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 Sunday 21 December 2025
Summer Solstice

The sun, as seen from earth, gets to be directly overhead ~23.5degrees South
tonight.
This is the southern -most latitude to see the overhead sun, so Monday is
the day of longest sunlight this year in the southern hemisphere. This
latitude is named the Tropic of Capricorn. It nominally divides the tropics
from the non-tropical temperate zone. The sun only gets overhead this
latitude once each orbital year and this instant is named the summer
solstice (the sun stops appearing to go south and starts appearing to go
north, so it "stands still"). When this latitude was named, the sun
appeared to be in the constellation of Capricorn, thus its name. However,
due to precession of the earth, the sun now appears to be in the
constellation of Sagittarius.

To mark the lengthening sunlight in the days after their winter solstice in
the northern hemisphere, the Germanic tribes named these days "Yule tide".
The Roman Empire set up a pagan festival on Dec 25th called Sol Invictus.
In the 4th century AD the Roman Christian church renamed this festival to be
the date for the nativity. However, Luke's Epistle mentions shepherds
watching their flocks at night during the nativity. In Bethlehem this would
be in spring when lambs were being born and needing attention. That's March
/April.

Season's Greetings
The following is from www.rolexsydneyhobart.com/race/2025/news/

Boxing Day Friday 26 December marks the start of the 80th Sydney to Hobart
yacht race, an annual 628 nautical mile race run by the Cruising Yacht Club
of Australia's and this year sponsored by ROLEX.
Light and variable winds are shaping as a key feature of this year's race,
with early forecasts pointing to a tactical opening phase rather than a
flat-out drag race south.
Bureau of Meteorology forecaster Edward Townsend-Medlock said the current
outlook for the 80th race, while still subject to high uncertainty, suggests
a relatively benign start on Boxing Day, driven by a high-pressure system in
the Tasman Sea.
"We're talking about something that's seven, eight or nine days ahead of us,
so the uncertainty is high," Townsend-Medlock said. "But there is good
consistency in the models around the main features."
At the race start on December 26, winds are forecast to be light east to
south-easterlies in the 5 to 10 knot range. The strength could vary slightly
depending on the exact position of a forecast passing high on the isobar
map, but the general direction appears consistent across models.
An alternative scenario - considered less likely at this stage - could see
stronger northerly winds if the high remains west of Tasmania.
As the fleet progresses down the New South Wales coast on the first day, the
forecast suggests winds may gradually shift toward the north-east and
strengthen into the 20 to 25 knot range, though confidence drops
significantly beyond the first 24 to 48 hours.
"After that, it's very difficult to say," Townsend-Medlock said. "Usually
one or two fronts will come through and bring stronger winds in Bass Strait,
but that's well beyond the forecast skill of the models at the moment."
Sea state is expected to be relatively orderly early, with southerly swells
of 1 to 2 metres at periods of 10 to 13 seconds at the start.
Townsend-Medlock said the swell is likely to be mature and well-organised,
generated far to the south and not especially significant.
Ocean currents may play a larger role, particularly in lighter winds.
Several eddies associated with the East Australian Current are forecast to
produce north-to-south flows of up to 1.7 knots off the New South Wales
coast and east of Tasmania, which could offer gains for boats able to
exploit them.
By December 27, conditions in Bass Strait - where the race leaders are
typically positioned - are forecast to be light and variable.
"At that time, Bass Strait is looking at light and variable winds, with no
major weather feature coming through," Townsend-Medlock said. "It's very
difficult to say with any certainty what the direction will be."
It seems this year the weather may bring a bit of everything, a race where
patience, precision and navigation may prove as decisive as outright speed.
TROPICS
The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu
and Tropical Cyclone Potential is from
www.ospo.noaa.gov/products/ocean/tropical/tcfp.html

Tonight Tropical depression NINE is travelling westwards to south of
Jakarta.
Last week Cyclone Bakung drifted harmlessly around parts of the eastern
Indian Ocean, south of Sumatra.


WEATHER ZONES
Weather Zones Mid-week GFS model showing isobars, winds, waves (purple),
rain (red), MT (Monsoonal trough), STR (Subtropical Ridge), SPCZ (South
Pacific Convergence Zone) CZ (Convergence Zone)

Rain accumulation this week from Windy.com

The South Pacific Convergence zone extends from PNG to northern parts of
Fiji and Tonga. Another convergence zone covers French Polynesia.

Wind accumulation this week from Windy.com

The wind accumulation shows areas of wind around a low L1 southeast of
French Polynesia, another low L2 east of NZ and another low L3 in the south
Tasman Sea.
LOWS and HIGHS
Subtropical Low L1 is expected to form around the Austral Islands next few
days and then zig to the southwest and zag to the southeast.
HIGH H1 south of the Cooks is expected to travel southeast around L1.
HIGH H2 south of Fiji/Tonga is expected to travel east-southeast along 30 to
35S
Low L2 and associated front is expected to cross NZ on late Mondy and
Tuesday /Wednesday followed by a westerly flow and then Low 3 and associated
front on Thursday night and Friday. Then a few days with a disturbed south
to southeast flow is expected over New Zealand
HIGH H3 is expected to reach Aussie Bight by mid-week and then travel east
across the Sydney-Hobart Yacht race late in the week.
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If you would like more details about your voyage, then check metbob.com to
see what I offer.
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.
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