Snow and sun angle contamination in the Flight Icing Threat

After a busy weather week, the sun has risen over a quiet morning in most of the country. Something noticeable, however, was snow issues. The FIT has been showing an area of high probabilities over NW Indiana into Illinois (below).

20160226 1345 UTC FIT index with PIREPs overlaid and the area of interest noted in the blue dashed circle

No icing PIREPs were located anywhere in the region and no other environmental parameters were indicating icing conditions. A look at visible imagery revealed the reason for these values.

20160226 1345 UTC visible imagery

Visible imagery clearly shows an area of snow on the ground, remnants of the winter storm that passed through the area yesterday. This high reflectance of this snow from the view of the satellite caused erroneously high values in the various cloud retrievals, and subsequently resulted in an area of high FIT index values.

Also contributing to the higher FIT values in this area and nearby in western Illinois is the time of day. The terminator is easily noticeable in the Midwest, indicating the switch of the algorithm from day to night. Because of the sun angles at this time of the morning (and just before dark), cloud property values are erroneously increased and result in the erroneous increase of FIT values. By 1415 UTC (below), FIT values were reduced significantly.

20160226 1415 UTC FIT with PIREPs overlaid