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Bob McDavitt's ideas for sailing weather around the South pacific

28 September 2014

BobBlog 28 Sep 2014

WEATHERGRAM
YOTREPS
Issued 28 September 2014
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.
Barometer jumps
A yacht skipper at anchor in a river on the NE end of Baie de Prony (at SE
end of New Caledonia) last Wednesday morning noticed
his barometer jump up 3 hPa in an hour and then down in the next hour, and
once again.
These jumps were superimposed upon the normal semi-diurnal atmospheric tide
that the barometer was recording as a series of bumps
that look like Sine waves. The STRANGE thing is that nothing happened to
the wind or cloud in the area (usually a change of this
magnitude is associated with an incoming gale or a nearby squall). Nothing.

Later he found out that another boat anchored 10 miles further south
observed much the same thing.
Does anyone have an explanation of such an observation??

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one
number. It is based on the standardised difference in the barometer
readings between Tahiti and Darwin.
It has been negative since July and dived below -10 (Australian units) for
much of September.
The amount of heat that is being stored in the sea in the Eastern Equatorial
pacific has also increased during September, as measured by the
NINO3.4index.
During an El Nino episode, weather patterns tend to be drawn closer to the
equator. The subtropical ridge in the southern hemisphere tends to be north
of its normal position and this weakens the trade winds.
The South Pacific Convergence zone tends to be tugged north and east of its
normal position. If this happens during the cyclone season then it increase
the risk of cyclone formation about and east of the dateline, especially
during February and March.
In itself an El Nino episode doesn't have much impact on cyclone activity
about New Caledonia, but it may reduce the risk in the Coral Sea.

Sea surface temperatures are currently warmer than normal in a zone from
Samoa to French Polynesia
so this is likely to be a zone of extra convection when visited by the SPCZ
in the next few weeks.

TROPICAL TOPICS
In the East Pacific we have RACHAL and in the west Pacific we have KAMMURI
The weekly rain maps show a shift of the heaviest rain from 100E to 140E.


WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is more homogeneous and more intense this week than it was last
week and is averaging its position across Solomons to Tuvalu, but there has
also been convection over Vanuatu last week. SPCZ is expected to shift
southwards of Solomons towards Vanuatu on 4/5 Oct. The convergence
zone/trough which is crossing Vanuatu tonight is expected to weaken over
Fiji on Monday and fade over Tonga on local Tuesday.

STR= Sub-tropical Ridge
The STR is now mainly along 25S across the South Pacific, perhaps somewhat
north of its normal position, and certainly allowing the roaring 40s to roar
over the mid-latitudes.

Departing westwards from Tahiti:
There may be some scattered convection about Suwarrow this week, nothing
organised. Otherwise all is looking good for sailing west from Tahiti to
Tonga this week.

Between NZ to the tropics
A jetstream is triggering a rapidly developing low to NE of North Island
tonight and this low is expected to travel SSE over the next few days,
sideswiping eastern North Island. Not good for approaching NZ but maybe Ok
for getting north. This development wriggles the upper air so that another
jetstream forms an even deeper low near 130W by Wednesday, It is when a
jetstream is switched from zonal to meridonal that we get these surface lows
developing.
The next trough in the Tasman Sea is expected to cross northern NZ on
Thursday, followed by strong SW winds and big SW swells in the north Tasman
Sea on Fri. Avoid.
There may be some big SW swells left over in the Tasman Sea around Norfolk
Island on Saturday. Then the next High from Australia should bring in
settled conditions on Sunday/Monday/Tuesday, so try and time your trip
accordingly.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

21 September 2014

BobBlog issued 21 Sept

WEATHERGRAM
YOTREPS
Issued 21 September 2014

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

1. Resolution
The global computer models take weather observations and transfer them onto
a grid of dots, thereby averaging the weather out. Features that are
occurring between the reports of the observation network are missed
altogether, but some data such as temperature is observed in a continuous
fashion by remote sensing from satellites and is covered well. The model has
to balance this matrix of dots to ensure that certain rules are observed,
for example all the computed upward motions need to be balanced by down ward
motions so that the total atmosphere remains intact. Once a captured and
balanced pattern is obtained, it is then pushed into the future using
dynamic equations in small time-steps. Then another balancing computation is
done and so forth. SO the GRIB data of a weather forecast is just a
mathematical idea based on the extrapolation of a captured pattern. In the
real world chaos is continuously jiggling the weather pattern in to a
different direction, so that the weather forecast deviates from the real
world.

You may already be aware of the limitations of GRIB data. Sometimes someone
will offer you GRIB data with a better resolution, and the question arises
if it will have better accuracy. The answer is somewhat mixed, as I hope to
show you today:

If you can, check my Weathergram graphical edition on the web at/metbob.
wordpress.com to see the comparison of a map showing normal GRIB data from
the GFS model with another from the 8km resolution Predictwind version of
the GFS model (PWG) as available (under registration) from
http://lnk.ie/TPYD/e=bobmcdavitt@hotmail.com/http://www.predictwind.com

The PWG model captures some terrain effectslowering the air pressure in the
interior of the main islands of Fiji in the heat of the day, and there by
forming a westerly sea-breeze around Nadi/Lautoka and causing an
acceleration of the wind on the SW end of Fiji near Navula Passage. All of
these terrain effects truly happen, even if they are not resolved by the
normal GRIB data.

These models with higher resolution and closer dots (and smaller time-
steps) require heaps more computing than the normal global models and give a
pattern that is somewhat closer to the real world. However there is no
increase in the basic weather observation resolution that goes into these
models, so they still have limitations. Their output is still just an
averaged idea.

2 Equinox

Welcome to the vernal equinox today. That was when the overhead sun shifted
from the northern hemisphere to the southern hemisphere. It is only on the
equinox that the sun rises due east and sets due west.
Astronomers, logically, time the seasons using the solstices and equinoxes
of earths orbit around the sun. By this reckoning, spring starts with
vernal equinox. Note that this makes our winter last 93 days and summer 89
days because the earths orbit is slightly elliptical and the earth speeds
up a bit during our summer.
For practical purposes climatologists measure climate data averaged over
each calendar month, and by this reckoning the months of September to
November are taken as spring. Note that by this reckoning summer lasts
90 days (91 days in a leap year). In New Zealand this method of marking the
seasons is more popular than using the equinox and solstice. Fair enough, it
adds 1 or 2 days to summer J You may have noted that the time between
sunrise and sunset is NOT 12 hours today, but around 7 minutes LONGER. This
is because sunrise is defined at the time the top limb of the sun is just
visible on the horizon, and similarly sunset is defined with the top limb of
the sun disappears below the horizon. Due to refraction of the atmosphere
the sun appears and disappears when is slightly below the horizon. This
explains the extra day light. The name used to describe the date when time
between sunrise and sunset crosses twelve hours is the equilux as described
at wikipedia.org/wiki/Equinox Over the next few weeks, as we get to notice
the longer days there will be extra warmth reaching the southern ocean. Sun
starts to shine reach Antarctica after six dark months. Just as the coldest
part of night for us is just after dawn, so it is that Antarctica gets is
coldest temperatures of the year around the equinox. It is the temperature
difference across the Southern Ocean that energies the westerly winds found
there, and so these at their most energetic, and expand to their furthest
north, around the equinox. This is the origin of the term equinoctial
gales, however I prefer the phrase gales of the Antarctic dawn.
Note that New Zealand will be putting their clock forward an hour next
Sunday (28 Sep) and changing to NZDT. New South Wales changes on 5 October.

Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one number. It is
based on the standardised difference in the barometer readings between
Tahiti and Darwin. It has been negative since July and dived below -10
(Australian units) at the start of September where it has hovered ever
since.  if it holds below -10 for another few weeks then we can call this a
full-blown El Nino.

TROPICAL TOPICS
In the east pacific we have POLO and in the west Pacific we have FUNG- WONG.
The weekly rain maps show a shift of the heaviest rain from India/Pakistan
to the Taiwan area with TC FUNG-WONG.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is fragmenting into several branches this week, and slowly
intensifying. One branch is expected to form in the northern Coral Sea
between Monday and Thursday, with a squash zone of enhanced SE winds on the
south side of this zoneavoid.
One of these convergence zones is expected to affect Fiji on
Friday/Saturday, and another to affect Vanuatu/ Loyalty Group on Sunday.

STR= Sub-tropical Ridge
The STR is at its normal latitude for this time of the year, along 20 to 30S
The HIGH over eastern Australia is expected travel across the Tasman Sea
from Tuesday to Thursday and northern NZ on Friday followed by a trough on
Saturday night and disturbed SW over NZ by Sunday.

Departing westwards from Tahiti:
A trough is travelling west along 20S followed by southerly winds and big
SW swells. It is expected to reach Niue by local Sunday, southern cooks by
local Monday/Tuesday, and fade over Australs by local Wednesday.
It is expected to be followed by a period of trade winds that look good for
going west , but might be a squash zone forming along 10 to 15S this weekend


Between NZ to the tropics
Vigorous SW/S flow over NZ on Monday as a Low from the southern ocean
sideswipes its eastern coasts. Then settled weather until the next trough
approaches with NW winds on Friday night/Saturday, then the trough itself on
Saturday night, and then a SW flow on Sunday 28th.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

14 September 2014

BobBlog issued 14 September

WEATHERGRAM
YOTREPS

Issued 14 September 2014
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.

1) One of my yachts encountered the South Pacific convergence zone that was
lurking to north of Fiji last Wednesday night/Thursday. He reported
(Abridged):
>>>>>>>>>>>>>>>>>>>>
I'm currently at 15 06S, 179 41E, on passage from Samoa to Port Vila,
Vanuatu. We left Savai'i Island on Sunday 7th Sep after hanging around for a
bit waiting for a window where the winds weren't going to be too strong.
After a day or so of 10 - 15 knot winds, they began to build from the SE
quite significantly.
2 days ago at around 7am, end of night watch, we were at 14 12S, 176 00W,
and with a GFS GRIB forecast of 12 knots we had winds around 21 knots. It
continued to build steadily from there, yesterday at 7am we were at 14 48S,
179 06W with winds around 28 knots (forecast 13), and at the moment we have
winds touching the bottom of the gale force range at a steady 36 knots
(forecast for here and now is 15 knots) with gusts to 44 or so. I'm looking
at the GFS file that I picked up a few hours ago, and I can see that just
ahead, about where we will be at 1pm today, there are forecasts around 22
knots. If I was a betting man then I would be putting my money on the actual
winds being steady in the 50+ knots range with perhaps peaks touching 60
knots.
I'm running at the moment with essentially a tri sail and storm jib and
although it's tempting to douse the tri sail there is also a swell that's
been building from 4 metres and the tri sail is helping to reduce the roll a
bit. I have suffered a bit of minor rigging and structure damage out at the
aft deck but nothing too serious.
I guess that my main concern is that in amongst all of this there is nothing
from the various weather bureaus warning of this. I think that if any sailor
is venturing out into waters where the wind is blowing 50 knots or more,
especially where we are which is in reasonably well trafficked waters just
north of Fiji, they would expect at least a gale warning if not a storm
warning. There are no warnings at all from Fiji Met Service and none from NZ
MetService for this area, despite the actual winds having been blowing in
the "strong wind warning" range for over 24 hours now, and we're now in the
range where I would expect a gale warning to be issued, right now, for the
area we are in.
I also have no real idea if the anomaly between forecast wind and actual
wind (about 100% difference or a bit over as of the last 36 hours) is due to
some kind of local conditions.
So, a few things.
Can we get a gale warning issued for these waters at the first available
opportunity please? As a gale warning would be expected for any area where
gale force winds are likely to happen, and as there are gale force winds
blowing right here and now I think it's fair to warn other boats.
Are these conditions likely to continue? We're going to try to hold a course
close to 245M / 257T to get us to Port Vila, Vanuatu. I'm now treating the
GFS GRIB data as a work of fiction, but if you weather boffins can give me
some idea whether these winds are likely to build up to and past 50 knots
that would be appreciated.
Are these conditions entirely local or is there somewhere we can go that's
likely to give us a bit of relief, if not from the winds then at least from
the swell which is continuing to build (with a lot of white caps and a
significant amount of foamy spray as well)? With the amount of swell we have
at the moment, not to mention waves breaking over the base of the mast and
as high as the lower part of the HF antenna, it's a wonder I can hear
anything and unsurprising that I can't be heard.
>>>>>>>>>>>>>>>>>>>
He was caught in some squalls on the south end of the South Pacific
Convergence zone and you can sense his frustration with these gale-like
winds and increasing swell. In this case the way out of the SPCZ was to go
south, and I was able to check the WINDSAT satellite and the animated
satellite imagery from to show him (when he finally got good Internet at
Vila) that this was indeed isolated squall activity and thus unlikely to be
given a gale warning on a high seas forecast, even though the local winds
were gale-force.

Let this be a warning to us  there are limitations to weather forecasts:
the weather computers that produce those GRIB files just average the
weather out, and DO NOT resolve the details in the South pacific Convergence
zone, also the High Sea forecasts do NOT issue gale warning for squalls
that come and go along the SPCZ.
>>>>>>
2) There is a rare double solar flare passing by our planet at present, with
stunning aurora and also some possible disruption to satellite
communications.
>>>>>
3) For readers with internet access: If you enjoyed last weeks experiment,
here is another go from some college students:
https://www.youtube.com/watch?v=JsoE4F2Pb20,
and another showing how to get an hard-boiled egg into and out of a bottle
https://www.youtube.com/watch?v=28TIyWdfxxc
>>>>>
Background influences
The Atmosphere: The Southern Oscillation Index SOI (30 day running mean)
sums up the weather pattern over the South Pacific as one number. It is
based on the standardised difference in the barometer readings between
Tahiti and Darwin.
It has been negative since July and dived below -10 (Australian units) a
week ago, and relaxed a little in the past week  if it holds below -10 for
a month then we can call this a full-blown El Nino.

TROPICAL TOPICS
In the ATLANTIC we have EDOUARD, off the Mexican coast there is ODILE and a
tropical depression, and in the Pacific NW we have KALMAEGI.
The weekly rain maps show that a rain burst has been hovering around
northern India for the past fortnight. Indeed, Kashmir seems to be the worst
affected area, with an estimated 200 deaths in India and another
200 in Pakistan.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is slightly weaker than last week, and by Wednesday is expected to
be hugging the 10S line from PNG to east of Samoa. It may shift a little
further south later in the week.

STR= Sub-tropical Ridge
The STR has reverted to its normal latitude for this time of the year and
this pattern may last for a week or two, with no sign now of the blocking
that was affecting the pattern over the past month or so. This week the High
cells with the STR are expected to travel east along 30S.

Departing westwards from Tahiti:
A convergence zone is expected to slide east across the southern parts of
French Polynesia during local Monday and Tuesday, followed by a period of
easterly winds. These easterly winds should be well maintained by a high
cell traveling along 30S, and should be good for going west.
The SPCZ may affect Suwarrow from time to time. Also there may be a small
squash zone of SE winds near 16S late in the week. And the north end of a
trough is likely to cross Southern Cooks late in the week. Otherwise it is
looking to be an OK week for sailing westwards good for travelling west.

Between NZ to the tropics
A trough is expected to be crossing Northland on Tuesday, followed by
another on Thursday night/Friday/Saturday. Avoid.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com Feedback to bob@metbob.com To unsubscribe, send
a reply email saying unsubscribe.

07 September 2014

BobBlog 7 Sep 2014

WEATHERGRAM
YOTREPS
Issued 07 September 2014
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

To help keep you advised of my blogs and other weather details I have opened
a twitter account. My twitter handle, if you want to follow it, is @MetBob2


For readers with Internet access: Lets start with an oldie but goodie
experiment to prove that 1000hpa= 10,000 kg (10 tonnes) of pressure:
https://www.youtube.com/watch?v=p3b9pK-O6cE also read the comments, (those
Sydney physicists make physics fun)

Background influences The Atmosphere:
The Southern Oscillation Index SOI (30 day running mean) sums up the
weather pattern over the South Pacific as one number. It is based on the
standardised difference in the barometer readings between Tahiti and Darwin.

It has been negative since July and in the past week the 30day running mean
has dived below -10 (Australian units)  if it holds this for a month then
we can call this a full-blown El Nino.
Interestingly, the amount of heat being stored in the ocean in the eastern
equatorial Pacific Ocean is also on the rise now, as measured by the weekly
NINO 3.4 index.

So some of you may be asking, IF an El Nino is kicking in , THEN how come we
are NOT getting lots of strong SW winds over NZ/Tasman Sea area? Well the
answer to this can be gleaned by looking at the variation from the average
weather map for August  the monthly isobar anomaly map shows an anomalous
HIGH over Australia New Zealand, but shows typical El Nino type weather
across the remainder of the South Pacific Ocean.
These anomalously high pressures have been thanks to some local blocking
around the 180th meridian recently. Blocking is an interesting factor that
plays around on a monthly scale with the seasonal pattern. The current
blocking has been around for six weeks. So , it is probably due to soon
move off and is there is pattern in the weather, then we can expect  after
this weeks generous looking Tasman High- a return to a month or more of
disturbed westerlies that are more El Nino-like over NZ area. Probably
about time too, as there hasnt been any decent rain in several parts of
western NZ (including Westland/Buller) for 3 weeks. And we all know pasture
grass dies after 3 dry weeks.

TROPICAL TOPICS
Hurricane NORBERT is an interesting cyclone off the Pacific NE coast , which
some computer models are picking may remain as a meteorological entity all
the across north America. Wow.
The other feature around at present is tropical storm FENGSHEN off eastern
Japan:
And there is a tropical depressions in the South China sea and a couple in
the tropical Atlantic ocean.

The weekly rain maps for the past fortnight show that rain burst that was
near India is shifting east across Burma/The Republic of the Union of
Myanmar, and there has been a weakening in the convection in the South
Pacific.

WEATHER ZONES
SPCZ= South Pacific Convergence Zone
The SPCZ is slightly weaker than last week, and a branch of it is expected
to affect the area between Vanuatu and western Fiji on Wednesday
10 September, along with strong SE winds on its southern side. Avoid.
Indeed there is likely to be a zone of strong SE winds on the south-side of
the SPCZ across the northern Coral Sea for the next week plus.

Departing westwards from Tahiti:
A trough in the southern ocean/mid-latitudes is expected to travel east this
week weakening the trade winds between Niue and Southern Cooks. It is
expected to deepen over and to the SE of the Austral Island (south of
Tahiti) late this week.
SO those in Tahiti intending to go west this week should take this trough,
and its associated convergence zone, into account.

STR= Sub-tropical Ridge
The STR has a wave-shape in its latitude across the map, dipping south over
NZ at present, but is north of normal over Australia and Tahiti.
The High that is expected to cross North Island next few days should travel
east along 40S to east of NZ from Wednesday. After this high, a return to
disturbed NW/South island W winds is expected to reach South Island on
Thursday and North Island on Friday.

Between NZ to the tropics
Good weather for getting north out of NZ on Monday.
Also good for departing tropics early this week to get to NZ: the next
period of SW head winds around Northland is expected to occur next Sunday
14 Sep 14 September, and these is not expected to be strongso come on in if
you are ready to go now.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
My website is at metbob.com  Feedback to bob@metbob.com To unsubscribe,
send a reply email saying unsubscribe.

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