This is a Doppler velocity image showing winds toward the radar (green) and high winds away from the radar in pink. The strong winds (toward and away) so close together indicate an intense rotation. It was rather close to the NOAA radar site so the beam intercepted the storm at just a few thousand feet off the surface.

An EF 3 tornado (winds near 240 km/hr/150 mph) hit Jefferson County Alabama early Monday morning (see previous post). Two are dead, and the damage is quite severe with over 100 injuries. The tornado hit in the middle of the night, when many people were asleep.

It was a good bet that there would be a deadly tornado somewhere late Sunday into Monday, because the upper level storm system was very energetic. If you draw a line through the trough, it has a backwards slant. Meterorologists call this a negative tilt trough and this tends to make the wind shear in the atmosphere very favourable for strong tornadoes.

Tornados require two main ingredients: wind shear and instability. Most often, a winter time outbreak has low instability and very high wind shear. That was the case in this event, and once agin confirms a forecast rule I was taught 30+  years ago. Always beware of a negative tilt trough.