Biblical Context:
The onset of the Flood is a pivotal event described in the Book of Genesis, chapters 6 through 8. This cataclysmic event marks a divine judgment upon a world steeped in sin and corruption. The narrative unfolds with God’s decision to cleanse the earth through a deluge, sparing only Noah, his family, and a remnant of animal life.

Scriptural Account:
The account begins with a description of humanity's moral decline: "Then the LORD saw that the wickedness of man was great upon the earth, and that every inclination of the thoughts of his heart was altogether evil all the time" (Genesis 6:5). In response to this pervasive wickedness, God expresses His sorrow and resolves to "wipe mankind, whom I have created, from the face of the earth" (Genesis 6:7).

Noah, however, "found favor in the eyes of the LORD" (Genesis 6:8). He is described as "a righteous man, blameless in his generation" (Genesis 6:9). God instructs Noah to build an ark, providing specific dimensions and materials, as a means of preservation for his family and pairs of every living creature (Genesis 6:14-21).

The Commencement of the Flood:
The onset of the Flood is marked by precise timing and divine orchestration. "In the six hundredth year of Noah’s life, on the seventeenth day of the second month—on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened" (Genesis 7:11). This dual source of water, both from the earth and the heavens, underscores the totality of the impending judgment.

Noah, his family, and the animals enter the ark as commanded by God. "And those that entered, male and female of all flesh, went in as God had commanded him. Then the LORD shut him in" (Genesis 7:16). The sealing of the ark by God Himself signifies divine protection and the finality of the judgment about to unfold.

Duration and Impact:
The rain falls for "forty days and forty nights" (Genesis 7:12), and the waters prevail upon the earth for 150 days (Genesis 7:24). The Flood covers "all the high mountains under the whole heaven" (Genesis 7:19), eradicating all life outside the ark. This comprehensive destruction serves as a stark reminder of the consequences of sin and the holiness of God.

Theological Significance:
The onset of the Flood is a profound demonstration of God's justice and mercy. While it serves as a judgment against sin, it also highlights God's grace in preserving Noah and his family. The Flood narrative foreshadows themes of salvation and covenant, as God later establishes a covenant with Noah, promising never again to destroy the earth by flood (Genesis 9:11).

Cultural and Historical Considerations:
The Flood narrative has parallels in various ancient cultures, suggesting a shared memory of a significant flood event. However, the biblical account is unique in its theological emphasis, portraying the Flood as a divine act of judgment and redemption.

Conclusion:
The onset of the Flood is a foundational event in biblical history, illustrating the gravity of sin and the depth of God's grace. Through Noah's obedience and God's covenantal faithfulness, the narrative sets the stage for the unfolding account of redemption throughout Scripture.