The coolest air of the season so far was present over northern Iowa on Monday morning with low temperatures dropping into the mid 30s for some locations. A number of locations, including Mason City and Sioux City, reached a low of 37°F for the first time this season. The featured map looks into the frequency of having such a cool low temperature by the 23rd of September. The percentiles shown indicate the proportion of years on record for a given site with as cool a low temperature by the given date. So a value of around 50 would imply that the 23rd of September is about average, which is roughly the case for the northern half of the state.

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