A cold front helped to fire late afternoon thunderstorms over the state on Thursday with some of these storms reaching severe limits. The NWS issued a few warnings valid for Polk County (Des Moines) last night and the featured chart looks at the frequency of Severe Thunderstorm Warning duration for the county. An individual warning is issued for a prescribed amount of time. This warning can be cancelled early if the conditions warrant. The two lines on the chart thus represent the frequency of warning duration at issuance and then the final duration based on any cancellation that was done. From the chart, about 50% of warnings are issued for a duration of 45 minutes or longer.

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