Areas of snow cover can often lead to areas of persistent fog and low cloudiness under the right conditions. And determining the areas of fog and low clouds from snow cover can be tricky during the daytime hours. Here we look at a case from 10 December 2018 over the Midwest the snow cover likely contributed to low cloudiness and fog that either was slow to clear or lasted through the day in some areas. We’ll use a couple of CIRA products for this case.
We begin with a look at the extent of low clouds and fog around pre-dawn (1202 UTC on 10 Dec) using the CIRA GeoColor product from GOES-16 displayed using the CIRA SLIDER tool (available at http://rammb-slider.cira.colostate.edu).
GeoColor is not an operational product, but it is available for display on AWIPS and is widely used across the NWS (if you don’t have this product at your WFO and would like to get this product on AWIPS send us an email). In the nighttime city lights are displayed in the background and low clouds (water clouds) or fog are colored blue, while higher (ice) clouds are white. Observations at this time (shown below) suggest that much of the blue area in MN into WI is fog, with more low cloudiness in the area farther to the east.
During the daytime hours GeoColor uses the visible band 2 with a true color background. A look at the GeoColor image at 1802 UTC on 10 Dec shows that it can be hard to distinguish snow from clouds or fog with the visible band during the daytime.
Here is the NOHRSC snow cover analysis for this day.
There are RGB products that can be used to help discriminate clouds from snow (such as the Day Snow/Fog product developed by EUMETSAT. CIRA has developed a product that also discriminates clouds and fog from snow but retains white as the color for the snow cover. The CIRA Snow/Cloud Layer Discriminator product for the same time as the image above is shown below.
The color scale is shown at the bottom of the image. This product is also experimental but should soon be available for AWIPS as well. In this version of the product there is a discrimination made between lower (water) clouds and higher (ice) clouds, adding additional information. This product is for use during the daytime hours, with the image for a couple of hours later showing some breaking up of the low clouds and fog in areas without snow cover but otherwise the low clouds and fog persisting, in fact through the day as shown in the GeoColor image for 2202 UTC.