Fujiwhara effect
On September 14, a low-pressure area formed near the southern Marshall Islands. Favorable conditions allowed the low to strengthen into a tropical depression on September 17, and to intensify into a typhoon early on September 20. Shanshan reached peak intensity on September 21 as a Category 4 super typhoon. Due to the Fujiwhara effect, Shanshan was weakened by an extratropical cyclone located south of Kamchatka Krai, and Shanshan merged with it and collapsed into a single extratropical cyclone.