Friday morning visible satellite loop centered north of the Bahamas. (NOAA)

Update: This system has developed into Tropical Depression Two and the National Hurricane Center is now issuing advisories. They expect the cyclone strengthen modestly into Tropical Storm Bonnie and make landfall in the Carolinas on Saturday night or Sunday morning. A tropical storm warning has been issued for the entire coast of South Carolina. Rain and associated flooding will be the biggest impacts.

Original post:

An area of thunderstorms has been bubbling near the Bahamas this week, and is poised to become the season’s second Atlantic tropical cyclone. If it strengthens into a storm, it will be named Bonnie, and 2016 would join 2012 as the only years since reliable satellite coverage began to have two named storms form prior to June 1.

Hurricane Hunters plan to fly into the disturbance early this afternoon to determine whether or not it has developed a surface circulation. If it has, the National Hurricane Center will begin issuing advisories.

This system sparked earlier this week along a stalled cold front that was draped across western Cuba and the Bahamas. It has been festering nearly stationary since then. Back on May 20, the newly-upgraded GFS forecast model was the first to predict this — quite remarkable for a tricky one-week forecast!


A 10-day forecast of surface wind and pressure from the GFS model. The forecast was made on May 20. (tropicaltidbits.com)

Then a day later, the Canadian global model accurately predicted the genesis timing and location of this storm. (Though, to be fair to the other models, the Canadian has a reputation for developing every sneeze into a hurricane.) Two days after that, the European model finally joined the crowd and began showing the same scenario.

The latest model guidance is in excellent agreement that this disturbance will develop into a tropical or subtropical storm — albeit not a strong one. Subtropical storms are a type of hybrid system, but they carry the same impacts as tropical storms.

Environmental conditions are marginal now, but they are expected to improve over the next 24 hours. Vertical wind shear is forecast to decrease dramatically on Saturday, and it will cross over the core of the warm Gulf Stream on Saturday as well. There is a chance it could reach tropical storm intensity and get the name Bonnie. At the very least, it will be Tropical Depression Two.


Track forecasts made Friday morning by a selection of regional and global dynamical models. (UAlbany)

The models also agree on the track: slowly moving to the northwest, then making “landfall” on South Carolina early Sunday morning. However, for such a weak system, landfall sounds more ominous than it really is — it will mostly involve a few days of moderate to heavy rain in the mid-Atlantic states, primarily South Carolina. Wind and storm surge impacts should be minimal.


Accumulated precipitation swath over the next five days from the morning WPC forecast. (NOAA)