A number of severe thunderstorm watches have been issued today by the Storm Prediction Center (SPC). None of these in Iowa though. Anyway, the featured chart presents the daily frequencies of SPC issued watches and the ratio between the number of severe thunderstorm versus tornado watches issued. Both have clear annual signals with their respective peaks happening at different times of the year. The largest number of watches (dots) occur during the strongest battles of the air masses during the spring season. The bars shows the ratio of severe thunderstorm versus tornado warnings. This ratio peaks out during the first two weeks of August as temperatures are the warmest over the United States and the cool air masses are well north leading to less wind shear in the atmosphere.

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Tags:   spc   watch