Alabama Tornadoes - NASA's First Hand Account

Tornado Tracks from the rare EF-5 tornado in late April 2011.  Full size picture link. Photo Credit: NASA Earth Observatory.

There is a really great story I found on the Science@NASA website called: A Story from the Tornado Zone.  It is an excellent read.  The author, Dauna Coulter, found herself in the middle of ground zero of the huge storm that spawned dozens of tornadoes as it ripped through Alabama and Mississippi.

Tornado Tracks by EF rating.  Photo Credit: NWS

Most amazing was the EF-5 tornado that traveled through the northwest corner of Alabama. From the article by Dauna Coulter:

"This kind of set up in the atmosphere is extremely rare. We have a number in meteorology called the EHI (Energy Helicity Index)⁴ that indicates the likelihood of tornadoes developing. With an EHI over 2 you can expect some tornadoes. Over 5 and you can expect some significant tornadoes. The EHI predicted by computer models the day before this event was 10, and that prediction was correct."  -UA-Huntsville meteorologist Tim Coleman

 So, what was the science behind this huge, dangerous storm?  Well, the story goes as follows:  A large area of cold air traveled to the southeast out of the Midwest, following a cold front which was associated with a ground level low pressure area.  The low pressure pulled hot, most air up from the Gulf of Mexico area. Remember, fluids flow from high to low pressure. In addition, there were extremely strong winds with strong wind shear above the moist air.  All of this combined to create wind rotation and an increase in speed as the air gained altitude.  This is the exact formula for tornadoes.

Some other links to check out:
- National Severe Storms Laboratory Picture of Rotational Tracks.  This is an amazing picture showing the spatial extent of this storm.  Truly awe-inspiring.

I will add more links as I come across them.