Bob Blog 15 Nov

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.

Compiled Sun 15 Nov 2020

Why the Napier deluge?

On Monday 9 November, Napier City (east coast of North Island, NZ) got 242mm of rain —463 per cent of its normal monthly rainfall — in just a few hours. This was Napier’s second wettest day in 150 years. It caused floods and slips that left more than 100 homes uninhabitable.

This was due to a “collision” of warm moist air arriving from the north and cool air arriving from the south. These airflows combined and stayed in place like a hand-held-hose, bringing a stream of intense rain. Places north or south of this stream didn’t get the deluge.


The Low over northern NZ was “cut-off” – it had light variable winds above it and, lacking a steering field, was very slow-moving. This also allowed near vertical ascent in its clouds, accentuating the rain.

Moisture in the incoming northerly flow can be tracked back to Papua New Guinea, via Vanuatu. This is a weather pattern typical of our current LA NINA and should recur several times during next few months,

The intensity of this event also indicates how climate change is affecting us— a warmer world allows these plumes of higher humidity and thus, when the moisture is cooled, zones of enhanced rainfall.

The Tropics

The latest cyclone activity report is at and Tropical Cyclone Potential is from


Cyclone ETA has moved offshore and weakened after leaving six dead in North Carolina and Florida, and is followed by IOTA now near central America.

Typhoon VAMCO brought extensive damage to Philippines, including Manila and is now making landfall over Vietnam.

Cyclone ALICIA in southern Indian Ocean is the first cyclone of the new Southern Hemisphere cyclone season.


Weather Zones Mid-week (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), STR, and SPCZ.


Predict wind CAPE mid-week gives two possibilities for SPCZ


SPCZ=South Pacific Convergence zone.

SPCZ is slowly shifting south to its normal position from Solomon Islands to Northern Vanuatu to between Fiji and Samoa, and is increasing in activity.

A trough is expected to linger about and south of the Tonga/Niue area early to mid-week.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH skirting around south of NZ today expected to travel NE to 40S 140W by mid-week and build to 1038hPa with enhanced trade winds on its northern side mainly around 20 to 30S.

Weak ridge of high pressure is expected to move into west Tasman Sea on local Tuesday and travel to NW of NZ by Friday/Saturday and then weaken away.

Another weak High should cross Tasmania on Friday and South Tasman Sea by 22 Nov.

NZ/Tasman troughs

Looks Ok to arrive in northern NZ any day this week/early next week, but avoid arriving around Fri 27/Sat 28 Nov.

Trough crossing South Island tonight and weakening of remainder of NZ on Monday.

LOW crossing South Tasman Sea on local Tuesday may deepen to 974 hPa then weaken and go southeast.

Associated trough expected to cross South Island on Tuesday night and North island on Wednesday, and the deepen into a low 1004hpa east of North Island and move off to southeast on Thu to Sat.

Another trough expected to reach South Island by Saturday 21 Nov.


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

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Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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