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

24 February 2019

Bob Blog 24 Feb

WEATHERGRAM

YOTREPS

Compiled Sun 24 Feb 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.

 

A TROPICAL BLAST

I took some snap shots of a cloud feature I spotted last Thursday in the Coral Sea, to the northwest of TC OMA:

It is as if something has triggered a blast from the centre of this cloud feature, causing squall lines to move out to west, south and east in a radial fashion. Fascinating to watch.

I made a video of the event using the one-hour time stepped imagery available from New Caledonia Met site: www.meteo.nc/nouvelle-caledonie/observations/images-satellite

This video is available at tinyurl.com/tropicalblast

 

THE TROPICS

TC OMA moved a little slower than the computer models were picking, and consequently it missed an opportunity to be steered to New Zealand --- however the upper winds did vent a lot of its higher clouds to New Zealand bringing the blessing of some much-needed rain.

TC OMA edged close to southern Queensland from Friday and brought gusty southerly winds.

Gold Coast Airport was gusting over 40 knots on Saturday. Gale winds affected exposed coastal places. OMA has been downgraded now, but may make landfall over New Caledonia as it goes north over next few days, and there are still strong winds and heavy swells long the southeast Queensland coast.

This month the moon was closest it will get this year to earth on Feb 19th (357,012km) and the resulting King tides a few days later, boosted by a strong onshore flow/ storm surge thanks to OMA brought the sea inundating Gold Coast streets.

See www.brisbanetimes.com.au/national/queensland/streets-flood-as-cyclone-oma-approaches-south-east-queensland-20190220-p50z0e.html

 

As mentioned in last week’s blog, TC OMA has triggered a twin cyclone in the northern hemisphere (spurned by the near equatorial westerly winds) This is TC WUTIP and it is expected to strengthen next few days to CAT 4 as it passes to southwest and west of Guam and then to weaken as it travel northwest towards the Philippines.

There is also a tropical depression forming to northwest of Samoa, TD 95P. Its future is uncertain at this stage.

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

The weekly rain maps for the past 2 weeks clearly shows the slow path of TC OMA and TC WUTIP. The South Pacific Convergence zone has had an active week between Samoa and French Polynesia.

See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The movement of TC OMA to the south has disrupted the monsoonal trough. The SPCZ now stretches from northwest of Samoa to south of French Polynesia. TD 95P is now on the SPCZ and its future uncertain, but if it deepens it is expected to dominant the South Pacific this week and travel south affecting Tonga. The SPCZ is on the northeastern edge of the monsoonal trough, and likely to stretch from Samoa to French Polynesia this week.

 

Subtropical ridge (STR)

HIGH in central Tasman sea on Monday is expected to cross central NZ on Tuesday.

Next HIGH is expected to travel eastwards across Tasmania on Wednesday and then onto northern NZ on Sun 3 March .

 

Australia/Tasman Sea / New Zealand

North of 30S, too much in the way of SE wind for departing east from Australia this week, but may be Ok for going westwards.

Further south it is looking OK for getting east.

 

Panama to Marquesas

There were only light winds around Panama last week, but the forecast is for more useful northerly winds from local Sunday to Wednesday, and then light winds again from Thursday to Sunday.

 

Port Vallarta to Marquesas

There may light winds for a Monday start, and better winds for a start on local Tuesday and Wednesday, then light winds again. Weak ITCZ likely between 5N and 2N, and there may be a “mirror CZ” near 3 to 4S.

 

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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 February 2019

Bobgram

WEATHERGRAM

YOTREPS

Compiled Sun 17 Feb 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.

 

SEA ROUGHNESS

The Beaufort Scale was developed by the Irish Royal Naval Officer Francis Beaufort in 1805, and first used on HMS Beagle, as an empirical measure that related wind speed to the observed conditions over an ocean (not near a shore).  The story goes that in those days a British Man-of-War could carry 12 sails, which is why the Beaufort scale goes from 1 to 12 (plus 0, for no wind).

 

From the Beaufort Scale, for a given wind speed, the height of the resulting waves offshore for a fully developed sea are given. But the impact of waves on navigation depends on the size of the vessel and factors such as the wave period, the water depth, and swells from other directions. Also, larger waves need time and distance to develop, so the wave fetch is big factor.

 

After World War One, in the 1920s, the first SHIP weather reporting code was being developed, and just one character was set aside to report the “state of the sea” and another for the “state of the swell”. Captain H.P Douglas (later Vice Admiral Sir Percy Douglas and hydrographer of the Royal Navy) devised two ten-point scales to do this.  These scales estimate the roughness of the sea and the swell for navigation.

   The Douglas scale for the “state of the sea” (Wind waves) is as follows:

Degree Height (m)          Height (ft)          Description

0                           no wave                                                         Calm (Glassy)

1                           0–0.10                 0.00–0.33                         Calm (rippled)

2                           0.10–0.50           0.33–1.64                         Smooth

3                           0.50–1.25           1.6–4.1                              Slight

4                           1.25–2.50           4.1–8.2                              Moderate

5                           2.50–4.00           8.2–13.1                            Rough

6                           4.00–6.00           13.1–19.7                         Very rough

7                           6.00–9.00           19.7–29.5                         High

8                           9.00–14.00         29.5–45.9                         Very high

9                           14.00+                 45.9+                                 Phenomenal

Interestingly, It is almost linearly related to the square root of the wave height

 

MetService forecasts give the wind speed to the nearest 5knots. 

MetService uses the following Sea State guide, based on the Douglas and Beaufort Scales,

relating wind speed, sea state and probable wave height:

 

Wins Speed (Knots)        Sea State                           Possible wave height (metres)

0-5                                     Smooth                             <0.5

10-15                                 Slight                                 0.5-1.25

20                                       Moderate                         1.25- 2.5

25-30                                 Rough                                2.5-4

35-40                                 Very rough                       4-6

45-55                                 High                                   6-9

60+                                     Very High                          9+

 

Note that sea state given in the MetService marine forecast is the maximum possible state for that wind speed.  So, if the forecast is for 25 knots, then the seas will be forecast to be rough, but may actually be smooth or slight within places of limited fetch such as the harbour.

For a more detailed discussion on this see the MetService blog at

blog.metservice.com/sea-state-and-swell

When it comes to working out the likely Height H and Period P of swell waves, the main chart used by marine meteorologists is the Bretschneider nomogram (1970) which, based on climatological observations, gives what waves result from a given wind + fetch + duration.    Examples of this are given in that MetService blog.

The Bretschneider normogram does NOT tell us much about the roughness/steepness of swell.

Looking at the cruising forums, there seems to be some agreement that wave Period affects the roughness of the swell

www.cruisersforum.com/forums/f90/wave-height-period-wind-speed-comfort-116480.html

www.cruisersforum.com/forums/f90/how-to-estimate-roughness-of-sea-206263-2.html

In summary, If P in seconds is 1.33 or more times H in feet then waves are manageable.  Any shorter a period and the waves are too rough.  

We use metres, so that translates to if P in seconds is more than a ratio of 4.4 times the H in metres then waves are OK.   I think that perhaps a simple factor of 4 may be a better pick, so a 2m swell is manageable if its period is 8 Seconds or more, and a 3m swell is manageable if its period is 12 seconds or more. However, this is just a rule of thumb that kind-of works for mid-range waves,

I came across an article at widecanvas.weebly.com/nature/waves-height-period-and-length

And here Dr. Dilip Barua offers us a graph which shows 7 roughness or comfort curves and how they sort out with swell given in Significant height in metres and Period in seconds for what he calls “longshore waves” .

Basically, his magenta roughness curve is close enough to acceptable comfort.

 It isn’t a straight line, but at H= 2m height is around P=7 seconds period (ratio of 3.5) and at 3m height it is near 9 seconds (ratio of 3) and at 4m it is at 10 seconds (ratio of 2.5).  The suggestion from this graph is that maybe the ratio between H and P for acceptable waves is not linear, and maybe it drops for the higher waves. I think this relationship requires more research. 

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THE TROPICS

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There is still a monsoonal trough stretching from the Coral Sea /TC OMA across Fiji to another tropical Low located south of French Polynesia.   TC OMA is expect to slowly drift southwest across NW parts of New Caledonia and then curve southwards into the Tasman Sea.  When it reaches 30S I expect it to encounter stronger WNW winds aloft that may knock off its top clouds and start its spin-down, or perhaps shunt the whole system east-southeast across northern NZ—ah well, we need the rain, and the surfers may get some interesting swells.   The models still haven’t latched onto this system and are offering different scenarios, and at this stage a wide variety of outcomes are still possible, so get updates. 

An interesting meteorological is very likely to soon occur north of OMA. The near-equatorial westerly winds are expected to help spin-up another tropical cyclone north of the equator which will have a life of its own.  This is called “cyclone twinning”, but in this case the spin-up is likely to occur when OMA is near 15S, and usually it occurs when the starter cyclone is a 5 to 10S.

 The weekly rain maps for the past 2 weeks clearly shows this monsoonal trough weakening and the slow path so far of an intensifying TC OMA

See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is on the northeastern edge of the monsoonal trough ,and likely to stretch from Samoa to French Polynesia this week.

 

Subtropical ridge (STR)

HIGH is travelling east across central NZ next few days and then further east along 40S.

Next HIGH is further south and likely to travel east across Tasmania and into South Tasman sea around Sat/Sun 2/24 Feb

 

Australia/Tasman Sea / New Zealand

Wait until after TC OMA.

 

Panama to Marquesas

Only light winds for starters over next few days, but  a reasonable northerly wind is expected next week, so may as well stay put this week.

 

Port Vallarta to Marquesas

There may be a few days of light winds for starters, and then looks Ok with a departure on Tuesday to Thursday, and then some more light winds until Saturday.  ITCZ likely between 5N and 2N, and there may be a “mirror CZ” near 3 to 4S.

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

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 February 2019

Bobgram

WEATHERGRAM

YOTREPS

Compiled Sun 10 Feb 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

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

The monsoonal trough that was over northern Australia earlier this month is now stretching across the Coral sea then in an arc around Vanuatu to north and then east of Fiji.

 

There are tropical depressions spaced out along this monsoonal trough, and there is a good chance that at least one of these may deepen into a tropical cyclone this week, but the models are having problems resolving the details, so it seems best to avoid sailing in this region for the coming week.

 

Sunday evening (Fiji time) weather map shows spaced out depressions, and ex TC NEIL between Fiji and Tonga with gale winds.

The rain map for the past 7 days clearly shows this monsoonal trough,

See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is draped around a monsoonal trough, and is likely to have at least one tropical low along its length this week. Avoid. The SPCZ the n stretches further to the southeast , touching southern parts of French Polynesia.

 

Subtropical ridge (STR)

Next HIGH is expected to spread from East Australia into Tasman Sea on Monday and then travel east along around 32S fading away to north of NZ on Wednesday.

 

Australia/Tasman Sea / New Zealand

With the Highs crossing central or southern Tasman Sea, the main wind flow north of latitude 35South is from the east, good for sailing from New Caledonia to Queensland.

Easterly winds are expected to get strong and seas rough near Northland from Thursday to Sunday local as a low from the tropics travels southwards to east of NZ.

 

Panama to Marquesas

Same as last week: OK to go any day this week with northerly winds for starters and a good tail current to current to 2N 85W. From there either peel off to the Galapagos Islands or follow the current along around 1deg north to 125W and then head for Marquesas Islands.

 

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

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

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

03 February 2019

Bob Blog 3 Feb

WEATHERGRAM

YOTREPS

Compiled Sun 03 Feb 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.

 

Late January brought some extreme weather, with the peak of a heatwave over Australia, some spots in NZ having a heatwave, an unpresented polar vortex over North America, and the continuing drenching monsoonal low over norther Queensland. A mighty meal for any meteorologist. Sadly, I was out of action, as I needed emergency surgery for a blocked gut.   I’m back home now, so here’s my regular blog once more.

 

REVIEW of JANUARY

Sea Surface temperature anomalies as at end of January may be seen at www.weatherzone.com.au/climate/indicator_sst.jsp?lt=global&lc=global&c=ssta

The eastern equatorial Pacific around Galapagos is the focal region for ENSO and the warm area there seems now to be shrinking. There is also a corridor of warmer-than-normal conditions between Samoa and Southern Cooks — this may add oomph to the South Pacific Convergence zone and act as a possible path for cyclones. The Tasman Sea remains warmer than normal, and ready to add extra rain to any passing lows. And the Southern Ocean is showing a lot of melt water, which is cooler than normal.

The Gulf Stream off the east coast of North America still stands out, which is unusual at this time of the year.

 

To see how the annual weather cycle and the seasons are working out, check the average isobar maps from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

The subtropical ridge in the southern hemisphere is almost at its southern most latitude and the anomalies show it is stronger than normal and maybe somewhat further south than normal.

Zooming into the NZ area, and comparing January with November (December data missing due USA govt shutdown) shows that to north of NZ, the 1010hP  isobar has shifted south from around 10S to 20S.  To south of NZ, the 1010 remains in much the same spot, but the pressure gradient shows stronger westerly winds in the Southern Ocean in January than in November.

 

The last 30 days of rainfall, and its anomaly are seen at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

This shows that during January, western South Pacific and Tasman Sea has been dry and it has been French Polynesia that has had the main share of the South Pacific Convergence Zone.

 

THE TROPICS

 

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There is a monsoonal low north of Mt. Isa, and this is expected to stay over land next few days, limiting its chances to develop to a tropical cyclone.

There is a convectively active phase of the MJO over the Western Pacific at present, and it is expected to weaken this week. It may remain in the western Pacific and intensify again by mid-February.  

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is like a monsoonal trough stretching from Australia to Fiji and then as a convergence zone from around Samoa to Tahiti. Tropical lows are likely to form and space themselves along this trough line.

 

Subtropical ridge (STR)

HIGH to east of central NZ on Monday expected to travel off to the east-NE.

New High is expected to travel east across the south Tasman Sea on Tuesday and then around eastern NZ on Thursday and after that to the east-NE.

 

Australia/Tasman Sea / New Zealand

With the Highs crossing the southern Tasman Sea, the main wind flow north of latitude 35 South is from the east, good for sailing from New Caledonia to Queensland.

Easterly winds are expected to get strong and seas rough near Northland from Wednesday in a squash zone with rising pressures to the south and falling pressures to the north.

Further south there are expected to be swinging winds from a passing front on Monday and Tuesday, and increasing NW wind from Friday to Sunday.

 

Panama to Marquesas

Looks OK to go any day this week with northerly winds for starters and a good tail current to current to 2N 85W. From there either peel off to the Galapagos Islands or follow the current along around 1deg north to 125W and then head for Marquesas Islands.

 

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

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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