He even has someone tracking an online radar for him.
"I keep weather.com up all the time," said Karen Poston, who works for Mr. Johnson's family business, Everett Johnson Roofing and Construction in Aiken.
Mr. Johnson said the reports give him "a heads-up peace of mind," noting that "about 2 o'clock in the afternoon they can pop up on you so quick."
Meteorologists say the phenomenon is pretty common with higher temperatures.
"Pop-up showers are the most common way to get rain in the summer," said David Schuetrum of the National Weather Service in West Columbia, S.C.
They result from moist air rising in the day's heat, dropping as rain and quickly dissipating.
"At a certain point in the warm afternoon, surface temperatures reach that magical point where the air starts to rise," Mr. Schuetrum said. Warm air holds "quite a bit of water," and as it rises it begins to form clouds.
"If the clouds are big enough and have enough water droplets, the droplets collect together and form even bigger drops," he said. "When the drops get heavy, they fall because of gravity."
"If there are no other factors to help promote and continue lift ... the shower will rain itself out and dissipate."
Mr. Schuetrum said many places have enough humidity and heat to have a shower nearly every day, but the moisture is not as abundant in this area.
They do occur frequently and haphazardly enough, though, to keep Mr. Johnson guessing.
"Lately, they've been coming dead north and running south, so we've been getting hit when Augusta doesn't," Mr. Johnson said.