First Cape Verde Storm of 2015 Possible This Week

A tropical wave in the central Atlantic has the potential to develop into a tropical storm over the next several days. Invest 96L was gradually organizing near 10.0°N and 28.3°W at 1800 GMT Sunday (2:00 pm EDT), moving west at about 15 mph. Showers and thunderstorms (convection) blossomed around the wave on Saturday night before weakening on Sunday. Convection often subsides during the daytime and redevelops at night over incipient tropical cyclones. Invest 96L has a fairly large shield of moist air around it, separating it from a large area of dust and dry area further north and west (see Figure 2 below).


Figure 1. Enhanced infrared imagery (4 km resolution) from the Meteosat satellite, showing Invest 96L at 2145 GMT (5:45 pm EDT) on Sunday, August 16. Image credit: RAMMB/CIRA.

The outlook for 96L
The National Hurricane Center boosted the odds of development for 96L on Sunday afternoon, with a 50% chance of a tropical depression within 48 hours and a 60% chance in 120 hours. The 0000 GMT Monday update of the Atlantic Genesis Index from RAMMB/CIRA (Colorado State University) shows a 96% chance that 96L will develop into a depression over the next 48 hours and a 99% chance by 120 hours. The main factors behind these high probabilities are converging air at low levels, climatology, and relatively low vertical wind shear. Shear affecting 96L is projected to average only about 9 mph over the next five days, compared to an seasonal average of 22 mph, although shear will be gradually increasing through that five-day period. Sea-surface temperatures along 96L’s path will increase from about 26°C to 28°C, close to the climatological norm for the region and more than adequate for development.



Figure 2. Water-vapor imagery from the GOES-East satellite, collected at 2215 GMT (6:15 pm EDT) on Sunday, August 16. Invest 96L is located at the far lower right of the image. Dry air covering much the central Atlantic (light patch near center of image) could eventually interfere with the evolution of 96L. Image credit: NOAA/NHC.

Both the GFS and ECMWF operational models, our most reliable models for tropical cyclone track, bring 96L along a nearly due-west track over the next five days, with a gentle west-northwest turn by late in the week. The latest GFS ensemble includes some runs that move the system more rapidly toward the west-northwest. If the operational models are correct, 96L will remain south of 15°N though the upcoming week, keeping it over warm waters and well away from midlatitude systems that might interfere with its development. An upper-level trough will be sagging from the western Atlantic toward the Bahamas, potentially affecting 96L toward the weekend, but no large, deep troughs will be in place to steer the system dramatically northward for at least the next 5 or 6 days.



The intensity forecast for 96L is a bit more uncertain. Of the two most trusted models for intensity, the 1200 GMT Sunday run of the GFDL model brings 96L up to weak tropical-storm strength by midweek before a subsequent weakening. The 1200 GMT Sunday run of the HWRF model failed to develop 96L, but the 1800 GMT run brings 96L to midrange tropical storm strength by late Wednesday. Given the overall favorable conditions and the apparent healthiness of 96L’s structure, I’d rate the odds as being at least 50-50 that we will have a tropical storm in the central Atlantic before Friday.

Cape Verde season gears up
If 96L becomes a tropical storm, it will be Danny, the fourth named storm of this year’s Atlantic season. Based on data from 1966 through 2009, the fourth named storm typically occurs around August 23, so a tropical storm this week would be more or less on schedule. It would also be the first named storm to form in the Cape Verde region, where some of the most powerful Atlantic hurricanes originate. An informal definition of the peak Cape Verde season is from August 20 to September 20, although tropical storm formation becomes increasingly common throughout August across the deep tropics of the Atlantic. Already, a string of increasingly potent waves has been shuttling from Africa into the eastern Atlantic. However, all of these waves have collapsed as they approach the western Atlantic and the hostile conditions fostered by El Niño, including very high wind shear over the Caribbean and relatively stable air over much of the North Atlantic. Tropical cyclone activity often tapers off prematurely in the Atlantic during El Niño years, so it may become even more difficult to get Cape Verde storms toward September and October. We’ll see if this week’s wave happens to encounter El Niño at a weak point.


Figure 3. An infrared image from the MTSAT satellite, collected at 2132 GMT (5:32 pm EDT) on Sunday, August 16, showing typhoons Goni (left) and Atsani (right). Image credit: CIMMS Tropical Cyclone Group.

Twin typhoons in the Pacific
As expected, a pair of typhoons has developed in the Northwest Pacific over the weekend, and at least one appear destined to make a run at super typhoon status. Typhoon Atsani, packing minimal-typhoon winds of around 75 mph, was located near 14.4°N and 159.4°E at 2100 GMT Sunday. Over the weekend, Atsani has been a lackadaisical mover, drifting west-southwest at 7 mph, but the Joint Typhoon Warning Center (JTWC) projects that Atsani will take a somewhat speedier course this week toward the northwest. The JTWC outlook brings Atsani to Category 4 strength in the next four days, but there is a good chance that the typhoon will recurve before threatening any major land areas. Further west, Typhoon Goni is set to pose a more serious threat to East Asia. With winds already above 90 mph at 1800 GMT Sunday, Goni was located near 16.0°N and 142.7°E, moving west-northwest at about 10 mph. While Atsani has a sprawling, less focused circulation, Goni already has a large, symmetric shield of strong convection and appears to be intensifying rapidly. The JWTC projects Goni to reach the threshold of super typhoon strength (130 knots or about 150 mph) in about three days, as it moves in the general direction of Taiwan and the northern Philippines. One very worrisome factor is the projected weakening of the upper-level steering flow later this week. As noted by the JWTC, this could leave Goni moving very slowly in the vicinity of Taiwan, a scenario that could lead to potentially devastating rainfall.

I’ll have a full update on the Atlantic and Pacific by midday Monday. For more on Invest 96L, including a wealth of imagery, see the update posted on Sunday afternoon by Steve Gregory. Steve has also posted an special in-depth look at the evolution of this year’s El Niño event, including comparisons with prior events and some of the mechanisms that brought it about. He’ll follow up on Monday with a Part II post on some of the forecasts for this El Niño event.

Bob Henson