This image was taken by the Japan Meteorological Agency’s MTSAT-2 satellite at 0730Z on September 20, 2013. (NOAA)

Super typhoon Usagi, 2013’s strongest storm on the planet, may have peaked in intensity, but remains an extremely dangerous cyclone as it continues on a collision course with southern Taiwan and, likely, Hong Kong.

The Joint Typhoon Warning Center says Osagi’s maximum sustained winds are 150 mph, the equivalent of a category 4 hurricane.  That’s down from at least 160 mph Thursday (category 5 level). But this is a mammoth storm, tropical storm force winds span 275 miles across it.

On Thursday evening, a satellite-based estimate of its minimum pressure was an astonishingly low 882 mb, which would have made it the deepest and most intense storm to exist on Earth since 1984 (tied with hurricane Wilma in 2005).

Look at this incredible high resolution satellite image of the storm from Thursday afternoon, revealing the textbook traits of a flawless cyclone:

Usagi infrared satellite view 12:33 p.m. ET Thursday (Colorado State University)

You see the cloud-free, distinct eye which is surrounded by tall thunderstorms on all sides.

Due to a re-arrangement of its internal structure since that time, known as an eye-wall replacement cycle, Usagi has lost some steam and its satellite presentation – while impressive – is less than perfect.

Usagi infrared satellite view 1:14 p.m. ET Friday (NOAA)

Its eye is somewhat ragged, and deep convection (a fancy term for thunderstorms) is less pronounced in its northeast quadrant.

What’s next for Usagi?

The Joint Typhoon Warning Center predicts additional weakening as Usagi’s circulation is disrupted by Taiwan. Usagi will batter Taiwan’s south and east coast with damaging winds, torrential range, massive waves, and a dangerous storm surge today into Saturday. The storm’s rain bands have already begun to lash coastal areas.

Link: Animated Taiwan radar loop

Usagi is then expected to cross the South China Sea, but further weakening is forecast.


Usagi forecast track (Joint Typhoon Warning Center)

The storm could potentially come ashore near Hong Kong late Sunday local time (Sunday morning EDT). Confidence in the overall track is high but small deviations could push the brunt of the storm to Hong Kong’s northeast or southwest.

“Usagi is still far away from Hong Kong,” writes the Hong Kong Observatory. “As there may still be changes to its track, its effects on Hong Kongs weather can be vastly different. Usagi is a mature tropical cyclone and may become the strongest storm affecting Hong Kong since this year. The public should remain vigilant, pay attention to the latest weather report, and take precautions against strong winds and flooding as early as possible.”

The Joint Typhoon Warning Center predicts Usagi will have maximum sustained winds near 100 mph around the time it would potentially make landfall in or near Hong Kong. If a direct hit were to occur, the storm would also produce copious amounts of rain and, depending on the angle of approach, potentially a devastating storm surge (given the storm’s size and intensity).

This is a very busy weekend in Hong Kong, as a major autumn festival is underway.

Hong Kong’s typhoon history

Direct and indirect hits from typhoons or tropical storms are very common in Hong Kong, happening annually – almost without exception and often on multiple occasions.

As the city, populated by 7.1 million residents, has considerable experience with these storms, it is usually well-prepared and has avoided mass casualties from recent events. Since 1990, 74 named storms have affected Hong Kong with about 44 total fatalities (based on data from the Hong Kong Observatory). Not since 1983, when Ellen claimed ten lives, has Hong Kong experienced a death toll in the double digits from a tropical storm or typhoon.

Number of people killed or missing from named storms to impact Hong Kong from 1960-2011 (Hong Kong Observatory)

In the more distant past, 15,000 and 11,000 lives were lost in storms that devastated Hong Kong in 1906 and 1937, notes a blog post at the Hong Kong Observatory.

The lesser impacts from typhoons in recent decades, however, is likely tied to a lack of direct hits, the Hong Kong Observatory notes. The number of instances of typhoon’s eye (center) moving directly over Hong Kong – resulting in the issuance of Signal 10, the most dire alert – has been few.

“The tropical cyclone activity in the South China Sea was in a relatively quiet phase during the last two decades,” the Hong Kong Observatory writes. “The frequency of Signal No. 10 [the eye of a storm passing over Hong Kong] has decreased since the 1980s. The Hurricane Signal was issued only twice in the last three decades in 1983 (Ellen)and 1999 (York).”

(Update: The above quote is from April 2012, before the third Signal No. 10 of the last three decades was issued for typhoon Vicente in July, 2012)

It adds: “With slightly less frequent tropical cyclones affecting Hong Kong in the last few decades, some people may wrongly think that the typhoon risk in Hong Kong has declined.”

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