Potential Tropical Cyclone 7 was located between the Bahamas and Florida during the pre-sunrise hours on 03 September 2018. Toggles between VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images from NOAA-20 at 0636 UTC and Suomi NPP at 0726 UTC are shown above (courtesy of William Straka, CIMSS).

The storm became better organized and increased in intensity, and was named Tropical Storm Gordon at 1205 UTC. Animations of GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) (below) showed Gordon as it moved across far southern Florida (where heavy rain and flash flooding occurred) and into the Gulf of Mexico during the daytime hours.

===== 04 September Update =====

1-minute Mesoscale Domain Sector GOES-16 “Red” Visible (0.64 µm) images (above) and “Clean” Infrared Window (10.3 µm) images (below) showed a series of widespread deep convective bursts within the northeast quadrant of the storm as it moved northeastward toward the Gulf Coast.

The GOES-16 Rainfall Rate/QPE product (below) indicated rainfall rates of 2-3 inches per hour were possible from this convection, peaking in the 3-4 inch per hour range just after 1300 UTC. However, Infrared cloud-top brightness temperatures warmed dramatically as the convection moved onshore after about 22 UTC — and the Rain Rate product responded accordingly, with a significant decrease in hourly intensity.

Metop-A ASCAT surface scatterometer winds of 39 knots were sampled just northeast of the storm center at 1616  UTC (below).

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* GOES-17 images shown here are preliminary and non-operational *

1-minute Mesoscale Domain Sector GOES-17 “Red” Visible (0.64 µm) images (above) showed the dissipation of fog and stratus across the San Francisco Bay area on 03 September 2018. However, as most of the fog/stratus dissipated or retreated westward toward the coast in response to daytime heating, a persistent narrow ribbon of cloud was seen flowing east-northeastward through the Golden Gate and back into San Francisco Bay — with this flow of moist marine layer air leading to the formation of a new patch of low cloud as it encountered the higher terrain of the Berkeley Hills / San Pablo Ridge. This fog/stratus behavior was also observed using 1-minute GOES-14 Visible imagery in August 2016.

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A low moved eastward across the Beaufort Sea on 01 September 2018, bringing a cold front southward across the North Slope and Brooks Range in far northern Alaska (surface analyses). A sequence of Suomi NPP VIIRS Infrared Window (11.45 µm) images (above) showed the clearing of high/cold clouds in the wake of the frontal passage.

The upslope flow of cold air helped to generate accumulating snowfall across that region — prompting a Winter Storm Warning to be issued for the eastern Brooks Range, where 4-8 inches was expected at higher elevations — and some of the resulting snow cover was seen on a Suomi NPP VIIRS Day/Night Band (0.7 µm) image at 1415 UTC or 6:15 am local time on 02 September (below). A comparison with the corresponding VIIRS Infrared Window (11.45 µm) image and Topography is also shown. The darker shades of brown on the topography image correspond to elevations of 6000-8000 feet in the Brooks Range.

Later in the day on 02 September, additional clearing of patchy low clouds revealed more of the snow cover, as seen in a toggle between VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Topography images (below). Supercooled water cloud droplets are efficient reflectors of incoming solar radiation, making patches of low cloud appear darker shades of gray on the Shortwave Infrared image (helping to identify low clouds over snow cover).

At 2124 UTC (or 1:24 pm local time), a 30-meter resolution Landsat-8 False Color Red-Green-Blue (RGB) image viewed using RealEarth (below) provided a more detailed view of a portion of the snow cover. Snow and ice appear as shades of cyan in this type of RGB image — which is created by combining Landsat bands 6 (1.61 µm), 5 (0.865 µm), and 4 (0.655 µm) as Red, Green, and Blue — and numerous small ice floes can also be seen off the coast.

On a side note, farther to the west an interesting pattern of contrails was seen in VIIRS Visible and Infrared Window images at 2046 UTC (below). On the Visible image, note that the darker contrail shadows cast onto the surface are displaced about 15 miles to the north (due to the low sun angle); the contrail features exhibited Infrared brightness temperatures of -10 to -15ºC. These contrail patterns were generated by military aircraft performing training exercises: similar features have been noted over California and North Dakota.

A curved portion of one of these contrails was seen on web camera images looking south from Atqasuk (below).

Webcam from Atqasuk earlier. @AlaskaWx pic.twitter.com/VJbbLaqtJm

— Brian Brettschneider (@Climatologist49) September 3, 2018

 

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A series of large Mesoscale Convective Systems (MCS) developed across Nebraska and Iowa during the nighttime hours before sunrise on 01 September 2018, which produced large hail and damaging winds (SPC storm reports). Storm-scale anticyclonic outflow aloft around the periphery of the decaying convection acted as a short-term barrier to the upstream southwesterly winds within the middle/upper troposphere, creating quasi-stationary gravity waves along their rear (westward) edges which persisted for several hours. These waves were most evident over eastern Nebraska and northeastern Kansas on GOES-16 Upper-level Water Vapor (6.2 µm) images (above).

6.2 µm Water Vapor images with plots of GOES-16 Derived Motion Winds (below) intermittently showed these high-altitude anticyclonic winds along the western edges of decaying convection — for example, at 0842 UTC, 0922 UTC, 0957 UTC, 1127 UTC, 1212 UTC and 1312 UTC.

The quasi-stationary waves appeared to coincide with a few pilot reports of high-altitude turbulence: Clear Air Turbulence (CAT) was mentioned over northeastern Kansas at 37,000 feet and 39,000 feet, and “mountain wave action” was reported over southeastern Nebraska at 43,000 feet.

Higher resolution views of the convection were provided by VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images from Suomi NPP at 0755 UTC and NOAA-20 at 0845 UTC (below). With ample illumination from the Moon (in the Waning Gibbous phase, at 67% of Full), the “visible image at night” capability of the Day/Night Band was well-demonstrated. The coldest cloud-top infrared brightness temperature associated with the MCS in western Iowa was -84ºC — and the effect of a similar “blocking wave” along the western/northwestern edge of that storm could be seen, which was effectively eroding the approaching high-altitude anvil cloud material from the Nebraska MCS. Note that the 0845 UTC NOAA-20 VIIRS images are incorrectly labeled as Suomi NPP.

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