【正确答案】 B

【答案解析】30-day forecast和30-day outlooks；analysis与examining是两对同义词，因此定位到第二段最后一句These 30-day outlooks are based upon an analysis of the upper air levels which often set the stage for the development of air masses，fronts，and storms．(这些以三十天为周期的气象图是在对上层空气水平的分析基础上编制的)，所以选[B] 。