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Tropical Depression Forms in the North Atlantic

By: Bob Henson 4:46 PM GMT on August 18, 2015

The fourth tropical depression of 2015 has developed in the central tropical Atlantic, and it could become the year’s first Atlantic hurricane by later this week. Advisories on TD 4 were initiated by the National Hurricane Center (NHC) at 11:00 am EDT Tuesday. The depression was located near 10.6°N, 36.5°W, moving west at about 13 mph. Top sustained winds were estimated at 35 mph, just below tropical-storm strength (40 mph). TD 4 is expected to become Tropical Storm Danny, the season’s fourth named system, by later tonight, according to the NHC. Interestingly, this is the first named system of the 2015 Atlantic season to begin its official life as a tropical depression. Ana began as a subtropical storm, while Bill and Claudette were tracked as tropical storms by NHC from the outset.

Figure 1. Infrared satellite image of a gradually organizing TD 4, collected by the GOES-East floater satellite at 1545 GMT (11:45 am EDT) Tuesday. Image credit: NOAA/NESDIS.

Figure 2. GOES-East floater visible image of TD 4 at 1545 GMT (11:45 am EDT) Tuesday. Image credit: RAMMB/CIRA.

The outlook for TD 4
As the first Cape Verde depression of the year, TD 4 formed well east of the Caribbean, where El Niño has been producing record amounts of vertical wind shear this summer. Even though the Atlantic tropics have been largely suppressed, as expected with this year’s strong El Niño event, TD 4 appears to be finding a window in time and space where some development is possible. Wind shear near the system is quite low, only around 5 knots (see Figure 3), and the region of stronger upper-level westerlies to the north of TD 4 could end up helping to support development by serving as an outflow channel. A massive area of Saharan dust and dry air lies just north of TD 4, but as mentioned in Tuesday’s morning’s NHC discussion, it appears that the depression is surrounded by enough moisture that it may be able to intensify even if it ingests some of this dry, dusty air.

Figure 3. Wind shear between upper and lower layers of the atmosphere across the North Atlantic. Lower values of shear, as shown above TD 4 (far right), support tropical development. Image credit: University of Wisconsin-CIMMS/NESDIS.

Figure 4. NHC’s outlook for TD 4 as of 11:00 am EDT Tuesday.

The 11 am EDT outlook from NHC brings TD 4 to hurricane strength by Friday morning and to Category 2 strength (sustained winds of 100 mph) by Saturday. This long-range forecast is consistent with the statistical models that show more skill than dynamical models at intensity prediction beyond 3 days. Of the two dynamical models most trusted for intensity forecasting, the HWRF has consistently called for TD 4 to develop into at least a strong Category 1 hurricane, while the GFDL has failed to develop TD 4, so the recently upgraded HWRF may end up closer to the mark in this case. Intensity prediction is still very challenging, so it is quite possible that TD 4 could be substantially weaker or stronger by this weekend than the current NHC forecast indicates. Over the last two years, the average 5-day error in NHC intensity projections was around 15 mph, or a bit more than half a category on the Saffir-Simpson Hurricane Wind Scale. This is substantially better than prior years: between 2000 and 2010, intensity errors at 5 days averaged more than 25 mph.

Looking ahead
TD 4 is many days away from any threat to the Leeward Islands. Most of the dynamical track models keep TD 4 moving west to west-northwest at a modest pace. The model consensus keeps TD 4 east of 50°W longitude until this weekend, when a building ridge to the north of TD 4 should help push it at a faster rate toward the islands. By that point, the system would draw on oceanic heat content that gradually increases along its path. It is far too early to predict with any confidence how much of a threat TD 4 might pose to the United States next week. Only a small change in trajectory this far out can have big implications for the track many days from now, and it remains to be seen whether dry air and dust will keep TD 4 from maximizing its potential for development.

Figure 5. Enhanced image from the MTSAT satellite, collected from the Northwest Pacific at 1532 GMT Tuesday, showing twin typhoons Goni (left) and Atsani (right). Image credit: NOAA/NESDIS.

Twin typhoons raking the open waters of the Northwest Pacific
Two intense, well-structured typhoons are churning their way toward Asia, posing no immediate threat to large land areas. Typhoon Goni, which surged to Category 4 strength on Sunday, has weakened somewhat as a result of an eyewall replacement cycle, although Goni remains a powerful, well-structured system. Now packing top 1-minute sustained winds of 115 mph, Category 3 Goni was located near 18.7°N, 132.9°E at 1200 GMT Tuesday, moving due west at about 18 mph. Typhoon Atsani is the tortoise to Goni’s hare: though it developed more slowly than Goni, it is now more powerful, with top 1-minute sustained winds of 140 mph. At 1200 GMT Tuesday, Category 4 Atsani was located near 17.0°N, 154.8°E, moving northwest at about 9 mph.

Goni is currently traveling near waters that were left slightly cooler in the wake of Typhoon Soudelor, which peaked at Category 5 strength in this region less than two weeks ago. Over the next couple of days, Goni will move into an area of progressively richer oceanic heat content (see Figure 6 below), which should support reintensification. Strong ridges over China and over the Northwest Pacific east of Japan will help keep Goni traveling on a due-west track until late this week, when the typhoon may attempt to recurve in between ridges. The latest JTWC outlook has recurvature occuring on a northward track just east of Taiwan, which would put the island on Goni’s weaker left-hand side; that would be good news for a population still reeling from Typhoon Soudelor. Meanwhile, Atsani is already traveling over rich oceanic heat content, and with upper-level shear relatively weak, Atsani is predicted in the latest JTWC outlook to reach Category 5 strength (around 160 mph) before recurvature and weakening begin over the weekend. It appears likely that Atsani will recurve before reaching Japan, but residents should not let their guard down just yet.

Bob Henson

Figure 6. Oceanic heat content over the western Pacific as of August 17. Image credit: NOAA/AOML.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.