A Louisiana mayor fears torrential rain, not a tidal surge, will overwhelm his city's drainage pumping system

The Long-Allen Bridge crosses the Atchafalaya River in Morgan City, Louisiana.

(CNN)It's all "blue sky and calmness," Mayor Frank "Boo" Grizzaffi tells CNN less than a day before Tropical Storm Barry is due to arrive in Morgan City, Louisiana.

The year's first tropical system is due to land Saturday west of the city of 12,000 people, ushering in a 3 to 6 foot tidal surge on the already-high Atchafalaya River. A flood warning has been issued by the National Hurricane Center.
Hurricane force wind gusts are expected to peak around 4 a.m. Saturday in Morgan City, about 70 miles southwest of New Orleans.
    But it's the very heavy rain, which could hang around awhile, that's worrying Grizzaffi.

    Not-so-calm before the storm

    "We're buckled down," says the mayor, who is concerned that his city's drainage pump system will be overcome not by the tidal surge but by torrential rain -- 20 to 30 inches predicted this weekend.
    As in other towns in south Louisiana, the entire city's drainage system relies on pumps to move water off the streets, so a downpour, if it is swift and hard, could spell trouble.
    "We can handle the first 5 inches, but after that, we can pump 1 inch per hour. If we get rain greater than that, it will exceed our capacity to pump it out," Grizzaffi notes.
    As of Friday morning, additional pumps from the state are on their way, but the extra pumping power may still not provide enough capacity to keep up with the deluge.
    Meantime, other preparations are happening.
    Water rises around Morgan City, as seen in this image captured via drone.
    "By noon, we'll have the city shuttered down," Grizzaffi says. That includes preparing to close the floodgates that stay open till the last minute as major oil companies that operate on nearby waterways shut down.
    Evacuations are voluntary and apply only to people living near Morgan City and south of the intracoastal waterway.

    Public workers prepare

    For city workers, it's "all hands on deck," Grizzaffi explains. At some point, non-essential employees will go home to prepare for the coming storm while a 24-hour crew will "man the phones" over the weekend at City Hall.
    Among the city employees still at t