BobBlog

WEATHERGRAM

YOTREPS

Issued 20 July 2014

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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Negative storm surge

In this blog last week we looked at the storm surge at Marsden Point (off Whangarei) in the storm that bothered Northland earlier this month. With vigorous on-shore easterlies in the past few days a similar storm surge of around .5m has been measured at Whitianga (off Coromandel) recently:

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One of my blog followers pointed out a corresponding drop in sea level at the south end of NZ at Green Island (off Dunedin).  During last week this  was as much as 0.6m and now it is still .4 of a metre. Imagine the embarrassment of boats taking shortcuts across  a mudflat when the real tide is .6m below the computed tide!

Can anyone offer an explanation for these strong “sea sucks” – I don’t think the winds at Dunedin have been strong off-shore for all of last week, so what’s the cause? It seems to mirror the shape and trend of the storm surge on the NE coast- is this a clue to its cause, or a coincidence?

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Sea level anomaly at Green Island from http://www.niwa.co.nz/our-science/coasts/tools-and-resources/sea-levels/green-island The Inverse barometric effect IB explained around -0.1m of the observed almost -0.6m ‘storm suck’ on 17 July.

TROPICAL TOPICS—

Typhoon MATMO is winding up and set to cross Taiwan on Wed 23 July 2014.

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From http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/wp1014.gif Refer to website to get the latest

The weekly rain maps for the past fortnight show heaps of convection over SE Asia weather and along the ITCZ. Also it shows to very wet weeks over Northland in NZ.

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Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing isobars, Lows and Highs, wind, waves, rain, current, SPCZ and STR . Purple lines are upper flow, light brown arrows are wave directions. .

SPCZ= South Pacific Convergence Zone

A weak convergence zone is expected to stay put over Solomons towards the Tuvalu/Tokelau/ Samoa area.

STR= Sub-tropical Ridge

The STR is behaving normally over Australia and in the eastern South Pacific, and is weak –and slowly returning to normal across the Tasman Sea and NZ area. The High departing eastern Australia on Wednesday 23 July is expected to travel east along 30 to 35S across the Tasman Sea on Thursday and Friday reaching northern NZ on Saturday 26 July.

Departing westwards from Tahiti:

North end of a trough in the middle =latitudes is expected to bring squally showers and variable winds to Tonga by Tuesday 22 July UTC and Southern cooks by Thursday/Friday 24/25 UTC. A voyage from Papeete to Suwarrow may work OK but trips to places further south should take this trough into account.

Departing from NZ to the tropics

A compact and intense Low has been travelling east past northern NZ over the weekend causing damage with heavy rain and strong onshore- easterly winds for a while. This low has now gone off to the east of NZ and has opened the way for a trough to form along 170W to east of the whole of NZ. Lows from the southern ocean are likely to be drawn into this trough and one should deepen there on Wednesday.

It looks to me that the coldest week of the year for NZ is very likely to be this week. I say this because the coldest time of the year is usually late July/early August, and during the coming week air from the southern ocean is likely to be shoveled onto New Zealand thanks to the southerly flow that is being directed by the High to the west and the Low to the east— what is sometimes called an eggbeater southerly.

See http://blog.metservice.com/2009/05/an-eggbeater-southerly/

The polar vortex parameter I use, the AAO, is expected to be negative this week, HOWEVER the weather forecast is NOT for a full polar outbreak over NZ this week, as the polar vortex is expected to remain intact (in our part of the world anyway).

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Antarctic anomaly AAO, a good proxy for the strength of the Polar vortex. When this is negative, as it is this week, the risk of a polar outbreak rises.

So, when to depart? The SW flow over Northland is expected to be too strong for comfort on Monday, but weather conditions should be OK for going north on Tuesday to Thursday. After Thursday there may be problems with the voyage as the High then in the Tasman is expected to travel east across northern NZ by the weekend, bringing light winds, and may then be followed by NE winds and another Low.

See my yotpak at boatbooks.co.nz/weather.html for terms used.

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

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