A strong high pressure ridge building over northern Ontario and the Great Lakes region during Friday, and a low pressure trough near the Atlantic coast, are causing a cool northerly flow across southern Ontario and all of the forecast regions with temperatures significantly cooler than seasonal values. Partly clearing skies are expected over most regions, except for mostly cloudy skies over eastern regions.
Similar conditions are expected during Saturday and Sunday, with the high remaining over the Great Lakes region and the low forecast to move very slowly northeastward, resulting in continuing north to northwesterly winds and temperatures gradually moderating, but remaining cooler than seasonal values under mostly sunny skies.
Air quality indices are forecast to remain mainly in the good category across the province during the next several days under the cool northerly flow. Later Sunday, winds may become light in northwestern and southwestern forecast regions and air quality indices may increase toward the moderate category, but air quality indices are expected to be mostly in the good category.
Smog, comprised of Ozone (O3) and/or Fine Particulate Matter (PM2.5), is primarily a summer phenomenon, occurring mostly in southern Ontario; however, PM2.5 can elevate smog levels during all months of the year. For this reason, the ministry reports the AQI hourly, seven days a week; and continues to provide air quality forecasts daily.