The Bureau of Meteorology has issued a severe weather warning for Sydney this afternoon, cautioning of a thunderstorm that might involve heavy rainfall, hailstones, damaging winds and flash flooding over the next several hours, from 4pm.

The storm was detected near Hornsby and Terrey Hills around 4.25pm, and is expected to affect Berowra, Glenorie and Mona Vale by 4.55pm, according to Weatherzone.

The State Emergency Service is advising residents to move their cars under cover or away from trees, secure loose items outdoors, keep clear of creeks and storm drains, unplug appliances and avoid using phones during the storm.

Follow the Bureau of Meteorology for more.