Bob Blog 22 Aug

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 22 August 2021

BREAKERS (on the coast) and swimming
Usually, it is the slope of the beach that determines the type of breakers:
Spilling, plunging or surging. These notes are from
www.seafriends.org.nz/oceano/waves.htm

. Spilling breakers: result from waves of low steepness (long period swell)
over gentle slopes. They cause rows of breakers, rolling towards the beach.
Such breakers gradually transport water towards the beach during groups of
high waves. Rips running back to sea, transport this water away from the
beach during groups of low waves. When caught swimming in a rip, do not
attempt to swim back to shore because such rips can be very strong (up to 8
km/hr). Swim parallel to the beach towards where the waves are highest. This
is where water moves towards the beach. The next group of tall waves should
assist you to swim back to shore. However, when launching (rescue) boats,
this is best done in a rip zone.

. Plunging breakers: result from steeper waves over moderate slopes. The
slope of a beach is not constant but may change with the tide. Some beaches
are steep toward high tide, others toward low tide. A plunging breaker is
dangerous for swimmers because its intensity is greatly augmented by
backwash from its predecessor. This strong backwash precludes easy exit from
the breaker zone, particularly for divers. Often a steep bank of loose sand
prevents one from standing upright. In order to exit safely, wait for a
group of low waves.

. Surging breakers: occur where the beach slope exceeds wave steepness. The
wave does not really curl and break but runs up against the shore while
producing foam and large surges of water. Such places are dangerous for
swimmers because the rapidly moving water can drag swimmers over the rocks.

When waves break, their energy is absorbed and converted to heat. The
gentler the slope of the beach, the more energy is converted. Steep slopes
such as rocky shores do not break waves much but do reflect them back to
sea, which 'shelters' marine life.

TROPICS
The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical
Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

It has been a busy week in the tropics. GRACE brought damaging winds to east
coast of Mexico, killing eight people. FRED brought flooding to North
Carolina, killing at least 2. The remains of LINDA are passing by to north
of Hawaii tonight. HENRI is heading for New York and New England, and OMAIS
is heading for South Korea. There is also a strong potential for possible
formation across the North Atlantic.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.

The SPCZ is expected to build and stretch from Solomona to Tuvalu to Samoa
to Southern Cooks.
Trough south of Southern Cooks expected to form a low near 25S south of
Palmerston/Aitutaki that will travel off to southeast.

HIGHS and LOWS

HIGH over 1034 east of NZ and north of Chatham Island is a slow-moving block
and should take all week to travel NE from 43S to 33S.

LOW down to below 994 forming off Newcastle on Mon/Tue and travel across
Tasman Sea and then stall and fade west of the North Island this weekend
(due the blocking High).

Then by Friday a multi-centred HIGH is expected to travel east with #1
centre to SE of Tasmania and #2 over New South Wales. These should continue
east with #1 going around south of South Island and to east of South Island
next week, and with #2 fading in north Tasman Sea.

Looks like we may be having more dominant HIGHS in Tasman Sea over the next
few weeks, good tropical trade winds.

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