Bobgram 21 July 2024

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 July2024

Rapid Cyclogenesis - a Weather Bomb
A Low which central pressure of 1003hPa on July 19 near New Caledonia
intensified to around 975hpa on 20 July to NE of NZ. That's more than
24hPa in 24 hour which makes it fit the definition of a weather bomb.

This is what happens when a Jetstream gets on top of a forming low. It
vents off the rising air like a spray gun, sucking air up and out and
thereby reducing the surface pressure.

As the cloud canopy associated with the jet stream peels off it
reveals the inverted comma shape of a new low centre . called an
instant occlusion as seen in this animation of satellite imagery from
Windy. youtu.be/oSGJPECSncs

As for the week ahead at Niue:
On local Friday 26 July the winds are forecast to swing from strong NE
to strong westerly, gusting over 30kt due to a passing low (call it
L3). And on Saturday 28th SWELL from the southwest then south is
forecast to rise to 5 significant metres.

L3 is followed by large SW swells from the southern ocean as shown
here for next Monday 29 July.
AVOID.

TROPICS
Gaemi and Prapiroon are near the China sea.
A minimal and unnamed tropical cyclone brought rain to central Vietnam
without being accompanied by high wind

WEATHER ZONES
The South Pacific Convergence zone is sitting over the northern Coral
Sea and extends a finger towards northern Vanuatu and Fiji. A Low is
expected to start forming in this zone near New Caledonia on Wednesday
and deepen into a LOW L3 near Niue by end of the week, and an intense
Low this weekend over Southern Cooks. AVOID.

As for sailing from Tahiti to Tonga. Need to wait a few days for winds
and sea now near Tahiti to ease, then there is an opportunity (maybe
on local Mon, but double check) for a yacht of average speed to get to
Suwarrow before the Weekend low and its large SW swells arrive. Other
possible spots such as Aitutaki/Palmerston/Niue will be no good as
stop overs in this weekend's low. So perhaps stay put this week.

HIGHS and LOWS
High H1 now well east of NZ and south of Tahiti continues to move
slowly east. Enhanced trade winds and 3m swells on its norther side
are affecting French Polynesia this weekend.

L2 is moving from Tasman Sea across NZ on local Monday and then going
off to the southeast following Saturday's weather bomb.

This is followed by High H2 from Australia to central NZ by end of
this week. Sea conditions off northern NZ should settle after L2 by
Wednesday to finally allow a good-looking voyage northward from NZ
(maybe by late Tuesday for some).

Low L3 is expected to become noticeable near Tonga on Thursday. As
explained above: avoid.

NOTE THAT I WILL BE UNAVAILABLE ON WEDNESDAY/THURSDAY (NZ DAYS) this
week as I plan to be away on village business.

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If you would like more details about your voyage, then check
metbob.com to see what I offer.
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Contact is bob@metbob.com or text 64277762212.
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