Forecast spread was substantially reduced with decreasing IC spread for all examined supercell features in all three cases. In cases where storm lifetime is only 3 - 4 h an intrinsic predictability limit (IPL; i.e., the lead time beyond which forecast uncertainty can no longer be reduced by decreasing IC uncertainty) may not exist. Upon reaching the IPL, the forecast spread can remain modest, allowing the practical predictability limit (i.e., the lead time beyond which the forecast becomes too uncertain to be useful) to exceed the IPL. Comparing to previous work, initializing post-convective initiation (CI) rather than pre-CI substantially improves predictability. Similar to tropical cyclones, storm location was far more predictable than a given feature’s intensity. The practical predictability of strong low-level rotation (a tornado potential proxy) increased from 30-90 min to 3 h when IC spread was reduced by 50%.
Impacts of IC uncertainty within vs. outside of the storm were also explored. Forecast spread for all supercell features in all three cases benefited greatly early on from reducing intra-storm ensemble perturbations to zero (e.g., storm certainty). Forecasts benefited more from eliminating uncertainty in the storm environment (e.g., environment certainty) later in the simulations, but the results were more case-dependent. Storm certainty reduced forecast location spread for lead times of up to 2-3 h. Low-level rotation was more sensitive to intra-storm perturbations rather than environment perturbations suggesting that tornado forecasts out to lead times of 1.5 – 2 h would benefit more from improving initial storm state certainty.