The student pilot reported that during the landing, as he began to flare, the airplane was, "hit with the first blast of wake turbulence", from a helicopter that was hovering on the helipad to the left of the runway. The airplane landed hard, exited the runway to the right, and collided with a runway sign. According to the meteorological terminal aviation routine weather report (METAR) at the airport, the temperature was 99 degrees Fahrenheit and the wind was variable at 4 knots. The helicopter pad was located over 400 feet to the east of the location that the pilot reported encountering the blast of wake turbulence. Due to the distance of the helicopter pad from the event location, the ambient temperature, and lack of significant wind, it is likely the student pilot encountered thermal related turbulence on short final.