NWS Des Moines issued two watches yesterday, a Severe Thunderstorm Watch and a Flash Flood Watch. The eventual storms yesterday did yield a few Severe Thunderstorm Warnings over the watch area and even a few tornadoes further east within Davenport NWS's area, but no Flash Flood Warnings. The featured chart looks at some IEM computed metrics on the conversion of watches into warnings. The red bars represent the overall frequency of converting an individual watch event into a warning (meaning getting at least one warning anywhere within the watch). The blue bars represent the individual county/zone conversion of a watch into a warning. For example, there is a 59% chance of getting at least one warning within the Flash Flood Watch yesterday and 19% frequency of an individual county within the watch getting one warning. The moral of this story is that not all watches are created equal. The actual size of the event the watch is for creates large differences in the frequencies. One item to note is that this plot is for a straight conversion and does not consider the situation of the eventual warning being of a different type. For example, some Winter Storm Watches become Blizzard Warnings.

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Tags:   nws   watch