Last week, I attended The National Weather Association’s 36th Annual Meeting in Birmingham, Alabama.  It was a fitting location, considering the super tornado outbreak the state experienced back on April 27th of this year.  Much of the conference focused on this event, and other events from this past Spring, which has sent the year 2011 into the weather record books.

2011 is currently the 4th deadliest year for tornadoes, with just over 540 deaths to date… and still a couple months to go (the deadliest year on record is 1925, when 749 people were killed).  One of the reasons for this spike in tornado deaths, is that this has been the year of the urban tornado… twisters have hit many metropolitan areas:  Raleigh, Tuscaloosa, Joplin, Birmingham and Nashville.

April 2011 is ranked as the most active tornado month on record with 753 tornadoes.  Today I will discuss April 27th, 2011… when several rounds of severe weather affected parts of Missippi, Alabama and Tennessee… and even us here in Ohio.  (That day there was a weak EF0 tornado in New Carlisle.)

But we had it easy, compared to what our friends to the south were dealing with.  There were basically three waves of severe storms for them–an early morning squall line, a late morning/early afternoon surge, and then the main show late in the day.  In Alabama alone, there were 61 tornadoes, and 238 fatalities.  Total area affected:  549-square miles, which is 1.06% of the state!  This may not seem like a lot, but it actually IS.

First, the early morning squall line.  At 3:31am, the NWS issued a special weather statement saying a severe weather event would be “rapidly unfolding” as VERY strong 500mb mid-level winds set up over the area.  Extreme shear was in place that day, which is a change in wind speed and direction with height in the atmosphere.  Surface winds over the area were out of the south-southeast, and the jet stream was blasting into the region around 100 mph out of the west-southwest.  There was a bullseye of 1km Bulk Shear over the area, and the Significant Tornado Parameter (STP) was extremely high… even that early in the morning!  The STP is an index used to highlight the co-existence of ingredients favoring right-moving supercells… which are capable of producing F2-F5 tornadoes.  CAPE (a measure of instability) was also high, and 1km Storm Relative Helicity (SRH) was at 328, a value that favors supercells.

So, ingredients in place… a strong squall line formed.  It’s sometimes difficult to issue tornado warnings on a squall line, as tornadoes in this type of situation can be hard to detect.  The ones that do develop can spin up very quickly and can be short-lived… possibly not even being on the ground anymore by the time the warning is actually issued.  Typical squall-line tornadoes would be EF0 or EF1 strength, but this was not a typical event.  On April 27th, EF3 tornado damage occured with the squall line, and it was definitely worth warning for.

Something interesting is the different types of tornado warnings that went out.  The Huntsville NWS office went with 30-minute polygon warnings… and the Birmingham office went with 60-minute warnings.  There are pros and cons to each… with the shorter-duration warnings, there is minimal overlap, so less confusion… but also increased workload on the meteorologists.  Not to mention you could lose lead-time on the next warning.  With the longer, 60-minute warnings, you definitely get that better lead-time, but you do get more overlapping warnings… different parts of different counties, and that can confuse people.  Also, there is some question as to if this is too much lead time.  Is there such a thing?  I will discuss this in a later blog.  Meteorologists from these NWS offices say that the time of day, population in the area, and confidence that there is actually a tornado help determine how long their warnings are issued.  For the longer-duration warnings, they will issue more special weather statements… with the wording “Tornado Emergency” in extreme cases.  On this date, the Huntsville office used that wording nine times, while the Birmingham office used it 38-times!

Here is a radar image of the morning QLCS (Quasi-Linear Convective System, aka Squall Line) event early that morning:

Now one big problem was that this line of morning storms knocked out power to much of the area.  This was extremely dangerous, as more severe weather would occur… and these people would be without TV’s to follow along when their local meteorologists were tracking even stronger tornadoes in the area.  Many ended up following the action on their cell phones–social media, including Facebook and Twitter–were HUGE tools during this event.  As many sat disconnected from the broadcast world, yet another line of severe weather developed during the late morning hours ahead of an approaching cold front.
But by far the worst episode of severe weather was the late-day event.  There was a deep negatively tilted trough at 300mb… winds were flowing quite strongly into an approaching area of low pressure… backing winds at the surface, and crazy shear in the area.  A capping inversion showed up at 700mb on sounding… which really allowed CAPE to build up (high instability).  There was also a thermal boundary at the surface, as Northern Alabama was cooler than the central part of the state.  The EHI… Energy Helicity Index…was through the roof!!  Anything over a “2″ value can mean supercells.. and values over 5 indicate that EF4/EF5 tornadoes would be possible.  That day 11-13 values were present!  Here is a look at the bullseye of 0 to 6 km helicity values that evening:

And a radar snapshot of a very historic… and deadly… moment in weather history:

You are looking at numerous supercell thunderstorms on the image above, some of which are producing EF4 and EF5 tornadoes, with winds up to 210 mph!  The images that we saw on TV were almost unbelieveable–huge metropolitan areas, like Birmingham in the foreground with big monster tornadoes in the background.  It almost looked like something from a movie:

The scars from these storms are still very visible today–even six months later.  It takes months and months to clean up storm damage, and while there has been much progress, there is still a long way to go. Below are some pictures taken just last week in neighborhoods affected by this deadly tornado outbreak:

The last shot is at least encouraging… new development, as people are rebuilding.  Again, it takes time.  It’s so easy to forget what’s going on down there when the national media turns it’s attention to the next big thing.  The people of this region are still dealing with the affects of tornadoes from back in early spring… their lives forever changed, and in some cases, lost.