Tropical Storm Danny is forecast to intensify into the season’s first hurricane over the weekend, but it faces significant hurdles. (NOAA)

Tropical Storm Danny gained some strength and speed overnight and could reach the Leeward Islands in the eastern Caribbean early next week. Although Danny is forecast to become a Category 1 hurricane over the weekend, the long-term future of the storm is questionable.

Danny’s sustained winds increased overnight to about 50 mph, which makes it a moderately strong tropical storm, tracking west across the tropical Atlantic at about 12 mph. Danny has lost much of its strong thunderstorm activity, and its low-level circulation is plainly visible on satellite — something that does not indicate an intensifying tropical cyclone.

The official forecast from the National Hurricane Center indicates that Tropical Storm Danny could become the first hurricane of the 2015 season on Thursday. The center is forecasting Danny’s winds to increase to 90 mph on Saturday, which would make the storm a Category 1 hurricane on the Saffir-Simpson scale. But beyond that, Danny faces significant environmental hurdles.


(National Hurricane Center)

Although the storm has been intensifying and will likely continue to do so, dry air might be the biggest challenge Tropical Storm Danny will need to overcome.

[Tropical trouble may be brewing for Hawaii]

On Wednesday, Danny was still located along a moist atmospheric boundary called the monsoon trough, and immediately to the north of that boundary lies a substantial plume of Saharan air, characterized by dusty, low humidity conditions in the low-mid levels of the atmosphere. The lack of moisture limits the growth and intensification potential of the storm. Some storms can remain relatively isolated from nearby dry air, while others ingest it and then weaken.


Visible satellite image of Tropical Storm Danny from 8:15 a.m. EDT, with the approximate boundary of Saharan air outlined in yellow. (NASA)

The three most critical parameters for tropical cyclone intensity are wind shear, sea surface temperature and mid-level humidity. In order for storms to intensify, they need low wind shear (less than 20 knots), warm sea surface temperatures (more than 26 degrees Celsius) and high humidity (more than 60 percent).

The forecasts of all three of those parameters are shown below from three different models: the GFS, HWRF and GFDL. These models suggest that wind shear and sea surface temperatures can likely support a hurricane, but the humidity around the storm will take a nose dive after Thursday, reaching extremely low levels by early next week. The intrusion of extremely dry air is unanimous among these models and might prove to be the death of Danny.


Intensity and track forecasts from three models (GFS, HWRF and GFDL), as well as the storm’s large-scale environment according to each model. (CIRA/RAMMB)

To see how previous storms have tracked in this region, we used the track from the National Hurricane Center to find those that have tracked close to Danny’s forecast. Each storm shown here tracked within an arbitrary distance of 200 miles from Danny’s forecast location.

There are numerous notable landfalls in this lot, including Hurricane Dora (1964); Hurricane Frederic (1979); Hurricane Andrew (1992); and Hurricane Dean (2007). There’s a very wide spread of storms that have tracked through Danny’s location — some of which recurved out into the open Atlantic without much landfall, and others that have stayed south and tracked straight through the Caribbean.

Interestingly, Hurricane Arlene of 1963 might be the most applicable analogue for Tropical Storm Danny. Arlene is the only example in this group that formed during a strong El Niño, although not quite as strong as the current one. El Niño tends to decrease hurricane activity by increasing wind shear over the Atlantic Ocean in the summer months, but Arlene (and Andrew in 1992) proves that hurricanes can form in El Niño years, despite the rotten environmental conditions.

It’s too soon to tell where Danny will track next week — or whether it will be a storm at all beyond Sunday.


Past August tropical cyclones that passed within 200 miles of Danny’s current position AND its three- and five-day forecast positions (all four criteria must be met to be counted).