Category 5 Meranti Threatens Taiwan; Tropical Storm Ian Forms in the Atlantic

Earth’s strongest tropical cyclone of 2016 thus far is heading for a potentially destructive encounter with Taiwan. A mere 50-mph tropical storm just two days ago, Super Typhoon Meranti was packing top sustained winds of 155 knots (180 mph) at 12Z (8:00 am EDT) Monday, using the 1-minute peak wind standard employed by the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center. (Outside of the U.S., most weather agencies employ a 10-minute wind average; by this standard, Meranti’s peak winds were 115 knots, according to the Japan Meteorological Agency.) Meranti has vaulted to Category 5 strength by taking advantage of nearly ideal conditions, including very warm sea-surface temperatures around 30°C (86°F), very low wind shear (below 10 knots), and a fairly moist mid-level atmosphere (60-70% relative humidity).


Figure 1. Enhanced infrared satellite image from Japan’s Himiwari-8 satellite of Typhoon Meranti at 1500Z (11:00 am EDT) Monday, September 12, 2016. Image credit: NOAA/NESDIS.

Meranti is the planet’s fourth Category 5 storm of the year, following Tropical Cyclone Winston (February, Southwest Pacific Ocean), Tropical Cyclone Fantala (May, Southwest Indian Ocean), and Super Typhoon Nepartak (July, Northwest Pacific Ocean), which struck Taiwan (see below). The globe averages between 4 and 5 Category 5 storms per year. Meranti has now tied Winston for the strongest winds of the year, and its central pressure of 905 mb, as analyzed by JMA at 12Z Monday, puts it just behind Nepartak (900 mb).

Forecast for Meranti
Southern Taiwan faces a serious threat from Meranti. Typhoons this strong will sometimes undergo an eyewall replacement cycle that can trim their peak winds for a day or so, but otherwise it appears Meranti will hang onto most or all of its power until it approaches Taiwan in a couple of days. The latest track forecast for Meranti reflects some major disagreement among models and forecast agencies. The 00Z Monday runs of the GFS and European models take Meranti across the southern tip of Taiwan, while the 00Z UKMET model takes the cyclone on a much more southerly track, which would keep it well offshore. Likewise, the JWTC forecast as of 12Z Monday brings Meranti’s center to the southern tip of Taiwan just after 00Z (8 am local time) Wednesday, while the 12Z Monday outlook from JMA keeps Meranti’s center about 100 miles south of the island. Given the uncertainty and the potential for disaster, southern Taiwan needs to prepare for the possibility of a landfalling super typhoon.


Figure 2. Forecasts for Meranti’s track issued at 12Z (8:00 am EDT) Monday by the Japan Meteorological Agency (left) and the Joint Typhoon Warning Center (right). Image credit: JMA and JTWC.

The GFS and European models project a slight northwest bend to the track that may allow Meranti to grind its way along the southwest coast of the island on Wednesday while maintaining at least Category 3 strength, as reflected in the JTWC outlook. Taiwan’s second-largest city, Kaohsiung, is located along the southwest coast. Taiwan’s southern tip is very sparsely populated, but there are close to 3 million people in the Kaohsiung area.

Taiwan’s second super typhoon of the year
Meranti’s projected track is very similar to that of another super typhoon that struck Taiwan back in early July. Super Typhoon Nepartak maintained Category 5 strength with sustained winds of 160 mph and a central pressure of 900 mb until it was just 12 hours from landfall in Taiwan on July 7, 2016. Nepartak made landfall on the southeastern shore of Taiwan as a Category 4 super typhoon with top sustained winds of 150 mph, as estimated by the the Joint Typhoon Warning Center (JTWC), with a central pressure estimated at 930 mb by the Japanese Meteorological Agency (JMA). Nepartak struck the sparsely populated southeast coast of Taiwan and moved offshore north of Kaohsiung, limiting the damage from the storm. Three deaths in Taiwan were blamed on Nepartak, along with $33 million in agricultural damage. National Taiwan University (NTU) buoy NTU2 (located about 170 km southeast of Taitung, Taiwan) recorded a surface pressure of approximately 897 mb as the eye passed over near 8 am EDT July 7. If verified, this may rank as the lowest surface pressure ever measured by a buoy in world history. A team from National Taiwan University is working to verify that the calibration of the pressure on this buoy was correct.

The encounter with the high mountains of Taiwan destroyed the inner core of Nepartak, resulting in the surface circulation separating from the circulation at mid-levels of the atmosphere. A much weakened Tropical Storm Nepartak made landfall in mainland China day later, causing torrential rains that triggered flooding that killed 108 people and caused over $1.4 billion in damage. The future of Meranti beyond Taiwan hinges in part on whether the storm’s core stays just offshore or whether it gets torn apart by Taiwan’s southern mountains--an outcome too close to call at this point. Track models agree that Meranti will continue on a general west-northwest to northwest track toward the China coast, but with important differences on where Meranti might strike along the coast and how it would behave after that point. Meranti’s size and strength give it the potential to be a catastrophic rainmaker in China.


Figure 3. Radar image of Super Typhoon Nepartak making landfall in southeastern Taiwan taken at 5:30 pm EDT July 7, 2016 (5:30 am local time July 8 in Taiwan.) Image credit: Taiwan CWB.

Another wrinkle now taking shape in the Northwest Pacific
Another factor in Meranti’s track--and a threat in its own right--is Tropical Storm 18, which is gradually strengthening about 300 miles west of Guam. TS 18W could be a Category 3 or 4 typhoon as it approaches Japan’s southern islands on a gradually recurving track. By Wednesday, it’s possible that TS 18W will be close enough to Meranti to trigger the Fuijiwhara effect--the process by which two tropical cyclones begin to rotate cyclonically around a point in between. Should this be the case, it would enhance 18W’s poleward motion--perhaps sending it toward Japan’s populous island of Honshu as a typhoon--while also acting to slow any recurvature of Meranti and potentially increasing its rain- and flood-making potential in China.


Figure 4. Latest satellite image of Orlene.

Yet another East Pacific hurricane: Orlene
The Northeast Pacific notched its 10th hurricane of the 2016 season with the arrival of Hurricane Orlene on Sunday night. This puts the East Pacific well ahead of its long-term average of 8 hurricanes for the entire season (1971-2009). Packing top sustained winds of 90 mph as of the 11 am EDT Monday advisory from NHC, Orlene has a good shot of attaining Category 2 strength later today or tonight as it benefits from extremely low wind shear (around 5 knots) and fairly warm SSTs (27-28°C). Working against Orlene will be cooler waters lurking just below the surface, which are increasingly likely to be churned up as Orlene’s northwestward motion slows to a crawl by Tuesday. A strengthening upper-level ridge should push Orlene westward after its expected midweek stall, nudging it away from land and toward cooler waters.


Figure 5. Latest satellite image of Ian.

Tropical Storm Ian forms in the central Atlantic
Tropical Storm Ian finally decided to spin up into a named storm late Monday morning in the waters of the Central Atlantic, about 1140 miles southeast of Bermuda. Ian is the ninth named storm of the Atlantic hurricane season, bringing our total activity at the approximate halfway point of the season to 9 named storms, 4 hurricanes and 1 intense hurricane. An average season has 11 named storms, 6 hurricanes and 2 intense hurricanes, so we are ahead of schedule for named storms and hurricanes, but near average for intense hurricanes and ACE index, as detailed in our Friday post. Ian is in an environment not conducive for intensification, with wind shear a high 20 knots. Shear is expected to weaken only a little by mid-week, making it unlikely Ian will ever attain hurricane strength as it moves northwards over cooler waters. Ian is not a threat to any land areas.

93L approaching Florida little threat to develop
An area of low pressure (formerly called Invest 93L) was located over the central and northwest Bahamas on Monday morning, and was headed west-northwest at about 10 - 15 mph towards central Florida. Satellite images and long-range radar out of Melbourne, Florida showed that 93L continued to have only a limited amount of heavy thunderstorms, with no sign of a surface circulation center. The disturbance was battling high wind shear of 25 knots and plenty of dry air. The disturbance should move inland over Florida on Tuesday without developing, but will bring some heavy thunderstorms to the central and northwest Bahamas on Monday, and central and northern Florida on Tuesday. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10%.

Bob Henson and Jeff Masters

At the risk of breaking twitter, Super Typhoon #Meranti at 500 m resolution every 2.5 mins, via #Himawari VIS pic.twitter.com/58HVQtvc0j

— Dan Lindsey (@DanLindsey77) September 12, 2016

Figure 6. CIRA/CSU's Dan Lindsey posted this mesmerizing loop of Typhoon Meranti on Monday. It features images collected by the Himiwari-8 satellite at 500-meter resolution every 2.5 minutes. Image credit: @DanLindsey77.