The main weather factors affecting air quality indices during the next several days include:
-a surface high is centered southwest of the province Thursday and is causing partially sunny skies across most of the forecast regions along with light northwesterly winds, however an upper trough is responsible for the cloudiness in northern forecast areas and scattered cloudiness elsewhere
-the upper trough is expected to continue to be over the province during Thursday and is forecast to cause increasingly overcast skies along with unsettled conditions developing for parts of southern Ontario and northern forecast areas
-near to below seasonal temperatures for late October are expected to continue along with light, clean northwesterly winds, which are expected to keep air quality indices generally in the good category
-a deep upper trough is forecast to swing across parts of the province Friday and is expected to cause continuing partly to mainly overcast skies and periods of steady precipitation
-a northeasterly flow is expected for most regions, except parts of southwestern Ontario, where a south to southeasterly flow is forecast, however winds are expected to become northeasterly late Friday in these areas also
-near to below seasonal temperatures are forecast to continue Friday
-air quality indices are expected to remain in the good category due to the development of northeasterly winds during Friday
-mainly overcast skies are expected to persist into Saturday with early day showers for most areas as the deep upper trough is forecast to continue to track eastwards over Ontario Saturday, however, drier conditions and clearing skies are forecast for northern regions as a strong high pressure ridge builds across northern Ontario and the US Midwest
-below seasonal temperature for early November are forecast in conjunction with a northerly flow, which is expected to keep pollutant concentrations low and air quality indices mainly in the good category
Air Quality Forecast:
Air quality indices are forecast to remain generally in the good category across the province during the next several days due to low background of pollutant concentrations and winds with a northerly component.
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.