Recent observations of rare cloud-to-stratospheric lightning discharges suggest the events are inherently “slow-rising”, with the emitted energy reaching peak values in about 10 milliseconds. Applying a dipole radiation model, we demonstrate that the emitted radio wave energy from such slow-rising events is strongest below about 50 Hz, and possesses a significant rolloff at higher frequencies. In the analysis, various current distributions are considered in order to determine the effect on the radio spectrum. Near 10 kHz, the emission from cloud-to-stratospheric lightning is significantly reduced as compared to the typical cloud-to-ground return stroke, with amplitudes as much as 50 dB lower. This result may explain the lack of detection of VLF signals from recently observed long-lasting discharge events.