From environment Canada:
Indication right now suggest that the winds will shift to more northwesterly on Monday. As a result, the snow squalls will likely drift southward into the Barrie, Orillia, Midland, Watford, Pinery park, eastern Lambton county, London, Stratford and Woodstock areas. Local snowfall amounts could reach 25 cm by Tuesday morning and even up to 40 cm are possible if one or more of the snow squalls become stationary for more than 6 hours. The strong and gusty winds will likely cause very low to at times zero visibility in blowing snow in areas affected by heavy flurries and snow squalls. Hazardous winter driving conditions are expected.
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My prediction is the squalls will start around 3:00 pm tomorrow.