Environment Canada has issued a fog advisory for Toronto and most of the GTA.
The advisory covers Toronto, Durham and York regions, as well as Mississauga, Halton Hills and Caledon.
Hazardous travel is expected due to near-zero visibility in many areas, the agency says.
A number of flights have been cancelled so far because of the fog and Pearson International Airport has advised travellers to check the status of flights before leaving for the airport.
The advisory comes as Toronto sees a wet day with showers on and off. A return to sunshine is expected on Wednesday with a high of 20 C expected. However the rain will be back Thursday and Friday, along with cooler temperatures between 9 C and 14 C
Environment Canada meteorologists are tracking a line of severe thunderstorms capable of producing very strong wind gusts, up to nickel size hail and heavy rain.
Thunderstorms with gusts to 110 kilometres per hour are moving southeastward through the regions and will continue southeastward over the next hour or two. Hail of 2 to 4 centimetres in diameter and heavy downpours are likely with these thunderstorms.
Take cover immediately, if threatening weather approaches. If visibility is reduced while driving, turn on your lights and maintain a safe following distance. Avoid areas of the building that could be affected by falling debris such as tree limbs. Lightning kills and injures Canadians every year. Remember, when thunder roars, go indoors!
Severe thunderstorm warnings are issued when imminent or occurring thunderstorms are likely to produce or are producing one or more of the following: large hail, damaging winds, torrential rainfall.
Please click the link to view the regions that will be affect by the thunderstorm.