Tornadoes Wreak Havoc on South

One of the Most Deadly Outbreaks in US History Takes Over 315 Lives

By now, most people have seen news coverage of the utter devastation across the Southeast US.  Alabama was hit especially hard and saw the lionshare of the casualties (210+).  This most recent outbreak, April 25-28 (with the most damage and fatalities on the 27th), surpasses the "Super Outbreak" of April 3-4, 1974 as the one of the worst tornado outbreaks in US history.  What I find most horrifying about that statistic is that weather warnings have improved immensely in the past 30-40 years and yet we still lost the same amount of lives.  This shows the continued need for improvement of warning systems as well as just how destructive the 2011 outbreak really was.  One of the major factors in the high casualty rate was the propensity for EF-4/5 (Enhanced Fujita Scale) tornadoes on April 27th 2011 to hit populated areas.  No matter the warning time, some people's living situations are not conducive for tornado survival.

I could go on at length about the awful destruction and fatalities but instead will focus on what meteorological ingredients came together on Wednesday April 27th, 2011 to cause this record breaking tornado outbreak.  This really is a once or twice in a lifetime event and from a meteorological perspective, it is extremely fascinating.

Synoptic Weather Set-Up

The United State is one of the most conducive continents for tornadoes.  These localized destructive storms are actually very uncommon in other parts of the world.  The right ingredients must be present to spawn a twister and to see an outbreak such as this past week's, everything must come together perfectly.

Severe thunderstorms originate when there is a clashing of airmasses.  In the southern US, moisture rich, warm Gulf of Mexico air is transported northward.  Farther north across the Rockies and southern Canada, cooler, drier air moves southward.  During the spring, Canada is still fairly chilly and the Southeast is warming up as the Gulf of Mexico warms.  When these two air masses collide, strong thunderstorms can occur.

Above is a weather map from the 8am EDT on April 27th.  Note the temperatures (red) and dew points (green) across the Southeast.  Both are in the 60s which means it is very moist at the surface, great fuel for storms.  Notice the temperatures and dewpoints in the 30s and 40s across the Midwest (cooler, drier air).   Lastly, notice that winds are from the Southeast across much of the region.

This second map (above) shows what was happening around 1500m above the surface (again, 8am on the April 27th).  The key takeaway from this map is the strong winds (~50 knots) across the Southeast and the southerly wind direction over western Georgia and south-southwesterly wind direction farther west over Alabama, MS, and LA.  If you compare these wind direction and speeds with the surface map (1st chart) you can see that the winds at 1500m about 40 knots stronger and at least 45 degrees (in some cases 90 degrees) different than the surface winds.  This change in direction and strength is called shear.

If we look at ~5400m above the surface, the winds have shifted even more to the west over the Southeast and are ~60-70knots.  Again, more shear.  Also, temperatures are 8 degrees F over the Southeast which is ~60 degrees less than the surface temps.  Cold air on top of warm air creates instability which aids thunderstorms.

From the above charts, we can conclude that the temperature rapidly decreased with height across the Southeast which is an important ingredient for thunderstorms.  However, the ridiculous amount of shear throughout the region is the key for tornado-genesis because it allows the storms to rotate.  The combination of these basic ingredients, as well as a plethora of more complex factors came together to create a combustible situation that ended up yielding close to 100 tornadoes (we don't know yet how many will be confirmed).

I'm hunting for a track map but can't seem to find it right now...I'll post it when I come across it.   It is remarkable.  The tornado that ripped through Birmingham and Tuscaloosa began on the AL/MS border and ended in northern Georgia!