Strong westerly to northwesterly winds in the wake of a departing winter storm were aiding in the formation of widespread mountain waves along and downwind of the Appalachian Mountains on 27 December 2012. A comparison of AWIPS images of 1-km resolution Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) with the corresponding 11.45 µm IR channel data at 07:39 UTC or 2:39 AM local time (above) showed how the DNB imagery could be used as a “visible channel at night” to aid in the detection and characterization of cloud features that are illuminated by moonlight. City lights could also be seen in cloud-free areas (or in areas covered by thin cloud layers) on the DNB image..

After sunrise, a 1-km resolution POES AVHRR 0.63 µm visible channel image at 14:48 UTC or 9:48 AM local time (below) showed how widespread the mountain waves had become across much of the Mid-Atlantic region of the US. At that time, surface reports showed wind gusts of 37 knots or 43 mph at Martinsburg, West Virginia (station identifier KMRB) and 39 knots or 45 mph at Jefferson, North Carolina (station identifier KGEV). The peak wind gust at Jefferson was 68 knots or 78 mph at 4:55 AM local time.

Comparisons of visible channel images with their corresponding false-color Red/Green/Blue (RGB) images from Suomi NPP VIIRS (above) and Aqua MODIS (below) demonstrated how RGB imagery can be used to aid in the discrimination of cloud vs snow cover — for example, there was a large area with snow on the ground (darker shades of red on the RGB images) seen in the Shenandoah Valley and the surrounding mountains of northwestern Virginia. Gaps in the clouds also revealed patchy areas of snow cover which extended northeastward into south-central Pennsylvania.

The appearance of mountain waves on satellite imagery — either on visible imagery, or on water vapor imagery — often suggests an enhanced potential for turbulence. While there were isolated pilot reports of light to moderate turbulence across the region throught the day (some of which were seen on the VIIRS and MODIS images above), there was one incident of severe turbulence encountered in Maryland at 18:49 UTC or 1:49 PM local time (below).

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A large and intense storm system moved across the southern US on 25 December 2012, producing a wide variety of weather that included heavy snow from Texas and Oklahoma into Arkansas (HPC storm summary), along with hail, damaging winds, and tornadoes from eastern Texas into Alabama (SPC storm reports).

AWIPS images of 1-km resolution Suomi-NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel data with overlays of surface reports and the corresponding surface analysis (above) showed great detail in the cloud structure, as well as the variety of weather conditions across the region at 19:41 UTC. At 19:48 UTC, a similar comparison of 1-km resolution MODIS 0.65 µm visible channel, 11.0 µm IR channel, and 6.7 µm water vapor channel images (below) also showed the cloud structure associated with the storm, in addition to the well-defined dry slot associated with an upper level cyclonically-curved jet streak that was wrapping around the base of the storm.

GOES-13 10.7 µm IR channel images (below; click image to play animation; also available as a QuickTime movie) included overlays of  SPC Storm Reports of hail, damaging winds, and tornadoes. This event may turn out to be one of the largest outbreaks of tornadoes on record for Christmas Day — and the EF-3 rated tornado that struck Pearl River County in Mississippi was the strongest tornado on record for Christmas Day.

GOES-13 6.5 µm water vapor channel images (below; click image to play animation; also available as a QuickTime movie) showed the dry slot (associated with the upper-level jet streak) as it approached the rear edge of the squall line which formed in eastern Texas and intensified as it moved eastward into Mississippi and Alabama.

===== 26 December Update =====

A “nighttime visible image” provided by the Suomi-NPP VIIRS 0.7 µm Day/Night Band (above) showed the hook-shaped signature of moonlight-illuminated snow cover across the Texas/Oklahoma Panhandle region and surrounding areas at 07:58 UTC or 1:58 AM local time on 26 December. The corresponding VIIRS IR brightness temperature difference “Fog/stratus product” image showed that while there were some patches of low cloud (yellow to red color enhancement) in the central and eastern portions of the satellite scene (as well as high cirrus clouds, black enhancement, approaching from the west), the majority of the bright areas seen on the Day/Night Band image were fresh snow cover.

Later in the day, a comparison of the 19:22 UTC VIIRS 0.64 µm visible channel image with the corresponding false-color Red/Green/Blue (RGB) image (below) demonstrated the value of using RGB images to help discriminate between snow cover (shades of red) and supercooled water droplet cloud features (shades of white). Storm total snowfall amounts in Arkansas were as high as 15.0 inches.

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A major winter storm impacted much of the central Plains and the Upper Midwest regions of the US on 19 December – 20 December 2012. This storm rapidly intensified as it moved northeastward from the Texas/Oklahoma Panhandles toward southern Lake Michigan, eventually producing widespread heavy snowfall and blizzard conditions across southern Wisconsin (for additional details, see the NWS Milwaukee storm summary).Storm total snowfall smounts included 10.0 inches in Gretna, Nebraska, 14.5 inches in Alkeny, Iowa, 20.1 inches at Beaver Dam, Wisconsin, and 19.6 inches at Gaylord, Michigan. A wind gust of 71 mph was recorded at South Haven, Michigan. In addition, Chicago and Rockford in Illinois received 0.2 inches and 1.4 inches of snow, respectively, each ending their record streak of 290 consecutive days without measurable snowfall.

An animation of AWIPS images of 4-km resolution GOES-13 6.5 µm water vapor channel images (above; click image to play animation) captured the latter portion of the storm’s life cycle, from 15:15 UTC on 20 December to 02:45 UTC on 21 December. Features that were evident on the water vapor imagery included a broad warm conveyor belt with a pronounced dry slot entering the back (western) edge, and what appeared to be the development of a pair of small-scale cold conveyor belts — one that moved over the Chicago area and the southern part of Lower Michigan, and another that later moved over the northern part of Lower Michigan and Lake Huron. A well-defined deformation zone could also be seen along the western edge of the “comma head” portion of the storm.

A closer look at southern Wisconsin using 4-km resolution GOES-13 10.7 µm InfraRed (IR) images (below; click image to play animation) showed the development of colder cloud tops around -40º C  (brighter green color enhancement) associated with multiple convective elements that pivoted northwestward over the area, as well as the band of colder cloud tops within the deformation zone along the rear edge of the storm system. There were a few lightning strikes with some of the stronger convective elements, producing thundersnow with enhanced snowfall rates.

A comparison of 1-km resolution Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images (below) showed a more detailed view of the numerous convective elements across the region at 17:55 UTC or 12:55 PM local time — the higher spatial resolution VIIRS image displayed cloud top IR brightness temperatures as cold as -50º C (darker orange to red color enhancement).

A comparison of a daytime Suomi NPP VIIRS 0.64 µm visible channel image with the corresponding false-color Red/Green/Blue (RGB) image at 19:35 UTC or 2:35 PM local time on 20 December showed that much of the northern and central Plains states had snow cover (shades of red on the RGB image); however, note that there was a large area of bare ground across the South Dakota-Nebraska border region. Even though moonlight illumination was not optimal, the Suomi NPP VIIRS 0.7 µm Day/Night Band on the following night at 07:52 UTC or 2:52 AM local time was able to show the contrast between the darker bare ground areas and the adjacent brighter areas of snow cover.

After more clouds had cleared on 21 December, another comparison of Suomi NPP VIIRS 0.64 µm visible channel and false-color RGB images at 19:16 UTC or 2:16 PM local time (below) showed better detail of the areal extent of the resulting band of fresh snow cover that stretched from Colorado eastward and northeastward into Wisconsin and Illinois. Note that the snow cover farther north across the Dakotas and Minnesota was from an earlier winter storm.

===== 22 December Update =====

A 250-meter resolution MODIS true-color image centered over Chicago (below) revealed a signature of enhanced turbidity within the nearshore waters over southern Lake Michigan on 22 December. This was a result of mixing by strong winds associated with the intensifying surface low as it passed over the area –  wind gusts across the southern Lake Michigan region were as high as 71 mph at South Haven, Michigan and 66 mph at Michigan City, Indiana.

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A large and intense area of low pressure was located in the Gulf of Alaska on 18 December 2012 (above), which was forecast to produce areas of hurricane-force winds west and southwest of the storm center.

A comparison of AWIPS images of Suomi NPP VIIRS 0.7 µm Day/Night Band images (below) revealed a large amount of Bering Sea ice motion just off the southwest coast of Alaska in the 101 minutes between the two images (21:59 UTC and 23:40 UTC), due to the strong northwesterly winds of 30-40 knots that had existed over that region.

The image comparison below shows false-color Red/Green/Blue (RGB) composites created using the 0.64 µm VIIRS visible channel (Red) and the 1.61 µm “snow/ice channel” (Green and Blue) which allowed an easier discrimination between the ice (which appeared as darker shades of red) and supercooled water droplet clouds (which appeared as shades of white).

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