A high pressure ridge spreading southward over the Great Lakes region and southern Ontario is causing skies to become mostly sunny during Tuesday, along with light northerly winds over most regions, and temperatures continuing below seasonal values. The northerly flow is the main factor that will be causing mostly good air quality during Tuesday. A plume of smoke, originating from forest fires in northern Quebec, is forecast to move southward over southern, central and eastern Ontario during Tuesday.
During Wednesday, the high is forecast to remain over southern Ontario and to cause continuing sunny skies with light winds.
During Thursday, the high is forecast to cause continuing sunny skies over southern Ontario, while a low pressure trough is forecast to move eastward across northern Ontario and to cause some cloudiness over northern, central and eastern regions. Winds are forecast to be southwesterly and temperatures are forecast to moderate toward seasonal values.
Air quality indices are forecast to be mostly in the good category during Tuesday, except for some indices increasing to the moderate category due to ozone in regions north of Lake Erie. Some indices may also increase toward the moderate category due to fine particles from the smoke plume in locations in southern and eastern Ontario. Wednesday's air quality indices are forecast to be in the good to moderate categories in southern regions, due to ozone, and generally in the good category elsewhere. Thursday's air quality indices are forecast to be in the good to moderate categories across the province due to ozone.
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.