Bob Blog

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 Sunday 07 July2024

Ten Reasons to hate a high

There is a 1036 HIGH travelling east along 45S past Tasmania.

I heard my good mate David talk about its implications on Gulf Harbour
radio ZMH286 on Friday evening. He broadcasts weather info and Pamela
handles roll calls daily at 0515UTC on 8752 (or 8779 or 8297) KHz an
stream the call at https://www.ghradio.co.nz/live-streaming.html.
David quoted from his own notes (as found on their website) and my
notes on 'Ten reasons to hate a high' Thanks Dave. This high is a
good reason to dust off those notes

When extra isobars are added to a weather map to accommodate an
intense High, this squeezes the isobars already on the map closer
together. making them crunch together in zones I call squash zones.
Hence my mnemonic "when the HIGH is over 1030, its rims are going to
get dirty"

And here, for the record are my notes on
Ten reasons to hate a high (anticyclone):

Anticyclones are usually associated with light winds and sunny skies,
but they also have some undesirable attributes,
especially when they get blocked and linger:

1. Near the centre are "dead" winds and usually an area of low clouds
causing dull days called "anticyclonic gloom", or dirty air which may
turn into fog.

2 Round the rim, winds are strong. If the central pressure is
over 1030, 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.

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 west-ern ("back") end of a high may become a breeding
ground for storms.

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. Some-times 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 frost.

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


. Hurricane Beryl killed at least six people while pummelling
islands of the Caribbean as the earliest Category-4 and Category-5
storm on record. Beryl was fueled by unprecedented oceanic warmth
across the Atlantic basin and a lack of wind shear.
. Short-lived Tropical Storm Chris brought rain to Mexico's central
Gulf of Mexico coast.

WEATHER ZONES

The South Pacific Convergence zone is sitting over the northern Coral
Sea and extends from Solomons to northern Vanuatu to Samoa and is
getting stronger this week.
There are some passing troughs from Southern Cooks to south of French
Polynesia. A Low-pressure system is located south of Easter Island
this week.

HIGHS and LOWS
High H1 over 1036 is travelling slowly along 45S this week crossing
central NZ on Tuesday to Thursday with a squash zone of enhanced SE
winds on its northern side and some squash zones with swells over 4m.
Avoid.
Once the High moves off and isobars relax, air is expected to pile up
on itself and produce Lows at the "shoulders" of the High. A trough
is expected to form along the eastern aussie seaboard from Wednesday
so that there should be a strong NE flow moving onto NZ this weekend
or early next week followed by a trough/Low. Avoid.

If you want to depart from Northland to the tropics, then there
should be a brief window mid-week that avoids the squash zones to the
north and the incoming NE winds.
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