There are several products to reference when planning to avoid thunderstorms. In the United States, the Aviation Weather Center (AWC) produces Traffic Flow Management (TFM) Convective Forecasts (TCF). These are high-confidence graphical representations of forecasted thunderstorms that meet specific criteria including coverage, intensity, and echo top height. TCFs are issued every 2 hours and valid at 4, 6, and 8 hours after issuance time.
Additionally, The Storm Prediction Center (SPC) issues convective outlooks five times per day that include the probability of severe thunderstorms. The categorical risks can range from general thunderstorms with no severe weather expected to a high risk where widespread severe thunderstorms are expected.