Bob Blog 29 Sep

WEATHERGRAM

YOTREPS

Compiled Sun 29 September 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

At present there are three active Cyclones LORENZA in the mid north Atlantic, NARDA heading for Baja California and MITAG near Japan, and a few areas of potential activity in North Pacific. ‘Looks like we are heading for a lull in the cyclone season.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from Solomon Islands to Tuvalu /Tokelau, similar to last week.

Passing trough 1012-16hPa over Tonga on Tongan Tuesday, Palmerston on Local Wednesday, and Southern Cooks and parts of Tahiti on local Thursday/Friday followed by SE winds.

Heads up that a bigger passing trough 1008-1012 hPa is expected the following week.

 

Subtropical ridge (STR)

The STR has retreated to 30S around NZ

HIGH to NE of NZ on Monday travelling east along 30S around 1024, so no squash zone.

Next HIGH forming in Tasman Sea by Tuesday travelling NE to 30S by Friday the fading but followed by another HIGH on Friday/Saturday.

 

Tasman Sea /NZ

Intense Trough over northern NZ on local Monday, followed by a SW/WSW flow until Thursday. swell until Friday, strong on Wednesday, easing by weekend.

HIGH in N Tasman Sea should ridge towards northern NZ on Thursday/Friday.

Another Front is expected over Southern NZ on Friday and central/northern NZ by Saturday, followed by a southerly flow.

Reasonable for voyages from tropics to NZ departing this week. Voyage will need waypoints.

 

Noumea to Aus: S to SSE winds at Noumea on local Monday Tuesday in wake of a passing trough then SE winds expected from Wednesday onwards , Ok for going to Aus,.

 

Tahiti to Tonga

The passing trough 1012=1016hpa later in the week doesn’t really have much wind swing at the surface, so can depart any date this week with mainly trade winds.

However a deeper trough 1008-1012hpa is expected to reach Tonga around Sunday 6 oct followed by SE winds, and then travel east, so may need to arrange some positioning waypoints for that.

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

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

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

 

Bob's Blog

WEATHERGRAM

YOTREPS

 

Compiled Sun 22 September 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.

 

Equinox is tomorrow NZ time. Monday at 7:50pm ( 25 0725 UTC)b

At this point in time the overhead sun is directly over the equator, shifting from the northern hemisphere to the southern hemisphere.

If you check sunrise /sunset times for your place for Mon 23 Sep, you will find that these compute to something like 12hr 7mins of sunlight. This is because sunrise and sunset are defined with respect to the top limb of the sun, rather than the centre of the sun, and that adds a few minutes of sunlight to the day. You’ll find (if you look) that the day that has closest to exactly 12 hours of daylight, called the equilux, is around three days earlier than the September equinox.

In New Zealand, the change to NZDT Daylight time occurs at 2am (becomes 3am NZDT) on the last Sunday of September, and so that’s 29 September. Clock goes forward I hour.

The change to daylight saving occurs on different dates for the nations around the South Pacific so I think it’s a good idea for you to have a summary for reference

 

Date Place Daylight saving time zone

29 Sep New Zealand UTC+13 (NZDT)

29 September Samoa UTC+14 (NZDT

29 Sep Chatham Islands UTC+13h45mins (CHADT)

6 Oct New South Wales, Tasmania UTC+11 (AEDT)

6 Oct Lord Howe Is. UTC+11 (AEDT) (30 minute ahead)

6 Oct Norfolk Island UTC+12 (AEDT)

10 Nov Fiji UTC+13 (Fiji Summer Time FJST)

No changes in Tonga or American Samoa or Vanuatu or Queensland/Northern Territory/Western Australia or New Caledonia.

 

As yachts start their end-of-spring migration, with the approaching South Pacific Cyclone Season, nominally on 1 November (starting at low risk). it is also useful to have a summary of the various national holidays, as these limit available departure dates.

 

From East to west

French Polynesia 1 Nov All Saints Day, 11 Nov Armistice day

Cook Islands: Mon 28 Oct Gospel day

Niue: Oct 19 to 22 Constitution day celebrations, Oct 25 is Gospel day.

Tonga Nov 4 Constitution Day

Fiji Oct 10 Fiji Day, Oct 28 Diwali, Nov 11 Prophet Mohammed’s Birthday.

New Caledonia Sep 24 National Day, 1 Nov All Saints Day, 11 Nov Armistice Day .

New Zealand Oct 22 Labour day.

Australia: OCT 7 Labour Day (ACT, NSW &SA), Queen’s Birthday (QLD)

 

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

 

At present there are five active Cyclones JERRY and LORENA and MARIO and KIKO and TAPAH and several areas of potential activity in North Atlantic and North Pacific. ‘

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from Solomon Islands to Samoa.

Squash zone of enhanced SE winds on north side of a passing HIGH on MONDAY from Vanuatu to Samoa/Tonga easing on Tuesday.

Passing trough over southern Cooks/ parts of Tahiti on local Sunday/Monday followed by fresh SE winds on local Monday.

 

Subtropical ridge (STR)

The STR briefly visited 40S over NZ last week. It is returning to north of NZ this week

HIGH to NE of NZ on Monday travelling east along 30S around 1024, so no squash zone.

Next HIGH forming in Tasman Sea by Wednesday to travelling NE to 30S then travelling east past northern NZ by Sunday.

 

Tasman Sea /NZ/Aus

Trough over northern NZ on Tuesday, followed by a SW swell until Friday, strong on Wednesday, easing by weekend.

Another Front is expected over northern NZ Monday 30 Sep followed by southerly winds reaching as far north as Fiji by Wednesday 2 October and Tonga by Thursday 2 October. No good for trips from tropics to NZ arriving next week.

For Noumea to Aus: OK at first but interrupted by a passing trough on Sunday Monday 29/30 Sep with light winds/

 

Tahiti to Tonga: Try and time your voyage after the passing trough (affecting parts of Tahiti on local Sunday /Monday) so depart from Tuesday.

 

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

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

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

Bob Blog 15Sep

WEATHERGRAM

YOTREPS

Compiled Sun 15 September 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.

 

SAM

 

We have had a taste of winter in New Zealand late August/early September, followed by a few weeks with disturbed lows and fronts, but this week is looking more on the HIGH side.

 

A good parameter to watch, to forecast southerly blasts is the 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

Showing a swing to positive SAM and likely HIGHS in the subtropics.

 

 

 

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

For those who want to see the President of US comment on DORIAN possibly hitting Alabama

See www.google.com/search?q=trumps+Dorian+track&oq=trumps+Dorian+track&aqs=chrome..69i57.220763j0j7&sourceid=chrome&ie=UTF-8

 

At present there are two Cyclones HUMBERTO and KIKO and several areas of potential activity in North Atlantic and North Pacific.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

 

The SPCZ is expected to stretch from Solomon Islands across northern Vanuatu to a weaker Convergence zone around Tuvalu and Tokelau.

 

Trough over Tonga on local Monday connected to a Low near 30S This Low is expected to travel east along 30S to 160W by local Wednesday. and then go south. Associated trough is expected to visit Niue on local Monday. Palmerston Island area on local Tuesday and affect Tahiti area with variable winds on local Tuesday night /Wednesday.

 

Subtropical ridge (STR)

The STR is expected to return to 40S thus week, but not until mid-week.

On Mon and Tuesday, a High is expected to travel east along 30S across northern NZ then fade.

Then from late Tuesday to Sunday another HIGH is expected to form near 40S in the Tasman Sea and take it’s time travelling onto South Island before going north to 30S and to East of NZ.

There may well be a squash zone in the tropics on the north side of this High NEXT WEEK>

 

Tasman Sea /NZ/Aus

Trough (between the HIGHS) over northern NZ on Tuesday, followed by a burst of a long-period SW swell from Tasman to north of NZ on Friday. Should be Ok for trips from Fiji/Tonga to NZ this week.

SE to East winds  OK from Noumea to Australia. No good going other way.

 

Tahiti to Tonga

Try and time your voyage taking into account the passing trough. It misses Suwarrow.

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

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

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

Bob Blog

WEATHERGRAM

YOTREPS

Compiled Sun 08 September 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.

 

Parts of a depression.

A depression is a zone where the isobars are depressed below normal. The average surface air pressure in 1013.25hPa and nearest isobar too that is 1012hPa—The 1012hpa is usually the straightest on the weather map, dividing anticyclones or anticyclonic flow with higher isobars -- from cyclones or cyclonic flows with lower isobars.

The same thing is called a Cyclone when referring to its winds rather than it’s isobars. It is also called a Low.

 

Lows come in many shapes and sizes and their intensity (of wind and rain) may be related to the central pressure. The lower this is. the more the intensity. Other factors are the cloud width and shape, and , for tropical cyclones there is Category ( a wind scale), or Dvorak code (size and cloud extent) or ACE (accumulated cyclone energy), Cyclone DORRIAN has a central pressure as low as 910hpa.

 

A low in the Tasman Sea today with a central pressure of 990hpa is a good demonstration showing the parts of a mid-latitude depression (not a tropical Cyclone). It occurs when a stream of cold air from the Sothern Ocean (high density air collides with warmer more humid air in the Tasman Sa (lower density air). At the cold front on the eastern side of the system, higher density air pushes the lower density air up and out to eastwards, creating rising air = cloud and rain

 

The Cold Pool is formed by the upper air reflection of the surface low and is usually found N or NE on the surface Low (in the southern Hemisphere), It can easily be detected from satellite imagery as a circular area of thundery showers, formed by warm surface air rising quickly due to its extra buoyancy when there is colder than normal air aloft.

 

Between the front (which arrived like and express train this morning) and the cold pool (which arrived late this afternoon/evening) there is a zone with partial l sunshine and no rain which is called the “Dry slot”. Most fronts are followed by dry slots and they can become a sailor’s friend, so long an you to brace for the cold pool that may follow.

 

After the Cold pool comes the “Back side” of the Low with its curl of cloud containing another dose of rain.

 

And on the south side of the low there is the frontal occlusion . This zone may become very slow-moving so that even if it only has moderate rain intensity , its volume of rain may be larger than normal over a small area.

 

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 DORIAN hovered around the Bahama for more than 2 days, making 3 landfalls, and since then has been skirting east coast of North America. It also marked a surge in Cyclone activity: there are now FIVE cyclones, and several areas of potential activity in North Atlantic and North Pacific. There is also a zone of potential development around the Solomon Islands, but models are quiet there.

 

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with quieter convection hovering around Fiji this week.

Passing trough, with northerly winds swinging to southerly winds, now crossing eastern Fiji on local Monday is expected to go SE and reach Tonga by Wed/Thu local date and then Niue the next day (Wed local date) but fade and not affect Aitutaki or Tahiti, but may affect Rarotonga on local Thursday/Friday..

 

Subtropical ridge (STR)

Yet another week with STR north of NZ.

Hight to NE of NZ is traveling east along 30 to 35S on Mon-Tuesday

Another High is expected to form off east of Sydney on Wednesday and then NE to 30S then east across northern NZ by local Friday.

 

Tasman Sea /NZ/Aus

Low down to 990 hPa in Tasman Sea is expected to cross northern NZ on Monday/Tuesday local, followed by S Wed/SW Thu//W/Friday. Then another low is expected to deepen in Tasman sea over weekend and cross over northern NZ on Mon 16 Sep.

Looks OK to sail from QLD to New Cal early this week in the winds following Monday’s low but no good from Wednesday when SE winds return, and it is then OK to go t’other way.

If sailing from tropics to NZ this week, then try and time your voyage to arrive in NZ between fronts – as is likely next week..

 

Tahiti to Tonga

Looks Ok to depart from Monday this week, may need to go north of the direct path to avoid lights winds. Early notice that next week may have a squash zone of enhanced winds.

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

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

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

Bob Blog 1 Sep 2019

WEATHERGRAM

YOTREPS

 

Compiled Sun 01 September 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.

 

REVIEW OF THE LAST MONTH

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

The main pattern changes are the ribbon of cooler than normal sea around the Galapagos. There is also a return to near normal in the Tasman Sea and a cooling off western Australia (an continuation of the Positive Indian Dipole).

 

To see how the annual weather cycle and the seasons are working out, we can check the average isobar maps for past 30 days and their anomaly from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

The southern subtropical ridge is much the same as last month. The monsoonal trough is a little further east. The trough around NZ has got bigger.

Zooming into the NZ area, and comparing monthly anomalies for end of last month with now, shows that the subtropical ridge STR is in much the same place but slightly weaker than a month ago. The anomalous trough over NZ has stronger SW flow over NZ than a month ago.

 

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 DORIAN is expected to hit Bahamas and then approach Florida, but may swing north and remain mainly offshore. For the latest see www.tropicaltidbits.com/storminfo/

 

There are several areas for potential development, especially around Bangla Desh, NW Pacific and west of Central America, --- as expected at this time of the year.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with another branch hovering around Tuvalu and Samoa.

Passing trough now crossing Tahiti is expected to be followed by a good SE flow on local Monday.

Passing trough, associated with Low approaching NZ, is expected to travel from New Caledonia today to Fiji on local Tuesday night to Tonga on local Wednesday night to Niue on local Wednesday night to Palmerston on local Thursday and then fade as it moves SE.

 

Subtropical ridge (STR)

Hight to E of NZ is traveling east along 30 to 35S and should hover to south of FP until end of the week.

There is a gap until the next high. It is expected to travel east into the Tasman Sea around Tuesday next week, but not before.

 

Tasman Sea /NZ/Aus

Low deepening in mid -N Tasman Sea on Monday SW flow is expected to travel SE across North Island on Tues/Wed/Thursday down to 990 hPa and then go south away from NZ deepening to 980hPa at 55S, followed by a SW flow. On Friday another Low may form in mid-N Tasman Sea and then travel east across North Island on Saturday. This may be followed by another Low on Monday.

Looks OK to sail from QLD to New Cal early this week in the winds following Monday’s low but no good from Thursday, from then OK for going from New Cal to QLD.

Sailing to NZ this week means skirting around several fronts.

 

Tahiti to Tonga

Looks Ok to depart from Monday this week, voyage is likely to encounter one passing trough, but should be able to skirt around it over the open sea.

 

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

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

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