This sentence is given in the simple future. We use the Spanish simple future to talk about what will or shall happen. It is also used to talk about the possibility of what someone may or might be doing in the present. When talking about the weather it's common to use the verb hacer and conjugated the simple future for the third person singular is hará. Finally:
<em><u>Luisa:</u></em><em> ¿Cómo estará el clima el jueves?
</em>
<em><u>Milagros:</u></em><em> El jueves </em><em>hará</em><em> calor, la temperatura superará los 30ºC.</em>