A weak low pressure trough over the central Great Lakes region and a high pressure ridge over the southern and eastern US during Tuesday are causing gusty west to southwesterly winds over most of the forecast regions and mostly cloudy skies with scattered precipitation. The winds are resulting in good ventilation and dispersion conditions and are the main weather factor resulting in air quality indices in the good category. A large and cold upper level trough over central Canada is forecast to move eastward over the province and to cause temperatures significantly cooler than seasonal values during the next several days.
During Wednesday, the ridge is forecast to be over the eastern US and the weak low pressure trough is forecast to move eastward over central and eastern Ontario, resulting in continuing west to southwesterly winds over southern and eastern regions ahead of the trough and mostly cloudy skies with scattered precipitation. Northwesterly winds are forecast over northern regions north of the trough.
During Thursday, another strong high pressure ridge is forecast to build eastward from the southern US and to cause westerly winds and partly sunny skies over southern and eastern Ontario, while cloudiness and scattered precipitation are forecast over northern regions. Air quality indices are forecast to remain generally in the good category.
In summary, air quality indices are forecast to generally be in the good category during the next several days across the province.
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.