After an extended, unexplained global “drought” of Tropical Cyclone activity, the West Pacific region spawned a series of Super Typhoons in October. The most recent of these fearsome storms was Super Typhoon Haiyan, or Yolanda as named by the Philippine Atmospheric, Geophysical and Astronomical Administration (PAGASA). I will refer to the Typhoon as Haiyan to limit confusion.
Haiyan quickly strengthened into a Typhoon, then Super Typhoon. A Typhoon is a Tropical Cyclone in the Western Pacific with maximum sustained winds of 64kts (74mph, 119km/h) or greater. A Typhoon becomes a Super Typhoon when maximum 1-minute sustained winds reach 130kts (150mph, 240km/h) or greater as advised by the JTWC, the JMA does not include this classification. The equivalent phenomena in the Atlantic is the Hurricane, where a Super Typhoon would be a category 4 equivalent Hurricane. Tropical Cyclones derive a vast amount of their energy from the warm sea surface waters they form over; the Western Pacific has the warmest waters on Earth and they stay warm enough to support Tropical Cyclones all year. The western Pacific is therefore no stranger to Typhoons and Super Typhoons with the 5 Super Typhoons that have formed this year not being far from normal. Fortunately the strongest winds of these storms are confined to a small region near their center, and they can rarely maintain such intensity for very long. However, Haiyan not only became a Super Typhoon and maintained that intensity for more than two days, but it made direct hits on Palau and at least 5 islands in the Philippines at that strength.
Meteorological reporting in the Western Pacific is much less streamlined than that in the Atlantic. The National Hurricane Center based in Miami, FL is the sole source of official information on Tropical Cyclones in the Atlantic and Eastern Pacific. However, in the Western Pacific, many countries have their own agency that produces official information regarding these powerful storms. Additionally, it is common for disagreement on storm intensity between agencies, not to mention that they use different units for wind speed, and different definitions for sustained winds. All these factors can make understanding the warning message and comparing them very difficult. It should be noted that the Japanese Meteorological Agency (JMA) is regarded as the official source for Tropical Cyclone information in the Western Pacific, but the American Joint-Typhoon Warning Center (JTWC), and PAGASA also issue their “unofficial” advisory products for this region.
To complicate matters a little more, there are very few (if any) in storm observations to help determine their intensity operationally. Therefore, to a large extent, the intensity quoted in advisory products is an estimate based on the storm’s satellite appearance. In the Atlantic and Eastern Pacific, the National Hurricane Center can rely on the Hurricane Hunters to investigate Tropical Cyclones to get a better ‘fix’ of their intensity, structure, and location. This service no longer exists in the Western Pacific and so unless a storm nears land, which allows land based radar and observations to be included, only satellite estimates can be used. Lets understand that these estimates tend to be very accurate as, operationally, a blend of different methods in addition to human interpretation are included in making the final estimate for storm intensity. However, this technique begins to break down when storms reach Super Typhoon strength.
With those thoughts in mind, Super Typhoon Haiyan passed to the north of Palau on Thursday, November 7th putting northern parts those islands in the storm’s southern eye-wall. The eye-wall is the small region of the storm that contains its strongest winds. The eye-wall, characterized by heavy rain and these violent winds, surrounds the often calm and clear eye (center) of a strong Tropical Cyclone. If intensity estimates were accurate, northern parts of Palau could have experienced 1-minute sustained winds of 115-130kts (130-150mph, 215-240km/h) for a period as Haiyan passed to their north. Yap also experienced the upper end of tropical storm conditions (34-63kts, 39-73mph, 63-119km/h) as Super Typhoon Haiyan passed to the south of those islands. A Personal Weather Station in Yap unofficially recorded sustained winds of 63mph with a gust to 66mph at the peak of the storm.
Haiyan was not done after Palau and Yap, it continued strengthening as it approached the Philippines. You may be wondering why this happened. A first guess would be to attribute this storm’s freakish intensity to unusually warm sea-surface temperatures. But, although warmer than normal, the ocean beneath Haiyan was not so far from normal to explain such intensification as the typhoon passed overhead. And remembering that Tropical Cyclones are very complex creatures of weather, a more accurate reason would be the combination of the typically warm sea surface temperatures and the near perfect mid and upper atmospheric environment that Haiyan was embedded within. The environment was so close to perfect that structural changes within the storm that would cause a Tropical Cyclone to weaken or even fall apart in a typical environment, had little to no effect on Haiyan’s intensity and led to further strengthening.
Haiyan then reached the Philippines. The JMA was analyzing Haiyan to have 10-minute maximum sustained winds of 125kts (145mph or 230km/h), or 1-minute maximum sustained winds of 170kts (195mph or 315km/h) according to the JTWC (keeping in mind the limitations with operational intensity estimation in the Western Pacific). Here the center of Haiyan crossed at least five different islands, while many other experienced tropical storm conditions. The first town to be hit was Guiuan, where the last observation was of 96mph sustained winds with a pressure of 976mb. That pressure indicates the storm was not quite there (as the minimum central pressure was estimated at a much lower 895mb), yet Category 2 hurricane conditions were occurring. The northern eye-wall of Haiyan likely delivered the strongest winds in the storm to this town and region. Catastrophic destruction is likely as a result of winds, but a significant storm surge (predicted to be 17feet above normal mean high water level by PAGASA) likely caused additional destruction. Based on the intensity estimates provided at this time, Haiyan may have been the strongest storm to make landfall since reliable records began in the 1960s. Very few Tropical Cyclones make landfall with winds over 140kts (160mph, 260km/h) and so it is hard to imagine what was experienced as the worst part of Haiyan came ashore.
Damaging Typhoon strength winds likely occurred across much of the Central Philippines along with heavy rains and coastal flooding from storm surge. Haiyan has since weakened from its Super Typhoon status due it interaction with land in the Philippines. As of 21UTC November 9th 2013, Haiyan has 10-minute maximum sustained winds of 85kts (100mph or 160km/h) with a minimum central pressure of 950mb from the JMA or maximum 1-minute sustained winds of 90kts (105mph or 165km/h) from the JTWC. This makes Haiyan a category 2 equivalent Typhoon. Haiyan is moving west-northwestward and is expected to make a final landfall in Vietnam near Hanoi as a Typhoon with 65kt (75mph, 120km/h) 10-minute sustained winds from the JMA, and 70kt(80mph, 130km/h) 1-minute sustained winds from the JTWC. Please follow latest advisories from the JMA and JTWC.