Bob Blog19 Feb

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 Sunday 19-Feb 2023

GABRIELLE as I saw it
GABRIELLE proved to be historically significant. Many rivers broke their
banks bringing extensive damage to Gisborne, Wairoa, and around Napier.
Landslips were noticeable about west Auckland and Coromandel.
A snapshot of power outage for Auckland area (from VECTOR) shows over 60%
out.

As explained last week, GABREILLE did an expected right swing in its track
past Aotea /Great Barrier.
Lowest barometer reading on land was at Whitianga was 968.3 at 4am Tuesday
14 Feb (Valentine's day) when GISELLE "kissed" Coromandel.
There have been lower readings on 26 July 2008 in a winter storm.
Since Whitianga was closest to the cyclone centre it had the highest
"inverse barometer" or IB effect. Its highest total Storm surge SS occurred
around half a day before the kiss. This shows that piling up by onshore
winds can be the larger contributor, as seen in its storm surge as seen by
NIWA.
Channel island sits in the middle of Colville Channel between Aotea/Great
Barrier and Coromandel and squirts wind thru a gap. Gusts as seen on
PredictWind got to 80 knots.
Winds around Northern NZ as GABRIELLE passed by can be seen at the following
video link: https://youtu.be/FPO35_NxyS4
Thanks to MetService (and Neil Gordon) I was able to download the isobar
maps for midnight at start of 14Feb and the following noon.
As usual, where winds meet rain peaks.

Regarding the 5 days accumulated rainfall map from MetService during
GABRIELLE. The actual amounts may be much higher along the mountain ranges
(data here is based on raw data from a rain gauge network at populated
places, and not corrected for orographic effects. Also, there is some
missing data from Hawkes Bay due power cuts)
When GABRIELLE left the tropics the clouds around it assumed a "Koru"
pattern
A "koru" is the name of the pattern of an unfurling fern in New Zealand
This koru cloud sheet lingered over eastern North island as GABRIELLE slowed
down during its kiss past Aotea, hence the heavy rainfall in the onshore
winds.
Uni of Waikato has some rural gauges that measured up to 476mm in two days.

COMPARISION WITH BOLA
In March 1988 Cyclone BOLA brought significant damage to Vanuatu and New
Zealand.
Lowest measured barometer over mainland NZ during BOLA was 980 hPa.
Its track was different from GABRIELLE but both cyclones lingered offshore
northern NZ as they changed direction.
This allowed their Koru clouds to dump heavy rain onto eastern ranges for
an extended period.
I found an accumulated rain map for the Gisborne area during BOLA with a
peak of 917mm

Basically, BOLA was a three-day event and GABRIELLE may have been more
intense but was a two-day event.

TROPICS
The positive boost of the MJO cycle continues to linger around northern
Australia.
The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu
and Tropical Cyclone Potential is from
www.ssd.noaa.gov/PS/TROP/TCFP/index.html
Still going since last week, Cyclone FREDDY is heading for Madagascar. There
is some potential for cyclone development around northern Australia in the
coming week, but not much.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ is expected to remain active across Vanuatu/Fiji/Tonga to a Low L1
that starts in the tropics and goes south-southeast. As usual the peak rain
does not correspond with the peak CAPE.
There has been active convection around western Fiji for a few days and a
flood alert has been issued.
There is expected to be some active convection around Samoa this week.

HIGHS and LOWS
After several weeks of blocking Highs east of NZ it is good to see a return
to the mobile westerly path for features in the mid latitudes. A fast moving
active front brought some squalls to Sydney on Saturday 18Feb, throwing
yachts around .. this is a sign that the pattern has changed.
So, the high H1 east of Aotearoa/NZ tonight is expected to travel east along
45S allowing a frontal trough L2 now near Tasmania to travel east across NZ
on Wednesday.
And this trough should be followed by another HIGH H2 along 45 to 50S.
The easterly winds north of H2 will hinder travel eastward from Australia
for a while.

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