"لا أُحِبّ اَلْطَّقْس اَلْمُشْمِس."
Translation:I do not like sunny weather.
Because this is an attributive adjective. An adjective that describes the noun, and does not tell some information or completes the sentence. Attributive adjectives in Arabic follow the noun in its status: definition, gender, number. Thus, since الطقس (the weather) is defined, then the adjective afterward which describes it must be defined (and masculine since طقس is a masculine noun): المشمس.