Bob Blog 10 July

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 10 July 2022
THE FORECASTING DISCLAIMER
I received an interesting email this week asking as follows:
We are planning this passage imminently but are getting conflicting
forecasts. Predict wind says 2-3 metre swell with up to 20 knots but friends
said they experienced 30 knots and 5 metre swell.
What is your analysis?
In my reply I inserted the following
Note that these waves are in "significant metres". The occasional wave,
around 4 times per hour, is 50% bigger, and wind gusts are usually 40 to50%
stronger than the average wind. Hence both the models and your friends are
correct at the same time. There is also the factor that the wind across a
sailor's face is the apparent wind rather than the real wind.
So, the wind and waves mentioned in the forecast are averages and need to be
"upsized" to reflect the peaks felt by the sailor.
On 27 December 1998, the Sydney to Hobart fleet of 115 yachts was hit by a
southwesterly storm as it entered Bass Strait. Winds gusting at times to
over 70 knots combined with heavy seas over a strong flowing East Australian
Current to exact the biggest toll ever on the race. Six lives lost, five
boats sunk and a further 66 boats retired from the race. The severe and fast
developing storm caught the fleet entering Bass Strait waters and resulted
in the biggest maritime rescue operation ever in Australian waters with 55
rescued in an operation involving some 25 aircraft, six vessels and
approximately 1,000 personnel.

One of the recommendations from the Court of Inquiry into this event was as
follows:
6) THAT WEATHER FORECASTS WHICH ARE SPECIFICALLY PROVIDED FOR YACHT RACING
FLEETS CONTAIN: -
(a) AS WELL AS THE AVERAGE WINDS EXPECTED, THE MAXIMUM GUSTS OF WIND THAT
ARE LIKELY TO OCCUR.
AND (b) AS WELL AS THE SIGNIFICANT WAVE HEIGHTS EXPECTED, THE MAXIMUM WAVE
HEIGHTS THAT ARE LIKELY TO BE ENCOUNTERED.
(See the report at
cdn.ussailing.org/wp-content/uploads/2018/01/Sydney-to-Hobart-Race-Coroners-
Report-1998.pdf)
Indeed, all marine forecasts issued by the Australian Bureau of meteorology
now carry the following disclaimer:
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Please be aware Wind and wave forecasts are averages.
Wind gusts can be 40 per cent stronger than the forecast, and stronger still
in squalls and thunderstorms.
Maximum waves can be twice the forecast height.
>>>>>>>>
TROPICS
BONNIE made it into the Pacific for a few days then faded and is now being
followed by TC DARBY.
There are Zones of potential development this week are around the
Philippines and Central America.

WEATHER ZONES
SPCZ=South Pacific ConvL1ergence zone and STR (Sub tropical ridge).
The SPCZ is active in the Coral Sea and should help breed lows L1 an L2 this
week.
There is also a convergence zone lingering roughly along 15S between Fiji
and southern Cooks
OK to travel from Tahiti to Fiji this week but be mindful of a burst of
southerly swell from the Southern Ocean on Bastille Day 14 July and a
possible squash zone near Niue around 19 July.
HIGHS and LOWS
High H1 east of NZ is expected to move off to the NE maintaining trade winds
be-tween Tahiti and Tonga this week.
L1 is expected to form south of New Caledonia on Monday and deepen as it
travels across NZ on Tuesday.
This is followed by L2 forming in north Tasman on Wednesday and deepening as
it travels across NZ on Friday, followed by a vigorous SW flow.
H2 is expected to stay over central Australia this week and send the tongue
of a ridge across north Tasman Sea this weekend.
L1 and L2 make it difficult to plan trans-Tasman travel this week.
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If you would like more detail for 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 txt 64277762212
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