Bob Blog 9 Sep

WEATHERGRAM

YOTREPS

Compiled Sun 09 Sep 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.

The “Olde Man” Southerly

When a low crosses the North Island and the deepens as it moves away, the resulting southerly gale thru Cook Strait lasts for around three days. Such an occurrence is nicknamed by many Wellingtonians as an “Olde Man Southerly”. Its steadiness feels belligerent , lingering longer than desired. At this time of the year the arriving air hovers just below 10 C, but when the wind chill is taken into account is feels less than 4C—colder than the inside of a fridge. This wind has been described as “blowing straight through you” for you feels it’s cold in your bones .

We had an Olde Man last week and I was in Wellington to experience it. Looking at the weather map (Noon Wednesday local, courtesy of MetService) we can see the low responsible.

image

Because this low is close to Auckland, I have heard some smart people claiming that the reason Wellington gets such cold southerly outbreaks can be summed up in two words: “Auckland sucks”. The map illustrates a classic “eggbeater” with a clockwise spinning wind around the low to the east of the North Island, and a counter-clockwise spinning wind around the High to east of the South Island.

image

Of course another contributing factor to this Cook Strait gale is the layout of the land, with mountain chains over the South Island and the North Island, Cook strait is the right sized gap to make a “river of wind” .

Thanks to windy.com we can see a 2D image of this river of wind, and also by clicking on Wellington airport see a time-lapse diagram of the temperature, wind and pressure profile during this “Olde man”

image

We can see how this river of wind crosses the isobars toward s low pressure. Zooming into those graphs some interesting points can be made


1) The switch from Northerly to Southerly is relatively quick, with no real “calm” in between. Also the barometer starts rising hours before the northerly winds fade. image

2) It takes around 12 hours for a SW wind to swing to a southerly and rise to a gale, along with a period of rapid rising pressure.

3) AFTER the Southerly gale starts , the pressure continues to rise, not so fast, but a steady rise from 1015 to 1025hPa over 2 days.

4) Once the pressure peaks and steadies, the wind relaxes just a little. It continues to stay strong for another day or more.

5) Temperature remains much the same throughout the event with no day/night signal. Showers continue until the air dries out and dewpoint drops to below 5C.

TROPICS

It is now “busy time” for the Atlantic Ocean. FLORENCE is likely to make landfall in North Carolina, and OLIVIA may visit Hawaii but is expected to weaken in the process. MANGKHUT is heading to the northern Philippines /Southern Taiwan area.

image

Looking at the weekly rain maps we can see the tracks of last weeks’ cyclones with JEBI across Japan and NORMAN just missing Hawaii. Note hoe the rain associated with the low east of NZ just touched the east coast of the North Island. See http://www.facebook.com/MetService/videos/891981477639175/

There has been very intense rain associated with a low just south of Tahiti – I’m not sure why this low was so wet.

image

image

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

WEATHER ZONES

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Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves (magenta). STR, and SPCZ. Pink area = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stay in the region from PNG to just south of the Solomons Islands to north of Fiji this week.

Low to south of Tahiti is expected to go south and fade next few days.

Passing trough to south of Fiji on Monday UTC and then over southern Tonga on Tuesday UTC and Southern Cooks on Thursday UTC, consisting of erratic winds and possibly a few showers.

image

Accumulated rainfall for next week from windyty.com. CZ= convergence zone.

Subtropical ridge (STR)

HIGH less than 1020hpa is expected to form near 25S South of Tonga on Monday UTC and travel east along 20 to 25S to be south of French Polynesia by Wednesday UTC.

HIGH over 1025hPa over New South Wales on Monday UTC spreading east along 30 to 35S reaching NZ around Thursday UTC and then spreading further east along 35S.

There may be a zone of enhanced SE wind so the north side of this High, mainly near 20S.

Around Tasman Sea, NZ.

Low is expected to cross northern NZ on Monday and southerly winds following this feature are likely to affect area between NZ and 20S on Tuesday –upsetting sailing between topics and NZ, but OK going t’other way. After that, with high pressures over NZ and trade winds to north of NZ, some good-looking sailing voyages from the tropics to NZ are likely.

Mainly easterly winds in North Tasman sea, so OK to sail New Caledonia to Australia, but not the other way. Traffic going from Australia to NZ may need to get to at least 35S to find a way eastward.

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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

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