Areas of dense fog impacted southern Colorado during the evening and early morning hours of 05 October 2017. A moist airmass and weak upslope flow allowed for low clouds and fog to quickly develop during the evening of the 4th in the vicinity of the I-25 corridor between Colorado Springs and Pueblo. GOES-16 Nighttime Microphysics imagery, discussed in previous blog posts, was utilized to monitor the development and evolution of the low clouds and fog (Fig 1).
There was, however, another tool available to forecasters to aid in fog analysis during a period of the very morning hours: The Suomi NPP VIIRS Near Constant Contrast Imagery. This channel, also known as the Day Night Band (DNB), uses primarily moonlight and starlight to allow for visible imagery at night. Because the DNB has a horizontal spatial resolution of 750 m, it provides more detail than is available from the 2 km GOES-16 IR nighttime products. The key negative of DNB is that because it is a polar-orbiting satellite, a given location over the CONUS will typically receive only one scan at night and one scan during the day. However, many locations will see overlap between scans on occasion, as was the case over eastern Colorado early on the 5th. Additionally, city lights make it difficult to diagnose some features such as clouds in urban areas.
VIIRS swaths were available over eastern Colorado at 0800 UTC and 0942 UTC on 05 October 2017 (Fig 2). Not only did the DNB allow for a higher resolution view of the low clouds and fog impacting southern Colorado, but the sequence of two images provided a glimpse into how the fog was evolving. One can see that the fog in the Colorado Springs area and along I-25 between Colorado Springs and Pueblo had dissipated, while fog had increased within the Arkansas River Valley from Canon City through Pueblo and east of Pueblo.
Comparing the VIIRS DNB with GOES-16 Nighttime Microphysics RGB at 0942 UTC, the higher spatial detail of the DNB more precisely pinpoints the position of the fog and better detects areas of very thin/shallow and narrow fog (Fig 3).
Figure 4 combines the VIIRS DNB images with the GOES-16 RGB imagery.
JPSS-1 will launch later this year and will provide similar capabilities to that of Suomi-NPP.
-Bill Line, NWS
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