Monday will be another warm day with afternoon temperatures heading well into the 80s, but cooler air of Canadian origin is headed toward the city. A cold front that marks the leading edge of the cooler air arrives Monday evening, accompanied by hit-and-miss showers and thunderstorms. Cold frontal boundaries usually serve to trigger considerable thunderstorm activity, but atmospheric conditions this time are not especially favorable for thunderstorm development. Only scattered storms are expected (though some could be vigorous) and it’s possible that rain will spare as much as 50 percent of the metropolitan area. Once in place by Tuesday morning, Canadian air — relatively cool and quite dry — will keep area temperatures in the 70s through mid-week. Following that, a strong warming trend begins Thursday, culminating in 90 degrees by Saturday.