Here's one weather forecast you can bank on: There'll be heavy rain every night through Sunday and on Saturday and Sunday afternoon, too.

That's the outlook for the Hobby Center, where Theatre Under The Stars is proving Mark Twain wrong. Twain said nobody "does anything" about the weather, but TUTS manufactures an onstage downpour at every performance of Singin' in the Rain.

Actually, it rains twice in each performance — first, as Michael Gruber (in the role immortalized on film by Gene Kelly) performs the title number to close the first act, and then as the ensemble reprises the tune in the finale.

What makes it rain? Well, forget everything you've heard about rising vapor and overstuffed clouds. It rains because there are two 250-gallon water tanks on the loading dock. Pipes carry the water to Hobby's Sarofim Hall, where it's pumped to 80 feet above the stage. The "rain" falls from two long, parallel pipes overhanging the set.

One, along the downstage perimeter, has a straight line of holes, creating a curtain of rain at the front of the set. The second, upstage, delivers a shower-head-style spray over the entire area.

The set's two-level floor collects the water, which is pumped back to the tanks.

"The number uses about 350 gallons at each show," says TUTS master carpenter Bob Guthrie. "The shorter version in the finale uses an additional 100 gallons. The tanks are equipped with heaters so the rain is 130 degrees, a comfortable, bath-water temperature."

Unlike all the show's other sets, this number's street scene is painted with water-resistant exterior paint. The vinyl dance floor is coated with clear acrylic vinyl, with sand mixed in to provide traction for Gruber's rubber-soled shoes. After he skidded a bit (and deftly recovered) on opening night, another coat was applied.

"It creates a sandpaper texture," Guthrie says. "For the sidewalk, the coating has crushed walnut shells. That color looks better than the black sand we used on the street."

The climactic title number comes right before intermission.

"It takes 10 minutes to remove that set and mop up (afterward)," Guthrie says. "The crew use mops and towels to dry everything off after the stage is cleared. Naturally, there's always some water that splashes out during the number."

Gruber loved the challenge of re-creating one of the best-loved numbers in movie musical history live at every performance — without the luxury of retakes.

"In terms of the basic steps, it's not hard," Gruber says. "But there are a lot of variables that make it tricky. Depending on the alignment of street and sidewalk with the pipes overhead, which can vary a bit with each performance, the heaviness of the rain can affect movement. If the line of heaviest rain is right over the path I take, it can make the umbrella break, as it did on opening night. It also affects how the umbrella flips when I toss and catch it."

The biggest risk is slipping as water collects, especially in the depressions where Gruber makes his puddle splashes.

"At the first preview I actually fell on my back," Gruber says.

The suit Gruber wears in the scene is not water-resistant, so he gets drenched to the skin at every performance. To prevent his microphone from shorting out, it's covered with a special cone, attached by waterproof tape.

"The water's warm, so it's not uncomfortable. The suit is a thin polyester that dries quickly and doesn't wrinkle or shrink. Actually there are two of them, for matinee days.

"The hat was a problem. At first I had a floppy felt hat that the water just destroyed. So we replaced that with a Totes rain hat."

This is Gruber's fifth time starring in Singin' in the Rain, which he first performed at New Jersey's Paper Mill Playhouse.

"It's always fun," Gruber says, "especially once you get the first week of performances under your belt and you've got your stamina up. The thing is to just roll with it and be as joyful as you can be. If something goes wrong, if you take a spill, you recover and go on. I first did the role when I was 28, so to me the physical accomplishment is thrilling — that I can still do it at 40."