The main weather factors affecting air quality indices the next several days include:
-an area of high pressure is spread over southern and parts of eastern Ontario and is causing partly to mainly sunny skies along with light southwesterly winds, while elsewhere a cold front is over northern and central forecast regions and is causing mainly cloudy skies, precipitation and light northeasterly winds
-the area of high pressure is forecast to weaken as a low pressure trough and cold front are forecast to move east across the province and are expected to cause partly to mainly cloudy skies across the regions along with scattered precipitation for most areas, except southern and parts of eastern Ontario, where dry conditions are forecast to continue
-a southwesterly flow is expected for most regions ahead of the cold front while a northerly flow is expected for northern regions and is also expected to develop following the passage of the cold front
-near seasonal temperatures are forecast for most regions, except northern regions where below seasonal temperatures are expected
-a surface high pressure region is expected to be centered over the province during Thursday and is forecast to cause clearing skies with a north to northeasterly flow and below seasonal temperatures for mid-September
-the surface high is forecast to track eastwards and is expected to cause partly to mainly clear skies early Friday across the regions, however, a low pressure trough is forecast to track east across Hudson Bay Friday while a cold front extending southwestwards from the low is expected to move across northern Ontario and is forecast to cause increasing cloudiness and precipitation across northern and parts of central forecast regions
-the surface high is expected to keep southern, eastern and parts of central forecast areas mainly clear and dry
-near to below seasonal temperatures and a south to southwesterly flow are forecast
Air quality forecast:
Air quality indices are expected to generally be in the good category for the next several days due to cool temperatures and the development of northerly winds.
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.