With a ridge of high pressure in place over the western Atlantic Ocean, GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) indicated the presence of dry air within the middle troposphere off the Southeast US coast on 23 January 2019.

GOES-16 “Red” Visible (0.64 µm) images showed that marine boundary layer stratocumulus clouds covered much of this region of the Atlantic — and due to minimal absorption by mid-tropospheric water vapor, these stratocumulus clouds were also very apparent in the corresponding GOES-16 Near-Infrared “Cirrus” (1.38 µm) images (below).

Terra MODIS Visible (0.65 µm) and Near-Infrared “Cirrus” (1.38 µm) images at 1513 UTC (below) also showed a clear signature of the stratocumulus clouds at 1.38 µm.

Cross sections of GFS90 model fields along Line I-I’ — oriented from Charleston, South Carolina to Bermuda — are shown below. Note the very dry air within the middle troposphere, with Specific Humidity values of less than 0.2 g/kg and Relative Humidity values less than 10% centered around the 500 hPa pressure level. In addition, the depth of the moist marine boundary layer was higher to the west at Charleston (2.6 km, at 746 hPa) than to the east at Bermuda (1.9 km, at 822 hPa).

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A large central/eastern US winter storm impacted much of the Northeast with rain, snow, freezing rain/drizzle and strong winds on 20 January 2019 (surface analyses) — and a post-storm comparison of GOES-16 (GOES-East)  “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images on 22 January (above) revealed a dark 1.61 µm signature (often indicative of significant ice accrual) in parts of New York, New Jersey, Connecticut, Rhode Island and Massachusetts. Since snow and ice are effective absorbers of radiation at the 1.61 µm wavelength — with ice absorbing even more strongly — those features appear as darker shades of gray in the Snow/Ice imagery.

A closer view using NOAA-20 VIIRS Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images at 1708 UTC is shown below (note: the NOAA-20 VIIRS image is incorrectly labeled as Suomi NPP). Ice accrued in thicknesses up to 0.60 inch at Meridan in central Connecticut and 0.40 inch at Newburgh in far eastern New York.

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

A comparison of GOES-17 Low-level Water Vapor (7.3 µm) images with topography (above) revealed that radiation being emitted by the higher elevations of the Brooks Range in northern Alaska was able to be sensed by the 7.3 µm detectors — in spite of the very large satellite viewing angle (or zenith angle) of around 75 degrees.

The GOES-17 ABI Water Vapor band weighting functions calculated using 12 UTC rawinsonde data from Fairbanks, Alaska (below) showed that the presence of cold, dry air within the middle to upper troposphere had shifted the peak pressure of the 7.3 µm weighting function downward to 753.63 hPa (corresponding to an altitude of 7053 feet) — which was at or below the elevation of much of the higher terrain of the Brooks Range. There was very little absorption of upwelling surface radiation by the small amount of water vapor that was present within the middle/upper troposphere, allowing the cold thermal signature of the higher terrain to be observed on the Water Vapor imagery.

On the following day (19 January), a very cold/dry arctic air mass was moving southward across the Upper Midwest and Great Lakes — the coldest temperature in the US that morning (including Alaska) was -42ºF at Kabetogama, Minnesota — and the outline of Lake Superior was very apparent in GOES-16 (GOES-East) Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor imagery; in fact, a portion of the northwestern shoreline was even faintly visible in Upper-level (6.2 µm) Water Vapor images (below).

Plots of the GOES-16 Water Vapor band weighting functions calculated using 00 UTC rawinsonde data from International Falls, Minnesota (below) showed some radiation contribution coming from near or just above the surface. As a result, a signature of the strong surface thermal contrast — between a relatively warm Lake Superior (water surface temperatures in the 30s F) and the adjacent cold land surface temperatures (generally -10 to -20ºF) — was able to reach the satellite with minimal absorption by water vapor aloft.

===== 21 January Update =====

In the wake of a large winter storm, arctic air spread across the eastern US on 21 January (minimum temperatures,– and the outline of the coasts of Maryland, Virginia and North Carolina could clearly be seen on GOES-16 Low-level (7.3 µm) Water Vapor images (above). In addition, the coast of the Albemarle-Pamlico Sound  and the Outer Banks of central North Carolina could even be seen for a short time on Mid-level (6.9 µm) Water Vapor imagery (for example, at 1502 UTC).

This cold/dry air mass set new daily records for lowest rawinsonde-measured Total Precipitable Water at Greensboro in central North Carolina (0.04 inch), Roanoke/Blacksburg in western Virginia (0.02 inch) and Wallops Island on the Eastern Shore of Virginia (0.05 inch). GOES-16 Total Precipitable Water product showed values in the 0.01 to 0.09 inch range in the vicinity of Roanoke and Greensboro. In plots of the GOES-16 water vapor weighting functions for those 3 rawinsonde sites (below), note the very strong contributions of radiation directly from or just above the surface for the 7.3 µm and 6.9 µm spectral bands.

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

An 1802 UTC GOES-17 “Red” Visible (0.64 µm) image with an overlay of the 12 UTC surface analysis (above) revealed a well-defined rope cloud which stretched for nearly 1000 miles, marking the cold front position at the time of the image. Rope clouds can therefore be used to diagnose the exact location of the leading edge of a cold frontal boundary between times when surface analyses are available. In this case, the cold front was associated with a Hurricane Force low over the East Pacific Ocean on 16 January 2019 (surface analyses).

An animation of GOES-17 Visible images is shown above, with a zoomed-in version closer to the rope cloud displayed below.

An even closer look (below) showed that the rope cloud was only about 2-3 miles wide.

When the 18 UTC surface analysis later became available, a close-up comparison with the 1802 UTC GOES-17 Visible image (below) indicated that the northern portion of the cold front (as indicated by the rope cloud) was slightly ahead of — and the southern portion slightly behind — the smoothed cold frontal position of the surface analysis product.

1-km resolution AVHRR Visible (0.63 µm) and Infrared Window (10.8 µm) images of the rope cloud were captured by NOAA-15 at 1617 UTC (above) and by NOAA-18 at 1710 UTC (below). Along the northeastern portion of the rope cloud, there were a few convective clouds which exhibited cloud-top infrared brightness temperatures as cold as -55 to -60ºC (darker shades of red) and were tall enough to be casting shadows due to the low morning sun angle.

Check out this “rope cloud” in this morning’s MN2 pass. Pretty defined. UltraTrack with Doppler compensation along with XHRPT. pic.twitter.com/Bc1xtk4hGj

— ??? ???¢?? (@usa_satcom) January 16, 2019

===== 17 January Update =====

On the following day, another rope cloud (one that was more fractured) was seen moving across Hawai’i as a cold front passed the island of Kaua’i — the southeastward progression of the rope cloud was evident on GOES-17 True Color Red-Green-Blue (RGB) images (above)  from the UW AOS site.

Surface observations plotted on GOES-17 Visible images (below) showed the wind shift from southwest to north as the cold front moved through Lihue on Kauwa’i around 00 UTC.

===== 18 January Update =====

Not all rope clouds are associated with cold fronts; with ample illumination from the Moon — in the Waxing Gibbous phase, at 90% of Full — a Suomi NPP VIIRS Day/Night Band (0.7 µm) image (above) provided a “visible image at night” of a rope cloud in the northern Gulf of Mexico which highlighted a surface wind shift axis.

A sequence of VIIRS Day/Night Band images from NOAA-20 and Suomi NPP (below) showed the movement of the rope cloud during a time span of about 1.5 hours.

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