Thunderstorm asthma events are proving "chaotic beasts" for Victorian meteorologists to predict, but new data is being used to refine the world-leading forecasting service for this year's grass pollen season.

Key points: Forecasts predicting the likelihood of thunderstorm asthma events in Victoria have begun for 2018

Forecasts predicting the likelihood of thunderstorm asthma events in Victoria have begun for 2018 Ten people died and thousands were affected during a 2016 thunderstorm asthma event in Melbourne

Ten people died and thousands were affected during a 2016 thunderstorm asthma event in Melbourne Meteorologists say they are improving the accuracy of the forecasting system each year

The three-day forecasting service was first launched at the start of last year's hayfever season, and uses eight pollen monitors across the state and a decade of weather data to predict the likelihood of a thunderstorm asthma event.

It was introduced after 10 people died when the weather phenomenon struck in 2016, after grass pollens burst into smaller particles during a thunderstorm, triggering severe asthma in thousands of people across Melbourne.

Bureau of Meteorology senior meteorologist Tony Bannister said a series of factors were required in order to bring about the potentially deadly phenomenon.

"[It's a combination of] high or extreme pollen forecasts plus a certain type of thunderstorm that we're looking for which is basically a very dusty thunderstorm," he said.

"That combination only really comes up a few times a year, maybe two or three times a year in Victoria.

"We don't have a good handle on how many times given there's been a drought or weather is very humid."

Pollen monitors across the state are used to predict thunderstorm asthma events. ( Supplied: Department of Health and Human Services Victoria )

Mr Bannister said thunderstorms were "chaotic beasts" which made it difficult to predict or forecast their arrival far in advance.

"[Thunderstorm asthma events] are really quite rare, and looking from past seasons, the actual forecasting of it is about two or three times a season, actually occurring probably less than one time a season, so they really are quite a rare beast," he said.

He said information from last year's forecasting service was being used to improve the accuracy of forecasts this year.

"The pollen information from the locations last season has been put into the pollen statistical model and some of the variables have been adjusted slightly from the work that was done last year," he said.

"We're on the start of a process of having a thunderstorm asthma forecasting service. It's one of a kind in the world … and we'll be learning things and trying to make it better each year."

What is thunderstorm asthma? ( Supplied: Department of Health and Human Services Victoria )

Asthma sufferers urged to prepare

The Victorian Government urged people with asthma to make sure they were managing their condition properly and prepared for an increase in allergens in the air.

"We're running public information campaigns about hay fever. That if you get hay fever, don't dismiss it if you're getting watery eyes and sniffing a lot," Health Minister Jill Hennessy said.

"What we know is that hay fever is one of the great indicators for undiagnosed asthma.

"We're saying to anyone who has hay fever go off and talk to a doctor about getting some Ventolin and a care plan in place."

University of Melbourne Associate Professor Ed Newbigin said the grass pollens — which were present in the 2016 thunderstorm asthma incident — are one of the more severe allergens.

"Most people in Australia who get seasonal allergies will have an allergy to grass pollen," he said.

"This is usually the time of year when most people get their sniffles and their sore eyes and start to sneeze a lot.

"It's also a bad time for asthmatics, grass pollen is one of their triggers."