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

06 March 2016

BOB BLOG 6 March 2016

WEATHERGRAM

YOTREPS

Issued 6 March 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.

 

February Weather maps

During February the averaged isobars show the Siberian high and the Aleutian low and the Bermuda High in their normal positions. The Highs in the Southern Hemisphere have been migratory, with average position of the subtropical ridge along 40S across the Australian Bight to east of central NZ, but closer to 30S in the South Indian ocean.

These may be seen at http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30.fnl.anim.html

Looking at the variations from normal, we can see that the subtropical ridge in the southern hemisphere is stronger and further north than normal, especially around Western Australia. Since TC WINSTON has been bothering the South Pacific for most of February, it has left its mark on the anomaly map too. This anomalous low combines with an anomalous high east of NZ to produce an anomalous NE flow onto northern NZ -- this can be seen at

http://www.esrl.noaa.gov/psd/map/images/fnl/slp_30a.fnl.anim.html

 

Early March

During early March the sun is directly overhead 7S to 4S (a table of the daily changes in sun declination angle may be seen at http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html)

This tends to trigger an extra convergence zone in those latitudes between Galapagos and Marquesas. Some call it a mirror of the ITCZ that forms around the March equinox. This can be seen forming now (see http://wwlln.net/WWLLN_movies/Movie_of_Lightning_in_Americas_BIG.gif) and offers an obstacle to vessels travelling between Galapagos and Marquesas over next few weeks.

 

The Tropics:

No tropical cyclone anywhere at present.

The remains of WINSTON made landfall on the Queensland coast in Australian on Friday 4 March (local). It started as TD09F from Feb 8th, was named as TC Winston on Feb 11th, crossed Fiji as a Cat 5 on Feb 20th, was downgraded Feb 25th, and so has been an entity for 25 days. TC REWA holds the record for longest cyclone duration in South Pacific Dec 1993-Jan 1994 26 days. See https://www.facebook.com/metbobnz/ for track maps.

During March the likelihood of formation of a tropical cyclone in the South Pacific remains high, however over the next few weeks it is expected to ease. It will take a few weeks for the South pacific Convergence zone to replenish itself after WINSTON. Meteo France (New Caledonia) indicate this with a drop of the probability of a cyclone to low levels by mid-March. This can be seen at http://www.meteo.nc/nouvelle-caledonie/cyclone/phenomenes-en-cours

 

Rainfall in the tropics over the past week shows that the South Pacific Convergence zone has been strong near French Polynesia in the past week (thanks to the brief appearance of TC Yalo). It also shows the formation of the "extra" convergence zone between Marquesas and Galapagos.

As seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

Panama to Galapagos:

Light winds around Panama for the start this week. May be better to wait for more useful NE winds next week.

 

WEATHER ZONES

 

SPCZ=South Pacific Convergence zone.

SPCZ is expected to spend this week regenerating in the Coral Sea. There is still a part of it lying around Tuvalu and Samoa, and a developing extra zone north of 5S between 160W and 100W.

 

 

STR= Sub-tropical Ridge

HIGH to east of NZ is expected to travel east along 35/40S and form a zone of enhanced trade winds on its northern side between 20 and 30S.

HIGH travelling east across Tasman Sea is expected to linger and fade in northern Tasman Sea on Thursday.

Next HIGH is expected to travel east along 50S into south Tasman Sea on Thursday, shovelling colder southerly winds onto the South island. This HIGH is then expected to cross southern Z on Friday and build east of the South Island on Sat/Sun 12/13 March.

 

For NZ and Tasman Sea:

Weak front on Monday/Tuesday followed by SW wind change.

Stronger front over South Island on Wednesday and North Island on Thursday followed by colder southerlies, turning SE/E across northern NZ by Saturday.

Rain with these wind changes.

>>>>>> 

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

See my website www.metbob.com for information on tailor-made voyage forecasts– Feedback to bob@metbob.com.

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

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

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