In “wa-lammā ṣafā al-jawwu waṣala,” what does lammā primarily do?

Supplementary Practice 8: Rhetorical Analysis of Five Connected Passages

• It merely coordinates with the earlier story, like wāw
• It necessarily supplies the same immediate consequence as fāʾ
• It links clearing weather to the time of arrival
• It directly names the subject of waṣala