Summer Weather and Bushfire Threats in SE Australia

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Summer Weather and Bushfire Threats in SE
Australia

• Tony Langdon VK3JED
• for the VoIP Weather Net
• November 5 2006

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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.

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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)
• Extremely hot and dry Often creates extreme
  fire danger.
• Warm, moist maritime to the NE (N Tasman Sea and
  Pacific Ocean)
• 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 in 20-30
  minutes.

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Major Air Masses around Australia in Summer
Courtesy Recreational Aviation Australia Inc
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Typical Summer Weather Patterns in Australia.

• Courtesy Australian Bureau of Meteorology

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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 (gt 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.

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Fire Seasons Around Australia

• Courtesy Australian Bureau of Meteorology

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Weather Factors InfluencingFire Danger

• Humidity Relative humidity lt 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.

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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.

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Effect of Cold Front on Fire Spread

• Courtesy Australian Bureau of Meteorology

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Example of High Risk Fire Day in SE Australia -
Ash WednesdayFebruary 16, 1983

• Courtesy Australian Bureau of Meteorology

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Example of High Risk Fire Day in SE Australia -
Ash WednesdayFebruary 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.

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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.

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Recent Rainfall
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Recent Rainfall

• Recent rainfall has been severely below average

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Rainfall Deficiency 2005-2006
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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.

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