Bob Blog 25 Sep 2016

WEATHERGRAM

YOTREPS

Compiled 25 September 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.

SOI The Southern Oscillation Index SOI 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.

SOI (30 day running mean) has slipped into La Nina territory during past two weeks. It was above 10 units on this Australian scale. If it can hold this high for a month in a row the we can say we are having a La Nina.

image

SOI trend (x10) since 2013 showing us in neutral territory as at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the widest and warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. When SST in the target zone (equatorial Pacific between dateline and Galapagos) are notably cooler than normal, this is called a La Nina episode.

image

Sea surface temperature anomaly as seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

We now have cooling seas in the target area, and there MAY be a weak La Nina in the next few months

The Tropics

image

TC MEGA is in the NW Pacific, there are two tropical systems between Hawaii and Mexico, and TC KARL and LISA are in the North Atlantic.

Cyclone tracks seen on ruc.noaa.gov/tracks/

image

image

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

Rain accumulation maps show an increase in convection over Indonesia and Philippines from the west, and an easing in convection over the South Pacific.

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.

image

SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stay quiet in the Solomon islands to Tokelau area

No squash zones. A Passing trough may affect from Tonga to Niue late in the week.

Subtropical ridge (STR)

This is expected to remain weak and along about 20 to 30S.It is NOT often this far north and is weakening all the trade winds, offering rare opportunities for getting east in an Island hopping fashion if you like.

image

Rain accumulation may be seen at windyty.com

Next High is expected to spread to east of NZ from the SOUTH after Wednesday, shovelling polar chilled southerly winds northwards so that they deepen a LOW near 35S 165W late in the week. Avoid travelling direct between NZ and Tahiti.

Travel from Tahiti to Tonga:

The weak winds that have been affecting Tahiti recently should be over from tomorrow. Go for it. No squash zones, and no strong convergence zones, but voyages are likely to encounter a passing trough with maybe some showers and varying winds for a day or two. It is looking OK to venture via Suwarrow this week.

Travel between the Tropics and NZ:

A series of LOWS continue to travel across Tasman Sea / Northern NZ area over next 10 days, with a brief break for a passing ridge on Thu/Fri.

These lows are not really making gales, but the fronts do have strong winds. Avoid arrival/departure on Sat 1 Oct or Thu 6 Oct.

Travel between New Caledonia and Australia:

Intense TROUGH crossing north Tasman sea on Mon 26 Sep, then OK weather for Australia to New Caledonia. Another intense trough in N Tasman sea on Thu/Fri 29/30 Sep to avoid.

As for getting form New Caledonia to Australia: not this week, too much headwind, and probably not next week. Maybe OK from around 8-10 Oct.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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

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

Tell anyone you like that to subscribe they should email me.

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.

To unsubscribe from WordPress: click the “unsubscribe” link on the bottom of the email. Or, if email wasn’t from WordPress then send a reply email saying LEAVE.