Japan Typhoon Season: Will Disaster Areas Be Hit?

March 24, 2011; 7:00 AM ET
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Play video For more details on the typhoon season and how Japan could be affected, click on this video.

While typhoon season doesn't typically ramp up around Japan until late summer, the threat of a tropical system affecting the fragile nation as it slowly recovers from the devastating earthquake and tsunami is a major concern.

Japan is located in the most active tropical basin in the world. Historically, tropical systems passing within 100 miles (160 km) of Sendai, Japan, an area hit hard by the earthquake and tsunami, have been observed as early as mid-June.

However, the threat of a tropical system affecting Japan usually becomes greatest in August and September, when ocean waters that far north are warmest, according to AccuWeather.com Expert Tropical Forecaster Dan Kottlowski.

In the western North Pacific Basin as a whole, typhoons and tropical storms can develop any time of the year, but the period from July through October is normally the most active.

This graphic displays the total number of tropical storms, as well as the number of those forecast to develop into typhoons and super typhoons, expected in the western North Pacific Basin in 2011 compared to the average number per year.

This year in the western North Pacific Basin, the AccuWeather.com Hurricane Center is forecasting a fairly average typhoon season with a total of 27 tropical storms, 16 of which becoming typhoons and 4 strengthening into super typhoons.

The definitions of a typhoon, super typhoon, tropical storm and tropical depression are given at the bottom of this page.

Odds of a Typhoon Hitting Japan Disaster Areas This Year

There are several good things going for the disaster areas of Japan in terms of the risk of impact from a typhoon. However, despite these positives, tropical systems do affect northern Japan with some regularity, and flooding rain tends to be the biggest problem.

One of the positives is that only a small percentage of tropical systems that develop in the western North Pacific Basin impact northeastern Japan.

Out of the estimated 1,757 tropical storms and typhoons that have formed in the western North Pacific Basin between 1945 and 2008, approximately 40 (2 percent) have tracked within 100 miles (160 km) of Sendai. Storm tracks are based on the movement of the center, or eye, of the storm.

Another positive is that, historically, typhoons that do affect northern Japan tend to be in a weakening phase and do not pose a significant damaging wind threat.

This is due in part to the fact that these typhoons have often already made landfall either in southern Japan or somewhere else in the western North Pacific Basin. It is also due in part to the fact that waters over which these systems track are cooler offshore of northern Japan.

Of the 40 tropical systems that passed within 100 miles of Sendai between 1945 and 2008, 16 were of typhoon strength at some point within 100 miles of Sendai. None of those typhoons were stronger than a Category 2 hurricane when they entered the 100-mile radius.

In fact, all but one of those 16 typhoons had a strength equivalent to a Category 1 hurricane during that time.

This image, courtesy of the U.S. National Oceanic and Atmospheric Administration's Historical Hurricane Tracks database, shows the tracks of tropical systems that moved within a 100-mile radius of Sendai, Japan, between 1945 and 2008. Light blue represents systems in the stage of a tropical depression, while green depicts a stage as a tropical storm. The other colors represent typhoons, with yellow being Category 1 hurricane strength, orange Category 2 hurricane strength and red Category 3 hurricane strength.

Because tropical systems that affect northern Japan tend to be weaker, the threat of wind damage is not a major concern. This is especially true for the earthquake and tsunami-ravaged areas of northeastern Japan, where the topography is such that these areas tend to be protected from damaging winds.

While the chance of any given typhoon, especially one stronger than a Category 2 hurricane, impacting Sendai (and other locations throughout the disaster area) is extremely small, tropical systems do affect that area with some regularity.

From 1945 to 2008, tropical systems have passed within 100 miles of Sendai 29 out of the 64 years, which is almost half the total number of years. The longest period of time that went by without a tropical system passing within 100 miles of Sendai was 4 years.

So even if the disaster areas of Japan were to escape impact from a tropical system this year, history would suggest that a system will affect these fragile areas within the next several years.

The last time a tropical system tracked significantly close to Sendai was in 2009. So if history repeats itself, one would expect another system to affect that area by 2013.

If or when that happens, flooding rain will be the biggest threat.

What Is a Typhoon?

A typhoon is essentially the same thing as a hurricane. Both terms are regionally specific names for a strong tropical cyclone with sustained winds of at least 74 mph (119 kph).

A super typhoon, as defined by the U.S. Joint Typhoon Warning Center, is a typhoon with maximum sustained winds of at least 150 mph (241 kph), which would be equivalent to strong Category 4 or a Category 5 hurricane.

The term tropical storm is a name for a weaker tropical cyclone with sustained winds between 39 and 73 mph (62-117 kph). A tropical depression is a tropical cyclone with sustained winds less than 39 mph.

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