Speaking strictly, as a weather forecast term, 一時曇り means it will be temporarily cloudy and the total length of cloudy weather is less than the quarter of the forecast period. For instance, if a forecast says 夜(from 6 p.m. to 12 a.m.)雨一時曇り, it means it will rain and be temporarily cloudy less than one and half hour. As for 時々~, it mean the weather is intermittent and the total length of the weather is less than the half of the forecast period. Thus, 日中(from 9 a.m. to 6 p.m.)曇り時々雨 means it will be cloudy, the rain will be on and off and the total length of raining will be less than four and half hours.