Cumulonimbus and towering cumulus clouds suggest a strong updraft, but they are common to thunderstorms and do not necessarily mean the storm is severe. Therefore these clouds are not reported to the NWS. Spotters are still encouraged to observe towering cumulus clouds, especially if the storm is headed their direction. A cloud with hard, well-defined edges implies a healthy updraft and a strengthening thunderstorm. Be alert to heavy rain, frequent lightning, gusty wind and/or hail if a strengthening cumulonimbus is headed your way.