Miracles were already predetermined during the six days of Creation. The prophets did not actually perform wonders and bring about a change in nature. Rather, the Holy One, blessed be He, told prophets in advance when a miracle would take place, and they in turn told the people.

For when God created this reality and set the laws of nature into motion, He included that these miracles would take place at an appointed time. The sign that a prophet would give was that God would tell him what would take place at the time the prophet claimed it would. But what would actually happen [was not brought about by the prophet,] but was placed in nature from the outset. (Rambam, Guide of the Perplexed II:29)

Further reading: For more on the great miracles that accompanied the exodus from Egypt, see A Concise Guide to the Torah, pp. 148, 169; A Concise Guide to the Sages, pp. 85, 97.