Issued 23 October 2011
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.
A La Nina episode is kicking in. From the atmosphere, the Southern Oscillation Index SOI is positive and hovering near 1: On 16th Oct it was 1.0, and on 22nnd Oct it was 1.2. Computer modelling suggests this La Nina will continue for rest of year and into the southern hemisphere cyclone season.
This helps intensify the subtropical ridges STR and knocks then pole-wards. The stronger highs in the STR helps make stronger trade winds. There is one of these squash zones of enhanced trade winds around New Caledonia and another to NW of Fiji at present, and there is likely to be more of these squash zones this week.
The South Pacific Convergence Zone SPCZ extends from northern Coral Sea across Northern Vanuatu to Wallis Futuna… slightly weak over Tokelau/Samoa the strong again over Northern Cooks. It had a brief welcome visit to Tuvalu mid-last-week, and is expected to drift SOUTH this week.
SUBTROPICAL RIDGE STR
This is a week of Big Fat Highs (BFH)
The one building east of NZ today is expected to make steady progress to the east along around 40 to 45S and grow to over 1030hPa when well south of French Polynesia FP from Wed 26 Oct.
A new BFH is likely to cross Tasmania on Mon 24 Oct and then squeeze around South Island on Wed/Thu 26/27 Oct and then build to maybe over 1035 as moves off to east of NZ from Fri 28 Oct to Mon 31 Oct.
A squash zone of enhanced trade winds is likely to form between this new BFH and the SPCZ=== most likely around 20S to 25S form Tonga to New Caledonia from Friday 28 Oct to Tuesday 1 Nov
TASMAN SEA/NZ AREA
A transitional trough between the two Highs is expected to be windy in the east and west in the west as it crosses South Island on Monday, and then weaken as it stalls over the North Island on Tuesday/Wednesday, 25/26 Oct.
Next trough should reach South Island around Sat 29 Oct --- as a LOW is likely to form off Sydney. This front should stall over central NZ as that Low move SE along the frontal zone. Fronts should then cross Northland around Tue /Wed 1 or 2 Nov, followed by a moderate westerly wind, and Thu 3 Nov followed by a moderate SW wind.
The BFHs are in charge this week. One leaves light winds over Northland until Wed 26 Oct then E or NE/NW winds over Northland from Thu 27 Oct to Tue 01 Nov. This provides a good window for arriving in NZ, especially for those leaving Tonga /Fiji/New Cal on Mon/Tue 24/25 Oct. AT this stage the winds over Northland on 2-3 Nov are expected to be mod W /SW, so this may be OK as well.
Because the BFH may be making a squash zone from Tonga to New Cal from Fri 28 Oct to Tue 1 Nov, we may as well treat next weekend as a stay ashore weekend, maybe a good time to complete the voyage planning rather than a target date for departure.