I couldn't write it any better so here is a great explanation and an example I found on The Weather Prediction website:
In the late fall and early winter months in the Southeast, cold front after cold front traverses the region, suppressing the truly marine air of the Gulf of Mexico further and further to the south. If this moisture is allowed to advect back northward over land and meets with a mid-latitude cyclone traveling over the Southeast at the same time, dangerous severe weather outbreaks can result. However, even a small amount of moisture coupled with ample shear aloft can result in severe weather over Alabama and Georgia in the winter months. These kind of events are the "sneakiest" of the bunch because they don't have the usual precursors such as sultry, humid air or extremely heavy rainfall accompanying them. In some cases, due to such lack of instability, there has been severe weather events, including tornadoes, without accompanying lightning or thunder. (http://www.theweatherprediction.com/weatherpapers/116/index.html)
The winter of 2006-07 was characterized by extreme cold in some parts of Alabama and Georgia, and as a result, moisture was slow to advect into the area after frontal passages. In this span of time, there were a few instances of high-shear, low-instability severe weather events in the area. The first such event occurred on November 30, 2006, in western Alabama. A low-topped line of convection approaching the border from Mississippi that night was energized by a high amount of shear in the atmosphere, as evidenced by the 00Z Birmingham sounding that yielded a 0-3 km helicity value of a whopping 273 m2/s2 and a 0-6 km shear value of 63 kt despite a CAPE value of just 162 J/kg
Despite the lack of lightning or thunder with the line of showers, they would go on to produce four tornadoes in Hale and Marengo counties, one of which was an F0 (this was before the advent of the Enhanced Fujita scale) and three of which were F1's, and several reports of wind damage in the Tuscaloosa and Birmingham areas. While the Hale County storms were warned by the NWS in Birmingham, there was no tornado warning issued for Marengo County, which ironically was the heavier-populated county of the two struck by tornadoes. The unique situation was forecast on the synoptic scale with a slight risk issued by the Storm Prediction Center, but the lack of resemblance to a "classic" severe thunderstorm prevented timely warnings.
What's great about 2015 versus 2006? The fact that the NWS has dual pol radar which means we can detect debris lofted from tornadoes which is helpful in the daytime but especially at night when it's difficult for storm spotters to see what's happening at the lowest levels.
Is Sunday into Monday going to be a similar type event? It could be but then again it may not be. Models are still trying to figure out the evolution of the trough aka dip in the jet stream. Will the wave stay open or will it close?
The latest data is indicating a better chance of this system closing off, which would slow it down. Thinking, which is the worst possible scenario seeing most of us will be sleeping or at least trying to that this event holds off until the wee hours of the morning on Monday.
FORECAST AS OF NOW:
A line of rain with very little thunder and lightning arriving late Sunday night across west Alabama and progressing eastward during the morning hours of Monday. Rain will be heavy at times and will impact travelers. Gusty winds will be possible with the strongest of the cells. Remember, it doesn't take much wind to knock down tree limbs and trees, especially dead ones. We can't rule out brief spin ups but it would be very localized. I barely see any instability around 12am Monday and then by 9am I see a little spike across east Alabama into west Georgia. There is a ton of wind shear with this system and so it's not going to take much instability to get a few strong or severe storms to form. We will be able to iron out the details in the coming days.
Please keep up with this changing forecast!
Jill Gilardi WBRC First Alert Certified Meteorologist