Bobgram 18 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 Sunday18 August 2024

POLAR VOTEX
The southern polar vortex is a ring of wind swirling over Antarctica
and this has been wobbling more than normal recently.

Our polar vortex is rather untidy, with a sheath of cold air extending
north into the Tasman Sea/South Island area, and another extending as
far north as 30S near Southern Cooks.
In July there was a SSW or sudden stratospheric warming event. This
was caused by a large-scale atmospheric wave, called a Rossby wave,
that was pushed up from below until it "broke" on top of the polar
vortex, causing the downwelling of a layer of air and thus a warming
of a large zone in the stratosphere. A SSW is a rare event -- last
three known events occurred in 2002,2010 and 2019. They disrupt the
polar vortex and encourage outbreaks .
SAM the Southern Annular Mode is a parameter we use to measure the
strength of the polar vortex. When SAM is negative the Polar vortex is
weak and allows polar outbreaks. I have access to AAO (The AntArctic
Oscillation) and use this as a proxy for SAM and it shows a long weak
period of the Polar vortex during July and August, but it is now near
normal and the forecast is for it to become positive in next few
weeks, retuning the Polar Vortex's health.
May be


TROPICS
Parts of northern Japan received record rainfall from Typhoon Maria. .
Maria was followed by Typhoon Ampil, which skirted the coast near
Tokyo and is now moving off to the northeast. . Tropical Storm
SonTinh dissipated near Japan's northernmost islands. . The Windward
Islands and Puerto Rico were raked and flooded by strengthening
Hurricane ERNESTO


WEATHER ZONES
The South Pacific Convergence zone is increasing in width and
intensity and stretches from Solomon Islands to Tokelau to Suwarrow.
A weak passing trough is crossing New Caledonia tonight and Tonga on
local Tuesday then fading sway.

HIGHS and LOWS
This coming week should see a steady procession of HIGHs and LOWs
eastwards in the mid-latitudes. The trough associated with Low L1 is
travelling east across Aotearoa NZ tonight and Monday, followed by a
cold southerly outbreak on Tuesday.
HIGH H2 is expected to cross Aotearoa NZ on Wednesday followed by the
trough associated with L2 on Friday and Saturday.
During the weekend on 24/25 August a trough is expected to stall
between NZ and Fiji and deepen into Low which may affect voyages next
week. So be aware of that and update accordingly.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>