This image was taken on September 21st at 7:03pm facing south-west from the Baseline and Broadway intersection in Boulder, CO. I believe this to be an altocumulus cloud based on it’s appearance and an estimate of its height. This is supported by the skew-t diagram which shows inversion in the 2-5km range and a slightly negative lifted index, indicating some slight instability at this altitude

Skew-T:

Unedited image: