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

15 July 2018

Bob Blog 15 July 2018

WEATHERGRAM

YOTREPS

Compiled Sun 15 July 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.

 

Kayaker Scott Donaldson made successful land fall in new Plymouth early last week after departing from Coffs harbour in early May.  See www.stuff.co.nz/national/105168174/Kiwi-Scott-Donaldsonreaches-Taranaki-the-first-person-to-kayak-the-Tasman-solo

 

I have been helping Scott with twice daily txt messages looking at currents, wind and swell. Scott is an adventurer motivated to raise money to help those with live with Asthma (such as he and his son)

If this success inspires anything in you then please show it by funding his donation page:

givealittle.co.nz/cause/tasman-kayak--- Do this soon as the page closes by end of July.

Surely such an adventure is worth a $10 donation?

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WINDY.COM have added a new layer, called THUNDERSTORMS, which comes from the global ICON model from DWD as a combination of rain and lightning density. More resolution than the CAPE parameter.

See community.windy.com/topic/6269/windy-is-the-first-weather-service-in-the-world-to-globally-forecast-thunderstorms.

Technical details are at confluence.ecmwf.int//display/FCST/45r1+new+parameters%3A+lightning+flash+density

Lightning is static electricity. Any object that spikes upwards from an otherwise flat horizon is asking to be struck by lightning. When updrafts in a shower cloud exceed around 25 km/hr (13 knots), air and water molecules are rubbed together, creating static electricity (making lots of free electrons). In the updraft, rain, ice, and snow are jumbled, and the lighter particles are lifted to the cloud top, whilst freed electron tend to attach to the heavier particles at base of the cloud. Like charges repel, and so this reservoir of electrons in the base of the cloud causes the free electrons in the ground beneath the cloud to go elsewhere. This forms an area of positive charge in the ground under the cloud. When the cloud gets blown around, this positively-charged area follows the cloud like a shadow. When the reservoir of charge in the cloud gets large enough, an explosive discharge occurs.

 

First, groups of electrons leap outwards and downwards in all directions towards the positive area near the ground. This happens in jumps, called leaders, each taking around a nanosecond, and at the end of each jump the pause may break the leader into two. When one of these leaders is within about 100 metres of the ground a group of positively-charged particles streams up from the earth to meet the leader. These are called streamers and are best made from a spike or tower (or tree). When the streamer meets the leader, the stored energy (100 million volts) is released suddenly enough so that the air in the stream is ionized. It turns to plasma, 20,000C+, or 10 times hotter than the surface of the sun for a fraction of a second. This ionization works its way up the streamer (so the light starts at the bottom and ends at the top), and this is called the return stroke. On its way up all the places where the streamer has divided get lit, hence the spread-out pattern of some lightning photos. After the first strike, the main pathway between the ground and the cloud reservoir of charge can go through another ionization episode (or strike). This is the flicker seen in a lightning strike.

 

A normal lightning path or bolt may be many kilometres long and is only as thick as a thumb. The speed of sound is 1 kilometre in about 3 seconds or 1 mile in about 5 second, and you can use the time difference between the flash and the bang to measure the distance between you and the strike. Lightning may be seen when it is 60 kilometres away, but thunder is only audible for maybe 10 to 20 kilometres. In the 19th century, thunder was thought to be caused by the implosive collapse of the partial vacuum of the lightning bolt. Nowadays, the consensus is that it is the shock wave of the sudden thermal expansion as the bolt forms plasma (air breaking the sound barrier). The rolling sound after the initial clap is sound arriving later from parts of the lightning bolt that are further away. Over land, rolling thunder also echoes off surrounding terrain.

 

80% of lightning discharges occur from top to bottom of a cloud, or between two nearby clouds (C2C or cloud to cloud). The ones that reach the ground are called C2G or cloud to ground.

 

I handle weather forecasts for around 400 yachts each year, and in the past year two have lost their navigation instruments due to lightning damage (both in tropical waters).  This seems to be a higher rate than the normal quote of 1 in a 1000 per year.

 

A yacht mast “spikes out from a flat surface” and thus tends to act like a lightning rod. Normally it is connected to the keel reasonably well by its rigging. Since lightning is static electricity it tends to flow on the outside of a surface, and so a metal mast and rigging should take most of the charge to the keel and surrounding seawater. Some extrapolate from that to say there is a cone of protection underneath the mast, but some say that’s a myth (see www.lightningsafety.com/nlsi_pls/cone-of-protection-myth.html). Installing a proper lightning rod and connecting this via copper plate to the keel may just divert some of the lightning charge so that it blows thru the copper plate, holing the keel. Opinions differ on this point.

 

When you see a lightning squall coming, the standard advice is to get the sails down and the engine going on auto helm, and standby in the companion way or below deck and cross your fingers. I’d like to add that you protect your electronic equipment: Disconnect your antenna, and gather any mobile navigation equipment such as GPS, satellite phone, tablet, laptop, etc., and toss them into the galley oven (or a microwave if you have one) as an approximation to a Faraday cage.

 

TROPICS

Tonight, we still have BERYL in the North Atlantic (now out of the tropics), and a “zone of interest” between Mexico and Hawaii.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Malaysia and Indonesia. This is part of a MJO event, a trend to increased convection spreading east into the NW Pacific. This is likely to trigger tropical cyclone activity around Micronesia over the next week or two.

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

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with a weak stretch across the Wallis and Futuna area.

A passing trough is expected to visit Tonga on Monday UTC and then travel east to visit Rarotonga on Thursday UTC and linger there until Sunday 22 July UTC.

 

Subtropical ridge (STR)

HIGH near 40S and east of NZ near 180 tonight is expected to slowly travel east along 40S this week. Associated squash zone of enhance d trade winds is expected to peak near 20 to 25S over Gambier Islands on wed to Fri 18 to 20 July UTC.

Next HIGH is expected to travel east along 25S across the north Tasman Sea on from Mon to Thu and then linger over northern NZ on Friday and Saturday UTC.

 

Around Tasman Sea, NZ to tropics.

Trough moving off NZ on Monday followed by a westerly flow. A weak trough is expected on Wednesday followed by a SW flow on Thursday.  Light winds with a passing ridge on Friday and then another trough is expected from the Tasman Sea on Saturday/Sunday.

 

Australia to tropics

A slow-moving ridge over the northern Tasman Sea this week.

 

Tahiti to Tonga

Winds around the Society island may reach a peak on local Sunday/Monday 15/16 July as a squash zone (associated with a passing HIGH) travels east.  For the remainder of the week there should be an OK east to ESE flow from Tahiti to Tonga with a minor squash zone along 25S.

South of 18S, A passing trough should reach Rarotonga area from Thursday/Friday UTC.

 

Galapagos to Marquess

SE to E to NE winds, mostly less than 20 knots. To use available currents, go to 2S 93W and then 6S 125W 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

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

 

08 July 2018

Bob Blog 8 July

WEATHERGRAM

YOTREPS

Compiled Sun 08 July 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.

 

Acceleration zones

Terrain effects near mountainous islands interrupts the wind flow and divides it into rivers of wind and puddles of calm. Today’s EC (9km resolution) model as seen on windy.com shows this in the trade winds over Fiji, with an onshore seabreeze developing around Momi Bay. The EC model data is running with only a few real world observations but its portrayal closely matches the observed data at Nadi Airport.

When I was a forecaster in Fiji back in the 70s I discovered that whenever the pressure difference across Fiji gets around 2 hPa or higher (isobars drawn 2hPa apart, the winds in the accelerated zones such as Vatu-i-Ra passage, Bligh water, and the SW end of Viti Levu may be funnelled to over 25 knots. Today the EC model shows orange-coloured speeds in these acceleration zones, for 21-averaged knots or more. These zones are strongest over the sea, and it would be good to be able to position some wind stations on nearby land.

The Fiji MetService has several local rules they use for compiling the Fiji Marine forecast. This evening’s edition responds to that timeless “isobar rule”:

Marine Weather Bulletin

Issued from the National Weather Forecasting Centre Nadi

at 7:30pm on Sunday the 8th of July 2018

A STRONG WIND WARNING REMAINS IN FORCE FOR ALL FIJI WATERS.

Situation:

A high pressure system to the south of Fiji continues to direct

strong east to southeast wind flow over the Fiji waters.

 

Forecast to midnight tomorrow for Fiji waters:

East to southeast winds 20 to 25 knots. Rough seas. Moderate southerly swells.

 

Further outlook: Easterly winds 20 to 25 knots. Rough seas. Moderate southerly swells.

===

Interestingly, the GFS model, with its 13km resolution, and .25x.25 degree grib distribution does not capture these terrain effects:

 

TROPICS

Tonight, we have Storms THREE and BERYL in the North Atlantic and MARIA in the NW Pacific.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Japan (severe flooding there in the past week). Also, the rain across the South Pacific has become more intense. It remains very intense around central America.

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

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to weaken during the coming week. It is expected to hover from Solomons to Samoa, and the zone over the Solomon Islands is expected to drift slowly south this week.

The trough which visited Samoa over the weekend is expected to travel SE across the Tahiti area on local Monday and fade over the Tuamotu islands on local Tuesday.

The next trough from the west is expected to reach the New Caledonia area on local Friday, Vanuatu on Saturday, and Fiji/Tonga/Samoa early next week.

 

Subtropical ridge (STR)

HIGH is travelling east along 30S to east of NZ this week. It should have a squash zone of enhanced trade winds on its Northern side, reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

Next HIGH is expected to travel across South Tasman Sea on Tue/wed UTC, northern NZ on Thu UTC and then to east of NZ along 35S from Fri UTC, fading near 170W by Sunday UTC.

 

Around Tasman Sea, NZ to tropics.

Large trough is crossing NZ on Monday and Tuesday with winds backing to SW then southerly and bursts of squally showers.

Too unsettled to depart on mon/Tue. OK to get away of Wed, but will need to allow for encounter with northerly winds ahead of the next trough

A Trough is expected to move off Australia and onto the eastern seaboard on local Thursday, and then deepen into a low between Lord Howe and New Caledonia on Friday. It is further is forecast to go southeast towards northern NZ over the next three-four days, turning in to a multi-centred system with centres below 1000hPa.

 

Australia to tropics

Stay put until after that low forms and moves away. Should be OK to go from around Sun 15 July.

 

Tahiti to Tonga

It may be better to stay put next few days – due to squash zone of enhanced trade winds reaching a peak near 15S between Tahiti and Samoa on Tue/Wed UTC.

A departure on Thu/Fri UTC may be able to get as far as Palmerston Island before the next trough arrives from the west, maybe Niue.

 

Galapagos to Marquess

SE to ESE winds, mostly less than 20 knots. To use available currents, go to 2S 95W and then 6S 125W and then direct.

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

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

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

01 July 2018

Bob Blog 1 July 2018

WEATHERGRAM

YOTREPS

Compiled Sun 01 July 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.

 

JUNE REVIEW

Sea Surface temperature anomalies as at end of June may be seen at www.ospo.noaa.gov/data/sst/anomaly/2018/anomnight.6.28.2018.gif

As has been the case for a few years now, the warm anomalies outweigh the cool, so there is a net discontinuity in our oceans. The eastern equatorial Pacific is the focal region for ENSO and is now on a slowly warming trend. Temperatures around Australia and into the Tasman Sea are about to below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America still stands out as warmer than normal.

Warm anomalies continue in the north Pacific indicate a busy cyclone season for next few months.

To see how the annual weather cycle and the seasons are working out, here is a quick look at the average isobar maps for past 30 days from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

 

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust. The North Atlantic and Northeast Pacific HIGHS are looking less strong than this time last month. The June Lows in the Tasman Sea have been less than the May Lows.

 

Polar westerly winds are stronger around Antarctica from NZ to South America but weaker than normal from South America to South Africa and around Australia to New Zealand. It’s complicated.

 

Zooming into the NZ area, the 1015hP (between blue and white) isobar has stayed put in the tropics and travelled east across NZ. It’s subtle, but observable. This indicates a change from a westerly flow to a (cooler) SW flow:

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

shows extra convergence in the ITCZ from Bangladesh to Philippines , across s the North Pacific Ocean, around the Mexican west Coast and across the equatorial Atlantic, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere there are large dry regions: Around Madagascar/ Western Indian Ocean, Vanuatu to Southern Cooks (the normal position of the South pacific Convergence zone), and the Amazon.

 

The Indian Monsoon 2018 arrived faster and earlier than normal, as seen at www.imd.gov.in/pages/monsoon_main.php

 

 

TROPICS

Tonight, we have a TC off the west of Mexico (FABIO) and in the China Sea (PRAPIROON). Also, the remains of EMILIA are travelling to the NW, leading the way for FABIO.

 

If we compare the past week’s rain with the previous week we can see that the intensity of the rain in the South Pacific Convergence Zone and around central America is intensifying.

– 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, to Samoa to Southern Cooks. A small low should form on the SE end of the SPCZ, to SE of southern Cooks, on Tuesday 3 July UTC and deepen as it gets further SE.

And a small but rather intense passing trough/tropical low os expected to form over Wallis/Futuna are on Thu/Fri UTC and travel over Vava’u are on Fri UTC and Niue/Southern Cooks area on Sat UTC. Avoid.

 

Subtropical ridge (STR)

New HIGH is expected to spread into the Tasman sea on Mon/Tue and then travel NE across northern NZ on Wed/Thu and further NE next weekend.

 

Around Tasman Sea, NZ to tropics.

Looks Ok for a voyage for NZ to tropics any day this week.

Small low is expected to form off Queensland coat on Monday and fade in situ by Thursday. Strong disturbed westerly flow is expected to spread east from Tasmania/southern New Wales from Wednesday, reaching South Island of NZ by Friday.

TROUGH that crossed NZ on Sunday is expected to deepen into a LOW over Chatham Islands on Monday UTC that is expected to go NE on Tuesday/Wednesday UTC and then east/southeast. Avoid this Low.

 

Australia to tropics

Not this week, too much head wind. But an opportunity may be appearing around Sat 7 July.

 

Tahiti to Tonga

A dose of strong SE winds is expected to affect northern parts of this area on Monday and Tuesday UTC. And on Fri/Sat UTC a small but intense low/passing trough is expected to affect the Vava’u to Niue area. Next week may be better.

 

Galapagos to Marquess

SE to E winds, mostly less than 20 knots. To use available currents, go to 2S 95W and then 5S 120W and then direct.

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

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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