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 31 May 2026 And they're off... The Solo Trans Tasman is now underway, and tonight the fleet are taking on an active front: www.solo-tasman.co.nz/race-live-2026/ And tonight's satpic shows what the challenge looks like The LOW crossing the Tasman Sea is followed by several shower bands. GLOBAL HEAT Extreme heat waves across South Asia and Europe caused deaths, overwhelmed hospitals and shattered temperature records as scientists warn that dangerous heat is becoming increasingly common and more intense in a warming world. In India, dozens of people died from heat-related causes. At the same time, Europe experienced unusually early heat for May, with Britain recording its hottest May temperature ever at 94.6 degrees Fahrenheit/35C. TROPICS Tropical Cyclone Jangmi is aiming for the east coast of Japan. WEATHER ZONES Wind and rain accumulation this week from Windy.com Rain accumulation from windy.com shows a rather weak and unorganised SPCZ. The Rain accumulation shows a rather weak SPCZ north of Samoa, and an intense group of rain bands around a Low L1 centred south of the Australs, and affecting the Southern Cooks. Wind accumulation this week from Windy.com shows strong wind areas associated with L1 and FRONT1. LOWS and HIGHS Low L1 south of the Australs is expected to move off to the southeast HIGH H1 is expected to remain east of NZ and near 160 to 170W/ Rain is associated the FRONT1 which is crossing North Island on Monday. It also brings some intense falls as far as Minerva on Tuesday night. In the Tasman Se it is followed by several lines of thundery showers each with NW winds. This is the challenge facing the solo trans-Tasman racers this week. Low L2 has deepened SW of Perth over the weekend and is expected to remain an intense system reaching Tasmania by mid-week. Its associated FRONT is expected to cross NZ next weekend and then L2 is expected to deepen around Chathams with an upper cold pool and cut off low next week. Following L2 a southerly flow covers NZ and then HIGH H2 which is expected to travel east along 35 to 40S. This is the classic good pattern for sailing from NZ to the tropics. Hopefully this ends the recent "low zonal index" (google this for more info) weather pattern that has been affected the South Pacific in May GFS is toying with the idea of a low forming near Fiji around 10-11 June, but other models don't agree with that idea. Just keep an eye on that if travelling. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Bobgram Tahiti to Tonga

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 24 May 2026 Tahiti to Tonga Now that many yachts are about to travel west from Tahiti to Tonga, it is time to review the weather, in general terms, for this route. The main players: 1. Getting from the Tahiti area to Tonga/Fiji means crossing the South Pacific Convergence zone, SPCZ. Like a dragon, it dances . This zone may be weak or may contain squally showers. In the models it is portrayed as a zone of light winds. To see a forecast for this zone, use windy.com and rain accumulation for a period over five days. This zone may linger in the north (it does this in an El Nino) or it usually hovers between Samoa and Southern cooks. To avoid it, choose a different latitude or aim for gaps in the zone. The SPCZ goes thru periods of activity and inactivity and is most active when there is the positive phase of an MJO (Madden Julian Oscillation). Occasionally a tropical low/passing trough will travel east/southeast along the zone, making for a burst of wind/rain travelling east from Fiji to Niue then SE across Southern cooks. These occur on occasions even at this time of the year and can be forecast beforehand and avoided. 2. Another thing to avoid is a SQUASH ZONE when a large High travels east along 30S. When the central pressure in the High is 1030hpa or above, it gets dirty (isobars get bunched together along its rim). So in the tropics the isobars marking the trade winds may get squashed together making a "squash zone" of enhanced winds and rough seas. These usually occur around 20 to 25 South and may last for several days. In Fiji this is called BOGIWALU, in Tonga AHO VALO, and in Tahiti the name used is MARA'AMU. They are reasonably easy to forecast and are worth avoiding. If caught in one, change your latitude rather than your longitude. Bogiwalu sweeps through palm leaves and pandanus; Salt spray and rain drive inland from the seas; Young August shields her face and droops her banners. And strips for flight, and with the wind she flees. July would fain forego her glorious gift of sunshine To breast a portion of the eight nights' strain. July's apparel-not for pain but peace time- Is armour poor against the wind and rain. Oh! linger not, July-with heart-warm kiss of sweetness: Go now-your golden sun-robe scarcely tom. Young August, swift and slim for greater fleetness, Plies through the years, and onward we are borne. V. M. P. COSTER. Suva Point, Fiji, 29/7/1939. (taken from PIM online) 3. The third thing to avoid is the PASSING TROUGH or passing low. The systems tend to form on the eastern end of the SPCZ in which case they travel southeast, propelled by upper NW winds as a steering field. OR they may form on the NW (back) corner of a travelling HIGH., in which case they may get shunted south or sometime to the southwest. They often have their own mini-squash zones (on their polar/southern side), and calm zones (on their equatorial/northern side). Isobars give a clue about the intensity of a passing trough: those above 1010hP are usually weak and those below 1007 are worth avoiding. There are three routes between Tahiti and Tonga, the northern, central and southern: 1. Sometimes the northern route via Suwarrow/Samoa is favored, and sometimes not. The northern route goes via Suwarrow to Samoa. Often this is covered by the SPCZ, but when the SPCZ is further south the northern route and its northeast winds may be best. 2. The middle-route offers stopovers such a Palmerston Island and Niue and is a good alternative when the SPCZ is further north. Watch out for passing troughs and squash zones . these come and go. The middle route allows short hops. 3. The southern route tends to get higher swells from the Southern Ocean SO, all in all, the "best voyage" changes every few days . sometimes it's worth waiting for something that suits, sometimes it isn't. Basically, avoid passing troughs and squash zones, and go thru the SPCZ where/when it is weak. This may mean taking short hops rather than getting direct from Tahiti to Tonga in one go. =-=-=-=-=-=-=-=-=-=-=- TROPICS No named storms at present WEATHER ZONES LOWS and HIGHS LowL1 over the Austral's tonight is finally expected to move off to the southeast after a week of travelling due eats. Low L2 has formed on western side of a cold pool aloft and thus been tracking to the NE . It is expected to weaken over Fiji and Tonga next few days but have lots of lightning in its squall clouds (over eastern Fiji and Tonga tonight). It is expected to travel ESE towards the Austral's this week, following L1. High Hi is quasi stationary over New Zealand. After a SW change along the east coast on Wednesday, it should be replaced by High H2 from Australia. Low L3 is forming under a cold pool over central Australia, so its rain clouds may be intense. Its wind and rain are expected to form a deepening low near Lord Howe by Friday which is expected to bring intense wind and rain from Fiji to New Zealand by Sunday (and that's the middle of a long weekend in New Zealand). Avoid. Rain accumulation from windy.com shows a rather weak and unorganised SPCZ. Also well-defined areas of rain associated with L1, L2 and L3. Parts of Australia have had their wettest May on record, and L3 takes another crack at Sydney. Wind accumulation this week from Windy.com shows a squash zone just north of NZ as part of L2 and the winds associated with L1 mainly about the Austral Islands, and winds from L3 spreading across the Tasman Sea with the approach of the Kings Birthday long weekend. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

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 17 May 2026 TEN REASONS TO HATE A HIGH WE have a 1040hPa anticyclone on our weather map this week. Time to review the ten reasons we should beware of Big Fat Highs. They have their undesirable attributes, especially when they stall. 1. Near the centre are "dead" winds and usually an area of low cloud - trapped below a subsidence inversion - causing dull days called "anticyclonic gloom", or dirty air which may turn into fog. T 2 Round the rim, winds are strong. "If the central pressure is over 1030, the rim may get dirty" --look for a gale somewhere on the outside of a high. 3. Highs intensify the trade winds in the tropics. It may take about a week for a high to travel eastwards past New Zealand, and during this time the stronger trade winds tend to give night-time rain to the eastern side of the larger tropical Islands. In Fiji this is sometimes called BOGI WALU. In Tahitian MARA'AMU 4. The bigger the highs are, the slower they move, blocking the fronts and lows that are trying to follow them. When this block is released, the western ("back") end of a high may become a breeding ground for storms. See www.abc.net.au/news/2026-05-16/most-widespread-may-rain-to-reach-all-austral ian-states/106685072 5. Intensifying highs tend to squash together the isobars between themselves and any nearby low pressure centres, creating "squash zones". 6. A deepening low-pressure system and an intense or lingering anticyclone get together like the arms of an eggbeater and create a zone of enhanced wind and rain. 7. As air flows around a high, it spins out across the isobars and speeds up until it is as much as 20% MORE than that indicated by the isobar-spacing. 8. If a range of mountains blocks the air flowing around a high, the air tends to squeeze around the mountains rather than flow over them. This splits the wind flow over New Zealand into rivers of wind and puddles of calm. Sometimes a narrow gap is made just above the mountains through which pent-up air may be suddenly released at a rapid rate. 9. In winter and spring, a high may bring unwelcome frosty fogs 10. In summer and autumn, a high may allow sea breezes to converge and, if it is cold enough aloft, this can form thunderstorms and hail. TROPICS There are no named storms this evening. WEATHER ZONES SPCZ is weak and scattered this week. There is also a convergence zone lingering around French Polynesia. HIGH H1 over NZ was over1040hPa this weekend - a BFG or Big Fat High. It is expected to slowly travel along 40S. Low L1 is expected to form south of Niue and then move south and east. A squash zone of rain and enhanced wind and swell should form on its south side. On the back end of the BFH there has been a build-up of tropical moisture over Australia, as evidenced by the intense recent rain. This week that moisture is expected to enter the Tasman Sea and produce a series of Lows. These lows , being on the back end of a BFH are likely to stall and rotate around each other. Avoid. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>>>

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 03 May 2026 Some good news:  Thanks to the persistence and generosity of Max on Y2K The link listing useful weather links for the South Pacific is back www.y2ksail.com/meteo-link.html Try this out and if you like it Buy him a beer (it’s the bottom link) A review of last month’s weather Here is a link to a YouTube clip giving an animated loop of the isobars and streamlines For last month is at youtu.be/GjGUycWtUYo Cyclone Vaianu breezed across NZ quickly on 11 April and a cold pool convergence zone flooded Wellington on 20 April The 500hPa heights and anomalies shows that prolonged upper low  over Tasman sea late in the month From http://www.cpc.ncep.noaa.gov/products/intraseasonal/z500_sh_anim.shtml Sea Surface temperature anomalies from psl.noaa.gov/map/clim/sst.shtml The waters over eastern equatorial Pacific are warming --- El Nino is coming. The cool zone around NW of Australia is relaxing . There is a well established warm zone between Hawaii and LA. The cool (dry) zone around NW Australia is shrinking. Average isobars for past month (below) The subtropical ridge in the southern hemisphere has further  intensified and shifted north. The HIGHS over Siberia, over the Aleutians and in the North Atlantic have all weakened . Pressure anomalies for past two months (below) The anomaly pressure pattern for last month still shows a low breeding area around Hawaii.   The southern subtropical ridge intensification is well developed east of NZ, but there is still a low breeding area between Tonga and NZ, and another over Australian Bight. The 1015 isobar has shifted north over Australia to Cairns and the 1020 spans from Melbourne to Brisbane. TROPICS No named storms around the planet this evening. WEATHER ZONES SPCZ is weak and scattered this week, but there is some severe convection associated with the clouds around L1, a tropical low that is expected to form on Wednesday around the Southern Cooks --- on the NW shoulder of the large High H1 that is travelling east along 40S and to east of NZ.  L1 is expected to deepen as it travels south near 160W this weekend and early next week. There are two lows and troughs crossing the Tasman Sea /NZ area this week. L1 is expected to weaken over central NZ on Thursday. Then L2 should be followed by a strong cold SW flow on Saturday. If L2 then moves off to the east without leaving behind a secondary development, and if the High following L2 travels steadily along 40S with a central pressure between 1025 and 1030 (no more, no less), then this pattern is consistent with what I call a Southerly flush…. The arrival of the cold air in Northland is such that the marinas are flushed empty of those yachts seeking the warmth of the tropics.  I’m not sure yet, and we may have to wait until Wednesday before the differing models start agreeing. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>  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. >>>>>>>>>>>>>>>>