Bob Blog

WEATHERGRAM

YOTREPS

Compiled Sun 17 Nov 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 winter in New Zealand late August/early September, followed by a few weeks with disturbed lows and fronts, but this week is looking more on the HIGH side.

A good parameter to watch, to forecast southerly 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

 

There was a Sudden Stratospheric warming over Antarctica, and that encourages streamers of cold air to break off Antarctica into the Southern Ocean and further north at times. The SAM index shows this with its negative values since last October, and even the forecast for the next week or so is for negative SAM. Yuck.

 

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TropicsTheThe 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

Active cyclones at present are KALMAEGI, FENGSHEN and RAYMOND

Large potential for development this week over the Guam area.

 

WATHER ZONES

SPCZ=South Pacific Convergence zone.

 

The SPCZ is expected to stretch from PNG to Samoa ---a tropical Low is expected to form NW of Fiji by Wednesday. Its future is uncertain, but there is a good chance it may onto Fiji or Vanuatu next week, so get some updates.

 

There is also a convergence zone over Australs and Gambiers this week.

 

Passing trough over Tonga on Friday, expected to travel east followed by light variable winds.

 

Subtropical ridge (STR)

HIGH in north Tasman sea is expected to fade on Tuesday.

Another HIGH is expected to move into South Tasman Sea on Wednesday and stall in the Tasman Sea until maybe Tues 26 Nov.

Tasman Sea /NZ/Aus

Trough crossing northern NZ on Monday, a brief ridge of light winds on Tuesday, then another trough on Wednesday. That second trough is expected to develop a Low east of NZ from Thursday to Monday, travelling NE. This maintains a southerly wind between NZ and Tonga, so not good for getting to NZ.

For Noumea to Aus, OK this week with trade winds.

 

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