By Bob Henson From Weather Underground

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.

For more on this story go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3073

UPDATE:

96L Slowly Organizing

By Bob Henson , 8:02 PM GMT on August 17, 2015 From Weather Underground

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 latitude 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.

For more on this story go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3074