Early on September 11, the Joint Typhoon Warning Center (JTWC) began monitoring an area of convection roughly 1,100 km (680 mi) east of Guam.

[1] A broad, low-level circulation center had formed within the system and became increasingly organized, showing signs of developing convective banding.

[1] A Tropical Cyclone Formation Alert was released later that day as convection was rapidly consolidating around the low, which was embedded within the eastern end of the monsoonal trough.

[11][12] Later that day the JMA reported that Typhoon Choi-wan had reached its 10-minute peak wind speeds of 195 km/h (121 mph) and a pressure of 915 hPa (mbar).

[13][14] Typhoon Choi-wan then intensified further and reached its peak 1-minute wind speeds of 260 km/h (160 mph) which made it the first Category 5-equivalent tropical cyclone on the Saffir–Simpson hurricane scale anywhere in the world in 2009.

[14][15] With the help of excellent poleward outflow and high ocean heat content, Typhoon Choi-wan remained at its peak intensity until early on September 17 when deep convection started to erode in the northwestern quadrant as the tropical upper tropospheric trough (TUTT) cell to the northwest was no longer providing good outflow.

[10][17] During September 19, Choi-wan rapidly weakened, as it moved into an unfavorable environment with higher amounts of vertical wind shear, causing the storm's deep convection to erode.

[23][24] The Tropical Storm Warning in place for Saipan was canceled during September 15 as Choi-wan intensified into a high-end Category 4 equivalent super typhoon.

On Saipan, 179 residents sought refuge in shelters by the afternoon of September 15, by which time heavy rain had begun falling.

[31] Early on September 16 the JMA started to issue Typhoon force wind warnings, for the Naha, Okinawa; however these were discontinued later that day.

Power lines were downed and sewer pipes were broken; however, crews began repairing damage once Choi-wan passed.

Roughly 260 gallons of fresh water, 50 lbs of rice and other health or comfort supplies were delivered by Helicopter Sea Combat Squadron 25 to both islands.

[42] On September 21, the Salvation Army announced that residents displaced by the storm would be received donated items, such as clothing, toys, pillows and baby formula.