R44 pilot reports a precautionary off airport landing due to airspeed indicator failure and thunderstorms in the area.

Narrative

I took off [after] briefing and checking METAR; NOAA; and TFR; isolated storms rising to northwest; to not affect my flight to southeast along the shore line until my destination. Eighty miles from departure I got on airspeed indicator stuck. I made a decision to turn back to the nearest airport; but all were near the thunderstorm. As soon as I was flying back I got moderate turbulence; so I decided to make a precautionary landing on a football field. Without an airspeed indicator and turbulence I [made my] approach to landing on the football field. I flew up twice to check the area and there were no people in the field or obstacles that could be a safety factor. As a precaution; as soon as I was on the ground safely; I called to mechanic for instruction or repair; and also I called the Inspector from FAA that I met one day before just to report that I landed there as a precaution. As instructed I just opened the dash checked wire; and check the pitot tube over the main mast. I was there for 1:40 hours until thunderstorm and rain finished over the area; as soon there everything done and working well; I took off and flew back to base.