Showers and thunderstorms have frequented Iowa over these past few days. Those of you that frequently monitor NWS RADAR may have noticed how strangely these storms have moved, or rather barely moved. The lack of movement is due to very weak winds aloft as illustrated by the featured chart. The chart presents wind speeds from the Wednesday evening NWS Davenport sounding (0 UTC 18 Sep is 7 PM 17 Sep CDT) expressed as percentile values compared against other 0 UTC soundings from the site during the month of September. For example, the 850 hPa (a few thousand feet aloft) wind speed value of 1.5 m/s (~3 MPH) ranks at the 1.2 percentile, so roughly the lowest value over a three year period. So while the atmosphere over Iowa has been unstable with plenty of available moisture, the weak winds aloft have caused the storms to be pulse and not move much at all. Some of the storms were able to attach to some surface or boundary aloft, which allows them to organize a bit more and produce gusty winds and/or hail. The slow movement has also lead to localized heavy rainfall events. You may notice the larger wind speed values above 100 hPa (stratosphere) and wonder why it is different. In some regards, what happens above the tropopause (boundary between troposphere and stratosphere) is detached from what happens below. Additionally, these values can be misleading when the troposphere is anonymously warm or cold and the mandatory levels occur at significantly different altitudes aloft. Anyway, the moral of the storm is to blame the weak winds for the strange storm activity!

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