Deadly Nocturnal Tornadoes; Secondary Cold Front

DEADLY NOCTURNAL TORNADOES

A pair of tornadoes each caused a death in the wee hours of the morning on Saturday November 15 in North Carolina. This brought the death toll for the year to 125. You have to go back to 1998 to find a deadlier year (130 tornado deaths), and then all the way back to 1974 (366 deaths).

The killer tornadoes also continued a trend for November -- deadly nocturnal tornadoes. 87% of the tornado deaths in November in the last 15 years have been at night. Nocturnal tornadoes tend to be deadlier than ones in the daytime at any time of year, with 42% of all tornadoes occurring at night, but causing 57% of deaths in the 1997-2006 period. People are often asleep and can't normally see the tornado coming. And residences are often of weaker construction than the buildings where many people work during the day.

Stu Ostro wrote a blog on November 15 showing the radar from the deadly EF2 Kenly, North Carolina tornado that struck shortly after 3AM on Saturday morning, November 15. The second killer tornado that night struck about a half hour later, an EF3 just west-southwest of Elm City, NC. Its radar precipitation pattern is shown below.

The tornado is on the southeast edge of that band of red, just west-southwest of Elm City. Notice the overall S-shape of the precipitation pattern, sometimes called a LEWP (line-echo-wave-pattern) known to sometimes be conducive to tornadoes. The storm-relative velocity pattern below shows a red-green couplet inside the circle that I've superimposed. This is the tornado signature, with red indicating eastward and green indicating westward. "Storm-relative" means that the storm motion has been subtracted. This often accentuates embedded rotation signatures like this one, making then easier to spot and warn on.

Following this event, a series of cold fronts have made it too cold and stable across the United States for severe thunderstorm and tornado activity. The image below shows one of those cold fronts, from Friday November 21.

SECONDARY COLD FRONT

This visible satellite image shows what we sometimes call a "secondary" cold front.
It is a "reinforcing" cold front, bringing in even colder air into an area that was already cool. I've drawn a yellow arrow to indicate the location of the secondary front -- that band of bright clouds running basically from west to east.

The orientation of my yellow arrow indicates winds from the north-northeast and bands of clouds called "cloud streets" in the cooling air behind the front. The cloud streets are formed as the cold air is heated and moistened over the warm Gulf of Mexico behind the front. Brighter, taller clouds at the arrowhead are enhanced by the convergence in the colliding air masses right at the front. Other bands of clouds south of the cold front may be remnants of previous fronts that moved into the area and weakened.

This secondary front is already showing such signs of weakening on the surface weather map below. I've drawn blue and red arrows to indicate the winds behind and ahead of the front. The relatively small wind shift suggests a weakening front, not atypical of secondary fronts that have gone this far south (away from their Canadian cold air origin.

The cold air behind this front promises to keep conditions too stable for a tornado threat through at least Sunday.