Good to moderate air quality expected

Weather Forecast: Issued on Friday, April 24, 2015

The main weather factors affecting air quality during the next several days include:

Friday:
-a high pressure ridge spreading over Ontario and the Great Lakes region is causing partly or mostly sunny skies over southern and northern regions, while a low pressure trough over Atlantic Canada is causing some cloudiness over central and eastern regions
- northwesterly winds are forecast with temperatures continuing cooler than seasonal values
-air quality indices are forecast to remain in the good category as a result of the good ventilation and dispersion conditions with the cool northwesterly flow

Saturday:
-the ridge is forecast to remain over Ontario and to cause sunny skies over most regions, except for some increasing cloudiness over southwestern regions ahead of a low pressure system which is forecast to move eastward over the midwestern US
-winds are forecast to be north to northwesterly over most regions, except east to northeasterly over southwestern Ontario with temperatures moderating toward seasonal values
-pollutant concentrations are not expected to increase significantly in the generally northerly flow and air quality indices are forecast to remain generally in the good category, except for regions north of eastern Lake Erie where light winds may allow air quality indices to increase to the moderate category due to ozone generation in the strengthening late April sunshine

Sunday:
-the ridge is forecast to move westward over the western Great Lakes region, while an upper level trough over eastern Canada and northeastern Ontario is forecast to circulate moisture southwestward over southern Ontario and most of the forecast regions
-partly sunny skies are expected with northerly winds and temperatures near or below seasonal values
-pollutant concentrations are not expected to increase significantly under the partly cloudy skies and the generally northerly flow and air quality indices are forecast to remain generally in the good category

Air Quality Forecast:
Air quality indices are forecast to be generally in the good category across the province during the next several days, except for indices that may increase to the moderate category during Saturday in some regions of southern Ontario.

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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.