Weathergram

WEATHERGRAM

YOTREPS

Issued 28 July 2013

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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Background influences

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 standardised difference in the barometer readings between Tahiti and Darwin. It maintained a value over 1 for most of June, hinting at a La Nina. During early July it dropped to 0.64 by mid-month and rose to around 0.7 between 21 and 28 July.

image

SOI as shown at http://www.farmonlineweather.com.au/climate, showing a hint of a hump and now steady.

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 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. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet. So far this year NINO3.4 has been on the cool side, but rather weak.

image

Sea temperatures along the equatorial Pacific as shown at http://www.farmonlineweather.com.au/climate, showing things to be near average.

GLOBAL TROPICAL TOPICS

TC FLOSSIE is heading for Hawaii and should make landfall there on Monday UTC.image

Low level average wind for past week , as seen at http://www.cpc.ncep.noaa.gov/products/Global_Monsoons/Global-Monsoon.shtml

Over Asia, SW/W Monsoon wind and rain are crossing Indonesia. These equatorial westerlies are likely to help rigger tropical cyclone development around Philippines early in August.

In the past week a large High/Anticyclone moved east along 40S to south of French Polynesia reaching an abnormal central pressure over1040hPa. This produced stronger than normal squash zone over French Polynesia.

imageimage

Weekly rain signatures for past fortnight,  as shown at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

image

Weather Zones (see text) as expected mid-week at 0000UTC on Thursday

South Pacific Convergence Zone SPCZ

The SPCZ changed its shape last week hugging the north end of the squash zone over French Polynesia. The squash zone is easing now, and the SPCZ is expected to drift slowly south this week, reaching Suwarrow around Thu 1 Aug UTC and Papeete around Sat/Sun 3/4 Aug UTC.

A low is expected to form at 30S to south of Rarotonga on Tuesday UTC and then deepen as it goes SE. This should ‘steal’ the wind from Southern cooks/French Polynesia making that area light and variable for a while. Another Low is expected to form over Norfolk Island area in the Tasman Sea around Thursday UTC and also deepen as it goes SE across the North island on Fri/Sat 2/3 August. Avoid these Lows.

Sub-tropical Ridge STR

This is getting weaker than last week. The High cell that was over the Tasman Sea this weekend is expected to move steadily east across North island on Monday and then to east of NZ along 40 to 45 South this week with a moderate squash zone on its northern side (not as severe as last week).

Roaring 40s and New Zealand

After the Low crosses northern NZ on Fri/Sat 2/3 August a change of weather pattern is likely. Disturbed westerly winds as far north as 30S are expected to move across the Australian Bight from Thu 1 Aug and reach NZ from Sat 3 Aug then spread to east of NZ. This may allow some weather that may be good enough for sailing away from northern NZ—next week.

Route Briefings

Tahiti to Tonga:

The SPCZ is expected to drift south onto this route from wed 31 Aug, so those who are doing this route at present should be able to finish before then. Others may as well wait for this convergence zone to go away again—maybe around 5 or 6 August

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

Weathergram with graphics is https://metbob.wordpress.com

Weathergram text only and translator is http://weathergram.blogspot.co.nz

Website http://www.metbob.com

Feedback to bob@metbob.com