This is the more logical because this is an adverb. Recall that an adverb is a word or group of words that describes or modifies a verb, adjective, another adverb or a whole sentence. The adverb apenas translates into barely and is used for saying that something only just happens or exists, or is only just possible. So this adverb modifies alcanzará (simple future) that comes from the verb alcanzar. Finally:
<em>Hoy la temperatura es de 10ºC. Y mañana </em><em>apenas </em><em>alcanzará los 5ºC</em>
