Although Nancy initially moved west, the system maintained a general westward course for much of its duration, striking Luzon on October 14 at peak intensity of 215 km/h (130 mph).

It weakened to tropical storm strength overland, but re-intensified to typhoon intensity over the South China Sea.

Typhoon Nancy originated from a large area of convection situated in the middle of the Pacific Ocean,[2] which, while drifting westward,[3] began to consolidate on October 8 within a favorable environment aloft.

The convection separated from an upper-level low embedded within a tropical upper-tropospheric trough (TUTT).

[4][nb 2] At 07:30 UTC, a Tropical Cyclone Formation Alert was issued by the JTWC while located 370 km (230 mi) north of Guam.

Nancy maintained its intensity for 24 hours while tracking westward before rapidly turning west due to a change in steering patterns.

[2] Meanwhile, the Philippine Atmospheric, Geophysical and Astronomical Services Administration also monitored the storm and assigned it with the local name Weling.

At 00:00 UTC on October 13, the same agency classified Nancy into a typhoon,[4] and subsequently, the storm developed a well-defined eye.

[2] At 0600 UTC, the JMA reported that Nancy reached its peak intensity of 185 km/h (115 mph) and a minimum pressure of 935 mbar (27.6 inHg).

[4] The storm rapidly weakened over land,[2] with satellite images showing that the eye had quickly disappeared.

[4] After entering the South China Sea,[2] the storm initially tracked west-northwest before slowing down and turning west.

[3] Meanwhile, JTWC upped Nancy to typhoon status as it entered an area with favorable mid- to upper-level winds.

[2] Continuing to gain strength, the JMA raised the intensity of Nancy to 130 km/h (80 mph) on October 16.

While passing slightly north of the Paracel Islands,[2] the JTWC estimated winds of 145 km/h (90 mph), equivalent to a high-end Category 1 hurricane on the SSHWS.

[4] Prior to Nancy's first landfall, typhoon warnings were issued for much of Luzon and many residents left for shelter.

A minimum pressure of 1009.4 mbar (29.81 inHg) was recorded at the Hong Kong Royal Observatory (HKO) on the October 15.