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

24 June 2018

Bob Blog

WEATHERGRAM

YOTREPS

Compiled Sun 24 June 2018

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.

 

Cyclone Slowdown.

In Nature Magazine this month there was an article about a gradual slowdown in the speed of movement of tropical cyclones. James Kossin (www.nature.com/articles/s41586-018-0158-3) studied data from 1949 to 2016 and showed tropical steering fields have reduced by 10% globally and by as much as 15 to 20% regionally (as much as 30% over land in NW Pacific). The slower a cyclone moves the greater the amount of rain it can deliver to the places it visits. Indeed the peak rain rates of storms have been measured to have increased by 30% over the past 60 years, see www.businessinsider.com.au/hurricanes-and-rain-getting-stronger-2018-6 and www.businessinsider.com.au/hurricanes-moving-more-slowly-causing-more-damage-2018-6

 

Marine Barometer.

I’ve been trying out a few smart apps on my mobile phone (iPhone 6S) to use its inbuild gps and motion/pressure sensors to measure air pressure (a barometer). Some can mimic a barograph, but need continual access to your location, and that drains the battery.

The one app I like the best, and I think will appeal to mariners, is the Marine Barometer (for iPhone or android) from Starpath (go to Apple store or Google Store and search for Marine Barometer). In particular it appeals to me for it has an averaging function over 14 seconds, so it’ll work OK on a yacht going up and down in a sea way (sea waves have a period of 7 or less seconds and swells have a period of 7 to 14 seconds). Also, it simply allows you to store your readings and thereby keep a log. See www.starpath.com/marinebarometer/about.htm.

 

TROPICS

All quiet at present, but there is a tropical depression off the Mexican west coast.

If we compare the past week’s rain map with the previous week we can see a general relaxation everywhere.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from northern Coral Sea /Solomon Islands to northern Vanuatu, and be weak over Fiji, and sporadic over Tonga. A passing convergence zone is expected from Niue area to south of Southern Cooks.

 

Subtropical ridge (STR)

A High is expected to travel east along 35S this week, entering Tasman Sea from New South Wales on Tuesday and Wednesday, and then onto central NZ on Thu/Fri/Sat, and then off to NE of NZ on Sunday/Monday. There is likely to be a squash zone of strong SE winds on the nort side of this high in the Coral Sea.

 

Around Tasman Sea, NZ to tropics /FP

TROUGH is expected to cross Northern NZ on Monday followed by strong SW flow on Tuesday with large swells to 25S. Conditions are looking OK for departure from northern NZ to the topics from Wednesday but may encounter strong SE winds north of 25S next week. 

 

Australia to tropics

Strong SE winds are forecast in the Coral sea this week, and there seem to be too many SE/E wind for a good trip form Brisbane to Noumea this week.

 

Tahiti to Tonga

A convergence zone is expected over Niue are on Tue UTC and Raro area on Wed UTC. Otherwise it looks like a good enough week—and be aware there is the possibility of a squash zone next week, so this week may be better.

 

Galapagos to Marquess

SE to E winds, mostly less than 20 knots. To use available currents, go to 5S 110W and then 5S 120W and then direct.

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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 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

17 June 2018

Bob Blog 17 June

WEATHERGRAM

YOTREPS

Compiled Sun 17 June 2018

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.

 

CAPE  addendum.

I was asked to try and track down who originated the CAPE index. I‘ve looked and not found—maybe someone who reads this can fill us in. The earliest stability indices where when we started to use weather balloons to measure the vertical temperature profile and plotted this data on what we call tephigrams.

 

See a tephigram at www.meteo.mcgill.ca/wxlab/ATOC-546/notes/lesson07.convection/ this us a Meteorologist’s lunch: 27  parameters listed on the right, and CAPE is the red area .

For teh latest tephigram see weather.uwyo.edu/upperair/sounding.html ( select New Zealand and  Gif Stuve  then NZWP )

Spot indices such as the Lifted index  LIFT (difference between Temperature of environment and temperature of adiabatically-lifted parcel at , say, 500hPa),  and Showalter index  SHOW (much the same but at 850hPa) can be computed quickly on a Tephigram and were the first ones used. CAPE can also be shaded-in on a Tephigram and takes into account the whole vertical range of the temperature profile (what we call the “sounding”) and so carries more finesse. When computer models started to go global we needed a good way to parametrise the  vertical temperature profiles  so they could  be compared around the planet. That’s when CAPE became popular.

 

The earliest paper I could find using it is M. W. Moncrieff, M.J. Miller (1976). "The dynamics and simulation of tropical cumulonimbus and squall lines". Q. J. R. Meteorol. Soc. 120 (432): 373–94. Bibcode:1976QJRMS.102..373M. doi:10.1002/qj.49710243208. At rmets.onlinelibrary.wiley.com/doi/abs/10.1002/qj.49710243208

 

CAPE is good, but not a universal convective index. You can see how it is always highest in the tropics. This is because when the air is warmer, CAPE can be higher.   However, thunderstorms in the mid latitudes triggered by air being forced to rise over a mountain range--- such as the Southern Alps or the Andes --- can have updrafts just as powerful as their tropical cousins –even if they have much lower CAPE values.

 

As we approach the SOLSTICE this week(Thu 21June 1007UTC), it is time to check the state of the ENSO = neutral

 

The Atmosphere:

El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather. The La Nina, caused by cooler than normal seas along the equatorial eastern pacific, shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

 

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number of isobars between them on the weather map. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

 

From June 2017 to May 2018 the SOI has been mostly between plus 0.5 to plus 1, sometimes higher, consistent with a weak but rather persistent La Nina. The subtropical ridge line was further from the equator than normal, and trade winds were stronger than normal.

However, over the past month, the SOI has settled into a near zero state, and has a NEUTRAL status.

Neutral conditions as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

(Note that in this graph on the vertical axis 10= 1 standard deviation)

 

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the warm seas of the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, then a warm period until July/Aug 2017, and then a cool period. That cool period seems to be coming to an end now, and the sea surface temperature are near normal.

Fading La Nina as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

 

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM during the rest of this year, but the average of the predications has only warming to 0.5 above normal--- not enough to be called an El NINO event (but closer to it that we have been for a while).Then again 3 of the models are hinting at an El NINO by end of the year.

CPC/IRI predictions are at iri.columbia.edu/our-expertise/climate/forecasts/enso/current/

 

Latest SST anomaly map shows a zone of slightly warmer sea from Galapagos along equator to dateline. Worth watching, if it grows we can expect the trade winds to weaken. The remains of the cooler seas of the recent La Nina seem to be further south.  Sea surface temperatures across the Pacific can be seen  at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

 

TROPICS

There has been an MJO in the North Pacific over past week or so, triggering several developments: MALIKSI bypassed Japan, followed by EWINIAR

AT present we have TC GAEMI off the south of Japan, also expected to peel off to the northeast, and this is likely to be followed by another tropical depression. Can be seen at www.metoc.navy.mil/jtwc/products/wp0818.gif

 

Off the western Mexico coat we had tropical storm ALETTA and BUD over a week ago and now we have TC CARLOTTA pounding the Mexican coast with potential flodding rain for Acapulco.. It’s the second-earliest fourth tropical cyclone in this region during the satellite era (since 1966), missing out on being the earliest by 6 hours.

Can be seen at www.nhc.noaa.gov/storm_graphics/EP04/EP042018_5day_cone_no_line.png

 

If we compare the past week’s rain map with the previous week we can see a build-up of activity over yanmar and in the China Sea, and a focussing of activity off western Mexico. Activity has eased across the South Pacific.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from Solomon island to north of Vanuatu, and be weaker than last week. There may be some rain from a passing trough other Austral island of southern French Polynesia.

 

Subtropical ridge (STR)

A High is expected to travel east along 25 to 30S from 180 to 150west this week. There is likely to be a small squash zone of enhanced trade winds and swell on its northern side, affecting mainly 17S from 150W to 170W /Niue mainly on Tue/Wed UTC.

Next HIGH from the west is expected to enter the south Tasman Sea around Thu 21 June and then spread across NZ on Sat/Sun 23/24 June.

 

Around Tasman Sea, NZ to tropics /FP

LOW1 is expected to cross central NZ on Monday /Tuesday, and Low2 is expected to travel from northern Tasman Sea on Tuesday to east of northern NZ by Thursday.

After Low2 there is a reasonable-looking weather pattern for heading off north to tropics or east to Tahiti. Might be able to depart on Thursday, but that voyage may have some strong winds off shore on its first night.

 

Australia to tropics

There is a good opportunity this week, on the back end of Low1/Low2.

However, strong SE winds are forecast in the Coral sea this week.

 

Tahiti to Tonga

South Pacific convergence zone is not really a factor of concern this week.

Passing trough over Australs on Monday 18June UTC and gambier Islands on Tuesday 19 June UTC.

There is expected to be a zone of enhanced ESE winds over 20kt and swells over 2.5m along around 17S from 150W to 170W /Niue mainly on Tue/Wed UTC.

Next trough from the west is expected to reach New Caledonia on Tue 19 June UTC, Fiji/Tonga on Thu 21 June UTC, and then Rarotonga around Sun 24 June UTC.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

 

10 June 2018

Bob's Blog

WEATHERGRAM

YOTREPS

Compiled Sun 10 June 2018

 

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.

 

CAPE  an index for thunderstorms and lightning

One of the parameters available from the www.windy.com sidebar is the CAPE index—the Convective Available Potential Energy.

Basically, this measures buoyancy of the air, the amount of heat released if there is any convection. On earth we normally have static stability. Sometimes parts of the atmosphere become unstable. The way we meteorologists study instability is called the “parcel method”: we look at a parcel of air starting off near the ground with temperature T(parcel) or Tp and compare this with the vertical temperature profile of the wider environment, called T(environment) or Te. If the parcel of air is lifted it will cool at the standard atmospheric lapse rate. So the “lifted-Tp” is less than starting Tp. However, the wider environment may have some unstable layers. In these layers the “lifted- Tp” gets HIGHER than the surrounding Te, or lifted-Tp>Te, meaning the parcel has gained buoyancy (or become less dense than its surroundings) and will accelerate upwards. It will continue accelerating upwards until it eventually finds a matching Te (neutral buoyancy) and then slow to a stop (or until deposited ice particles stick together, but that’s another story). This is the convective process, and you can see it happening in the rising turrets of active cumulus clouds.

Modern weather models have many vertical dots in their matrix, and thus store a vertical profile of the atmospheric temperature or a Te. With aid of a rather complex equation (which I won’t show here), we can turn this Te profile into a single number that shows how much energy is released (in joules per kilogram) if a parcel of air is triggered to rise from the surface. This is called CAPE and if it is zero then the parcel just drops back again. If CAPE is positive then the parcel will rise, and the higher the CAPE, the faster the rise.

It has been suggested (but not yet measured) that the updraft speed in a thunderstorm needs to be over around 25 km/hr (13 knots) in order to be able to be strong enough to separate the charged particles and produce lightning discharge. This seems to correspond with a CAPE of 2000, so this is the threshold I use for the pink area of CAPE in my weather map for these blogs.

 

There was a squally front over the Lau group of Fiji and southern Tonga last Thursday night.  It occurred in an area well east of the main convergence zone and its sudden onset, and six hours duration, and ferocious content was downright dangerous and, in some cases, damaging.

Computer models don’t have enough incoming data to be able to get a good enough “grip” of the atmosphere to resolve such squall lines, and all they were showing was a zone of light winds. The satellite imagery showers jetstream cloud, mainly further west, and the weather maps mainly showed a convergence zone to southwest of Fiji. The jetstream cloud could be used as a warning sign, for these squalls are known to occur near the equatorward side of the the entrance region of a jetstream.

I have used the earth.nullschool.net archive to zoom in on a surface wind map of Fiji area, with a white overlay of CAPE. The CAPE is high all the way between Fiji and Samoa, so does seem to over some warning of squalls, albeit rather vague and imprecise. I suspect that if we had more incoming data, the CAPE view would also be more precise. In the meantime, all I can offer to those afflicted by that thumping, is that weather is a mix of pattern and chaos. Our following of the pattern is vaguer than the reality of the chaos.

 

TROPICS

Off the western Mexico coast we have tropical storm ALETTA and BUD.

Travelling northeast past Japan we have TC MALIKSI and in the China Sea we have Tropical depression EWINIAR

 

If we compare the past week’s rain map with the previous week we can see a build-up of activity over India to Indonesia/Vietnam, along the ITCZ across the North Pacific and Panama, and also along the SPCZ especially near Fiji.

– see trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay put from Solomon island to Vanuatu to southern Fiji. There may be an arm of extra convection from Tuvalu to Tokelau and possibly to Tuamotu Islands. It is expected to have another quiet week between Tahiti and Tonga

 

Subtropical ridge (STR)

A High over southern New Zealand on Monday is expected to travel northeast to east of NZ towards 35S 120W by end of the week. There may be enhanced trade winds between 10 and 20S to north of this high from mid-week.

Next HIGH from the west is expected to enter the Tasman Sea around Thu 21 June—a long way away

 

Around Tasman Sea, NZ to tropics /FP

LOW1 approaching NZ from the north is expected to curve westwards and cross the Auckland area on Tuesday. It has strong winds and rain tied up with its warm front in its dangerous quadrant (southeast of centre) and this is expected to affect the Gisborne area, which is vulnerable and flood-prone due to heavy rain last week.

There is another Low, Low2, near Lord Howe Island tonight, and this is expected to go northeast and fade.

 

This means that after Low1 gets south of Northland, there is a reasonable window of opportunity for sailing north to the tropics---the voyage will encounter a zone of lights winds near 30S.

Another trough is expected to travel east across the Tasman Sea on Saturday reaching South Island on Sunday and North Island on Monday 18 June. Preceded by strong NW winds—finally a good direction for kayaker Scott Donaldson attempting to Kayak across the Tasman Sea (with me watching the weather) Sadly Scott got caught in an eddy loop last week and it has taken a week to get out of it. Follow Scott’s progress at tasmankayak.com/

 

Australia to tropics

There is a good opportunity this week, with LOW2 until is around until Tuesday and with the trough approaching from the west until Friday.

 

Galapagos /Marquesas

From Galapagos area to Marquesas, departure can be any time this week. Best path for avoiding direct down-windsailing is to go to 10S 130W, then go direct.

 

Tahiti to Tonga

South Pacific convergence zone is well to the north this week, and route is clear of any strong convection. Expect ESE winds over 20 knots between 10 and 18S from 12 to 15June with 2m+ swells as HIGH passes to the south.

 

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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 6427 7762212

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

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