Weather Questions

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So, just what am I talking about when I mention the NAO and Greenland Block?

The North Atlantic Oscillation has to do with the water profile and surface pressure, primarily over the North Atlantic. When we go into a negative phase of the NAO, a trough USUALLY likes to lock into the East, meaning much colder than normal conditions- especially late in the winter.

The Greenland Block is when a massive area of high pressure develops over Greenland. This then "blocks" the air pattern from being "progressive," usually buckling a deep trough into the Eastern half of the country. More times than not, the Greenland Block leads to a prolonged period of colder than normal weather over the East.

What is Northwest, Upslope Flow? This occurs quite frequently after cold frontal passages in the winter. Because the wind behind the front is usually from the northwest, it is pushed up our mountains, thus it cools and condenses leading to the formation of clouds and precipitation (usually snow.) Sometimes, if we get into an extended period of moist, upslope flow we recieve significant accumulations.

What is training? Training often occurs in the spring and summer when rain and thunderstorms repeatedly move over the same location. Johnson County has experienced "training" several times over the last few years, resulting in our recent flash floods. In the most extreme cases, training can lead to several inches of rain in as little as a few hours.

What is the coldest temperature ever recorded in Tennessee? 32 degrees below zero which occured right in Mountain City on December 30, 1917. Pretty impressive, huh?

Why do you sometimes see wintry precipitation when the temp is above freezing? Factors that can produce "warm" snow events include: high temperature lapse rates, due to the cold air aloft and various factors warming air near the ground, and dry air below cloud base, to cool melting snowflakes.

Why do we sometimes see rain when the temperature is below freezing? The reason we sometimes see rain when it's below freezing is because the air just above the surface (sometimes only 500ft. to 1,000ft. above the surface) is above freezing. The reason? Warmer air aloft is moving in, however, because cold air is so dense and heavy this warmer air has problems completely scouring out the cold air locked in at the surface. Because the air above the surface is above freezing it causes the snowflakes to melt, however, when the rain hits the surface, where temperatures are below freezing, it re-freezes. This is known as freezing rain. A perfect axample of this is when Mountain City recieves freezing rain, areas such as Stone Mtn., Snake Mtn., and other "higher elevation" areas are only seeing rain.

How much snow equals 1 inch of water? Typically, ten inches of snow equals 1 inch of rain, however, there are exceptions to this rule. The colder the air temperature the higher the snow/rain ratio.

What is a severe thunderstorm? A severe thunderstorm is a thunderstorm with winds of 58 MPH or greater, and/or, hail .75'' or larger. Extremely vivid lightning, and torrential downpours typically are associated with severe thunderstorms, as well. Keep in mind, severe thunderstorms, are capable of producing tornadoes. If a severe thunderstorm warning is issued, take shelter immediately.

Bill's Top 5 Storms (Experienced since 1995.)

1. Blizzard of 1996: This storm occured Jan. 6-8, 1996. At my house I recorded 28 inches of snow. Drifts were as high as 4 feet!

2. Snowstorm of 1998: Jan. 14, 1998. So far, this was the most UNREAL and fun storm I've ever had to forecast. Initially we were expecting only a few inches of snow that morning then quickly change to rain. Little did we know the next morning we'd awake to 2 feet of heavy, wet snow. I remeber going to bed the night before, looking out in the distance at transformers blowing under the weight of the snow. The next morning, Johnson County looked like a war zone! Power crews were called in as far away as Florida to help restore power to the county!

3. Snow and Bitter Cold of Feb. 1996: Feb. 5, 1996. Up to 8 inches of snow followed by a low the next morning of 22 below zero! Brrrrrrrrrr. . .

4. Flood of July 2001: July 29, 2001. Over 6 inches of rain fell over a period of 36 hours resulting in significant flooding across the county. Several homes were destroyed and many others were evacuated. Also, there was a major mud slide in Forge Creek.

5. Christmas Upslope Snow Storm: Dec. 18-20, 2003. This went down in the record books as the biggest snow of its type. . .no Gulf/ Atlantic Low. 11 inches of the white stuff fell in Mountain City, but as much as 22 inches fell in Trade.