Bob Blog 18 Aug

WEATHERGRAM

YOTREPS

Compiled Sun 18 August 2019

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.

SAM

We have had a taste of wintery weather in New Zealand during past two weeks, A good parameter to watch, to forecast these blasts is the Southern Annular Mode (SAM), or its proxy called the Antarctic Oscillation (AAO),

SAM describes the north to south movement of the westerly wind belt that circles Antarctica, so it can be used as a predictor for cold air reaching our mid-latitudes.

When SAM is positive, the belt of strong westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over mid latitudes, limiting the northern extent of cold fronts.

A negative SAM event indicates an expansion of the belt of strong westerly winds into the mid-latitudes. This weather pattern results in low pressure systems over mid-latitudes such as New Zealand, with more (and stronger) storms.

We can see the latest forecasts for the AAO (a good proxy for SAM) at

www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html

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This shows a gradual rise from negative mode towards maybe something more positive, but there is a large amount of variation in the ensemble. We are still in the grips of winter.

THE TROPICS

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are currently no tropical cyclones. LEKIMA made landfalls in southeast Zhejianng province (Cat 2) followed by KROSA hitting mainly southwest japan and killing three.

Activity in the SE Asia area is expected to continue during the next week or two due to a Madden-Julian Oscillation event transiting the region.

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WEATHER ZONES

Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)

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SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from Solomon Islands across northern Vanuatu to northern Fiji, with another CZ hovering from Tokelau to Suwarrow as a passing trough.

Passing trough over Niue on local Sunday, to Palmerston/Aitutaki / Suwarrow local Monday, and Papeete on local Monday to Wednesday, with a swing to NE then SE winds.

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Accumulated rainfall for next week from windyty.com.

Subtropical ridge (STR)

The STR remains north of NZ this week, but a new High is expected to spread into the Tasman Sea near 30S from Wednesday, passing across the north of NZ on the weekend. This may be followed by another High across the Tasman early next week.

Tasman Sea /NZ/Aus

Trough is expected to visit NZ on local Tuesday, followed by strong west to SW winds in the Tasman until Friday, with SW 4m swells reaching as far north as Fiji by Saturday. Peak swell in western Tasman late Thursday over 8m. Trans-Tasman and NZ to tropics voyages this week should wait out those swells.

Tahiti to Tonga

Looks good to go this week with rather weak winds. May want to go north of rhumb line to avoid light winds, and larger swells.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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