Bob Blog 4 Dec 2016

WEATHERGRAM

YOTREPS

Compiled 4 December 2016

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.

Any visitors to New Zealand who are thinking of doing some sailing around the Hauraki Gulf this summer: please note that I am President of CANANZ, a group of yachties in AUCKLAND who are interested in cruising and navigation. We are also interested in helping visiting cruising yachts with a little hospitality, and willing to answer questions about Auckland and the Hauraki Gulf. See our webpage at http://www.cananz.org.nz for more details.

Last Month

The averaged isobars for November shows pattern, with the normal weather features in the normal places: The Siberian high, the north pacific low and its “Pacific” or “Marine” high between California and Hawaii, the Azores high and the high that is quasi-stationary in the South pacific, which I call the ”Andes High” for it is struck west of South America by the Andes mountains. The monsoonal trough extending in November from Indonesia to Darwin. The low/brief rainy seasons over Kilimanjaro, and the start of the low/rainy season over the Amazon.

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Averaged isobars for November from www.esrl.noaa.gov/psd/map/images/fnl/slp_30.fnl.anim.html

By looking at the variations of these isobars from there long term November values we can see a measure of the chaos the was operating last month, the anomalies. That “Marine high“ in the NE pacific id more intense than normal and located closer the Hawaii, making a squash zone of stronger than normal easterly winds just south of Hawaii. The Azores High is displaced towards UK so there is a cold northerly flow over UK.

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And the pressures are much lower than normal in the Southern Ocean, breeding stronger westerly wind onto south Australia and all of NZ

November pressure anomalies as seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

A look at November’s rainfall and its anomaly shows some wet patches in the tropics from a few depressions and cyclones. One interesting feature these maps shows is the at there is a detectable shift AWAY from the equator of the Intertropical Convergence zone. This is a La Nina trait, and there does seem to be a trend towards a weak La Nina pattern in the east pacific anyway. The La Nina causes weather patterns in the tropics and subtropics to moves away from the equator. The implication of this in the Australia /NZ area is that the subtropical ridge may shift southwards—no sign of this yet, but we may well see a southward shift of the highs that travel along the subtropical ridge during December. That’s my prediction anyway.

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Rainfall for November as seen at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

The Tropics

The two depressions that were in the South pacific last week faded away OK.

No cyclones around at present and none are expected in the South Pacific this week, but this next increase in activity is likely after 15 Dec.

Rain accumulation weekly maps shows wet patches in the tropics from passing lows. The Intertropical convergence zone seems to show increasing activity.

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Rain for the past fortnight from trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, swell black arrows / Sig wave height purple lines, swell and wind waves, SPCZ and STR.

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SPCZ=South Pacific Convergence zone.

A weak trough is expected to pass off to southeast of Fiji on Monday UTC and pass across the Tonga Niue area on Tuesday /Wednesday UTC, then deepen into a low and it travels off to the southeast. Trade winds should return to Fiji by Tuesday UTC and to Tonga from late Thursday UTC.

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Tropical accumulated rainfall for next week from windyty.com

Subtropical ridge (STR)

The subtropical ridge spread out across NZ /Tasman Sea tonight should travel off to east along 40S on Mon/Tues and continue along 40 to 35S.

A new HIGH is expected to form east of New South Wales late in the week and remain slow-moving in the north Tasman sea at around 30S next week, stretching a tongue out eastwards along 30S. This is a light-wind barrier to tropical/NZ or Aus sailing, but its low swells should be good for power vessels.

Tasman Sea/NZ Area troughs.

Two troughs to avoid this week. First is expected to cross Sydney on local Monday night/Tuesday, mid Tasman and South Island on wed, and then weaken over the North Island on Thursday local, preceded by NW winds accompanied by rain and followed by disturbed SW/W winds.

Second trough is expected to reach New South wales area by late Thursday, mid Tasman on Friday and NZ area by Saturday local with STRONG winds and squally showers.

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