The main weather factors affecting air quality indices the next several days include:
-a surface high pressure ridge over the province is causing mainly clear skies across southern, central and eastern forecast regions, while a low pressure trough over Hudson Bay is causing some cloudiness over some northern forecast regions
-an upper trough over the province is forecast to cause partly sunny conditions during Thursday afternoon
- a northwesterly flow, less humid conditions and near to below seasonal temperatures are expected
-mainly clear skies are expected as the centre of the ridge moves southeast of the province, however increasing cloud cover is expected to develop in northern, central and southwestern forecast regions ahead of a low pressure trough that is forecast to be centered over northwestern Ontario
-the risk of showers is also expected for some northern and southwestern forecast regions
-a southwesterly flow is expected to develop and temperatures are expected to moderate to seasonal values
-partly to mainly cloudy skies along with showers or thundershowers are expected as a low pressure trough is forecast to move across the forecast regions
-a southwesterly flow is expected to continue along with near to below seasonal temperatures
Air quality forecast:
Air quality indices are forecast to generally be in the good category across the province Thursday due to a northwesterly flow. Forest fire smoke over northern forecast regions is not expected to affect air quality at ground level. Air quality indices are expected to be in the good to moderate categories in southern Ontario Friday and Saturday due to ozone and fine particles.
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.