eastern Pacific, affecting those sailing between Galapagos and Marquesas.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to be most active this week across the north Coral Sea to northern Vanuatu to south of Fiji. There is also expected to be a convergence zone over Nauru and Tuvalu.

 

Subtropical ridge (STR)

HIGH in the central Tasman Sea on Monday is expected to travel east across northern NZ on Tuesday and then further east along 30 to 35S.

Next HIGH is expected to travel east across Tasmania on Thursday and then across the South Island on Saturday.

 

Australia/Tasman Sea / New Zealand

Lots to avoid. Low to east of NZ is travelling slowly south-southeast from Monday to Wednesday, giving a southerly flow over North Island mainly on Monday. Next trough is expected to cross central Tasman Sea on Tuesday, the South Island on Wednesday, and then deepen over North Island on Thursday, with a Low near Cook Strait, and then a southerly flow over the North island on Friday and Saturday.

 

Panama to Marquesas

Gulf of Panama now has SW swells. There are reasonable northerly winds for departure until local Tuesday, and again from next local Saturday, otherwise variable light-ish winds. Now looks better to go around south end of Galapagos rather than the north end, to avoid adverse winds/currents.

 

Port Vallarta to Marquesas

OK winds for departure until local Tuesday, then light winds.

Weak ITCZ likely between 7N and 3N, and another convergence zone near 3 to 4S

 

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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

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