BobBlog 31 Aug 2014

WEATHERGRAM

YOTREPS

Issued 31 August 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

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 has been negative since July and has been hovering around -5 (Australian units) for the past month – enough to be called a weak El Nino. —

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SOI as shown at http://www.farmonlineweather.com.au/climate, showing we are now having a weak El Nino.

TROPICAL TOPICS

After a busy period, activity has relaxed  for the moment in the tropics and there are no Tropical Cyclones around the planet at present.

The weekly rain maps for the past fortnight show a continuing burst of rain around India. Activity between Mexico and Hawaii has eased, and there has been a well-defined band of rain across the SW Pacific. It also continues to be wet in the northern Tasman Sea.

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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

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing isobars, Lows and Highs, wind, waves, rain, current, SPCZ and STR . Purple lines are upper flow, light brown arrows are wave directions. .

SPCZ= South Pacific Convergence Zone

The SPCZ is now well established expected along 10S from 165E to 165W and is likely to drift southwards towards Samoa later this week. A weak branch of the SPCZ is drifting eastwards across French Polynesia this week and is expected to help in the formation of a low near 30S 135W from Wednesday 3 Sep UTC.

Departing westwards from Tahiti:

The convergence zone that has been lingering over French Polynesia over recent days should travel southeast off the group allowing trade winds to move onto the area by local Tuesday, good enough for departure.

The SPCZ may travel slowly south towards Samoa by late this week. If this track continues into next week then it may reach northern Tonga after 7 Sept, so keep a watch out for that

A squash zone of strong SE winds is likely to form between Tahiti and Samoa on Thur/Fri/Sat UTC and then drift to the southeast- this squash zone is associated with the northeastwards travel of a large high along 40 to 30S in the southern ocean. Uncomfortable.

STR= Sub-tropical Ridge

The STR is well south of its normal position this week. The intense high referred to in my blog last week did indeed linger over southern NZ at over 1034hPa, with lots of low cloud and drizzle under its inversion affecting various coastal parts of NZ (a dirty high). It is now east of NZ and travelling eastwards along 45S. This high is well supported aloft and slow to move, a good example of what is sometimes called an omega block.

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Blocking index for past few months showing blocking highs near NZ at a roughly 6 week interval .

From http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/real_time_sh/real_time_index_nrm.shtml

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500hPa map and anomalies for 30 Aug UTC, showing why this type of blocking high is supported by the upper air flow and is sometimes called an omega block because the 500hPa pattern resembles the shape of the capital Greek letter omega (shown here in green).

Between NZ to the tropics

Not a good week for travelling south to NZ.

There is a squash zone of enhanced NE to easterly winds between the high east of NZ and a Low tonight over northern NZ.

On Monday, Low is expected to move slowly across Northland (at 1008 it isn’t very intense but because its rain bands are slow-moving they have a high accumulative affect).

On Tuesday, that Low should finally move off to the east allowing a day of SW winds that may be good enough for setting sailing off to the north (but may encounter a period of strong winds on Thursday night near 30S).

However another Low is expected to form in western Tasman Sea on Tuesday, rapidly deepening to 990hPa on Wed/Thu and then traveling onto North Island on Fri/Sat. Its associated warm front is likely to cross North Island on Thursday and be preceded by strong northerly winds and followed by strong squally westerly winds near NZ. Avoid.

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

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

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