Why Tropical Storm Ana will likely weaken

Tropical Storm Ana, the first named storm of the 2015 Atlantic hurricane season, was moving toward the U.S. Southeast coast with maximum sustained winds of 60 miles per hour.

Winds from Tropical Storm Ana, the first named storm of the 2015 Atlantic hurricane season, picked up onshore in the Carolinas on Saturday and beachgoers were warned of expected dangerous rip currents and heavy rain.

Ana was moving slightly faster toward the U.S. Southeast coast with maximum sustained winds of 60 miles per hour (95 km/h) after it transitioned overnight into a tropical storm, the Miami-based National Hurricane Center said.

The storm was positioned Saturday morning about 115 miles (185 km) south of Wilmington, North Carolina, and about 100 miles (165 km) southeast of Myrtle Beach, South Carolina, the weather agency said.

Moving toward the north-northwest at 5 mph (7 km/h), the center of the storm was forecast to be "very near" the coasts of North Carolina and South Carolina by Sunday morning, according to the hurricane center.

It is predicted to begin weakening as it moves over cooler waters near the coastline.

The National Hurricane Center 11 a.m. Saturday advisory says:

Ana will be moving over the cooler waters to the northwest of the Gulf Stream latertoday, and water vapor imagery shows a belt of upper-level northerly flow advancing toward the tropical cyclone.  The decreasing sea surface temperatures and increasing northerly shear should cause Ana to weaken as it nears the coast.

 Global models continue to predict that the blocking mid-level ridge to thenorth of Ana will shift eastward and weaken over the next couple of days.  These models also show a broad trough moving from the central to the eastern U.S. over the next 72 hours or so.  This should result in the cyclone turning northward and north-northeastward with a gradual increase in forward speed.

The 11 a.m. National Weather Service advisory forecasts that top winds will drop to 45 m.p.h. within the next 24 hours. 

Tropical storm warnings were in effect for areas along a 275-mile (443 km) swath from South Santee River in South Carolina to Cape Lookout in North Carolina, at the southern end of the Outer Banks.

The U.S. National Weather Service office in Wilmington said a wind gust of 40 mph was recorded at the local airport, the highest its staff had observed so far on land from Ana.

Emergency management officials and forecasters urged caution for beachgoers and boaters, saying it was best to stay out of the water when the rip currents were strongest.

The hurricane center said the storm would bring anywhere from one to five inches of rain (2.5-12.7cm), and the storm surge could bring flooding of up to two feet (61 cm) in some coastal areas.

Ana's formation is the earliest appearance of a named storm in the Atlantic since a previous incarnation of Subtropical Storm Ana on April 20, 2003, said Jeff Masters, chief meteorologist for Weather Underground, a commercial weather service.

The Atlantic hurricane season typically runs from June 1 through Nov. 30. (Reporting by Harriet McLeod; Writing by Colleen Jenkins; Editing by Toby Chopra)