So far this warm season, Iowa has mostly avoided oppressive levels of humidity coincident with the numerous days of abnormally warm temperatures. The lack of humidity has been the subject of recent IEM features regarding low dew points during much of May. This lack of humidity has kept heat index values in check with the year to date maximum for Des Moines being about 90°F this past Saturday. The forecast for today has the first real muggy heat of the season arriving with heat index values expected to exceed 100°F for much of the state. This would represent a ten degree jump in year to date maximum heat index value and that jump is the subject of today's IEM feature info-graphic. Based on unofficial hourly archives, the table presents the largest jump in heat index for the first 100+°F heat index of the year. If the forecast for Tuesday works out, it would be the third largest jump on record since about the early 1930s.

Update: Based on the hourly reports, Des Moines topped out at a heat index of 96°F. So this featured jump did not materialize!

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