Weathergram

WEATHERGRAM

YOTREPS

Issued 24 Feb 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.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific in one number. It was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has recovered to minus 0.65 on 23 February. So it is unsteady with a negative tilt.

The Ocean: Sea surface temperatures across the equatorial Pacific act as a thermostat for the planetary weather engine. When they are different from normal we get a change in clouds around the equator and in the latitude zones of weather across the whole Pacific.

Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality. This shows up clearly in the monthly change of SST anomalies as seen at the Climate prediction centre at www.cpc.ncep.noaa.gov

image

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring

/enso_advisory/index.shtml

This shows a slight relaxing in the negative anomalies near the equator over the past month.

WEATHER ZONES

The weather is getting busy in the South Pacific over the next few weeks.

A Madden Julian Oscillation of enhanced convection is moving from the Indian Ocean onto northern Australia and towards the Coral Sea, and this is likely to increase the risk of tropical cyclone formation during the next few weeks.

At present TC HARUNA is moving away from Madagascar, there is a tropical storm over NW Australia heading for Port Hedland (landfall expected by 27 0600UTC (Wednesday), and a tropical depression off to the west of NW Australia, heading for Cocos Islands area. A low of topical origin deepened off Queensland and made landfall over northern New South Wales.

In the coming week, there are early indications in the models of a tropical low forming in the Coral Sea and moving onto New Caledonia on Fri/Sat/Sun 1/2/3 March. Also a Low is expected to form south of Fiji on Mon/Teas and then maybe fade or move south off to the east of NZ on Wed/Thu/Fri.

The South Pacific Convergence Zone SPCZ has weakened during the last week and at present extends over north Vanuatu, Fiji and Tonga, with another branch active over French Polynesia. The cloud over Fiji is started to rotate and that is likely to produce a low between Fiji and NZ by Wed 27 Feb.

image

Situation at around 5pm/0400UTC NZDT Sun 24 Feb 2013 showing SPCZ less than last week and split into two and a STR mainly between 40 and 45S. Read text for a decode of the terms

The Sub tropical ridge STR extends from Aussie Bight across the Tasman Sea and southern NZ and then eastwards along 45 to 40S. The centre over NZ is blocked by a low east of NZ and is expected to finally move off to the east on Wed and Thu 37/28 Feb. along about 35S. Between this High and the Low near Fiji there is a squash zone of enhanced SE winds around the north end of NZ.

Another High is expected to skirt around south of Tasmania on Sat 2 March and then progress NE across NZ next week.

image

http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html. This shows the anomalously high pressure over NZ in past month caused by a series of blocked anticyclones.

There has been a series of blocking anticyclones over NZ during the past month, leaving the country HIGH and DRY.

image

http://nrfa.fire.org.nz/fire_weather/fwi_today/index.htm

image

http://www.niwa.co.nz/climate/daily-climate-maps

The two diagrams above show Fire weather index and Soil moisture deficit, two ways of looking at the impact of the lack of rain—Northland is worst affected, followed by parts of Waikato/Gisborne/Hawkes Bay/Manawatu.

TASMAN SEA/NZ TROUGHS

There is one front between those two highs. It is expected to move onto over the South Island on Fri/Sat 1/2 Feb and might affect the North Island on Sunday 3 Feb, but not much rain in it.

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 http://weathergram.blogspot.co.nz

Website http://www.metbob.com

Feedback to bob@metbob.com

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