Tropical storm warnings have been issued for the Southeast coast

By Hannah Gard and Haley Brink, CNN

Tropical Depression Four formed Monday morning off the coast of South Carolina and Georgia.

Four is forecast to make landfall as Tropical Storm Danny, the fourth named storm of the 2021 season and the second to impact the United States.

Tropical storm warnings have been issued for parts of the South Carolina coastline, from Edisto Beach to the South Santee River. That zone includes Charleston.

“A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area, in this case within the next 12 hours,” according to the National Hurricane Center.

Four has a 70% chance of formation within the next 48 hours, according to the NHC. A hurricane hunter aircraft is scheduled to fly into the system Monday afternoon to provide more data on its strength.

Tropical Depression Four has sustained wind gusts of 35 miles per hour. Winds are expected to strengthen to 40 miles per hour before landfall. The criteria for a tropical storm classification is sustained winds of 39 to 73 miles per hour.

It is moving west-northwest at 15 to 25 miles per hour and will bring a few inches of rainfall to the Georgia and South Carolina coasts through Tuesday.

“Impacts associated with it should come in the form of localized heavy rainfall, gusty winds, enhanced rip currents, and a low threat of tornadoes,” the National Weather Service in Charleston said.

The Storm Prediction Center (SPC) is tasked with monitoring the tornado risk. If the tropical depression intensifies, the tornado threat could increase as it makes landfall, prompting a marginal risk of severe weather to be issued.

“For now, the potential appears too conditional and low for a categorical risk area,” says the SPC.

The bulk of the impacts from the storm will lie in southeastern Georgia as thunderstorm activity ramps up.

Some areas in southern South Carolina and southeastern Georgia could see up to three inches of rainfall. The flash-flood threat could be mitigated by the speed of the system, as it is expected to move quickly inland without lingering.

A marginal risk of excessive rainfall has been issued by the Weather Prediction Center (WPC) for the coastal regions of Georgia and South Carolina due to the rainfall potential from Four.

“Guidance continued to show rain rates of 1-2 inches per hour, which could produce isolated flash flooding, especially in any coastal urban areas,” the WPC said on Monday.

Conditions are expected to improve overnight into Tuesday as the system weakens as it moves father inland.

Typical early season tropical activity

Hurricane season officially began on June 1, but storms in the early season have become more persistent.

The typical initiation point for these early season Atlantic Basin storms in June is within the Gulf of Mexico, where moisture is ripe, according to the NHC. As July rolls around, initiation in the Atlantic and off the northeast coast of South America becomes more prevalent.

Tropical systems don’t typically develop off the East Coast in June; that is more likely in July and onward.

As the season progresses into August, storm initiation begins to shift toward the mid-Atlantic and off the western coast of Africa, where tropical waves make their way northwestward, typically strengthening as they enter the Caribbean. Hurricanes become more likely, and much more frequent.

The peak of hurricane season typically lies in September for the United States, when landfalling hurricanes in the Gulf and up the East Coast ramp up significantly. Initiation happens within the Gulf of Mexico, as well as off the west coast of Africa.

Separately, a tropical disturbance in the central Atlantic has been producing thunderstorms and showers since last week.

Tropical storm initiation in the mid-Atlantic is fairly rare in the end of June, with one recorded initiation point occurring in the area from 1851-2015, according to the NHC.

Formation likelihood is low for the next 48 hours at 20%, with a 40% chance of formation over the next five days.

The system is expected to slowly develop over the week, moving northwestward at 20 miles per hour. It will likely make its way to the Lower Antilles by Wednesday night.

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