The Bureau of Meteorology is forecasting the city's record dry spell to run until Friday when it predicts 10-20 millimetres will fall.

Before that, two mild and partly cloudy days with pleasant top temperatures in the mid-20s should extend the number of consecutive days with less than 2 millimetres of rain to 76 days.

The previous run of such days was 64 days during August and October in 1989, the bureau said.

"The rain is part of a low-pressure trough," Kim Westcott, a meteorologist with Weatherzone, said. Behind it, a cold front will keep conditions relatively cool for a couple of days.

Friday's range of a minimum of 18 degrees - four degrees above average - will be followed by a top of 21, just a degree or so cooler than the October average.

The rain itself is likely to fall mostly during the morning on Friday before conditions start to dry up, Ms Westcott said.

Saturday's top will reach just 20 degrees, with showers lingering over the following day or so.

The last time Sydney received 20 millimetres in a day was on June 10. Since then, even showers have been rare, with September's 0.2 millimetres the lowest rain tally on record for Sydney.

The city's temperatures should start to climb again from about Tuesday next week, Ms Westcott said.

The return of the warmth is likely to dry out rapidly the moisture from Friday's falls.

During the extended dry spell since July, the city's rainfall has totalled about 37 millimetres at Observatory Hill, while evaporation has been in the order of 575 millimetres, bureau data show.

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