وَلَمَّا صَفَا الْجَوُّ وَصَلَ.
And when the weather cleared, it arrived.
“Lammā” introduces a temporal relationship: once the weather cleared, the arrival took place. The initial wāw connects this stage to the preceding narrative, while the unexpressed subject of “waṣala” is understood from the earlier “al-barīd.” These grammatical elements have different functions.
Replacing the initial wāw with fāʾ would foreground a closer sense of consequence or sequence. The two particles should not be treated as identical simply because both can link clauses in a narrative.