Mañana en Moscú (hacer) ... frío.

The sentence is in Spanish and describes a future event. The verb "hacer" needs to be conjugated in the future tense to agree with "Mañana" (tomorrow) and the implied subject (it).

The correct conjugation of "hacer" in the future tense for the third person singular is "hará".

Therefore, the completed sentence is: Mañana en Moscú hará frío.