A U.S. Air Force Reserve Hurricane Hunters aircraft flew on a reconnaissance mission Monday to investigate the brewing tropical storm along the Georgia and South Carolina coastlines.

The Hunters were scrutinizing the weather system to give forecasters more data, noting in a tweet: "We have a crew on their way to #TD4 (Tropical Depression Four) before it makes landfall on the East Coast. Remember to heed warnings and stay weather aware!"

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"NHC tasked the storm yesterday and then upgraded it this morning as our aircraft was en route to TD4," Lt. Col. Mark Withee of the 53rd Weather Reconnaissance Squadron told Fox News. 

The aircraft is taking direct measurements from the storm environment, which Withee said validated the predicted strengthening and allowed the National Hurricane Center to upgrade it to Tropical Storm Danny.

He added: "What you see now is the aircraft flying a standard alpha pattern that we fly in tropical storms and hurricanes that gathers data on all four quarters of the storm. That data is relayed back to NHC via satellite in real time which allows them to integrate it into the forecasts and facilitates the issuing of weather watches and warnings where it is expected to make landfall."

Parts of the South Carolina coast were under a tropical storm warning Monday as forecasters expect the storm to strengthen before it makes landfall and drop large amounts of rain along the coastlines.

The storm formed close to South Carolina’s coast and had top sustained winds of 40 mph with higher gusts.

The tropical storm warning stretches from Edisto Beach, South Carolina, northeastward to South Santee River, according to the National Hurricane Center.

The NHC gave Tropical Depression Four the name Danny after the system strengthened further into a tropical storm; it is the fourth named storm of the 2021 Atlantic hurricane season.

At 3 p.m., the storm was about 45 miles southeast of Charleston, the hurricane center said. 

A weather station at Folly Beach — just outside Charleston — recorded a wind gust of 41 mph.

Maximum sustained winds were near 35 mph with higher gusts.

The fast-moving system, known as a "homebrew" storm because it develops not far off the United States shoreline, could reach the Georgia and South Carolina coasts as early as Monday night, forecasters said.

Tropical storm-force winds are expected to be felt along the South Carolina coast as early as Monday afternoon, the hurricane center said.

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The storm could produce between 1 and 3 inches of rain with higher amounts in some coastal areas.

The Associated Press contributed to this report.