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

28 July 2019

Bob Blog 28 Jul

WEATHERGRAM

YOTREPS

Compiled Sun 28 July 2019

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.

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SAM

It is now mid-winter and we are sometime open to blasts from the chilly South. A good parameter to watch, to forecast these blasts is he Southern Annular Mode (SAM), or its proxy called the Antarctic Oscillation (AAO),

 

SAM describes the north to south movement of the westerly wind belt that circles Antarctica, so it can be used as a predictor for cold air reaching our mid-latitudes.

 

When SAM is positive, the belt of strong westerly winds contracts towards Antarctica. This results in weaker than normal westerly winds and higher pressures over mid latitudes, limiting the northern extent of cold fronts.

 

A negative SAM event indicates an expansion of the belt of strong westerly winds into the mid-latitudes. This weather pattern results in low pressure systems over mid-latitudes such as New Zealand, with more (and stronger) storms.

 

We can see the latest forecasts for the AAO (a good proxy for SAM) at

www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_index_ensm.html

This shows a sudden drop from Positive mode to negative mode in early August, perhaps more extensive than the southerly event of mid-July, especially for Southern Australia and New Zealand. This does have an impact into the tropics, weakening the trade winds, bringing periods of SW swell, discouraging squash zone, and possibly encouraging passing troughs.

 

THE TROPICS

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

TC ERICK is expected to pass by south of Hawaii (just)—but may prove troublesome with its swell.

There is more potential than last week, with probable areas in the NW and NE Pacific, and now also in the Atlantic.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Samoa area.

There is a trough passing over Tonga /Niue the next few days, then travelling off to the southeast so not reaching Palmerston Island. Wind change with trof at Niue is likely to be from NE to SE by Wednesday, so should be OK in its anchorage. Best not to arrive in Niue during this trough because of changing winds.

This is expected to be followed by a ridge between 30 and 25S, typical of a negative phase of the SAM parameter.

 

Subtropical ridge (STR)

The High that has been just east of NZ for the past week is expected this week to travel east along 40S.

The next High is expected this week to shift from Aussie Bight to Aussie interior, poking a ridge across the Tasman Sea along 35 to 25S.

 

Tasman Sea /NZ/Aus

In NZ, may as well stay put this week. Weak trough crossing the north next few days. Deep Low south of the Tasman Sea 986 hPa is expected by Wednesday to trigger a Low 980hPa with strong winds and swells over 7m in the Tasman Sea, followed by strong SW flow from Thursday 1 to Tuesday 6 Aug.

Noumea to Australia voyages OK for a few days thanks to a ridge forming across Tasman Sea along 35 to 25S, but swells from the SW are expected to be over 3m from Wednesday or Thursday to Saturday.

 

Tahiti to Tonga

Looking OK to depart from Tahiti on local Mon to Wed, with trade winds weak at times, but after that there may be a 3m southerly swell for  few days. Also need to avoid the passing trough that is expected to reach Tonga around Wed 7 Aug.

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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

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

 

21 July 2019

Bob Blog 21 July

WEATHERGRAM

YOTREPS

 

Compiled Sun 21 July 2019

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.

=-=-=-=-=-=-=-=-=-=-=-

Why do weather models differ so much from each other?

I’ve been asked to try and explain why the output from the EC and GFS often differ as seen at windy.com

Many top nations develop and produce their own global weather model, each with its own equations that take data from the WMO observing network and massage these numbers into forecasts. The EC is still top for accuracy, and the other model mainly accessible is the GFS which was revamped last March.

First problem is that the weather observing network is not 100%. Some data may be incorrect due to measurement error, and the data is spread out and fixed to places on earth so doesn’t gel with the matrix of dots in the computer model. There are huge gaps.

Then there is the matrix of data points. Different countries select different resolutions. Even if the dot separation is small, such as the 9km EC model, the accuracy of the forecast depends on the amount of observations it is based on.

In the tropics, away from good observations, the portrayal of a passing trough may be a zone of averaged out light variable winds, but in the real world there maybe squalls .

From www.researchgate.net/figure/WMO-exchanged-scores-from-global-forecast-centres-RMS-vector-wind-error-over-tropics-at_fig9_312093998

is a graph that is a few year’s old now but gives a comparison of performance of some main weather models including EC from WMO.

Sometimes some models handle the weather pattern better than others. Also, you can usually tell if a particular model goes “off track” as its future scenarios start looking very usual (sometimes ending is unlikely extreme weather).

When the differences are minor, try and work with the average and make allowances that cover both scenarios.

 

THE TROPICS

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

There is potential for another tropical system to form in the Indian Ocean or east of the Philippines or west of central America.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Tokelau/Samoa.

There is a trough passing over Niue on local Sunday 1008 hPa and Rarotonga on local Monday followed by a day or so of W/SW winds. It may be squally as it is associated with a compact low further south deepening as it moves off to the SE. Should be weaker over Tahiti by local Wednesday with a period of N winds followed by a SE wind change.

This is expected to be followed by a ridge travelling east along 30S (along with a High along 42S) from local Wednesday onwards.

There may also be a passing trough between Tuamotu Islands and Marquesas.

 

Subtropical ridge (STR)

Weak High up to 1022 hPa in Tasman Sea from Mon to Fri this week, quasi stationary.

Another High, east of NZ, is expected to travel north from 50S to 40S along 165W from Thursday to Saturday

 

Tasman Sea /NZ/Aus

Low is expected to travel from Tasmania on local Tuesday, deepening to 10008hPa by Thursday, then fading over Northern NZ around Saturday 27 July

This means it’s OK to depart NZ for the topics on Monday/Tuesday.

Also it should bring a zone of light winds between Noumea and Brisbane or vice-versa, mainly from S/SW, and OK. SE winds further north for a voyage to Bundaberg. Peak swell around 2.8m on Friday.

 

Tahiti to Tonga

Maybe wait for a passing trough to clear Tahiti around local Wednesday with a switch from N to SE winds.

After that should be SE winds for week or so, OK to go.

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

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

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

14 July 2019

Bob Blog 14 July

WEATHERGRAM

YOTREPS

Compiled Sun 14 July 2019

 

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.

=-=-=-=-=-=-=-=-=-=-=-

Status of the El Nino

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.

 

Since March 2019 the SOI has been mostly negative, and in past few weeks it has been on the verge of an El Nino. The southern hemisphere subtropical ridge line has been further north normal, and trade winds have been inconsistent, and often interrupted by passing troughs

On the verge of an El Nino, 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 that NINI3.4 has been warmer than normal for the past year , and verging on El Nino values since last October, but recently has been mostly trending downwards. This means the atmosphere and ocean are moving out of phase with each other.

Moving away from El Nino as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

 

The warm waters in the NINO3.4 area are just near the surface, and a temperature/depth graph shows cooler waters near-by. This speaks against any swing to El Nino in the near future.

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 are likely to stay as they are for the rest of this year. But one model is predicting a swing to the cool side.

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

 

Latest SST anomaly map shows a mix of cool and warm in the target area (around Galapagos). The Northern Hemisphere is mostly above normal. In the Sothern hemisphere there is a spread of cool water around eastern South Pacific. Cool water around Indonesia and Western Australia is linked to a positive phase of the Indian Ocean dipole (with stronger than normal trade winds), and is even cooler in the Australian Bight. Around New Zealand and Fiji, seas are continuing warmer than normal.  Sea surface temperatures may be seen at www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

 

THE TROPICS

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

TC BARRY has made in inland near New Orleans along with a large storm surge.

There is potential for another tropical system to form east of the Philippines.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to Tuvalu/Tokelau/Samoa.

There is likely to be two passing troughs this week. The first should clear Tonga at 1012 on local Tuesday and then weaken by the time it reaches Niue on local Tuesday.

The second trough  at 1011hPa is expected to bring a swing to N then S winds around 20 knots, reaching Tonga on local Saturday, then Niue a day later on their local sat night, and going on to Rarotonga at 1015hPa by local Sun/Mon 21/22 July.

 

Subtropical ridge (STR)

There is a quasi-stationary High near 40S to south of French Polynesia this week. It should peak at 1038 on Tuesday UTC, but should be far enough away so s not to produce a squash zone.

Next High is staying over Australia until Sat and then expected to cross the Tasman and reach NZ next week.

 

Tasman Sea /NZ/Aus

No real Highs this week, but maybe 5 passing troughs, with deepest at 998 hPa on Tuesday night local.

This means it’s a stay-put week in NZ.

Mainly southerly winds between Australia and New Caledonia, good for going either way, but avoid 3+ swells on Tue to Friday

 

Tahiti to Tonga

Mostly easterly winds this week, so Ok to depart Tahiti on Monday, but this voyage is likely to encounter a passing trough near 169W on local Sunday 21st— can either sail around this or  maybe stop at Palmerston Island and wait out that trough.

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

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

>>>>>>>>>>>>>>>>>>>>>>> 

07 July 2019

Bob Blog 07 July 2019

WEATHERGRAM

YOTREPS

Compiled Sun 07 July 2019

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.

=-=-=-=-=-=-=-=-=-=-=-

THE TROPICS

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

TC BARBARA appeared in the North Pacific,  weakening as it approached the south of Hawaii. There is potential for another tropical system COSME to develop after and behind Barbara.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ should stay over Solomons to northern Vanuatu to Fiji, with a small convergence zone north of Samoa.

There is likely to be a weak passing trough (1012hPa) reaching Niue around local Thursday and going on to Rarotonga by local Friday/Saturday. 

   

   Subtropical ridge (STR)

The big fat high and its squash zone have finally eased away.

There is a High 1030 in Tasman Sea on Monday, and this is expected to weaken below 1020 and travel east along 30S. 

 

Tasman Sea /NZ/Aus   

With a ridge along 30S, OK this week for sailing from New Cal to Australia and not the  other way.

Weak trough is expected to cross NZ mid wee, and a bigger trough on Friday night/Saturday, and a bigger again trough on Sunday, followed by disturbed SW flow early next week.

Departures from NZ to tropics are OK on Monday and Tuesday, A Wednesday or later departure is likely to encounter a passing trough. 

 

Tahiti to Tonga

OK to depart next few days. Anticipate a weak passing trough 1012 near Niue by local Thursday.

 

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

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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