The main weather factors affecting air quality indices during the next several days include:
-a high pressure ridge extending across southern, central and northeastern Ontario is causing partly or mostly sunny skies over most regions, except for cloudiness over eastern regions caused by a low pressure storm near the Atlantic coast, and some cloudiness over southern and northern regions caused by a low pressure trough moving over northwestern Ontario
-temperatures are forecast to continue warmer than seasonal values and winds are forecast to be light and west to northwesterly over most regions of southern, central and eastern Ontario, except south to southwesterly over southwestern Ontario and the northwestern forecast regions
-increasing concentrations of fine particles in southwestern regions are causing air quality indices in the good to moderate categories while elsewhere, air pollutant concentrations are not expected to increase significantly and air quality indices are forecast to continue to be in the good category
-a low pressure trough and cold front are forecast to move eastward across the province and to cause partly cloudy skies over most regions and cloudy skies over northern regions with scattered precipitation over most regions ahead of the front
-winds are forecast to be southwesterly ahead of the front, followed by gusty westerly winds over most regions and northwesterly winds over northwestern forecast regions, with temperatures continuing above seasonal values
-air quality indices are forecast to be in the good to moderate categories in southwestern regions due to fine particles during the morning until increasing winds provide improving ventilation and dispersion conditions later in the morning
-the cold front is forecast to be east of the province and a high pressure ridge is forecast to move eastward over the Great Lakes region, causing partly sunny skies over most regions with brisk west to northwesterly winds
-good ventilation and dispersion conditions are expected to result in low concentrations of air pollutants and air quality indices in the good category
Air Quality Forecast:
Air quality indices are forecast to be in the good to moderate categories in southwestern Ontario during Friday and Saturday, and generally in the good category elsewhere. Sunday's air quality indices are forecast to be in the good category across the province.
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.