From the highlighted temperature and dew-point, notice
how dry the air was. At 2pm local time, it was 39 degrees.
How could it snow? It's above freezing. However, due to the dry air
(do you see how low the dew point is), the precipitation would
evaporate. Through latent heat, this conversion from liquid/snow
to gas requires energy from the ambient air. Thus, the
evaporation is causing the air temperature to cool.
As the evaporation continues, notice how the
temperature continues to drop until the surface air is almost
saturated.
How much snow accumulated in Atlanta's
Hartsfield-Jackson International Airport ? 0.4" -- while parts in
northern GA had 3" of snow. This was enough to close all the
schools in a
dozen counties. The last time there was snow? January 2006.
This storm was developed from a Gulf Low -- If you
recall from Monday's class, this was the surface analysis:
Notice the stationary front just past Florida, which
trails into Mexico. This boundary allowed for the formation of a
low. With so much available moisture, the storm can turn massive
as shown on Wednesday:
Additionally this storm has an mid-upper level jet to
work with as shown at 300mb in the next image:
If you believe in the jet stream and if
you knew a storm would form in the southeastern part of the US, from
Monday's maps, you
could predict a storm later this week. Sure enough, today,
Friday (1/18) it is raining!
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