Winter storm warning, Dec 15-16, 2008.

“The National Weather Service will issue a winter storm warning when a dangerous combination of heavy snow, with sleet and/or freezing rain, will occur or has a high probability of occurring within the next 12 hours.”

Severe winter weather events are notorious to forecast. Our TV weathercasters had a range of predictions from a half-inch of ice accumulation to 5 inches of snow. The National Weather Service (NWS) had a similar forecast. After the event, we had a lot of variability across the region, so in part, all of the predictions were somewhere correct. I think the NWS made a good prediction on where the bad weather would happen, but the severity was hard to calculate.

The above map (shown in greater detail below) shows low temperatures across the U.S. and highlights the problem with forecasting such a large storm. The huge temperature change from -20° F in the northern states to 60° F in the southeast indicates two vastly different air masses in close proximity. The greater the difference, the more difficult the predictions. When the air masses begin to interact and do their dance, that’s the problem.

It’s easy to forecast calm conditions, but when the weather becomes bad, the severity is determined by how different the air masses are and how much they interact. Two similar air masses doing a “big dance” will have little impact. Two vastly dissimilar air masses just interacting slightly will produce enormous weather. Though we can measure the characteristics of air masses, the problem is that we can’t predict how they’ll interact.

The basic setup we had here was that cold polar plunged southward, and displaced the warmer, more moist tropical air. As the warmer, less dense air rode over the advancing colder, denser air, precipitation fell as rain. As it traveled through the colder, ground layer of air, it froze on surface contact. That’s freezing rain. However when upper layer of air approaches the freezing point, precipitation could fall as snow or sleet. As this boundary zone between air masses changes, the vertical temperature profile changes and so does the type precipitation that hits the ground.

Can you feel it for the weathercaster trying to predict this stuff?

Watches and Warnings Map, Dec 16

Above map is the Watches and Warnings for the U.S., December 16, 2008. Compare this map to the below map, which forecasts low temperatures for this morning. Note the axis of warnings from Texas to Pennsylvania which overlays a sharp temperature contrast between the polar air to the northwest and tropical air to the southeast. This steep temperature gradient is almost 90° F and is zone of interaction.

Low Temperature Map, Dec 16

The below map shows snow and ice totals for the Bluegrass region.

One of the most obvious and striking winter weather conditions is the atmospheric influence of the Great Lakes. When high pressure builds to the west of the Great Lakes and there’s a prominent northwest-to-southeast surface wind flow, weather is significantly different downwind of the lake compared to upwind. This often produces lake effect snow that can truly bury southeastern shore cities. However, the same influence can reach far south into Kentucky and the Appalachians.

The difference in weather can be outstanding, as people who live near the lakes know very well. Observe weather on both sides of Lake Michigan. The red number indicates the temperature in °F and the circle indicates cloud cover. A solid circle is completely cloudy, and an empty circle represents clear skies. The barb indicates wind speed and direction (see below for legend).

Compare Lake Michigan’s west shore with its east shore. The west shore of Lake Michigan is cold, below zero, and cloudless. Weather on the east shore of Lake Michigan is 20°F warmer, it is cloudier, and snow is falling, as represented by the green asterisks.

The cause of these conditions is simply the great reservoir of heat that the lakes contain and transfer to the atmosphere through evaporation. Though the lakes are slowly cooling through winter, they are significantly above freezing across a large area. When a colder, dry air mass blows over the lakes, evaporation will transfer heat and moisture into the air mass. As the rising, warmer air cools, if often condenses resulting in cloudiness and heavy snow; the lake effect snow machine. Also note the how far south the warmer air has penetrated compared to other locations at the same latitude.

For example this station here: is reporting that the current temperature is 55 degrees F, the sky is clear, and the winds are blowing from the north at 18 miles per hour.

And this station is reporting completely overcast conditons, winds from the north at 24 miles per hour and the temperature is 58 degrees F.

This sundog occurred Friday night over Lexington and there is a companion sundog to the left, but it is out of the frame. The bright spot to the far right is a reflection off our “Big Blue” building.

Sundogs form when a couple unique conditions are present. Cirrus clouds are the atmosphere’s highest clouds and are composed of small ice crystals. These ice crystals are were the sun creates the show.

When the sun is close to the horizon and there’s a significant layer of cirrus clouds, which should be noted are curving towards the horizon, sunlight will do 3 things: 1) be reflected at a predictable location causing the bright spot, 2) refracted at the same spot causing a rainbow, and 3) scattered throughout giving the sundog a general reddish cast.

While not rare, sundogs are uncommon and have been heralded as both good and bad omens by ancient peoples.