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.

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