Thunderstorm asthma. A combination of words that you wouldn't usually associate with each other, yet when put together, creates a monstrous phenomenon that can hospitalise hundreds. 

In southeast Australia, particularly Victoria and New South Wales, this phenomenon usually occurs from October to December, but not every year. As we wrote here, this period is also when some of Australia's strongest storms occur. 

So, we are in peak thunderstorm asthma season. During this time, grass pollen from pasturelands is released into the air, travelling long distances and into populated areas.  

Cold air downdraughts from certain types of thunderstorms can initially 'rupture' the pollen from increased internal pressure. These are then concentrated in the 'gust front', or the outflow from the storm, with the pollen then being carried and spread out from the warm air updraught (Figure). 

Figure: Example of a squall line thunderstorm, which is an ideal setup for thunderstorm asthma events in southeast Australia (Weatherzone, 2023). 

It's important to know what to look out for when predicting, and then responding to thunderstorm asthma events. On November 24th, 2023, VicEmergency issued a thunderstorm asthma warning, with Mallee and North Central districts receiving high risks, and much of the central districts receiving moderate risks.  

This was all associated with a high likelihood of severe thunderstorms, mostly about western Victoria, issued by the Bureau of Meteorology on the morning of November 24th. 

Figure: Thunderstorm forecast for heavy rainfall and damaging wind gusts issued at 10:39am on November 24th for Victoria (Bureau of Meteorology, 2023). 

Hours later the severe thunderstorm warning was adjusted to include large hailstones. Since hail is associated with a downward motion, there was an implied increase in thunderstorm asthma risk as the gust front will have been much more efficient at concentrating particles with this downdraught.  

Figure: Severe thunderstorm warning for heavy rainfall and large hailstones issued at 2:43pm on November 24th for western Victoria (Bureau of Meteorology, 2023). 

Until the end of the year, particularly if you're vulnerable to hay fever or respiratory conditions, it is important to be mindful of such warnings. Keep an eye out for them here.  

VicEmergency also issues daily thunderstorm asthma risk forecasts that can be viewed here. 

When these occur, be sure to stay indoors as much as possible, keep windows and doors closed, and for asthmatics, review your asthma action plan.