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. >>>>>>>>>>>>>>>>