A storm system swept across the mid-section of the country last week (Feb 20-21), dumping heavy snow on portions of the southern and central plains into the midwest region. In its wake, as clouds cleared, the heavy snow cover could easily be seen in daytime visible imagery. However, differentiating between snow and lingering cloud cover can be difficult in standard visible imagery. The False Color RGB can help to differentiate between snow and cloudy areas as shown in the image below.
In the image above, red colors indicate areas of snow, blue-green indicates bare ground, while white shows areas of clouds. The pink colors indicate areas of thin clouds over snow-covered ground. The immediate question this type of imagery begs is how far does the snow extend under the cloud obscured regions? While the clouds may linger during the day, the day/night band visible imagery from current VIIRS and future GOES-R instruments may help with these types of questions. Notice in the next image below from the VIIRS instrument aboard the Suomi NPP satellite, that the clouds have mainly cleared the area during the overnight and the snow field can now be more easily observed. In particular, notice the area lacking snow on the ground in portions of SE Colorado, along the Arkansas River Valley.
In the VIIRS Day/Night Band Radiance RGB image above, the areas of heavy snow cover are white, extending from the TX panhandle across NW Okahoma and areas to the north and east. Higher clouds appear as bluish-white (cities are bright yellow areas). To differentiate between clouds and ground/snow more easily, the Nighttime Microphysics RGB (Image 3) can be incorporated into a assessment strategy.
In Image 3 above, high clouds in portions of NE New Mexico, SE Colorado and the Ok panhandle appear as red-magenta. Low clouds further to the east in eastern Kansas, Missouri and Arkansas appear as white-aqua and salmon colors. This allows for quick verification of snow cover when compared with the previous image (Image 2). Now, notice the area in SE Colorado. In Image 1, it is not readily apparent the extent to which snow covers the ground, with clouds blocking the view. However, when Day/Night Band imagery became available later that night (Image 2), it was apparent that indeed a gap in the snow cover existed over the Arkansas River Valley. Additionally, the extent of the snow cover could also be more readily observed over northern TX and OK. With some mild warm air advection across the region on the 22nd, it was interesting to see the expansion of the snow “hole” over SE Colorado, which carried into the night. The next couple of images show the expansion of the snow free area in SE Colorado and some melting along hte edge of the snow pack. Some clouds obscured this area in the daytime False Color image (Image 4), but could be seen more easily in the Day/Night Band image later that night (Figure 5).
Further images from the afternoon of the 23rd and the early morning hours of the 24th show further melting in this region. I have constructed a loop of the VIIRS Day/Night Radiance RGB images and the Aqua False Color images available during the afternoon (Image 6).
What the VIIRS Day/NIGHT imagery is showing here, is a potentially great tool for forecasters to assess snow melt or perhaps snowfall in the wake of a fast-moving storm during nighttime situations (where there is no obstructing cloud cover). This information can be useful when forecasters are trying to assess appropriate forecast ranges for daytime highs or morning lows where the presence of snow cover can play a significant role.