Disappearing snow cover in southern Minnesota

One aspect of our warm-up this week to watch is snow cover trends across Minnesota.

Tuesday’s NOAA snow cover analysis shows there’s little or no snow on the ground basically south of the Twin Cities.

That bare ground is much more effective at using the sun’s rays to heat the lower atmosphere compared to snow-covered areas.

Here’s why.

Fresh snow can reflect as much as 90% of the sun’s rays back into space without heating the ground and air above. But bare ground turns the sun’s energy into longwave radiation that is effective at heating the air near the ground where we live.

As a result, snow-free areas can easily be 10 degrees warmer than snow-covered areas in the same air mass on sunny days.

The difference in snow cover is reflected in the forecast high-temperature maps for Minnesota this week. Some locations in southern Minnesota will be as much as 20 degrees warmer than snow-covered areas in the Red River Valley Wednesday. Part of that is air mass differences, but part is also the difference in snow cover.

The snow depth at MSP Airport was at 1 inch early Tuesday. Snow cover will be scarce around much of the Twin Cities by late Wednesday afternoon. There is still deeper snow in the northern Twin Cities that will take longer to melt.

Watching the snow melt in March is good therapy for the winter-weary in Minnesota.