1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed the formation of Potential Tropical Cyclone 3 in the Gulf of Mexico on 17 Jun 2021. A low-level circulation could be seen near the storm center on Visible imagery, but deep convection was only occurring at some distance from the center.

Even though the tropical disturbance was forming over warm waters (SST | OHC), a plot of modest to high amounts of deep-layer wind shear from the CIMSS Tropical Cyclones site (below) indicated that the rate of intensification might be relatively slow.

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A favorable orbit geometry on NOAA-20 on 17 June 2021 meant that portions of the upper Midwest received NUCAPS soundings 90 minutes apart. This is an ideal way to monitor destabilization on a day when SPC has predicted an enhanced risk of severe weather, as shown below and at this link.

Compare the sounding at 42º N, 92º W from ~1800 UTC to the one from ~1930 UTC, below.  Destabilization is apparent;  the later profile has a lower LCL and lower LFC, and moisture has increased.

This surface plot from 2000 UTC suggests that the NUCAPS profile at ~1930 UTC has a boundary layer that is too cool and too dry.  When the sounding values are edited — the lowest 3 layers were warmed and moistened — to better match the plotted observations, the sounding CAPE increased, and the LCL/LFC dropped even farther.

The convection developed rapidly.  The every-90-minutes toggle below (GOES-16 Visible Imagery at 2000, 2130, 2300 UTC) shows that, and the Day Cloud Phase Distinction RGB at the bottom (from 1901-2356 UTC) does too.

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A low-level water vapor (GOES-16 Band 10, 7.3 µm) infrared imagery animation, below, from CSPP Geosphere (here is a link that will show the animation in CSPP GeoSphere until about the end of June), shows evidence of a northwestward-propagating gravity wave that might have initiated convection.  This feature first interacts with convection over far southeastern Iowa, and then with a southwest-to-northeast line of convection that developed.

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Mostly clear skies over the Great Lakes early in the morning on 16 June allowed the VIIRS instrument on Suomi-NPP an unobstructed view of the surface waters.  Advanced Clear-Sky Processing for Oceans (ACSPO) Lake-surface temperatures, above, show the temperature distributions in the five lakes.  The warmest region is western Lake Erie where water temperatures are already in the 70s.  Saginaw Bay in Lake Huron (and Green Bay in Lake Michigan — although clouds at this time prevented a good view) are also in the 70s.  Much of Lake Superior remains around 40 F;  Lake Michigan is unusually warm — with a large region of 60+ — following an early-season heat wave (graph from GLERL’s Lake Statistics webpage).

Clear skies continued into the afternoon of the 16th;  the true-color image, below, derived from VIIRS data from NOAA-20 shows cloud over Lake Huron and some sun glint over Lakes Michigan and Superior. A zoomed-in view of Lake Erie (link) shows remarkable detail to the water color in the lake.

Suomi-NPP and NOAA-20 VIIRS data were downloaded at the CIMSS Direct Broadcast site and processed using CSPP software. Great Lakes SST fields and VIIRS imagery over CONUS are available via LDM feed to National Weather Service Offices. True-Color imagery can be found at VIIRS Today and at the CIMSS direct broadcast ftp site.

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True-color imagery derived from GOES-16, above, shows a low-level cyclonic circulation associated with then Tropical Depression #2 to the east of Cape Hatteras over the western Tropical Atlantic. The satellite imagery suggests strong southwesterly shear: ongoing deep and vigorous convection is far removed to the east and northeast of the near-surface circulation visible in the imagery. Indeed, a shear analysis from the CIMSS Tropical page, below, shows values in excess of 50 knots over the storm. Despite the strong shear, this depression was upgraded to Tropical Storm Bill at 0300 UTC on 15 June 2021.

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Scatterometry winds helped with the intensity determination.  The 2328 UTC 14 June overpass, below, shown in a toggle with the ABI Band 13 infrared imagery, shows the circulation with 40-knot winds on the south side.  Included in the image is the 1056 UTC 15 June image, showing the quick northeastward progress of the storm, and also showing the degradation in the storm’s symmetry.

Suomi-NPP overflew the storm at 0626 UTC on 15 June. The Day Night Band imagery, below, overlain on top of derived ACSPO SSTs and toggled with the VIIRS I05 11.45 µm window channel imagery (obtained from the Direct Broadcast site at the University of Wisconsin-Madison), show evidence of lightning northeast of the storm center. SSTs within the core of the Gulf Stream are 81 F.  The core of the storm at 0626 UTC as depicted in the infrared imagery below is far more symmetric than in either of the two times, 2326 UTC on 14 June, and 1056 UTC 15 June, in the toggle above.

Refer to the National Hurricane Center website for more information on Bill.

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