Bob Blog

WEATHERGRAM
YOTREPS
Issued 12 April 2015
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.

The Atmosphere:
The Southern Oscillation Index SOI sums up the weather pattern over the
South Pacific as one number and is based on the standardized difference in
the barometer readings between Tahiti and Darwin (30 day running mean).
It switched briefly blue and positive at the start of March but has now
gone very negative again. If it stays this negative for 2 more weeks then we
can call this an El Nino event.
SOI is shown at
http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekl
y

The Ocean
The amount of heat that is being stored in the sea in the Eastern Equatorial
pacific has been above normal since late 2014, as measured by the NINO3.4
index.
NINO3.4 is shown at
http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi,
confirming a trend towards El Nino.

During an El Nino episode, weather patterns tend to be drawn closer to the
equator. The subtropical ridge in the southern hemisphere tends to be north
of its normal position and this weakens the trade winds. The South Pacific
Convergence zone tends to be tugged north and east of its normal position.

TROPICAL TOPICS
In last week s weathergram we looked at the possibility of a tropical
cyclone forming in the Coral Sea by 20 April: well, this happened earlier
than expected and TC SOLO moved quickly across the Loyalty island
group today. There may still be one more cyclone forming around 20 April,
to finish off the season, and now it is more likely to form around the Timor
Sea, rather than the Coral Sea.
Observations from http://www.wunderground.com for Noumea Airport as TC
SOLO went past show that basically it was a gang of squalls.

In the Indian Ocean, east of La Reunion Island, Tropical cyclone JOALANE is
traveling south and intensifying.

The rain map for the last two weeks show the paths of two cyclone sin the
Indian Ocean, an d the build-up of activity in the past week in the Coral
Sea. In the past week the ITCZ has restored to normal intensity and
position, and there is a mirror tropical Convergence zone near 4 to 5S
from near 100W to 130W. Weekly rain signatures may be seen at
http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES
Panama to Galapagos
Maybe too late now to get a good voyage along this route. The head
component of a S/SW wind is spreading north across the zone. There is a
good south going current especially near 82W from 5N to 2N.

Galapagos to Marquesas:
This week a convergence zone is likely between 4 and 5S from 100W to 130W,
avoid this by heading for 6S between 95and 100W. Trade winds are south of 5S
and may be unlikely to get any further north because of that convergence
zone have been moderate to strong for the past week, but if the El Nino
continues to kick in then winds may ease a lot in the next week or so. There
is a good west going current along 5S from 105W to 120W.

Winds around Galapagos, from www.windyty.com, show bands of counter-wind:
west and SW winds near the equator kicking in by Thursday: an El Nino sign.
The trade winds are staying south of 5S.

SPCZ=South Pacific Convergence zone.
SPCZ is reasonable active and spread out into different branches. Lows are
likely to form over or around Pitcairn Islands on Wednesday and Friday.

STR= Sub-tropical Ridge
The STR extends from Australian Bight across Australia to Tasman Sea then to
north of NZ and then cycles north and south across the South Pacific. Next
HIGH to cross Tasman Sea on Tues/Wed 14/15 should fade over northern NZ by
Fri 17 April.

Departing from Northern NZ for the tropics.
It will certainly feel cold enough for departure by mid-week, as a Low
deepens east of NZ, directing air from the Antarctic ice shelf to NZ.
And when these SW winds start easing on Wednesday there is a good
opportunity for any early-birds who would like to get off to the north or to
Australia On Friday and Saturday another cold front is expected to usher in
another southern change across the Islands.

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
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