Final answer:
The storm, characterized by powerful Etesian winds and violent waves, is the natural phenomenon that prevents Ysolte from reaching Tristan in time.
Step-by-step explanation:
The natural phenomenon in question is a powerful storm.
The passage provided "So when Etesian blasts from Thrace downpour Sweep o'er the blackening main and whirl to land From Ocean's cavernous depths his ooze and sand,
Billow on billow thunders on the shore" vividly describes the storm with Etesian winds from Thrace that churns the ocean, causing waves to crash violently upon the shore with such force that it brings up the ocean's sand and ooze.
This description indicates that it is the storm's intensity, with its strong winds and massive waves, that prevents Ysolte from reaching Tristan in time to save him.