That should be said with a wide open mouth, a grating tone of voice, and a swooping series of notes.
"Whaaaaat?" was how my sister and I read the Little Golden Book "The Three Little Kittens" when the mother cat questioned the kittens about the loss of their mittens.
It was formative. Once, when Bernie and I were in San Francisco, walking along the Embarcadero, I said, "Whaaaaat?" in just that way when he told me about some salacious gossip he'd heard. A passing family heard us, and the little girl with her parents imitated me, "Whaaaaat?" earning a cuff and reproof from her mother. I only hoped that she would continue to use the sound in the future.
So at 7:45 am, when my phone on my bedside rang, I woke groggily, and then jumped and sprang out of bed as a very loud "BOOOOOOOM!!!!" echoed through the room. "An explosion," I thought. "No, wait, thunder? Whaaaaat? In September??"
The phone was Alex trying to wake me to find out where the hell I had my car keys, as the windows of my car were open and it was POURING. I found the keys and flung them at her, then ran to the back patio to bring in the rotissierie oven, which should have been able to live outside until the end of October, at least.
Then we retired to the garage, opened the door, and watched the storms roll across the area, one after another, lightning and thunder and rain and wonderful clouds. Bernie, with only four hours of sleep since he had come home from work, was too fascinated by the weather to sleep, and got up for coffee and doughnuts in the garage as well.
It was beautiful, especially when, right after we'd all gathered in the garage to watch the sky show, a double rainbow appeared.
Neighbor kids and Lillian ran around in glee when it wasn't thundering, rejoicing in a warm and very unusual rain.