16 December 2010

How Meteorologists Know to Forecast Ice Instead of Snow

Posted by Dan Satterfield

We had a significant ice storm in parts of Alabama and Tennessee on Wednesday. It started as sleet and then changed to freezing rain. Driving was a mess in many spots, and it was not an easy forecast.

The easy part was the type of precipitation.  I knew it would not be snow but mainly freezing rain. Take a look at the image below. It’s the weather balloon (rawinsonde) sounding from Nashville Wednesday night.

The red line is the temperature curve and the green is the dewpoint. 850 millibars is around 1700 meters above the surface. Notice that the temperature here is above freezing.

The set up was indeed classic. A shallow layer of cold dense Arctic air covered the region. A strong low pressure system started pumping warm air from the Gulf rapidly northward, and this warm air lifted over the cold air. You can see the warm layer of air in the sounding above. The winds are blowing SW at over 25 knots as well.

Notice the dramatic temperature difference across the warm front in Mississippi. In less than an hour, you could go from a temp. of 36F to a balmy springlike 63º. Now that's a warm front! Chart from NOAA HPC.

Another factor was the dry air near the surface. Notice the green dew-point line is well left of the red temp. line. Once the precipitation started the freezing rain evaporated as it fell into the dry air . It takes heat to evaporate liquid, and this cooled the layer even more.

This evaporational cooling actually caused the cold layer near the surface to thicken some at the start of the event, and many areas reported sleet. As the warming aloft continued, the layer of cold air slowly eroded, and it changed to all freezing rain.

There’s more.

When the water droplets condensed above the 800 millibar level (around 2km) heat was released into the air, and this warmed the layer even more. Since it takes heat to turn liquid water to vapor, when the vapor condenses back to liquid this heat is released. The scientific term for this is the latent heat of condensation. In this case it warmed the air aloft and then the evaporating rain near the ground did just the opposite!

Snow will only occur if nearly the entire sounding is below freezing.

Isn’t physics grand!