@CIMSS_Satellite: Any idea what the relatively small, darker area/depression was going over Grundy County (west of Waterloo) from 1232z to 1342z? Most prevalent at 1312z. Shows up in all GOES bands. Channel 7 also shows a corresponding darker area just north of that area.

— Andrew Ansorge (@ajsorge) June 18, 2018

It’s always good to get a question that lends itself well to the “What the heck is this?” blog category. The answer, as is often the case, relies on an examination of imagery from a variety of GOES-16 ABI bands.  To begin, note the darker feature seen on 1-minute Mesoscale Domain Sector GOES-16 “Blue” Visible (0.47 µm), “Red” Visible (0.64 µm) and Near-Infrared “Vegetation” (0.86 µm) images (below), which was moving northeastward across eastern Iowa and passing just to the west of Waterloo (KALO) on the morning of 18 June 2018.

To explore the initial hypothesis that this might be a shadow from a higher-altitude cloud feature, GOES-16 Near-Infrared “Cirrus” (1.37 µm), Mid-level Water Vapor (6.9 µm) and Upper-level Water Vapor (6.2 µm) images were examined (below), which did indeed reveal a small cloud element aloft that was drifting in the same direction as the darker feature seen above.

Finally, a comparison of GOES-16 Near-Infrared “Cirrus” (1.37 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.3 µm) images (below) showed that this small (and likely thin) high-altitude cloud exhibited no signature in the Shortwave Infrared, but did exhibit a 10.3 µm brightness temperature as cold as -20ºC (cyan enhancement) at times.

12 UTC rawinsonde data from Davenport, Iowa (below) showed southwesterly winds and an air temperature just below -20ºC at an altitude of around 9.6 km.

Thanks to Andrew Ansorge (NWS DMX) and Rich Mamrosh (NWS GRB) for alerting us to this interesting feature!