The Bureau of Meteorology issued a warning on Friday afternoon of severe thunderstorms in parts of the Blue Mountains and Hawkesbury areas west of Sydney.

The bureau said large hailstones and damaging winds were likely as the thunderstorms moved south.

In Canberra, a library was closed as a result of the storm, while the ACT State Emergency Service had received 56 calls for assistance related to flooding, hail damage and fallen trees by 4.20pm.

The bureau continues to warn of severe thunderstorms with large hail and damaging winds for a large band of eastern NSW on Friday afternoon.

The bureau issued the warning early on Friday afternoon as a tropical low-pressure system moved across the state with reasonably strong winds detected in mid to upper levels of the atmosphere.

"There's quite a lot of favourable conditions for thunderstorm development, resulting in warnings over quite a lot of eastern NSW," bureau forecaster Rose Barr said.

"Winds with thunderstorms can be gusty and erratic, and may well be damaging, which is classified as 48 knots or 90km/h or higher."

At this stage just the far western metropolitan area of Sydney is likely to be impacted, but "the situation can evolve quite quickly", Ms Barr said.

Weatherzone meteorologist Graeme Brittain said that on Saturday, Sydney could expect showers, with 5mm to 15mm in the west, and 1mm to 5mm in the city.

"It will be drier on Sunday generally, still a few showers, and they're more likely to be evenly distributed," he said.

"But generally just a few millimetres at most."

Sydney can expect maximums of 23 to 24 degrees over the weekend with lows of 18 to 19 degrees.

The State Emergency Service advises that, to prepare for a thunderstorm, you should:

• Move your car under cover or away from trees.
• Secure or put away loose items around your house, yard and balcony.
• Keep at least eight metres away from fallen power lines or objects that may be energised, such as fences.
• Report fallen power lines to either Ausgrid on 131 388, or Endeavour Energy on 131 003 or Essential Energy on 132 080, as shown on your power bill.
• Unplug computers and appliances.
• Avoid using the phone during the storm.
• Stay indoors away from windows, and keep children and pets indoors as well.
• For emergency help in floods and storms, ring the SES on 132 500.

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