[citation needed] These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean.

Tropical Storms formed in the entire west pacific basin were assigned a name by the Joint Typhoon Warning Center.

Walt reached peak intensity on May 12, before showing a distinct eyewall replacement cycle lasting four hours from late May 13 to May 14.

Subsequently, the eastward building of the subtropical ridge produced unfavorable vertical wind shear that weakened Yunya to a minimal typhoon before hitting Dingalan, Luzon early on the 15th.

The ash cloud that normally would have been dispersed across the oceans was redistributed over Luzon by the cyclonic winds of the typhoon, greatly exacerbating the damage caused by the eruption.

The water-laden ash fell over the evacuated Clark Air Base, as well as the rest of Luzon, resulting in downed power lines and the collapse of flat-roofed buildings.

Yunya exited Luzon through Lingayen Gulf as a weak tropical storm and then turned north toward a break in the subtropical ridge.

It then brushed the southern coast of Taiwan as a tropical depression and finally dissipated before it could complete full recurvature into the mid-latitude westerlies.

[2] 145 mph Typhoon Amy, having developed on July 12 over the open Western Pacific, brushed southern Taiwan on the 18th.

As Caitlin turned to the north the storm passed 60 miles (97 km) to the west of Kadena AB, Okinawa at peak intensity of 110 mph.

Typhoon Caitlin then began to accelerate northwards and passed through the Korea Strait before turning extratropical in the Sea of Japan.

Typhoon Fred organized from a monsoon trough situated to the east of the Central Philippines, a depression formed on August 11.

Typhoon Ivy formed from a broad monsoon trough situated near Kosrae in the eastern Caroline Islands.

Ivy began to rapidly intensify and reached typhoon strength as the storm passed 130 miles (210 km) east of Tinian and Saipan in the Northern Mariana Islands.

Typhoon Ivy continued on a northwesterly path and reached peak strength of 130 mph (210 km/h) on September 7 prior to recurving to the northeast.

As Typhoon Ivy made its closest approach to Honshū, Tokyo and surrounding areas were buffeted by high winds and heavy rains.

As Kinna moved towards the northwest the storm began to gather strength and reached typhoon intensity on September 12 just prior to turning north threatening Japan.

When the other two storms were far enough away, Mireille rapidly intensified, reaching super typhoon strength on the 22nd with a peak of 150 mph (240 km/h) winds.

Mireille continued to the northeast, and became extratropical later that day, after causing 52 casualties and heavy crop damage amounting to $3 billion (1991 USD).

The depression began to slowly intensify as it moved towards the west-northwest and Tropical Storm Luke was named on September 15.

Luke reached peak intensity of 60 mph (97 km/h) prior to recurving to the northeast and weakening due to increased shear.

The resulting flooding and landslides killed 8 people and left 10 others missing prior to Luke turning extratropical east of central Honshū Island.

Typhoon Orchid formed from a broad monsoon trough that moved through the Northern Marianas and became a tropical depression on October 4 to the west of the islands.

Orchid then began to rapidly intensify and reach peak strength on 130 mph (210 km/h) on October 7 prior to turning to the northeast and accelerating.

Ruth made landfall on October 27 on northern Luzon Island with winds of 115 mph (185 km/h) before weakening to a tropical storm.

Thelma, though a weak storm, caused dam failures, landslides, and flash flooding, resulting in a horrendous death toll of 6,000 people.

Super Typhoon Yuri was the most powerful storm during the season, with winds reaching up to 120 knots (220 km/h), and a minimum recorded pressure of 895 mbar.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration uses its own naming scheme for tropical cyclones in their area of responsibility.

Due to extensive damage and a high death toll, the JTWC retired the names Mireille and Thelma, which were replaced by Melissa and Teresa.

This table summarizes all the systems that developed within or moved into the North Pacific Ocean, to the west of the International Date Line during 1991.