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Warning Proportions

14 Jul 2025 05:30 AM
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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Flash Flood Warning Issuance

25 Jun 2025 05:30 AM
On Tuesday morning, National Weather Service Des Moines issued a Flash Flood Warning over a portion of southern Iowa due to an ongoing heavy rainfall event that brought three inches plus overnight. This warning was issued during the 6 AM hour and the featured chart today looks into the timing of the warning issuance based on text product archives dating back to 2002 for the office. The chart summarizes the number of warnings issued by hour and week of the year. The right hand side numbers present the hourly totals. The early evening hours are the most frequent time of day as late afternoon thunderstorms driven by the heating of the day have a number of hours to produce enough rainfall to lead to flooding. The early afternoon hours are the least frequent as storms are typically yet to get started by that time. The seasonality is clear as well with the largest frequencies from late May through July as storms tend to not move as quickly and thus produce larger rainfall totals over more isolated areas.

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Tags:   ffw   nwstext