This research was performed while the first author held a National Research Council Research Associateship Award at the National Severe Storms Laboratory. The authors thank the Storm Prediction Center’s Andy Dean for providing the convective outlook dataset. The constructive comments and suggestions made by the three anonymous reviewers helped improve the manuscript.
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The SPC also issues COs for days 4–8, but these products only indicate areas with a probability of severe weather that exceeds 30%. Since these COs differ substantially from those of days 1–3 and have been issued for a shorter period of time, they are not included in this study.
The National Weather Service currently defines a severe thunderstorm as one that produces a tornado, a wind gust of at least 50 knots (kt; 1 kt = 0.51 m s−1), or hail with a diameter of at least 1.0 in. Prior to 5 Jan 2010, the min hail size criterion was 0.75 in. (National Weather Service 2014), but was increased due to research suggesting that damage to older roofs begins with hailstones that have a 1 in. diameter (Marshall et al. 2002).
The annual mean value of each verification measure is calculated using a 2 × 2 contingency table that represents the totals for a calendar year.
As in Hitchens et al. (2013), 365-day running means are computed by constructing a 2 × 2 table that sums all 365 forecasts centered on each day. In the case of CSI values from outlooks, the 2 × 2 table associated with each day’s CSI value is used in the construction of the table for the 365-day period.
Beginning in 1999, the SPC began issuing “see text” areas within COs. These areas have no defined spatial extent, and are used when severe weather is expected but falls below the threshold of a slight risk.
Also included in the frequency of “improved skill” are consecutive outlooks with positive relative skill values that stayed the same. While not strictly an improvement, we believe a forecaster should be given credit for not changing a skillful forecast.