Mid October is typically when we see our first frost of the season. Our stretch of mild weather will break sharply Thursday and will be coldest so far this season by Friday morning. So far the coldest temperature this season has been 41° in South Bend, we could see 30s by the end of the week. 

Temperatures will drop sharply Wednesday evening from the 60s and 70s to the 40s and 50s early Thursday. Temperatures will struggle to make the mid 50s Thursday afternoon and that sets the table for a cold start in the 30s Friday morning and the potential for frost. 

Frost is a highly conditional, localized phenomena, that can depend on cloud cover, clam wind and temperature. Clear skies and light wind along with cold temperatures are key factors in causing frost to form.   

White hoar frost forms when water vapor in the air freezes on contact to an object ( tree limbs, grass, car windshield). This can happen even when temperatures don't officially drop below freezing because the air just above the ground could be below freezing or a surface like a meta car hood or windshield could draw heat away from the air and lower the temperature to the frost point. When temperatures drop below 36 we start to forecast the potential for frost