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The product of convective available potential energy and precipitation rate is used as a proxy for U.S. cloud-to-ground (CG) lightning. The proxy is computed from numerical weather prediction model outputs and thus provides a simple method for producing CG lightning threat forecasts. Here three types of forecasts are considered: number of CG flashes, spatial extent of CG lightning occurrence, and lightning/no lightning maps at 1 degrees x 1 degrees resolution. Forecasts have statistically significant skill for lead times up to 15 days depending on the type of forecast. Verification of the forecasts over the United States provides a predictability baseline against which other approaches can be compared.

Plain Language Summary Many studies of weather predictability have examined variables related to atmospheric circulation. Here we examine the predictability of cloud-to-ground (CG) lightning, which is the defining feature of thunderstorms. Forecasts for numbers of CG flashes and their spatial extent over the United States are constructed from the output of a state-of-the-art numerical weather prediction model and compared with observed CG lightning. The skill of the lightning forecasts is statistically significant for lead times up to 15 days, depending on the type of forecast, time of year, and region. The proxy-based forecasts provide a baseline for the predictability of thunderstorms against which other approaches can be compared.