On Monday, the Bureau of Meteorology forecast a sticky and hot top of 32 degrees on Sunday with a medium chance of showers and a thunderstorm in the afternoon and evening.

Christmas showers looked less likely as the week progressed, but on Friday senior forecaster Simon Louis said the chances had climbed back up to one in two.

"Looking at it this morning, it seems the medium chance of a thunderstorm is back, unfortunately," Mr Louis said.

"Basically we've got a fairly slow moving high [pressure system] out in the Tasman [sea] that is feeding a lot of humidity into the area in the next couple of days," he said.

The warm and humid conditions will increase the likelihood of thunderstorms late on Christmas Eve and on Christmas Day.

"You'll wake up and see quite a bit of sunshine around and then you'll see the storm clouds but because they are slow moving it's hard to know if they will reach Canberra," he said.

If they do, Mr Louis said the city could see up to 20 millimetres of rain, but it was more likely to have about 10 millimetres.

If the temperature reaches the forecast top of 32 degrees, it will be the hottest Christmas in Canberra in 13 years and five degrees above December's average.

Bureau of Meteorology climatologist Acacia Pepler earlier said the last time Christmas day exceeded Sunday's expected top of 32 degrees was in 2003, when the mercury hit 32.7.

The stingy humidity was tipped to hang around for the week following Christmas.

"It looks like Canberra's weather will stay fairly warm and unsettled through the middle part of next week and again you could see thunderstorms or showers in the arvo," Mr Louis said.

Maximum temperatures were forecast to stay in the early thirties all throughout next week, with mild nightime lows of between 15 and 19 degrees.