Snow at 48 degrees?

Right before noon at Tuesday I was outside and noticed a light snow/graupel mix that came down for about five minutes. At around 3:00 pm, there was an additional brief burst of snow and sleet. While it is not unusual to get wintry weather in Bowling Green in February, what made this a rare event was that the temperature at the time of the snow was 48 degrees!! So how does this happen?

I have only seen snow at temperatures in the upper 40s one other time. When I was a forecaster for AccuWeather in the mid 1990s, there was a May afternoon where light flurries fell on a day that was also around 48 degrees. What both events had in common was early morning sunshine, cold air aloft, and a steep low-level lapse rate.

On Tuesday, the morning started cold with a temperature of 22 and a dewpoint of 17 at 7:00 am. By 11:00 am, bright early morning sunshine had popped the temperature up to 48 with a dewpoint of 20. This created a steep low-level lapse rate and led to low-topped convection that caused the sky to cloud over by noon. The snow that was falling from the clouds remained in sub-freezing temperatures until just near the surface. Despite the 48 degree air at the surface, the low relative humidity and shallowness of the warm layer near the surface allowed the precipitation to remain as snow.

By late in the day, continued warm advection had moistened the low-levels enough that by 8:00 pm when the bulk of the precipitation arrived, the dewpoint was above freezing and all precipitation fell as rain.

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