People often dismiss harsh warnings until disaster finally strikes, and a drastic shift in public attitude toward prophetic messages is bound to happen, transforming outright mockery into absolute recognition. The exact nature of this impending event is understood in a few ways. It refers to the actual realization of the punishment the prophet warned about [רש"י, מצודת דוד, מלבי"ם]. Alternatively, it means the fulfillment of the prophecies themselves [רד"ק], or the arrival of the terrible news of destruction [ביאור שטיינזלץ].
The primary approach among commentators is that there is a firm promise that this disaster is prepared and will happen very soon. However, another perspective suggests this reflects the sheer shock of the people, crying out in amazement that the warnings have actually come true [מצודת דוד]. Up until that terrifying moment, the people treat the prophet's words as a joke. They view the messages of doom as a mere routine, something they do not have to take seriously, and they completely ignore the calls to change.
When the disaster finally hits, a fundamental shift occurs. The people will suddenly understand that God had placed a true prophet in their midst. They will realize that the words spoken were truly from God and not a laughing matter [רש"י, רד"ק, מצודת דוד, ביאור שטיינזלץ]. Tragically, this late realization offers no practical help to save the nation. Since the people failed to repent and prevent the disaster, they cannot undo the past once the punishment arrives. The only result of this delayed awakening is the stark, undeniable knowledge that a genuine prophet of God was standing right in front of them all along [מלבי"ם].