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

14 January 2018

Bob blog 14 Jan 2018

WEATHERGRAM

YOTREPS

Compiled Sun 14 January 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.

 

Addendum to last week:

Regarding the details of the Niwa SEA LEVEL site--- the bottom graph compares the total SS in blue with the IB component in red, and last week it showed that the IB component made up most of the SS. In other words, the main component to the observed risen sea-level was the IB or inverse barometer effect, not the spring (or king) tide NOR the storm surge.  The incoming low may have brought an associated “dome of raised sea”, and as the Low travelled, so too did the dome, but the sea itself did not travel (just rose and fell as low went by).  The storm tide of piled-up and pushed-around sea water, and the king tide, provided lesser contributors.

 

In the past few days a HIGH has been crossing southern NZ with offshore winds across Foveaux Strait (Dog Island) and this produced a NEGATIVE storm surge (offshore wind)  and a negative Barometric effect lowering the sea level. The NIWA graph show that while these drops lowered the level of the sea (especially notable on the beach and over the sandbanks) to a level lower than expected by the normal “quoted tide”, they did NOT lower it to what it was at the spring tide earlier in the week.  As seen at www.niwa.co.nz/our-science/coasts/tools-and-resources/sea-levels/dog-island

  

Weather trend over the last month.

Sea Surface temperature anomalies may be seen at www.ospo.noaa.gov/data/sst/anomaly/2018/anomnight.1.11.2018.gif

The SST anomaly image shows a typical LA NINA with cooler than normal seas straddling the eastern equatorial Pacific.  Also note the warmer-than-normal zone across the Tasman Sea – this can activate any Tasman Low to produce more than normal wind/rain.  We saw an example of this on Thu/Fri 4/5 Jan and may see another (lesser) example on Tue/Wed 16/17 Jan 

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

Note the strong three-wave pattern in the northern hemisphere with the deepest lows preferring to form near Japan, eastern North America and western Europe— this is linked to the air aloft and its jetstreams.

During December the subtropical ridge over the South Pacific shrunk, and is now focused across NZ and to west of the Andes. The 1015hP isobar shifted has spread from the Southern Ocean to the southern South Island, and from near Brisbane/Noumea to near Norfolk Island. It looks like the subtropical ridge may have given NZ an early start to summer, and now that it is fading we can expect an early start to Autumn=mist/low clouds. 

    

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

This shows that both the ITCZ and the SPCZ have an average position last month further AWAY FROM the equator than normal.  This is typical of La NIA. Interestingly in the Atlantic, the ITCZ is closer to the equator than normal—this shows that there is more happening than can be explained by just examining LA NINA.

 

TROPICS

TC JOYCE went inland to NW Australia on Friday and is now expected to get out to sea again by Tuesday and then travel south so that its rain visits SW Australia.

TC BERGUITTA is in the Indian Ocean to east of Madagascar nd expected to travel SW and then S , avoiding making landfall.

 

Looking at the weekly rain maps from last week and the week before, we can see that the main convective rain over e past week has been in the Indian Ocean and with TC JOYCE (before landfall). The cluster of extra convection in the Indian Ocean is expected to make its way eastwards and start appearing in the Coral Sea area around late January/early February.  This is called an MJO oscillation

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

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is hovering from Coral Sea to northern Vanuatu to Fiji/Tonga then to Southern Cooks, and is of weak to average intensity this week.  It is expected to become more active NEXT week, especially in the Coral Sea.

Tropical accumulated rainfall for next week may be seen at windyty.com

 

Subtropical ridge (STR)

HIGH to southeast of NZ s expected to travel northeast to 40S 170W and then east.

Next High is expected to struggle and finally get into west-central Tasman after Thursday this week, and another into southern Tasman Sea (SE of Tasmania) by Sunday 21 Jan.  These Highs should travel east – to north and south of NZ- early next week.

 

Around Tasman Sea

Trough is slowly crossing.  Low in this trough to SE of Lord Howe on Monday is expected to deepen until Wednesday and travel SSE then escape its jet and weaken and travel E/NE to Northland on Saturday 20 Jan.   Worth avoiding mid-Tasman mid-week.  And its fronts are expected to cross NZ on Tuesday night/Wednesday (occlusion/avoid) and then on Fri/Sat (cold/ho-hum).

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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07 January 2018

BobBlog 7 Jan 2018

WEATHERGRAM

YOTREPS

Compiled Sun 07 January 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.

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That low over northern/central NZ last Thursday night to Saturday brought lots of wind damage and some sea-inundation damage. Indeed worth avoiding.

Yes, it did deepen quickly on Thursday, but not quite steeply enough to satisfy the definition of a meteorological bomb. Its central pressure dropped from 1000 to 981 in 24 hours at average latitude 36S, that’s 19 hPa, or 23 hPa when corrected to reference latitude of 45S. SO,it reached a max of 0.95Bergernon, not quite 1B.

 

LOW was 1000hpa at around 33S on Thursday 1am local, then LOW was 981hpa at 1am Friday near 37S.

 

The term “bomb cyclone” has been given plenty of mention in the past week because of the intense low over eastern north America directing cold air for the chilled Arctic. A good write up of how the original term was coined is at www.washingtonpost.com/news/capital-weather-gang/wp/2018/01/04/this-researcher-helped-coin-the-term-bomb-cyclone-he-did-it-to-keep-people-safe/

The lower than normal air-pressure around northern NZ last Friday lifted the sea level (inverse barometric or IB) at the rate of 1cm for every hPa below the norm of 1012 —so for 995 hPa that’s a lift of 170mm for Whitianga on Friday. The raw tides were more extreme than normal last Thursday/Friday because the full moon occurred close to perigee. The stronger NE winds completed the tri-factor and brought a large storm surge of around 200mm in on top of a larger than normal high tide and the inverse barometer effect.

Adding it all together, there was a 350mm rise to the sea level measured at the Whitianga tide gauge last Friday as seen at the NIWA plot at www.niwa.co.nz/our-science/coasts/tools-and-resources/sea-levels/whitianga-wharf. On their plot the full moon (on Tue 2 Jan) is shown as a yellow circle and the time of perigee is shown as a purple circle.

 

It should be pointed out that the low wasn’t of tropical origin and went thru its deepening process in the 30 to 35S latitude band. Basically, the surface low managed to have an upper trough move over it so that it found itself between two jetstreams—on the polar side of a jet exit and on the equatorial side of a jet entrance. In such a zone the surface air rises and is carried away by the jetstream. In this case the air was blown off faster that it could rise, and so, as air was removed from the low, its central pressure dropped “explosively” the jetstreams got energized by the release of heat as the clouds around the low turned to rain. This is a classical mid-latitude low—NOT a tropical cyclone. Once in a while (maybe as often as once per month in winter) we see a low develop like this in the Tasman Sea, but few are as serendipitous in the mixing as this one, and we haven’t seen one like this for several months.

 

Let’s hope the next one is after summer.

 

 

The Tropics:

Looking quiet in the South Pacific for the next week or two.

There are two tropical cyclones in the South Indian Ocean at present: IRVING near 15S 85E is travelling SW and expected to turn off to the south before affecting La Reunion.

And TC AVA, as seen at www.tropicaltidbits.com, is further west and fading as it travels south along the east coast of Madagascar

 

Looking at the weekly rain maps from last week and the week before, as at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif, we can see pockets of intense convection over Indian Ocean and northern Australia, and an easing of activity in the Coral sea to Fiji area.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is still active between Samoa and Southern cooks, and rather weak but building over the Coral Sea and Vanuatu/Fiji/

 

Subtropical ridge (STR)

HIGH1 is expected to travel east from Tasman Sea across northern NZ on Wednesday and Thursday and then off to the east of NZ along around 30 to 35S.

The next high , HIGH2, is expected to spread east across Tasmania on Wednesday and across the South Tasman sea on Thursday and then build to northeast of NZ from Friday, moving along 40S from the weekend into next week.

 

Low crossing central Tasman Sea.

Trough is forming across the central Tasman Sea this week.

A low should form in the trough around mid-Tasman by Wednesday and then travel southeast towards the South Island on Tuesday, but is likely to stall west of the southern alps and fade away on Friday.Trough should remain in central Tasman Sea and perhaps brew another low next week.

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Feedback to bob@metbob.com or txt 6427 7762212

I’m on Facebook at /www.facebook.com/metbobnz/

Weathergram text only (and translator) is at weathergram.blogspot.co.nz.

Weathergram with graphics is at metbob.wordpress.com, Click FOLLOW at bottom right.

To unsubscribe from WordPress: click the “unsubscribe” link on the bottom of the email.

Or, if email wasn’t from WordPress then send a reply email saying LEAVE.

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