A slowly-moving storm was migrating northeastward along the East Coast of the United States on 25 September 2015. The GOES-13 water vapor animation, above, shows the evolution of this system between 22 and 25 September 2015. The center of circulation in the water vapor imagery has wobbled around the state of Georgia for the past three days as a strong jet along the United States-Canada border has has maintained a series of surface High Pressure systems northeast of the storm, effectively blocking the Low Pressure’s exit from the southeast US.

Moisture available to this storm is depicted in the animation of MIMIC Total Precipitable Water, below. Moisture that moves southwestward towards the southeast US along the Gulf Stream is being joined by moisture moving northward from the tropical Atlantic Ocean.

This system has generated flooding rains over South Carolina. The GOES-13 Visible animation from 24 September 2015, below, shows persistent inflow off the Atlantic Ocean. Click here for a media report; Columbia SC reported 2.84″ of rain on 24 September, a record for the date.

The thunderstorms associated with this system also caused a brief EF-2 tornado near Johns Island in Charleston County, SC (SPC Storm Reports). An animation of auto-detected Overshooting Tops, below, captures an overshooting top sequence persisting near Charleston at about the time of the tornado (0445 UTC). The toggle between Terra MODIS (0315 UTC) and Suomi NPP VIIRS (0618 UTC) 11 µm infrared data shows the general northwestward motion of the thunderstorm that produced the tornado (GOES-13 Infrared animation). The coldest cloud-top IR brightness temperature from MODIS was -75º C, with -76º C detected by VIIRS.

The pressure gradient between the storm over the southeast US and the High Pressure over the Canadian Maritimes had caused long-fetch onshore winds over much of the mid-Atlantic coast. This (and a Full “Super” Moon on Sunday) presages a weekend of coastal flooding. The animation of ASCAT (from METOP-B) winds below, shows 20-25 knot winds over a large region of the Atlantic Ocean between the Gulf Stream and North America. That long fetch will help generate large waves. The surface circulation off the coast of Georgia is also apparent in the 0220 UTC image.

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Three Semi-Truck/Trailers collided with a school bus in fog in Richland County North Dakota just before 9 AM CDT (1400 UTC) on 25 September 2015. The GOES-13 Visible animation, above, shows an eroding fog bank over southeastern North Dakota. Richland County is outlined in the imagery. The school bus was traveling on Highway 46, which hugs the northern border of Richland County; the accident was about 3 miles west of Interstate 29 (Press Report 1 ; Press Report 2).

During the preceding nighttime hours, Suomi NPP VIIRS “fog/stratus product” IR brightness temperature difference images at 0756 UTC (2:56 am CDT) and 0937 UTC (4:37 am CDT), above, showed an increasing signal of fog/stratus – yellow to red color  enhancement – over the region during that time period (when surface visibilities also began to rapidly decrease at Wahpeton KBWP, the county seat of Richland County).

GOES-R IFR Probability fields, above, around the time of the accident, showed a high probability of IFR Conditions over southeast North Dakota. High values were present over the area before sunrise (Sunrise in Fargo on 25 September is at 1318 UTC). Values at 1400 UTC show largest values over northern and western Richland County. The Low IFR Probabilty field for 1400 UTC, below, immediately after the time of the crash, shows a maximum across northern Richland County where the accident occurred.

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A severe hail-producing thunderstorm moved over northeast Nebraska before noon on 22 September (SPC Storm Reports). The region hit was just south of a Marginal Risk of Severe Weather (The update at 1630 UTC included the region of severe weather). The GOES-13 visible animation, above, shows the initial development occurring along a subtle cloud line aligned mostly east-west.

The NOAA/CIMSS ProbSevere model produces a probability that a developing thunderstorm will initially produce severe weather within the next sixty minutes. It consistently supplies information with a good lead time, and the storm on 22 September was no exception. The animation below shows the product for about an hour before the first storm report at 1408 UTC. The storm out of which the hail dropped was, at 1300 UTC, flagged as having a ProbSevere under 10%; values exceeded 10% at 1314 UTC and then jumped to 60+% at 1336 UTC (the first time that the value exceeded 50%) Values fluctuated between 60 and 80% between 1336 and 1400 UTC. After 1400 UTC, values increased into the mid-80s. The first report of hail was at 1408 UTC, 32 minutes after ProbSevere jumped above 50%. A severe thunderstorm warning for hail was issued at 1412 UTC.

The GOES Sounder Lifted index product, below, (also available here) showed the instability that was present over the central Plains.

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The Chinook Arch (a stationary cloud formation downwind of the Rockies) should move E and diminsh this aftn. #mtwx pic.twitter.com/UdfRRjOnBF

— NWS Great Falls (@NWSGreatFalls) September 20, 2015

As pointed out in a Tweet from NWS Great Falls (above), a “chinook arch” mountain wave cloud feature had formed in response to strong westerly winds interacting with the high terrain of the Rocky Mountains of western Montana on 20 September 2015. GOES-13 visible (0.63 µm) images (below; also available as an MP4 movie file) showed the development and evolution of the lower-altitude parallel bands of mountain wave clouds, as well as the larger patch of higher-altitude cloud immediately downwind of the eastern edge of the highest terrain (comparison of Suomi NPP VIIRS visible image and terrain) .

The corresponding GOES-13 infrared (10.7 µm) images (below; also available as an MP4 movie file) revealed that the large patch of high-altitude cloud (sometimes referred to as a “banner cloud”) began to grow in areal coverage after about 12 UTC, eventually exhibiting cloud-top IR brightness temperatures in the -50 to -55º C range (yellow to orange color enhancement).

The GOES-13 water vapor (6.5 µm) images (below; also available as an MP4 movie file) did not show the common signature of a jet streak axis (a well-defined moist-to-dry gradient) until later in the day.

A comparison of Aqua MODIS visible (0.65 µm) and “cirrus detection” (1.375 µm) images at 1949 UTC (below) demonstrated how the cirrus channel imagery can be used to better discriminate between the high-altitude ice clouds (brighter white features) and the low-altitude water and/or supercooled water droplet clouds.

A comparison of Suomi NPP VIIRS visible (0.64 µm), infrared window (11.45 µm) and shortwave infrared (3.74 µm) images at 2057 UTC (below) showed the high-altitude banner cloud feature as it was beginning to enter its dissipation phase. The 11.45 µm cloud-top IR brightness temperatures were as cold as -57º C; however, note that the 3.74 µm shortwave IR brightness temperatures were significantly warmer (in the +5 to +10º C range). These warm shortwave IR temperatures indicated that the banner cloud feature was composed of very small ice crystals, which were effective at reflecting incoming solar radiation back toward the satellite sensors.

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