Bobgram

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. Compiled 26 Apr 2026 In my Ten Tips last week I referred toY2K Weather Links hub www.y2ksail.com/meteo-link.html Sadly this web page is currently offline-the server put charges up beyond reason.. Max is finding a new server for it now Our cyclone season nominally ends on Thursday. For those hoping to set sail from New Zealand to the tropics in early May, the usual idea is to go with a "Southerly flush" When the chilly southwest wind changes reach north of Auckland that can provide a good weather pattern to hop on the back end of that trough and get carried off to the tropics. That trough which crossed Northland late last week had a cold cut-off upper low attached, bringing squalls and downpours. That was the wrong pattern for departure. There was another cold upper-air cutoff early last week over Wellington. Both these patterns were "left overs" brought about the remains of Cyclone VAIANU as it came south the previous week. We now have a HIGH pushing east along 40to 45South. This is NOT conducive to a "Southerly flush". SO, we may have to wait a week or more for that typical "Southerly flush" pattern. Having said that, the easterly winds on the northern side of this HIGH may get tweaked to southeast enough to allow a reasonable voyage to the tropics. A trough is expected southwest of the Cook Islands by end of the week and that may also help twist the pattern in our favour. As for getting from eastern Australia to Noumea.: That HIGH makes for head winds for this route so stay put until a trough forms in the north Tasman Sea.then there MAY then be a OK opportunity. As winter kicks in, the voyages from NZ and Australia to the tropics will be dominated by the position of the STR or Sub tropical ridge. So here is a description of this feature.. Weather is a mix of pattern and chaos, and meteorologists concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell. Recipe for the ITCZ 1. Because the sun is most directly overhead near the equator, that's where the warmest seas are, and this causes rising air. 2. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. 3. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds. 4. The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W, or south of Easter Island. It is quasi stationary, just like the High between California and Hawaii, and has a gyre that collects a rubbish heap just as badly (see Henderson Island: blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-i n-paradise/). Recipe for the SPCZ 1. There are easterly winds on the north side of this "Andes" High: they are dry due to continental outflow from off South America. These easterly winds travel well to west of the dateline along around 10 to 15S. 2. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. These South/SE winds come and go according to the migratory high and are usually found around 15 to 25S. 3. The convergence zone between these easterly and Southeasterly winds is called the South pacific Convergence Zone, or SPCZ. It is typically located from the Solomon Islands south-eastwards to the Southern Cooks, and is around 1 to 5 degrees latitude wide, but sometimes may have large gaps or be very quiet. It is affected by many things: . the strongest cycle is the ANNUAL cycle as earth orbits the sun, making the seasons. . the Pacific Decadal Oscillation or PDO which takes many years to switch. . the El Nino/La Nina which lasts a year or so. . and the MJO which comes for a week or so every six weeks or so. Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone I have found that the easiest way to determine the position and severity of the SPCZ is to use satellite imagery, and the easiest way to decide what it may do over next few days is to use the 5day rain accumulation parameter on windy.com. The Sub Tropical Ridge STR This is the zone that separates the trades winds from the "Roaring Forties" and is where high-pressure areas (anticyclones) travel generally from west to east. In the north Atlantic these are called the "horse latitudes'. So called because, in the past, sailing ships would become becalmed in this zone and, when short of water, dump their horses when passing islands. When these Highs cross the prairies of North America during the Autumn seas, the settled weather turns the air hazy allowing the local tribes to hunt bison--- thus the phrase "Indian Summer". This week, New Zealand weather can be described as "Indian Summer". In the South Pacific these highs usually end up merging with the semi-permanent high which is found west of the South America Andes. The area east of New Zealand and south of Tahiti is a place where these Highs often becomes "blocking Highs" and slow-moving. The anticyclone track is, in summer (say, Nov. to April), usually across the Southern Australian Bight and New Zealand latitudes. The track migrates northwards in winter, following the sun. Because of the seasonal variation of land-sea temperature differences, the winter track is further north over Australia than New Zealand. Each anticyclone's strength and size change daily, growing to a maximum in about three days then decaying. Often, as one collapses, a new one forms somewhat to the southwest. It is common for an anticyclone to slow down west of New Zealand whilst a new nose of high- pressure forms east of the Southern Alps. This new centre then moves off to the east while the old one dies. WEATHER ZONES The SPCZ is active from New Guinea to Vanuatu. A weaker convergence zone is sitting over Tuvalu and another between the equator and Marquesas. The trough over Austral Island sis weakening away. The trough over New Caledonia and south of Fiji tonight is expected to drift across Tonga early this week and then go southeast and deepen into a low south of Cook Islands by end of the week. High H1 in central Tasman Sea is expected to slowly travel east across central New Zealand this week. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> If you would like more details about your voyage, check metbob.com Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom). Weathergram archive (with translator) is at weathergram.blogspot.co.nz. Contact is bob@metbob.com or text 64277762212. >>>>>>>>>>>>>>>>