Bob Blog

WEATHERGRAM

YOTREPS

Issued 30 November 2014

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

I’ve just been introduced to another website presenting the weather data in an interesting way http://www.meteoearth.com/

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Screenshot from MeteoEarth showing track of Tropical Depression SINLAKU and winds and a burst of social media reports about showers in Japan

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 standardized difference in the barometer readings between Tahiti and Darwin.

It has been negative since July and dived below -10 (Australian units) for much of September, relaxed for a while in October, and dived below -10 early in early November, but is now relaxing again.

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SOI as shown at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi, showing that we are now having an at-least-weak El Nino.

We know that in an El Nino event the latitudes of the normal weather zones are drawn closer to the equator, causing the South Pacific Convergence zone to shift further north and east of its normal position.

Well, as November draws to a close we can look at the average weather map mid-month, thanks to NOAA and this shows Highs have been stronger than normal in the north Tasman sea, and just south of South Africa, and lows in the southern ocean have been deeper and more north than normal in south Indian Ocean, and to SE of NZ, and to SE of South America. The extra highs in north Tasman Sea and lows to SE of NZ have combined to give extra SW winds over NZ.

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Isobars averaged over 30 days to mid-November, and their anomalies, a s seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30.fnl.anim.html and http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

NIWA have issued a news release about New Zealand’s November weather at http://www.scoop.co.nz/stories/SC1411/S00083.htm

They say there has been more SW winds than normal over New Zealand during November. The rainfall, as measured by the SPI (Standardized Precipitation Index) has been close to normal in most places but the extra SW winds show as patches of extra rainfall in the W and south of the South Island.

The extra SW winds are causing soils to dry out faster than normal in eastern North island places from Northland thru Coromandel and Bay of Plenty to Gisborne/Hawkes Bay/and the southern quarter of the North Island, and the eastern South Island.

They also say, and I quote “It should be noted that this rainfall pattern is commonly associated with El Niño events and while El Niño has not been officially declared, it may be an indication that such an event is imminent.”

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The Standardized Precipitation Index on NZ for 30 days ending 30/11/2014 as shown at

http://www.niwa.co.nz/static/climate/last30daysspi.png?1234

TROPICAL TOPICS

Last weekend Cyclone Adjali was the first of the new season in the Indian Ocean and was heading towards Mauritius before it weakened. The Volvo Race fleet diverted clockwise around another system now near 20S 60E and one of fleet, Sailing Vessel Team Vestas Wind, ended up sailing onto a reef (at night) in the Cargados Carajos Shoals around 400 km to northeast of Mauritius. Ouch.

At present the remains of Cyclone SINLAKU are fading over China.

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Another area of interest is developing in the NW Pacific and is expected to become a Cyclone over Micronesia by Tuesday and travel west towards Philippines late this week.

There are a couple of areas of interest in the Indian Ocean, one near 20S 60E and another near 11S 90E – I suppose we can declare the Southern Hemisphere cyclone season to now be officially open.

Weekly rain maps over the past fortnight show an increase in convection over the past week in the Indian Ocean and in the zone from north of Solomon Island along to Samoa. There was flooding in Samoa early last week, but the feature responsible then faded as forecast in last week’s weathergram.

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Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

This increase in convection in the Indian Ocean is possibly part of a MJO cycle that will gradually make its way into the Pacific Ocean over the next few weeks. By mid December this is expected to increases the risk of a tropical cyclone generating in the area north of Vanuatu.

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As shown at http://www.meteo.nc/cyclone/coin-des-experts

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS/WW3/NOGAPS models) showing isobars, Sig wave height green lines, swell direction arrows SPCZ and STR.

SPCZ=South Pacific Convergence zone.

SPCZ may not be as strong as it was last week, and is expected to be located near the north end of its normal range, stretching from north of Solomon island to northern Vanuatu and the east across Tuvalu to Samoa/Tokelau, with scattered convection also over French Polynesia.

A burst of northerly swell reaching 2 to 3metres is expected to spread south and east over the next few days – it should be most noticeable about and just north of the SPCZ and roughen the sea there. Not the best for good diving or fishing between northern Vanuatu and Samoa this week.

STR= Sub-tropical Ridge

The STR is also strong and well defined and mostly at its normal latitude for the time of year, but now north of normal in zone east of NZ. It has been north of normal in the NZ area but is expected to be closer to normal latitude this week as the HIGH tonight in the south Tasman Se is expected to travel NE across the North Island on Tuesday night/Wednesday and then be slow-moving off the NE of NZ until next week, so that approaching troughs stall in the Tasman Sea.

HIGH near 35S 130W, to the south of Pitcairn Island, is expected to be slow-moving and peak in intensity over 1032 hPa by Tuesday UTC and is expected to stay put until Wednesday and then slowly weaken and move NE. There are likely to be strong easterly and NE winds on the northern side of this high, especially between 15 and 20S until Friday.

Between the tropics and NZ

Good weather this week for sailing from the tropics to NZ.

No good arriving on Monday 1 December because that’s when a cold trough is expected to cross the North Island. Then there is an easterly flow expected on the north side of the high that is expected to cross the North Island on Tuesday night/Wednesday.

After Wednesday there is expected to be a useful N to NW flow over northern NZ, good for arrival. This NW flow may get strong and unsettled on Tuesday 9 December, as a trough travels east across southern NZ. The NW flow should be Ok again on Wednesday 10 Dec for arrival, and then northern NZ may become unsettled as troughs start to arrive from the Tasman Sea.

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

Weathergram text only (and translator) is at weathergram.blogspot.co.nz

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

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