96L Slowly Organizing; Two Powerhouse Typhoons in NW Pacific

While Invest 96L takes its time developing in the tropical Atlantic, typhoons Goni and Atsani haven’t been wasting much time in the Northwest Pacific, both becoming major typhoons in the last 24 hours. Neither typhoon poses an immediate threat to land, although Goni could spell big trouble for Taiwan or neighboring areas by week’s end (see below).


Figure 1. Visible image of Invest 96L, collected by the GOES-Floater satellite at 1745 GMT (2:45 pm EDT) on Monday, August 17. Image credit: NOAA/NESDIS.

David vs. Goliath? 96L fights the El Niño factor
As of 2:00 pm EDT Monday, Invest 96L was located near 10°N and 31°W, moving west at about 10 mph. Located along a broad monsoon trough that coincides with the Intertropical Convergence Zone, 96L remains only loosely organized, with a large but unconsolidated area of showers and thunderstorms. Vertical wind shear is light (less than 10 knots), and 96L will encounter warmer sea-surface temperatures as it moves west-northwest (up to 28°C, or 82°F, by later this week), so the large-scale conditions favor gradual strengthening. The National Hurricane Center has been increasing the odds that 96L will develop: in its 8:00 am and 2:00 pm EDT updates, NHC gave the system a 50% chance of becoming a tropical depression in the next 48 hours and a 70% chance over the next 5 days. The RAMMB/CIRA Tropical Genesis Index is also maintaining high odds for development. Among the favored models for intensity, the 1200 GMT Monday runs of the statistics-based LGEM and SHIPS models, which rely heavily on climatology, bring 96L to Category 2 strength by Thursday. The dynamics-based HWRF and GFDL models, which simulate tropical systems within nested high-resolution grids, diverge on the future of 96L. The HWRF develops 96L into a Category 1 hurricane by Thursday, while the GFDL fails to develop 96L significantly. As we discussed in last week’s post on tropical cyclone modeling, HWRF features a dramatic increase in resolution this year, so it will be interesting to see if it correctly pegs the fate of 96L.


Figure 2. A large swath of dry air and dust from the Saharan Desert dust is sweeping across the Atlantic just to the north of Invest 96L. Image credit: University of Wisconsin-CIMMS and NOAA Huricane Research Division.


While it seems that 96L has a reasonable shot at becoming a tropical storm (which would be named Danny), it also faces some obstacles. Foremost is a huge area of dry air and Saharan dust that extends across the tropical Atlantic just north of 96L’s path. As the system grows in size and strength, it would become more likely to ingest some of the dry, dusty air, which would hinder shower and thunderstorm activity. 96L may also encounter an increasing amount of vertical wind shear as it approaches the longitude of the Leeward and Windward Islands this weekend, assuming it survives up to that point. Over the northern Caribbean, shear has actually lessened from the near-record values observed earlier this summer, although shear values of 20 to 40 knots continue to prevail across the southern Caribbean. The ever-strengthening El Niño favors westerly wind at upper levels across this region, though it’s possible that the relative lull in shear over the northern Caribbean will continue as 96L approaches. A weak upper-level low is forecast to become pinched off near the Bahamas, south of a building ridge over the northwest Atlantic; this low could become a growing influence on 96L’s track and intensity as it moves west of longitude 60°W.

So far, a rare storm-free August
It’s been a very quiet August so far in the tropical Atlantic. Chris Dolce at weather.com points out that if we go another two weeks without a named storm, this will be the first such August since 1997—which happens to be the last year that featured an El Niño ramping up as strongly as the current one. Prior to 1997, the last August without any named storms was 1961 (which wasn’t an El Niño year).



Figure 3. Enhanced infrared image of Typhoon Goni from the MTSAT satellite at 1901 GMT (3:01 pm EDT) Monday, August 17. Image credit: NOAA/NESDIS.

Goni goes gonzo
The rapid strengthening of Typhoon Goni was a sight to behold on Sunday. As with Typhoon Soudelor, Goni wrapped an intense core of convection (showers and thunderstorms) around a tiny eye, allowing it to intensify at a phenomenal rate. Goni zoomed from tropical-storm strength (1-minute sustained winds of 65 mph) to Category 4 status (135 mph) from 0000 GMT Sunday to 0000 GMT Monday, according to the Joint Typhoon Warning Center. Most of that strengthening occurred in just 12 hours (1200 GMT Sunday to 0000 GMT Monday), when JTWC bumped up Goni’s estimated sustained winds from 75 mph to 135 mph. (There have been even more rapid intensifications: in 1983, Typhoon Forrest went from 75- to 173-mph sustained winds in 24 hours.)

Over the last 12 - 18 hours, an eyewall replacement cycle has put at least a temporary brake on Goni’s headlong strengthening. The original tiny eye has dissolved within a larger, somewhat elongated feature, and JTWC has held the estimated peak winds at 135 mph. Goni’s overall structure remains potent, with ample strong convection at its core and extensive outflow aloft. Goni will be traveling in a low-shear environment over the next several days across very warm waters with relatively high heat content, so there is every reason to expect further intensification after the eyewall replacement cycle is complete. JTWC brings Goni’s sustained winds up to the super-typhoon threshold (130 knots, or about 150 mph) by Thursday, and I would not be surprised at all to see Goni reach Category 5 status, perhaps sooner than Thursday. Although Goni is projected to weaken somewhat after that point, it would still remain a formidable typhoon. Steering currents are also expected to weaken, which leaves a great deal of uncertainty over where and how Goni might affect land. The most recent JTWC outlook has Goni moving slowly northward just east of Taiwan as a strong typhoon toward the end of this week, a track roughly consistent with the 1200 GMT Monday runs of the operational GFS and HWRF models. Any slowdown in Goni’s progress near the mountainous terrain of Taiwan or nearby islands could lead to torrential, destructive rainfall. Taiwan’s worst natural disaster was in 2009, when Typhoon Morakot (which peaked at Category 2 strength) slowed just as it approached and moved over the island. Morakot brought Taiwan its all-time rainfall records for a 24-hour period (1403 mm or 55.2” at Weiliaoshan) and for a 48-hour period (2327 mm or 91.6” at Alishan).


Figure 4. Projected track for Typhoon Goni from 1200 GMT Monday, August 17, to 1200 GMT Friday, August 22. Image credit: Joint Typhoon Warning Center.


Figure 5. Infrared image of Typhoons Goni (left) and Atsani (right), collected by Japan’s Himiwari-8 satellite at 1920 GMT on Monday, August 17. Image credit: JMA/MSC.

While nearby Typhoon Goni dazzled typhoon watchers with its rapid strengthening on Sunday, the larger, slower-growing Typhoon Atsani more than held its own, transitioning from a minimal hurricane (75 mph) to high-end Category 2 status (110 mph) from 1200 GMT Sunday to 1200 GMT Monday. At 1200 GMT Monday, Atsani was located near 15.0°N and 158.0°E, moving to the northwest at about 7 mph. Having completed a well-predicted turn toward the northwest, Atsani should continue heading in that direction over the next several days, with a gradual increase in forward speed and at least gradual strengthening until at least Thursday. The latest JTWC outlook has Atsani peaking in tandem with Goni at 1200 GMT Thursday, both with sustained winds of around 150 mph. Japan will need to watch Atsani closely, as any delay in recurvature beyond the 5-day period would increase the potential risk.

Will the Fujiwhara effect influence the twin typhoons?
Given how close they appear on satellite, a sharp-eyed observer might wonder if Goni and Atsani will be influenced by the Fujiwhara effect, where two closely spaced hurricanes or typhoons begin rotating around a point midway between the two. In a website on the Fujiwhara effect, the Hong Kong Observatory explains that two tropical cyclones need to be located within at least 1350 km in order for the effect to be a major factor. Goni and Atsani are currently about 2000 km (1250 miles) apart, and their projected tracks only increase that distance, so the two typhoons will most likely remain free agents.

Bob Henson