Typhoon Nangka Continues on Course towards Japan: AIR Analysis

July 15, 2015

Catastrophe modeling firm AIR Worldwide has released an analysis of Typhoon Nangka, which is currently a Category 3 storm “with a central pressure of 950 mb and maximum 10-minute sustained wind speeds of 80 knots (~106 mph 1-minute sustained). The storm is located approximately 1,250 km (775 miles) from Iwakuni, Japan, Nangka, and is moving northward at 15 km/h (9 mph).

“Once the intensity of a Category 4 Super Typhoon, Nangka has weakened and maintained its current intensity for the last 72 hours,” said Dr. Anna Trevino, scientist at AIR Worldwide. “It is located in a favorable environment with very little vertical wind shear and warm sea surface temperatures, but nearby dry air is being wrapped into the center, preventing further convective growth and intensification.

“High pressure to the northeast is steering Nangka toward Japan, and the typhoon is forecast to make landfall as a Category 2 or weak Category 3 storm between the islands of Kyushu and Shikoku Thursday (local time) and rapidly weaken. Because landfall is not for at least 48 hours, there is uncertainty in the intensity of the storm at landfall. This is the first typhoon landfall in Mainland Japan has seen in 2015, with the exception of Typhoon Noul, which quickly weakened before making landfall as an extratropical storm in May.”

According to the Japan Meteorological Agency (JMA), Nangka is expected to slightly strengthen over warm ocean waters during the next two days before landfall, causing rough seas, anticipated to impact shipping.

AIR explained that “typhoons are the most frequent type of natural disaster to cause property loss in Japan. Winds are the predominant driver of loss, although Japan does have strict and well-enforced construction codes.”

AIR also pointed out that wind damage in Japan “is typically automatically covered under standard fire insurance policies, but flood damage is not, despite the fact that Japan regularly experiences ‘wet” storms that deliver extreme precipitation and flooding that contribute substantially to damage. Property owners who want flood coverage can purchase it as an add-on to a standard policy or select a comprehensive policy.”

Dr. Trevino continued: “As Nangka approaches landfall on Thursday, local time, Nangka is expected to bring increasing rains across Japan, including the metropolitan area of Tokyo, which may receive 50 mm (2 inches) of rain within a 12- to 24-hour period.

“On Thursday night, local time, strong winds, flooding rains, and inundating storm surge are expected to affect Shikoku prefecture and parts of the adjacent prefectures of Kyushu and Honshu as Nangka makes landfall at an intensity equivalent to a Category 2 or weak Category 3 hurricane.

“Other cities that could be affected are Oita, Koichi, Okayama, and Osaka. Nangka may bring as much as 150 to 300 mm (6 to 12 inches) of rain to Shikoku and adjacent prefectures, with significantly higher amounts of precipitation possible in localized areas, which could cause flash floods and mudslides in mountainous areas.”

Nangka’s storm surge is another serious concern, the report said; “especially near the site of landfall and east of it. Depending on how Nangka continues to track, it is possible that the storm’s surge could be directed into Kii Channel and even Osaka Bay, which could cause severe flooding to the coastal areas of the Kii Peninsula.

“Nangka is also expected to brush southeastern South Korea later this week, bringing heavy rains and strong winds.”

According to AIR, at the expected wind speed levels, “non-engineered structures may experience some damage to roof coverings. Some poorly constructed wood frame homes may experience moderate to high-level cladding and roofing damage, involving loss of roof coverings and removal of porch coverings and awnings.

“Some homes may even be destroyed. Masonry homes and well-constructed wood frame homes could have some damage to roof coverings (tiles or shingles), wall siding, soffit panels, and gutters. Some poorly built and poorly maintained industrial buildings may lose roofing and siding, especially from windward corners, rakes, and eaves and may even collapse. For engineered structures, structural damage is not expected. Some apartment building and shopping center roof coverings could experience moderate levels of damage, and wall siding may also experience some moderate levels of wind damage.”

AIR added, however, that a greater damage threat may come from “from precipitation-induced flooding and storm surge, which will vary by construction type. For a given flood depth/effective surge depth, a residential wood frame building is expected to sustain more damage than a residential masonry building.

“Concrete construction is less vulnerable to flood than steel or masonry. Commercial and apartment buildings usually have stronger foundations than residential buildings, and are thus better able to resist flood loads. Water damage to machinery and contents drives most flood-related loss; because damage is usually limited to the lower stories of a building, high-rise buildings will experience a lower damage ratio than low-rise buildings as smaller proportion of the building is affected.”

Source: AIR Worldwide

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