It happened shortly after three o'clock this morning along Stagecoach Road.
The clerk told officers the man was armed with a semi-automatic handgun. Besides covering his head with a towel, the clerk said the man also had a piece of black fabric over his nose and mouth.
He came behind the counter, the clerk told police, and ordered her to open the register before grabbing money from the cash drawer then running outside.
Police describe the suspect as a black man, 20 to 30 years old, 5'8" to 5'10" tall, and weighing about 150 pounds.
He was wearing a white t-shirt, grey jeans, and white and black tennis shoes.