20
BAMOS
Jun 2019
Charts of the Past
with Blair Trewin
28 June 1986
An unusual rain event occurred over large parts of Australia
in late June and early July 1986, through the combination
of a slow-moving cut‐off low and a northwest cloudband
originating from a low near Sumatra.
A low formed near 90 °E west of Western Australia on the 23rd,
and by the 24th was within a few hundred kilometres of the
southwest of the state, but then became cut-off and very slow-
moving. It was still centred near the southwest on the 28th and
29th, by which time it became a very large cut‐off circulation,
with a strong ridge to the south and east, covering much of
eastern Australia.
The low finally moved into the Great Australian Bight on the
30th, with a trough extending northwards from it. This trough
interacted with moisture moving southeast from the Sumatran
low to produce an extensive northwest cloudband ahead of it,
which moved gradually east as the trough moved eastwards
across the continent in the opening days of July. There was
no real break in the pattern until the circulation over southern
Australia turned more westerly on the 4th.
The initial cut-off brought heavy rain to the west coast of
Western Australia. Perth had 77 mm on the 25th with the initial
front, and then followed this with at least 20 mm on each of
the following four days—the only instance of five consecutive
days above 20 mm there—with a five-day total of 169.6 mm.
The heaviest rain was mostly in the Perth area, with the highest
daily total being 94.6 mm at Karragullen, although there was
widespread rain over much of the southwest. There were
widespread frosts in much of eastern Australia (including
Tasmania) under the blocking high during this period, including
−12.1 °C at Woolbrook (near Armidale) on the 26th, but there
was warm air ahead of the approaching trough, with 33.7 °C at
Richmond in north‐central Queensland on the 30th.
The northwest cloudband brought extensive and unseasonable
rain to large parts of northern and central Australia, initially in
the east Kimberley where 76.0 mm fell at Bedford Downs on the
30th, then in the Northern Territory and western Queensland.
The 1st was the wettest winter day on record at Alice Springs
(76.8 mm), followed the next day by several falls above 100
mm near the NT–Queensland border, including a Territory July
record of 142.2 mm at Brunette Downs. Rain fell as far north as
the Top End (although not in Darwin itself ). Combined with a
second major rain event later in the month, the rains contributed
to the wettest July on record for the Northern Territory, and the
second-wettest for Australia, along with the lowest July mean
maximum temperature on record for Western Australia.
The cloudband brought widespread rain to eastern Australia
in its later stages, with the heaviest falls on the northwest side
of the Alps. Several Alpine stations had more than 100 mm
on the 3rd, including 159 mm at Thredbo Top Station, while
Mount Buffalo had 324.6 mm in four days from 2–5 July. It was
a relatively warm system throughout and the precipitation on
the 3rd fell almost entirely as rain, but heavy snowfalls later
in the week restored the mountain snow cover. The moist air
eventually cleared after this, although not before heavy falls in
southern Australia on the 4th, including 80.0 mm at Aldgate
and 75.4 mm at Bridgewater in the Adelaide Hills.
Synoptic chart for 0000 UTC, 28 June 1986