Good to moderate air quality expected

Weather Summary: Issued on Wednesday, March 17, 2010

A large high pressure ridge over the western Great Lakes region, extending eastward over southern Ontario during Wednesday, is causing a light northwesterly flow over most regions except for very light winds over southwestern Ontario, with mostly sunny skies and temperatures continuing significantly warmer than seasonal values.

During Thursday, the high is forecast to move southward over the southern US and a cold front is forecast to move southward over northern Ontario ahead of a low pressure trough. Partly or mostly sunny skies are expected over southern and central regions, and cloudiness is expected over northern and eastern regions with west to southwesterly winds over all regions.

During Friday, the low pressure trough is forecast to move eastward over the central Great Lakes region. Increasing moisture and cloudiness is forecast to spread eastward across most of the forecast regions, and light westerly winds are expected over most regions.

Air quality indices are forecast to be mostly in the good to moderate categories in southern, central and eastern Ontario during Wednesday due to ozone during the afternoon and due to fine particles in some locations during the morning. Emerald-shaded regions on the forecast chart for northern regions indicate air quality indices mainly in the good category due to the upper level northwesterly flow over northern regions. During Thursday, air quality indices are forecast to be in the good to moderate categories in southern and central regions, and mostly in the good category in cloudier northern and eastern regions. Friday's air quality indices are forecast to be in the good to moderate categories in southwestern regions, due to fine particles and ozone, and generally in the good category elsewhere across the province.

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Smog, comprised of ozone and/or PM_2.5, is primarily a summer phenomenon, occurring mostly in southern Ontario; however, PM_2.5 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.