Sunny skies and temperatures in the 20s may be welcome news but with the first warm days of summer the season's first smog alert was also issued.

The Ontario Ministry of the Environment issued the alert Tuesday as temperatures reached up to 27C. When an alert is issued, it means there is a strong possibility of poor air quality in the next 24 hours.

Being blamed for the alert is an east-moving high pressure region that is causing a warm southwesterly flow of air over Ontario.

"With the southwesterly flow of polluted air from the U.S. and local emissions, smog is expected to increase during Tuesday across parts of Ontario," the advisory reads.

The advisory includes "parts of southern, central and eastern Ontario."

Southern Ontario can expect "moderate to poor" air quality in some areas on Tuesday and Wednesday.

By Thursday, weather conditions are expected to change, clearing the air somewhat and moving the air quality to the "good to moderate" range.

The Ontario government reports on the Air Quality Index every hour of each day, year-round. While smog can occur at any time of the year, it forms most frequently during the summer months.