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Golfball-sized hail has smashed parts of Sydney's west as two fast moving severe thunderstorm cells swept northeast across the city.
The Bureau of Meteorology (BOM) on Sunday night issued a severe thunderstorm warning for Sydney, the Blue Mountains, the Hawkesbury and Central Coast, advising people to prepare for large hailstones and heavy rainfall.
The cell is "extremely quickly moving," rumbling across the greater Sydney area at between 60-80kmh, a BOM spokesman told AAP.
"Severe thunderstorms were detected on weather radar near Parramatta, Katoomba and Richmond. These thunderstorms are moving towards the north," the BOM's warning, issued at 8.20 pm (AEDT), says.
"Large hailstones and heavy rainfall that may lead to flash flooding are likely."
In 30 minutes 35mm of rain fell at Box Hill and the largest downpour was recorded at Wilmot, where 46mm came down in about the same time.
At Erskine Park 20mm reportedly fell in 10 minutes.
The storm is expected to pass by midnight on Sunday.
No storms are expected around Sydney on Monday but the BOM is predicting some very light showers.
Roofing on the Blue Mountains hospital and the Katoomba sports and aquatic centre collapsed during the storm but no one was hurt, NSW Fire and Rescue Superintendent Paul Johnstone said.
Evacuation wasn't necessary but four fire crews were sent to the hospital to help relocate ten patients.
No one was at the swimming centre.
Fire & Rescue NSW received about 50 calls for help.
"It's for everything from flooding to wires down," Supt Johnstone said.
The SES says it's "experiencing high call volumes" and has asked people to be patient.