Translator

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

16 September 2012

BOBGRAM issued 16 Sep 2012

WEATHERGRAM
YOTREPS
Issued 16 Sep 2012
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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

The Ocean: Only small changes have occurred in the sea surface temperature anomalies in the equatorial mid Pacific during September so far. Generally things have stalled at around the plus 0.5C mark. So the recent trend towards El Nino is now on hold.

The Atmosphere: The Southern Oscillation Index or SOI is rebounding after August's dip. On 25 Aug its 30-day running mean was -1.01, and since then it has risen to plus 0.39 on 16 September. So the atmosphere keeps oscillating about and is not ready yet to dive into an El Nino episode. Even so, computer models are indicating that El Nina may slowly arrive by end of October.

Tropical cyclones: Not as busy as last week but things are still ticking over with NADINE in mid Atlantis, LANE and KRISTY off the west coast of North America, and SANBA over southern Japan.

In the South Pacific, the Convergence Zone SPCZ is slowly reactivating after a quiet period. It is stretching from Solomons to the Rotuma /Wallis region to south of Samoa and then towards the southeast. It is not active all along this stretch but is likely to become more active this week. There is another convergence zone along 7 to 10S from 150 to 165W.
On Fri 21 Sep a convergence zone is likely to form over Fiji and this is ten expected to combine with an incoming trough from the west and produce a trough that crosses Fiji and NZ on Mon and Tue 24 and 25 Sep. Avoid.

SUBTROPICAL RIDGE STR
High H1 east of NZ near 33S 170W tonight is expected to travel steadily eastwards along 33 to 35S this week, and maintain a squash zone of enhanced trade winds to its north at around 15S as it wanders eastwards.
High H2 in Tasman Sea near 30S 160E tonight is expected to fade and stretch south and reform over and then to SE of NZ by Thursday 20 September. This weekend it is likely to travel NE and then, next week, it is likely to follow the track of H1.

NZ/Tasman Sea
This week is looking reasonable for travel between Northland and the Tropics and vice versa, with lightest winds over Northland on Wed 19 Sept. There is likely to be strong to gale NE and SW winds in places when the next trough crosses Northland on Mon and Tue 24 and 25 Sep. Avoid.

SAILING TO/FROM NORTHERN NZ.
If you are planning to sail from Tonga/Fiji/Vanuatu/ New Caledonia to NZ, October/ November, and looking to buddy with someone, then you may be interested in having a look at the ICA's "All Points Rally" to Opua, see http://www.islandcruising.co.nz/ for this voyage.

THE BEST TIME TO SAIL FOR NZ???
Well, some Insurance companies do not cover you vessel for any storm damage incurred in the tropics during cyclone season. Some companies say the SW Pacific cyclone season starts on 1 November and hence the early November rush. Other companies say the season starts 1 Dec. usually the first cyclone appears around mid to late December... but there have been very damaging cyclones such as BEBE as early as 10 Oct.
The Australian and South Pacific Cyclone RISK is still minor in the six weeks after the equinox (late Sept and during Oct) and 'acceptable' in October but rises rapidly in November and reaches the rate of around 1 per month by the end of December.
The strength of the disturbed westerlies near NZ is another factor in your timing calculation. These roaring 40s winds feed off the temperature difference between the tropics and the Polar regions. At the equinox (14:29 UTC next Sat 22 Sep.) the sun as viewed from earth is directly overhead the equator. This is when sunshine starts to return to the Antarctic Polar circle after 6 months of darkness. So Antarctica air temperatures are then the coldest of the year. So that temperature difference between tropics and pole is at the strongest of the year, So the roaring 40s are at their strongest and at their furthest north, covering NZ-- and they sort of hold this peak over NZ for the next six weeks.. These are called equinoctial gales, but I like to call them the gales of the Antarctic dawn.
Anyway the Roaring 40s generally start easing in November and have down so by December. So if you wish to time your voyage to NZ by this factor then you'll wait in the tropics for as long as you can.
The most popular time for yachts to take this spring migration is from mid-October to mid-November. It is a good idea to allow 2 or 3 weeks to the voyage and then watch the weather and depart the tropics so as to avoid the worst of the passing parade of troughs and fronts associated with the roaring 40s. These troughs and fronts are usually 5 to 7 days apart, and the journey usually takes around 9 days, so you have to go through at least one trough at some stage. Not a good idea to do so at the start or near the end of the trip, so that means usually deliberately arranging to go through one mid voyage, say at around 30S where these fronts usually are less intense that elsewhere. That's a good general strategy but it needs to be adapted according to the actual weather pattern at the time.
Cyclones tend to be triggered during an episode called of enhanced convection on the South Pacific Convergence Zone. These tend to come and go in a rhythm known as the Madden Julian Oscillation ... named after two meteorologists who identified it independently. The current state of the MJO can be seen at web sites such as http://cawcr.gov.au/staff/mwheeler/maproom/OLR_modes/h.6.MJO.S.html. This web site shows the intensity of convection of the convergence zones between 2.5S and 17.5S. Just remember that blue is bubbly (intense convection) and yellow is mellow (relatively clear skies). An MJO shows itself as a pulse of blue that travels from west to east. During the cyclone season they tend to occur once every 4 to 6 weeks. This is one of the parameters I watch as I prepare my weathergrams.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<
It seems I made a typo a few weeks back with my mention of how to download and view the Fiji weather map via email, so here's the corrected version:
First: download and install PhysPlot from http://www.pangolin.co.nz/downloads/phys_su.exe
You can read more about this program at http://www.pangolin.co.nz/physplot. This program only works on computers using Windows operating systems.
Second: When you like you may download the Nadi Fleet code by sending an email to query@saildocs.com, no message header, with subject text "send nadi-fleetcode" (without the quote marks).
When the email arrives with the code, cut and paste it into a file called (for example). Fleet.txt on your desktop. Then use file>open in the PhysPlot program to open the fleet.txt program and the code is translated into a weather map.
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at http://metbob.wordpress.com/
Weathergram text only http://weathergram.blogspot.co
Website http://www.metbob.com
Feedback to bob@metbob.com

No comments:

Blog Archive