Download vkradio.com

Summer Weather and Bushfire
Threats in SE Australia
Tony Langdon VK3JED
for the VoIP Weather Net
November 5 2006
1
Summer Weather and Bushfire
Threats in SE Australia
●
●
●
●
●
●
●
●
●
Major Air Masses Around Australia in Summer.
Typical Summer Weather Patterns in Australia.
Fire Danger in Southeastern Australia.
Fire Seasons Around Australia.
Weather Factors Influencing Fire Danger.
High Risk Fire Weather Patterns in SE Australia.
Effect of Cold Front on Fire Spread.
Example of High Risk Day - “Ash Wednesday”.
Prognosis for 2006/2007 and Recent Climate.
2
Major Air Masses around Australia
in Summer
●
SE Australia is influenced by 3 major air masses
during summer.
–
Hot, dry continental to the NW (Australia)
●
–
Warm, moist maritime to the NE (N Tasman Sea and
Pacific Ocean)
●
–
Extremely hot and dry – Often creates extreme fire danger.
Warm to hot and humid conditions, may be a factor in severe
storms.
Southern Maritime (Southern Ocean)
●
Cool, moist conditions – may cause drizzle showers. Cold
fronts can cause dramatic temperature falls – as much as 20C
3
in 20-30 minutes.
Major Air Masses around Australia
in Summer
Courtesy Recreational Aviation Australia Inc
4
Typical Summer Weather Patterns
in Australia.
Courtesy Australian Bureau of Meteorology
5
Fire Danger in Southeastern
Australia
●
●
●
●
●
●
●
Vegetation includes a number of highly flammable species (e.g.
Eucalypts and dry grasses).
Many species have evolved to recover quickly after fires, and some
even depend on fire to reproduce.
Fire is a natural part of the Australian environment (though human
activities have increased their frequency over the past 40,000 years)
Mild (by US standards) winters allow a long growth season.
Summer temperatures often exceed 35C (95F), often accompanied
by low humidity (under 20%) and high winds (> 20 knots).
Strong, dry frontal changes can greatly increase the length of a fire
front and drive the fire into unburnt fuel.
Fringe areas of major cities are the most vulnerable to fire damage,
due to high population density extending into bushland.
6
Fire Seasons Around Australia
●
Courtesy Australian Bureau of Meteorology
7
Weather Factors Influencing
Fire Danger
●
●
●
Humidity – Relative humidity < 20% causes fuel
to dry out, making it more easily flammable.
Wind – High winds provide more oxygen to feed
the fire and increase speed of spread. High winds
can also pick up burning embers and carry them
considerable distances to start new fires ahead of
the main fire. This is known as “spotting”.
Drought – Causes heavier fuels to dry out,
increasing the risk of a fire starting and more
intense fires.
8
High Risk Fire Weather patterns in
SE Australia
●
●
●
Drought, as occurring in 2006 greatly increases
fire risk.
Northwesterly winds preceding a cold front bring
hot, dry air from central Australia. These winds
are often strong as well, due to tight pressure
gradients near the front.
Cold front brings high winds and a sudden 90
degree change in direction, which causes flank of
fire to become a broad front.
9
Effect of Cold Front on Fire Spread
●
Courtesy Australian Bureau of Meteorology
10
Example of High Risk Fire Day in
SE Australia - “Ash Wednesday”
February 16, 1983
●
Courtesy Australian Bureau of Meteorology
11
Example of High Risk Fire Day in
SE Australia - “Ash Wednesday”
February 16, 1983
●
●
●
Pre-frontal trough intensified northerly winds and brings
hot, dry area from central Australia. Winds exceeded 30
knots for much of the day, temperature in Melbourne
exceeded 43C (110F), with extremely low humidity.
Cold front and trough caused a wind change from NW to
SW, causing fires to spread rapidly on large fronts. Wind
increased to over 50 knots during the passage of the front.
Over 400,000 hectares (1 million acres or 1550 square
miles) were burnt across 2 states. Reported rates of fire
spread exceeded 60mph at some stages.
12
Prognosis for 2006/2007
●
●
●
Persistent below average rainfall (10 years), culminating
in a severe drought means fuel will be dry.
Current El Nino conditions make significant rainfall
unlikely before March. El Nino is associated with drier
than normal conditions across SE Australia. El Nino also
associated with an increased number of extreme fire risk
days (i.e. Hot, dry and windy).
BoM forecasting 60% probability of above median
maximum temperatures and 65-85% probability of above
median minimum temperatures during summer.
13
Recent Rainfall
14
Recent Rainfall
●
Recent rainfall has been severely below average
15
Rainfall Deficiency 2005-2006
16
Prognosis for 2006/2007
●
Personal opinion is that there is a significant risk
of major fire outbreaks during the coming summer,
due to the drought conditions during 2006 and the
expectation of hotter and drier than average
conditions during summer.
17