Soudelor Makes Beeline for Taiwan; CSU, NOAA Update Atlantic Hurricane Outlooks

Although its peak sustained winds have dropped from 180 mph to 105 mph over the last several days, Typhoon Soudelor may embark on a final burst of intensification before striking Taiwan on Saturday. At 1500 GMT (11:00 am EDT) Thursday, the Joint Typhoon Warning Center (JTWC) positioned Soudelor at 21.3°N, 127.5°E, which is about 550 miles east-southeast of Taiwan’s largest city, Taipei, on the north end of the island. Soudelor is moving just north of west at about 11 mph. The JTWC’s 1500 GMT Thursday forecast track brings Soudelor onshore across central Taiwan just before 0000 GMT Saturday, or about 8:00 am Saturday local time. The projected track, arcing slightly toward the northwest over time, is a classic trajectory for this latitude, and is consistent with the GFS and ECMWF operational model runs and ensemble averages. The GFDL model bumps the track a bit more to the northwest, which would take Soudelor into the northern part of Taiwan, closer to Taipei.


Figure 1. An enhanced infrared image of Typhoon Soudelor from 1550 GMT Thursday (11:50 am EDT). Image credit: RAMMB/CIRA.


A bigger question mark is how strong Soudelor will be when it reaches Taiwan. Soudelor currently has a rather large eye (roughly 55 miles across, close to half a degree of longitude), with an intensifying solid ring of convection surrounded by a large field of spiral banding. Mesovortices (small-scale circulations) could be seen spinning within the large eye in visible satellite imagery. Soudelor’s brief stint as a super typhoon earlier this week was interrupted by an eyewall replacement cycle, with the much larger eye succeeding the typhoon’s originally tiny eye (about 5 miles in diameter, one of the smallest on record for a landfalling system, when Soudelor struck Saipan). Since that point, Soudelor has had some trouble rebuilding a large, solid core of convection, despite its excellent overall structure and the gradually increasing size of its envelope of moisture and banding. Dvorak imagery from the MTSAT satellite over the last few hours shows that very cold cloud tops (a sign of intense convection) are beginning to wrap around Soudelor’s eye once again, and temperatures within the eye are warming, another sign of intensification. Soudelor is now moving over slightly warmer SSTs (29-30°C, or 84-86°F, about 1°C above average) as it approaches Taiwan, and wind shear will be low (only around 5 - 10 mph).

Dynamical models generally agree in bringing Soudelor back to at least Category 3 strength before landfall in Taiwan, and I would not be at all surprised to see Soudelor pass the Category 4 threshold. The nation’s urbanized areas are well prepared for high typhoon winds, though widespread power outages and transportation disruptions are likely. Massive amounts of rain will fall where Soudelor slams into the north-south mountain range that spans most of Taiwan, and substantial local flooding and mudslides can be expected. Fortunately, Soudelor’s steady movement will help keep its rainfall totals below the truly prodigious amounts that slower-moving systems such as 2009’s Typhoon Morakot can produce. Morakot was only a Category 1 storm, but it moved in a leisurely cyclonic loop across northern Taiwan, prolonging the widespread intense rainfall. Morakot caused more than 450 deaths and some $3.3 billion US in damage.


Figure 2. This 3-D view of Soudelor’s eyewall and convective banding was produced using infrared and visible imagery from MTSAT combined with data from the Dual-Frequency Precipitation Radar aboard NASA’s Global Precipitation Satellite at 0006Z GMT on Thursday, August 6. The highest storm tops with Soudelor--extending up to more than 14.7 km (48,000 feet)--were located southwest of the typhoon's eye. Image credit: NASA.


Figure 3. Soudelor is likely to bring at least 8” of rain to most of Taiwan and parts of east-central China, according to these projections of precipitation from the 0600 GMT Thursday ensemble of the GFDL hurricane model. Much higher amounts can be expected in and near mountainous areas of Taiwan. Image credit: NOAA/GFDL.

Guillermo peters out north of Hawaii
Barely a tropical storm, Guillermo continues its slow decline as it tracks north of the Hawaiian islands. At 6:00 am Thursday HST (noon EDT), Guillermo was located about 225 miles east of Honolulu, heading west at 12 mph, with sustained winds of just 40 mph. Tropical storm warnings were in effect offshore, with high surf warning across the east- and north-facing shores of the Hawaiian islands. Guillermo is expected to weaken to tropical-depression status before sweeping just north of Honolulu; it could make landfall on Lihu’e, the northernmost major Hawaiian island, but little impact is expected other than localized heavy rain, gusty winds, and high seas.

Well to the southeast, newly christened Tropical Storm Hilda was located more than 1500 miles southwest of Cabo San Lucas with sustained winds of 40 mph. Hilda is wrapped in a moist environment and already showing a healthy amount of convection. Low wind shear (5-10 mph) and warm water (28-29°C) favor strengthening, and the National Hurricane Center projects Hilda to become a hurricane by Saturday. Hilda will move in the general direction of Hawaii, although it is far too soon to know whether it might pose a threat to the islands late next week.

Meanwhile, the Atlantic remains quiet, with any waves coming off Africa struggling against dry air, Saharan dust, marginal sea-surface temperatures, persistently high wind shear, or some combination of these.

CSU and NOAA: Atlantic to be even quieter than predicted
Unsurprisingly, both Colorado State University and NOAA have reduced their projected seasonal totals for 2015 tropical cyclones in the Atlantic below the predictions released in May/June, which were already among the lowest in years. The elephant in the room (and in the Atlantic) is the ever-strengthening El Niño, whose hurricane-suppressing effects on the deep atmosphere over the Atlantic are in some cases unprecedented for this time of year. For a full rundown on why El Niño is so hostile to tropical cyclones in the Atlantic, see last week’s post by new WU contributor Dr. Phil Klotzbach, lead author for the CSU hurricane seasonal outlooks.


Figure 4. Rising motion associated with El Niño across the eastern tropical Pacific has led to strong westerly winds at high levels across the Caribbean and deep tropical Atlantic, helping to suppress hurricane formation. This image shows vertical wind shear as measured by the difference between winds at 200 mb (about 40,000 feet) and 850 mb (about 5000 feet). Values of greater than 16 m/s (about 35 mph) dominate the main development region (MDR) of the tropical Atlantic, shown in the green box. Within the red box, shear values for June-July 2015 were well beyond record values for those months in a database extending back to 1970. Image credit: NOAA.


The CSU forecast (see PDF), issued on Tuesday, calls for the following end-of-season totals, including this year’s activity to date (Tropical Storms Ana, Bill, and Claudette), as compared to its start-of-season outlook issued on 1 June:

Named storms: 8 (no change)
Named storm days: 25 (down from 30)
Hurricanes: 2 (down from 3)
Hurricane days: 8 (down from 10)
Major hurricanes: 1 (unchanged)
Major hurricane days: 0.5 (unchanged)
Accumulated cyclone energy (ACE): 35 (down from 40)
Net tropical cyclone activity: 40 (down from 45)

Landfall probabilities from CSU for various sections of the Gulf and Atlantic coast from August onward are generally only about one-third of their full-season averages for the past century. For the entire U.S. Gulf and Atlantic coastline, CSU pegs the odds of a major hurricane landfall at 23%, compared to a full-season, century-long average of 52%. (Even that 23% may be on the high side, considering that no hurricanes have made a U.S. landfall at Category 3 or stronger since Wilma in 2005.)

NOAA’s outlook, issued on Thursday, also brings down all of the forecast indices previously released in their pre-season May outlook. Below are the new NOAA numbers, again incorporating this year’s activity to date. For each range, the likelihood assigned by NOAA that the numbers will fall within the range is 70%, based on years with conditions similar to those now present.

Named storms: 6-10 (lowered from 6-11)
Hurricanes: 1-4 (lowered from 3-6)
Major hurricanes: 1 (lowered from 2)
Accumulated cyclone energy (ACE): 25-70% of average (lowered from 40-80%)

NOAA Is calling for a 90% chance of a below-average hurricane season, the highest odds for a relatively quiet year assigned at any point since the outlooks began in 1998 (right after 1997, the last season that saw an El Niño comparable in strength to the current one.) NOAA reminds us: “For the U.S. and the region around the Caribbean Sea, tropical storms and hurricanes can and do strike even during seasons with El Niño.” For evidence, one need look no further than Hurricane Andrew, a catastrophic landfall in South Florida that occurred during El Niño. As with Andrew, any substantial hurricane this year would be most likely to intensify at or near subtropical latitudes, rather than in the deep tropical Atlantic, where the sinking air and strong shear produced by El Niño will be at their strongest.

For a recap of seasonal hurricane outlooks issued by other entities earlier this year, see the Jeff Masters post from May 27.

Bob Henson