Radar Gaps and Tornadoes: Worse in Winter

The point of this article: in winter, it is more important than ever to have your tornado sheltering area ready to go and to move quickly to shelter when a tornado warning is issued. 

Posted by Bill Murray

Several meteorologists and others have recently posted newspaper images of stories pertaining to tornadoes that have occurred in early winter. The Vicksburg tornado occurred December 6, 1953. There was no warning. The tornado killed 38 and injured more than 250. 

A warning might not be optimal if a similar winter tornado occurred in that region today because the area west of Vicksburg (not Vicksburg itself) is in a National Weather Service radar gap (see map below).

A number of times over the past year we've discussed issues with storm warnings, which include gaps in radar coverage (example, here) where proper tornado monitoring cannot occur. With the potential for tornadoes as we get into the weekend, I want to discuss how the gap issue worsens during the winter. 

White and blue areas are serious radar coverage gaps.
Purple arrow is the gap west of Vicksburg. 
Red arrow is the Baring gap.

The blue and white areas on the map are serious gaps in National Weather Service radar coverage (note: the map does not show areas, such as Charlotte, that are covered by Federal Aviation Administration weather radars). The purple arrow is the gap west of Vicksburg and the red arrow on the map points to the radar gap in northeast Missouri. 

Last August, a destructive tornado struck Baring, MO which is situated in one of the gaps. There was no tornado warning at all. I discuss that case, here. While there was a hint of the developing tornado (circled, below) I do not fault the NWS for its failure to warn in this case. The "hint" is in the form of a "mesocyclone" which is an area of lowered pressure and circulating air above and near the tornado. The distance was so far from the St. Louis NWS the situation was somewhat unclear. 

Here's the problem: Even that hint almost certainly would not have been visible in winter. The radar gap issue gets even worse during the coldest three months of the year. 

Here's a diagram that explains. I'm going to skip over the meteorology of weather radar except for one element: as distance increases from the radar, the height of the radar "beam" above the ground increases. Diagram below. 

Radar, at left, sends out a beam of energy to detect storms. The beam
rises higher the farther it is from the radar. 
Base art: The Pennsylvania State University

The diagram shows (greatly simplified) how a tornado is topped by the "mesocyclone." A mesocyclone is an area of low pressure and swirling winds above and/or near the tornado. 

There are times in late spring and summer when the mesocyclone, which is a tell-tale sign a tornado may be occurring, extends up 15,000+ feet into a thunderstorm that tops out around 50,000 feet. But, in winter, the same type of thunderstorm may top out at 35,000 feet and the mesocyclone at only around 6,000 feet. Meaning that, in the radar gaps, it is likely there won't even be a "hint" a tornado is forming or in progress. As in the diagram above, the radar beam will shoot right over the mesocyclone. 

One other problem with tornadoes in winter: they very often move quickly.

My purpose in bringing this up is that it is vital to be extra vigilant if a tornado watch is issued during the winter months. You may well have less time to take shelter than if the same tornado occurred in spring or summer. 

Meteorological Explanation

Thunderstorms flatten out at the top when they reach the tropopause.

Thunderstorms develop in the lowest layer of the atmosphere (the "troposphere") when the air is "unstable," meaning it has a tendency to rise, sometimes violently. However, the height of thunderstorms is constricted by what is called the "tropopause" which is the boundary between the troposphere and the next layer up in the atmosphere, the "stratosphere." Think of the tropopause as the layer of icing in a slice of 2-layer cake. 

So, when the rapidly rising air of the thunderstorm strikes the tropopause it is like a ceiling and the thunderstorm tops out. 

In the winter, the troposphere (the "icing") is much lower to the ground than in the summer. The consequence is that tornado-bearing thunderstorms are lower-topped and, thus, harder for meteorologists to spot in radar gaps.