Weathergram

WEATHERGRAM

YOTREPS

Issued 23 June 2013

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.

Background influences

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin. It rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, reaching 1.2 from 9 to 16 June, and relaxing to 0.8 by 23 June. So the signal coming from the Atmosphere has been erratic and at present is hinting at a La Nina.

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SOI as shown at http://www.farmonlineweather.com.au/climate, showing a hint of a hump to a La Nina.

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus the Sea surface temperatures SST across the equatorial Pacific may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet. So far this year NINO3.4 has been on the cool side, but not strongly so, and continues in this trend.

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Sea temperatures along the equatorial Pacific as shown at http://www.farmonlineweather.com.au/climate, showing things to juts cooler than average. .

Tropical cyclones and Monsoon

No tropical cyclones around at present. The Monsoon has been generally quieter than normal for the past month over India but a deluge over RUDRAPRAYAG in northern India has recently produced damaging landslides. This shows up as a blue area near Nepal in the anomaly rain map here.

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As shown at http://trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

The monthly rain anomaly map clearer shows a pole-ward shift of the ITCZ. This is a La Nina trait

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday

South Pacific Convergence Zone SPCZ

The SPCZ is extending eastwards along 15S, with a branch over Vanuatu and another over Northern cooks to French Polynesia. The branch that is likely to be over Vanuatu until mid-week is expected to move east across Fiji and Tonga by the end of the week on Fri/Sat 28/29 June.

Sub-tropical Ridge STR

All that remains of the STR east of 180 is a thin line mainly along 25 to 30S.

A new high cell with central pressure over 1030 is moving east over Tasmania tonight and is expected to be deflected southeast to south Tasman by Wednesday 26 June and then northwards so that it may cross the North Island on Sat/Sun 29/30 June.

Roaring 40s and New Zealand

The eggbeater southerly that was mentioned in last week’s Weathergram was a remarkable Low, bringing snow for several days to the South Island so that there are now 1 to 2 metre drifts of snow on the upper Canterbury plains. Rain caused landslides in Dunedin and flooding in Christchurch, Wellington had a stormy night and swells rose to significant height of 7 metres (occasional 11 and maximum 15 metres) thru Cook Strait, and Auckland had some lightning and hail damage. The swells rotating around this system have reached Fiji/Tonga/Niue (surfs up) and are expected to reach Southern Cooks on Monday and French Polynesia around Tuesday/Wednesday –and these may well be exacerbated by spring tides on 24th to 26th June of tonight’s perigean FULL MOON tonight (some call it a super moon, and it is indeed the largest looking of 2013). Check it out if you can.

This week another low is expected- but this time it is forming NORTH of the STR rather than in the southern ocean. Its clouds are already spinning off the east Australian Coast and it is expected to move quickly along 30S- reaching 180 by Thursday 27 UTC. Another may form in the Tasman Sea next Monday 1 July and travel more slowly eastwards.

Route weather picks

Tahiti to Tonga:

The swells from the SW should ease again after Wednesday. The SPCZ is expected to cross southern parts of this route from Sunday 30 to Tuesday 2 July. SO If you go, take the northern route via Suwarrow.

NZ to the Tropics.

A dying southerly on Monday and light winds on Tuesday- but if you depart on these days you are likely to encounter strong head winds on Tuesday night and Wednesday. A more comfortable voyage may happen with a Thursday departure, as this should get far enough North by mid-next-week to avoid the next low.

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is https://metbob.wordpress.com

Weathergram text only and translator is http://weathergram.blogspot.co.nz

Website http://www.metbob.com

Feedback to bob@metbob.com