Bob Blog 11 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 11 July 2021

In 2016 I introduced a series of blogs on the topic of "buffering" in Goggle
maps and how sometimes it shows open ocean where there are reefs. This data
saving tactic is called "Google lacunae"

Art of SV Feeling Good introduced us to OVITAL MAP which allows downloading
satellite images from various sources. Sadly, I have now been informed by
Eve of SV Auntie that Ovital Map has taken itself out of this business as it
now only uses Chinese satellites in which French Polynesia isn't covered.

Tom of Yacht Adina wrote a detailed account of navigating with satellite
imagery for Yachting Monthly's May 2016 edition. This article is still
online, and he kindly gave me permission to share a link to his blog with
you at
www.yachtingmonthly.com/sailing-skills/how-to-make-navigation-safer-using-sa
tellite-images-34665. (Note that my malwarebytes software regards gdayii.ca
as having possible vulnerabilities).

SAS Planet still allows comparison of various satellite imagery. A review of
how to use this as given in 2016 by Max of Fluenta,
sv-fluenta.blogspot.com/2016/11/using-satellite-imagery-to-assist.html

Here is a list of 5 "best" Marine Navigation Apps as judged by "discover
Boating" at
discoverboating.com/resources/marine-navigation-apps:

C-Map Embark
iNavX
KnowWake
Navionics
SeaPilot

Weather regimes - SAM

Us humans love finding patterns in things. Our orbit around the sun is so
repeatable and predictable that we have "seasons". Watching weather maps
long enough, one can see that sometimes the same weather system may repeat
in clusters. We like to watch for signs that predict changes.

Tonight, I'll blog about SAM, the southern annular mode. This is a measure
of the average speed of the westerly winds around the Polar vortex (the ring
of westerly winds that circle the planet between 50S and the Antarctic
circle (66 S). The value of SAM alters the north-south movement of this
vortex. A high positive value of SAM occurs when the air pressure over
Antarctica is lower than normal, and westerly winds in the polar vortex are
stronger than normal (note, the actual isobars over Antarctica are always
higher than those in the polar vortex, but SAM works with the anomaly
values, not the actual values). So, in a high positive SAM the polar vortex
is shifted southwards, and pressures is the mid-latitudes are higher than
normal, with weaker winds and settled weather.
As seen in blog.metservice.com/Southern-Annular-Mode

However, when SAM is negative, the polar vortex is weaker, and may spread
outwards, so that west to southwest winds over the mid-latitudes are
stronger than normal--- the chances are that there may be a POLAR BLAST.

SAM refers to surface winds 50S-66S. A proxy of SAM is available, namely the
AAO or Antarctic Annular mode (average 700hPa Z wind component 20S to 90S)
at
www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_inde
x_ensm.html

This has had a negative jump this week. This indicates a good chance of a
polar blast somewhere (not necessarily affecting NZ). Looking at EC data of
the surface air temperature on Windy tonight it seems that the polar blasts
this time are at 160W and 0 longitude as far north as 45-50 South-not much.


TROPICS
ELSA damaged the eastern seaboard of USA last week, and a downpour flooded
New York subways,
washingtonpost.com/nation/2021/07/09/new-york-flash-floods-subway/
No cyclones around at present, but there is a moderate build up of potential
development off the southwest of Mexico and around Cocos Island in the
Indian Ocean.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
The SPCZ is expected to be about average stretching from Solomon Islands to
Tonga, and minimal further east.
Two passing troughs for the South Pacific this week. The first one is
tonight east of New Caledonia and expected to cross Fiji/Tonga on local
Wednesday and fade over Southern Cooks on local Thursday. The second trough
is expected to cross the northern Tasman Sea on Friday and reach New
Caledonia on Saturday.

HIGHS and LOWS
HIGH 1022 east of NZ and south of southern Cooks traveling east along 25
to30S.
LOW 1000 near Norfolk Island tonight is expected to move SE over Northland
as 985 on Monday then off to the SE of NZ.
Next HIGH 1022 is expected to travel along 30S off Coffs around Tuesday and
past north of NZ on Thursday then further east.
Low 980 expected to form in south Tasman Sea by Friday and cross the South
Island on Saturday.
This recurring High-Low pattern is typical for winter.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>