While thunderstorms can form at any time of the year, the best chance is during the spring and early summer. This is because thunderstorms thrive in warm, moist air, which is most available during these times of year. Also, shearing winds and other factors that help storms get started are most prevelant in April, May and June.
Gusty thunderstorms can also occur in the early fall. This is because, like spring, warmer air is switching places with cooler air during autumn and storms usually form along the zones separating the two air masses. Additionally, the jet stream is often in the area, which helps to promote uplift in the atmosphere, making it easier for strong storms to form.