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WEATHERGRAM

Bob McDavitt's ideas for sailing weather around the South pacific

23 April 2017

Bob Blog 23 April 2017

WEATHERGRAM
YOTREPS
Compiled Sun 23 April 2017

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 state of the ENSO
ENSO is currently neutral and forecast to move towards an El Nino over next few months.

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 standardized difference in the barometer readings between Tahiti and Darwin. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

El Nino and La Nina are tropical influences on the weather: the La Nina shifts the subtropical ridge away from the equator and the El Nino draws the subtropical ridge closer to the equator. This affects the seasonal weather all around the planet.

From May 2015 to May 2016 we had an extreme El Nino. Then there was a weak La Nina in October 2016, and since then we have had only a weak SOI.
See www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

The Ocean:
The eastern equatorial Pacific Ocean acts as a heat storage area during an El Nino, or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns. A good measure of the heat is the NINO 3.4 index

The diagram link given below compares the extreme El Nino of 2015/2016 with the previous extreme El Nino in 1997/1998. It also shows there was a period of cooler than normal sea over the past few months, and now things are slightly above normal, but not enough for this to be called El Nino.
Neutral territory is seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction model. This shows that the average of all models is for the NINO 3.4 anomaly to rise for the remainder of 2017 but no more than 1.0--- a rather weak El Nino. As usual, there is a large spread in the model data output, so low confidence.
CPC/IRI predictions are at iri.columbia.edu/our-expertise/climate/forecasts/#ENSO_Forecasts

The Tropics
On 16 April, Cyclone MAARUTHA made landfall on Myanmar with a central pressure of 996hPa. It was the first Northern Hemisphere cyclone to be named for 2017.
Another system, tropical depression CRISING, (4th tropical depression in the NW Pacific for 2017) formed over Palau on 13 April and made landfall over the Philippines, causing some fatalities and washing away buildings.
At present, there is another tropical depression near Palau and another near the south coast of Papua New Guinea. See ruc.noaa.gov/tracks/
Rain from last week, as seen at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif, compared with previous week, shows the South Pacific convergence zone is weakening. The ITCZ is about the same, and the convergence zone between Tuvalu and Galapagos is slowly fading.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is weak and mainly over northern Coral Sea this week. There is still a convergence zone between Tuvalu and Galapagos and another near Southern cooks/ French Polynesia, but these zones are slowly weakening. Tropical accumulated rainfall for next week may be seen at windyty.com

Subtropical ridge (STR)
High in Tasman sea on Monday and Tuesday is expected to travel east across North Island on Wednesday and then fade to east of NZ.

Departing NZ for the tropics:
On Monday, a LOW is expected to be travelling SE off to the east of NZ, and may produce an uncomfortable easterly swell. Tuesday is a public holiday. For the remainder of the week there are likely to be a series of troughs approaching northern NZ from the Tasman sea, and that can be uncomfortable.
Next week, on 1 May, an upper trough is expected to move off to the east of NZ and that is likely to bring a good weather pattern for sailing north from NZ.

Australia to New Caledonia:
Passing surface troughs are too close together next few days, then an upper trough is expected to move off to east of eastern seaboard on Sun 30 April, and that is likely to bring a good weather pattern for sailing to New Caledonia.

Mexico to Marquesas:
A moderate to strong Tehuantepec NE blast of wind near 95W should last until end of 24 April.
Further north, it should be useful north to NW flow for departure until 2May and then lighter WNW winds.
The ITCZ is likely to be between 9 and 6N, and light winds between 3 and 1 N. Perhaps more wind if you go a bit further west of the rhumb line. Maybe aim for 9N130W then direct.

Panama to Galapagos
There are light variable winds at Panama for much of the coming week, but forecast is for moderate southerly winds from 28 April to 5May = not so good for departure.

Galapagos to Marquesas
There is now a zone of light winds on the direct path, and a convergence zone near 5s from 100 to 110W, so recommended 5S 100W to 10S 125W then direct.

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