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

17 December 2017

Bob Blog 17 Dec 2017

WEATHERGRAM

YOTREPS

Compiled Sun 17 December 2017

 

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 Tropics:

We are going thru a period of increased risk for development this week, caused by a passing MJO or Madden Julian Oscillation. That is a period of increased convection that spreads east from Indian Ocean to Pacific Ocean.  Accompanying this MJO there is a burst of equatorial westerly winds  EW near Papua New Guinea and this can help  trigger cyclones at either 5 degrees  North latitude or 5 degrees South latitude – sometimes at both 5N and 5S at the same time (a process called twinning).

 

Tropical cyclone Kai-Tak is travelling west across the Philippines tonight, and tropical depression 97W is forming near 5N, also a Low is within the South Pacific Convergence zone north of Fiji.  hese features are loosely linkes with some equatorial westerly winds.

 

The South Pacific Convergence Zone SPCZ is active over northern Vanuatu to northern Fiji/Tonga/Niue /Southern Cooks, and looks ripe to be able to produce a tropical depression this week.  That feature, if it deepens, is likely to travel southeast along the SPCZ, and may get a cyclone name for a while but may only briefly reach gale /storm force. Worth watching if you are in the vicinity.

 

After next week the SPCZ should settle down, and the cyclone risk is expected to lower from late Dec to mid-January.

 

The weekly rain maps at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif ,from last week and the week before, show pockets of intense convection around Philippines and north of Papua New Guinea, corresponding to Kai-Tak and 97W.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ has SE trade winds on its south side and NW winds on its northern side, and is intense across north part of Fiji to between Samoa and Tonga to Southern Cooks. The zone has a tropical low on it, to north of Fiji tonight. This Low may deepen over next few days and then travel southeast, taking with it a lot of the extra convection. Avoid.

 

Subtropical ridge (STR)

HIGH in the north Tasman Sea is expected to fade away by Sat 23 Dec.

Another High is expected to travel from Aussie Bight to South Tasman Sea on Wed/Thu and then travel east across South Island on Friday and off to the east of New Zealand from Sunday.

 

Sailing to NZ:

A southwest flow is expected over northern NZ until that High in the north Tasman Sea fades away on Friday.

After the High in the South Tasman Sea travels off to the east of NZ (early next week), there is an opportunity for a cold front to travel east across the Tasman Sea, preceded by a NW flow that may help some yachts to get to Northland. However, during next week that front is expected to stall and fade, allowing a NE flow to establish over northern NZ and that may be tricky to deal with.

 

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