The main weather factors affecting air quality indices the next several days include:
-a surface low pressure trough is over the forecast regions and is causing partly to mainly cloudy skies across most regions along with scattered showers in parts of eastern Ontario and light winds
-the low is expected to weaken and gradually move south of the province and is expected to cause continuing partly to mainly cloudy skies along with scattered showers or thundershowers
-a west to northwesterly flow is expected to develop with the passage of the trough across southern regions Thursday, while elsewhere an east to southeasterly flow is expected
-near to below seasonal temperatures are expected
-an upper trough is expected to be over most forecast regions Friday, which is expected to cause continuing cloudy skies along with scattered showers or thundershowers, however some sunny breaks are also expected as an upper ridge is forecast to build over western forecast regions
-near seasonal temperatures are forecast along with an east to southeasterly flow and continuing high humidity
-an upper ridge is expected to build while a surface high pressure ridge centered over Quebec spreads over the forecast regions and is forecast to cause partly to mainly clear skies, however some cloudiness is also forecast to develop in advance of a low pressure trough south of the province
-an easterly flow is forecast
-near seasonal temperatures and humid conditions are expected to continue for most regions
Air quality forecast:
Air quality indices are expected to generally be in the good category for most regions the next several days. However, sunny breaks, humid conditions and calm or light winds may allow increasing concentrations of ground level ozone and fine particles in southern regions, which may cause indices in the moderate category.
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.