The circulation of the low pressure system designated as Invest 91L strengthened slightly on Thursday.  At 8:00 p.m. EDT on Thursday the center of Invest 91L was located at latitude 27.4°N and longitude 70.9°W which put it about 410 miles (660 km) east-northeast of Great Abaco Island, Bahamas and about 675 miles (1090 km) south-southeast of Charleston, South Carolina.  Invest 91L was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to near 45 m.p.h. (70 km/h).  The minimum surface pressure was 1013 mb.  The center of Invest 91L was near NOAA buoy 41047 which reported observations consistent with those numbers.

The organization of Invest 91L improved slightly on Thursday.  A few thunderstorms formed northeast of the center of circulation and they persisted throughout the day.  However, there are few thunderstorms in other parts of Invest 91L.  The circulation became slightly more circular although it is still elongated.  The surface pressure decreased slowly, but steadily throughout the day.

The environment around Invest 91L is only marginal for intensification.  It is over water where the Sea Surface Temperature is near 26°C to 27°C.  However, an upper level low near Cuba and an upper level ridge near the east coast of the U.S. are combining to create southerly winds over the top of Invest 91L.  Those winds are causing moderate levels over vertical wind shear which is inhibiting the development of the system.  The magnitude of the wind shear decreased during the past 24 hours and it could decrease a little more on Friday as the upper level ridge moves farther east.

If more thunderstorms develop near the center of circulation, the system could be classified as a tropical depression or tropical storm.  The National Hurricane Center (NHC) is indicating that there is an 80% probability that the Invest 91L will become a tropical cyclone or subtropical cyclone within 48 hours.  NHC has tentatively tasked a reconnaissance plane to investigate the system on Friday.

The vertical wind shear is limiting the strongest part of the circulation of Invest 91L to the lower half of the troposphere.  Thus, the system is being steered toward the west-northwest by a surface pressure system over the western Atlantic Ocean.  That general motion is expected to continue with a slow northward turn.  On its anticipated track Invest 91L could approach the southeast coast of the U.S. in 48 to 72 hours.

The primary risks with Invest 91L remain coastal erosion, rip currents and locally heavy rainfall.  Some wind damage will be possible, but it is likely to be minimal.