Weatherzone meteorologist Brett Dutschke said Canberra has reached temperatures of at least 18 degrees every day since May 11 with Saturday being the warmest day at 21 degrees.

“In the fortnight to [Sunday[ the average maximum temperature was 19.3 degrees, which is the warmest fortnight this late in the autumn since the week ending May 23, 1947," he said.

But Mr Dutschke said Canberra is unlikely to beat the warmest May record due to a cold snap at the start of the month that brought frosts and freezing overnight temperatures.

“The warmest May on record was 2005 with an average maximum temperature of 18.4 degrees, while at the moment we’re running at an average of 17.4 degrees,” he said.

Mr Dutschke said overnight temperatures have been warmer than usual this month with temperatures not dipping below freezing since May 15.

But Canberra’s dry spell, which has seen only 0.2 millimetres of rain recorded since May 5, is forecast to come to an end on Tuesday afternoon as a north-west cold front is drawn into a low pressure trough over the ACT.

“Rain will develop tomorrow and it looks as though it could be fairly significant - perhaps the heaviest rainfall since early April - and we should see widespread falls from anywhere between 15 and 30 millimetres across Canberra.

But despite the heaviest falls in nearly two months, Mr Dutschke said daytime temperatures will remain warm, around 19 to 20 degrees.

“Wednesday will be the coolest day of the week with a top of 15 degrees which is still warmer than the May average of 14 degrees,” he said.

Mr Dutschke said rainfall is also expected on the weekend with anywhere between 15 and 30 millimetres forecast to fall across the two days.