@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!

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NOAA-20 VIIRS Day/Night Band (0.7 µm), Shortwave Infrared I04 (3.75 µm), Shortwave Infrared M13 (4.05 µm) and Longwave Infrared (11.45 µm) images (above) showed signatures of the ongoing lava flows from the Lower East Rift Zone of the Kilauea volcano on the Big Island of Hawai’i at 1225 UTC (2:25 am local time) on 18 June 2018.

Note how the central ribbon of hottest lava flow (which continues its active ocean entry) saturated the I04 3.75 µm image, causing a “wrap-around” effect to display cold brightness temperatures (white pixels) — although the M13 4.05 µm band has a lower spatial resolution, it saturates at much higher temperatures, and sensed brightness temperatures in the 480 to 557 K range. The Infrared images also showed evidence of steam clouds flowing southward over the adjacent offshore waters.

A webcam image from near Kapoho (PGcam) around the time of the NOAA-20 VIIRS images is shown below. The active Fissure 8 is near the center of the image.

VIIRS imagery and webcam capture courtesy of William Straka (CIMSS).

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Persistent convection over northern Wisconsin, Minnesota and upper Michigan late Saturday (16 June)/early Sunday (17 June) caused significant flooding.  The animation above shows GOES-16 ABI “Clean Window” Infrared Imagery from 0200-0600 UTC on 17 June.  Note the persistence of the cold overshooting tops over western Bayfield County in northwestern Wisconsin! A longer Infrared animation (0110-1200 UTC) which includes hourly plots of precipitation type (yellow) and SPC storm reports of damaging winds (cyan) is available here. 7-day precipitation departures in some areas were 4 to 8 inches above normal for that period (or 600% of normal).

This link from Wisconsin Emergency Management shows aerial pictures of the flood damage. Of note is the break in US Highway 2 to the west of Ashland WI.

The heavy rains also affected runoff into Lake Superior. MODIS imagery, below, from the MODIS Today site (also available from RealEarth: Link), shows considerable offshore flow of sediment on 19 June (a similar image from 18 June is here, with a toggle between the 2 days here).

A Landsat-8 False-Color image, below, showed pockets of flooding (darker blue) adjacent to the Nemadji River in Superior WI on the morning of 19 June — water also cover a portion of US Highway 2/53. The Nemadji River had crested in Superior at a record 29.5 feet on the evening of 17 June (NWS Duluth summary).

============================ Added 22 June ============================== 

NOAA’s Hydrometeorological Design Studies Center (Link) created an Exceedance Probability Analysis for this event at 6-hour, 24-hour and 72-hout time spans, available here (from this link). Probabilities suggest this was an exceedingly rare event.

The continuation of sediment flow into Lake Superior could be seen in a series of daily MODIS True-Color images here.

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1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (above) showed the development of thunderstorms over southern Wisconsin during the afternoon and evening hours on 16 June 2018. There were reports of hail, damaging winds and 1 brief tornado (SPC storm reports | NWS MKX summary). The pulsing of short-lived overshooting tops is evident in both Visible and Infrared imagery; above-anvil cloud plumes can also be seen in Visible imagery, drifting southeastward from the more robust overshooting tops. Note at 2000 UTC the boundary that was oriented approximately north-to-south, with widespread cumulus clouds to the west and generally cloud-free conditions to the east: this was a lake breeze boundary that had migrated inland from Lake Michigan. Many of the storms appeared to intensify as they interacted with this boundary.

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