Annual Weather Summary: November 2017 to October 2018 Winter will be colder than normal, especially in the south, with the coldest periods from late November into early December and in late December, mid-January, and early February.
It is an adjective that serves to modify nouns. Nouns modified by every are singular and must be written in such a manner. For example: Every <person> involved in the crash died. The reason for this singularity is because the word every literally means each, and refers to a single entity. Moreover, another reason for this is every is derived from the terms everything and everyone.. Notice here that obviously thing and one are singular objects.
People often behave in the same manner, doing one thing in unison with the other members of their group. When these people are part of a collective noun, that noun becomes singular and requires singular verbs and pronouns. Every afternoon the baseball team follows its coach out to the hot field for practice. Team = singular; follows = a singular verb; its = a singular pronoun.