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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GOES-15 (GOES-West) and GOES-13 (GOES-East) Visible (0.63 µm) and Shortwave infrared (3.9 µm) images (above; click to play animation; also available as an MP4 movie file) showed the smoke plume and “hot spot” (dark black to red pixels) associated with a large and fast-burning grass fire in north-central Colorado on the afternoon of 18 September 2015. The smoke plume was more apparent in the GOES-13 visible images, due a more favorable “forward scattering” sun-satellite geometry. The fire burned an estimated 12,669 acres, and dense smoke forced the closure of Interstate 76 for about an hour in the afternoon.

On the following day, the fire burn scar could be seen in a comparison of Suomi NPP VIIRS  true-color and false-color images from the SSEC RealEarth site (below).

Due to the darker color and the lack of vegetation, the grass fire burn scar also exhibited a much warmer signature on the Terra MODIS Land Surface Temperature (LST) product (below) — LST values were as high as 112º F (darker orange color enhancement) within the burn scar, compared to LST values in the 80s and 90s F in surounding areas.

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Tropical Depression Nine in the Atlantic Ocean, above, shows characteristics of a heavily sheared storm (longer sunrise-to-sunset animations: gif | mp4). The low-level circulation is displaced south and west of the strongest convection (which is vigorous enough to produce occasional overshooting tops, below, as shown on this page).

Metop-B overflew Tropical Depression Nine’s circulation just after 1300 UTC on 17 September (Orbital tracks for Metop-B are here). The image below shows a circulation with 20-30 knot winds displaced to the south and west of cold 10.7 µm brightness temperatures indicating high cloud tops detected by the GOES-13 Imager.

Total Precipitable Water images from MIMIC (above) suggest the circulation of Tropical Depression Nine is on the northern edge of deep moisture in the Intertropical Convergence Zone. Its projected path to the north and west is towards dryer air. The projected path (below) does take the storm towards warmer waters; however, it also moves it towards a region of even higher deep-layer wind shear (plots below were taken from the CIMSS Tropical Cyclones site). Only the warmer waters argue that this storm will persist; everything else suggests a struggle to survive. Several tropical cyclones this season have succumbed to wind shear while over the tropical Atlantic. Tropical Depression Nine may add to that total.

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Some locations in southern California received record daily precipitation on 15 September 2015. A time series plot of weather conditions for Los Angeles International Airport is shown above; the 1.80 inches of rainfall received there made it the wettest September day on record.

As mentioned in the NWS Los Angeles record event report, the moisture source for this heavy rainfall event was the remnants of Hurricane Linda (storm path); the MIMIC Total Precipitable Water product covering the period 06-15 September (below) showed the evolution of the moisture associated with Linda, which originally tapped rich moisture from the Intertropical Convergence Zone (ITCZ).

A closer look at the MIMIC Total Preciptable Water product (below) showed how the moisture was becoming more concentrated just offshore, in advance of an approaching upper level trough and a decaying surface cold front.

GOES-15 Infrared (10.7 µm) images (below) revealed that the cloud-top IR brightness temperatures were actually quite warm (> -20º C) over coastal southern California during the period of the heavy rainfall event.

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