Thunderstorms are possible in the ACT on Friday afternoon with a health alert issued for a potential thunderstorm asthma event.

"Thunderstorm asthma can be triggered by a combination of high grass pollen levels and a particular type of thunderstorm, which causes some people to develop severe asthma symptoms over a short period of time," a statement from ACT Health said.

"Where possible avoid being outside during a thunderstorm - especially in the wind gusts that come before the storm. Go inside and close your doors and windows. If you have your air conditioning on, turn it onto recirculate."

In other news:

Bureau of Meteorology forecaster Helen Kirkup said storms late on Thursday night had increased humidity, with the moisture hanging around to create a foggy start on Friday morning.

She said it would likely lift by mid-morning.

The weather will clear from Saturday as the sun returns with a warm top of 23 degrees and a low of 8.

Throughout Sunday and Monday summer heat will make an early appearance, with a "warm burst" of 30 expected to kick-off the week.

Ms Kirkup said that was at least 5 degrees higher than average for this time of year, brought on by a warm air mass ahead of the next change which will bring temperatures back down later next week.

A sunny day with a top of 25 degrees and a low of 10 is forecast on Tuesday.