24
BAMOS
Sep 2019
Charts of the Past
with Blair Trewin
10 September 1991
August and September 1991 were exceptionally dry months in
Queensland, at the peak of a significant drought that affected
much of southeast Queensland over the preceding year.
Brisbane had its longest rainless spell, 64 days from 24 July to 26
September. Averaged over the state, August–September 1991
was the driest two‑month period on record, with only 0.9 mm,
less than half the next driest (July–August 2009).
Spring is the peak of the fire weather season in southeast
Queensland. With the antecedent conditions so dry, there
were several severe fire weather events during September
when strong westerly winds affected the area. One of these
occurred around 10 September. A strong cold front crossed
the southwest of Western Australia on the 8th before moving
east. A cutoff low formed on the front west of the southern Eyre
Peninsula on the 9th, before moving east to be centred near
Mildura on the 10th and off the southeast coast by the 11th.
Strong west to northwestly winds developed north and east of
the low, peaking over southeast Queensland on the 10th. By the
11th, a front associated with the low had cleared the east coast,
leaving southeast Australia in southwesterly flow.
In the dry offshore flow, temperatures in coastal southeast
Queensland and northern New South Wales reached generally
well above average, although some coastal sites (such as
Brisbane Airport) were tempered by sea breezes. Amberley
reached 30.6 °C, and a number of sites in northeastern NSW
(including Lismore, Casino, Grafton and Murwillumbah) were in
the range 29−30 °C, high for so early in the season although
well short of record levels. Humidity was also low, although
dewpoints generally remained slightly above 0 °C, and winds
were gusty, reaching 65 km/h at Amberley and 83 km/h at
Cape Moreton. These all contributed to dangerous fire weather
conditions. The worst fire outbreak was in the Sunshine Coast
hinterlands, where a firefighter was killed by a falling tree. The
strong winds also led to widespread raised dust, principally in
southern inland Queensland on the 10th before spreading east
on the 11th.
Further south, the low and front brought substantial rain to
much of Victoria and central and southern New South Wales.
Most areas on and northwest of the ranges from Tamworth
to northern Victoria, and as far inland as Dubbo, received at
least 10 mm. The highest falls occurred in the NSW Central
Tablelands, where Mullion Creek had 80 mm on the 11th and
a number of other sites around Orange exceeded 50 mm.
Minor flooding developed on the Lachlan River, as well as on a
number of northeast Victorian rivers, the Loddon and the upper
Yarra. Earlier, there had been falls of 25 to 50 millimetres in the
Adelaide region on the 10th, and in parts of the far southwest of
Western Australia on the 9th.
Minor wind damage was reported from around Adelaide and
Melbourne, and locally severe thunderstorms developed in
northern Tasmania in the system's wake, including one that
produced hail to golf‑ball size in the Launceston suburb of
Trevallyn on the 11th. Temperatures over the southeast states
were well below average from the 10th to the 13th, and snow
fell on the NSW Central Tablelands.
Severe fire weather was a regular feature of September 1991
in southeast Queensland; this was only one of several events
during the month, with major fire outbreaks also occurring on
the 1st, 14th–15th, 19th–22nd and 29th. In the second half of
the month, this extended to eastern New South Wales. This
proved to be the drought's peak in the region, with average
to above‑average rainfall returning from October onwards,
although dry conditions persisted through the summer in
northern Queensland (and in the tropics more generally).
Synoptic chart for 0000 UTC, 10 September 1991