The past few weeks have been very active over Iowa and beyond with severe weather and flooding rains prompting the NWS to issue numerous warnings over the state. The featured chart looks into the normalized frequency of Tornado, Severe Thunderstorm, and Flash Flood warnings by week of the year. The top panel presents data for the five weather forecast offices (WFO)s covering Iowa and the bottom panel is nationwide. There's a number of nice annual signals that appear within the two plots. Since we are normalizing between these three warning types, drawing individual inferences needs some consideration. Generally, the plot shows the effect of vertical wind shear and storm motion speed. Tornado warnings have the lowest relative frequency during July and August as the aforementioned vertical wind shear is the weakest and wind/hail severe storms are most common. The cold season months having the highest relative tornado warning frequency is interesting as shear increases and larger scale storm systems become more common. One note of caveat for Iowa during the winter months is that warning counts are much smaller, so the data is more noisy.

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Tags:   ffw   tor   svr