Penny-sized hail stones rained down on some parts of Toronto and surrounding areas Saturday morning as a severe thunderstorm tracked eastward through the region.

Thunder and lightening were reported as winds gusted up to 90 kilometres per hour.

The storm began to move towards Hamilton late Saturday morning. Severe scattered thunderstorms, high winds and lightning was expected to hit the area at around noon.

Environment Canada warned about reports of hail stones the size of golf balls and the possibility of a tornado.

"Emergency management Ontario recommends taking cover immediately when threatening weather approaches," the weather agency says on its website.

Forecasters predict between 10 and 15 millimetres of rain will fall in the city and between 30 and 40 mm of rain in the Greater Toronto area.

The heavy rain has prompted the Toronto and Region Conservation Authority to issue a water safety bulletin.

"The forecasted rainfall amounts may result in a rapid increase of flows and water levels in all our ponds, rivers and streams, creating dangerous conditions," the warning says.

Minor road flooding has been reported in several spots across midtown Toronto, including Yonge Boulevard and the York Mills Road and Leslie Avenue area.

At around 8:30 a.m., Environment Canada issued a severe thunderstorm warning for Brampton, Mississauga and Toronto after a storm system dropped hail over parts of Georgetown.

As of 10:00 a.m., the warning had been lifted, but rain and wind gusts of up to 50 km/h were expected to last throughout the day. A cold front will also send temperatures dipping to 6 degrees Celsius in the evening.

Meanwhile, residents to the west of the GTA are facing another storm system, which is looming northwest of London, Ont.

A severe thunderstorm warning has been issued for London-Parkhill-Eastern Middlesex County, Stratford-Mitchell-Southern Perth County and Woodstock-Tillsonburg-Oxford County.

The storm is moving eastward toward the St. Mary's Area with hail and high wind gusts predicted.

"Persons in or near this area should be on the lookout for adverse weather conditions and take necessary safety precautions," the warning said.