Watch the Cullman tornado, which some people describes as a "demon, reaching down." Watch the entire thing. Evil.
One thing that had people on edge, but probably saved many lives in the Bessemer and Birmingham area, is that as we watched the tornado tearing up Tuscaloosa, we knew that we were in its path. We knew to prepare. Again, watch the entire video.
So why did this happen?
Here are some descriptions from the ABC33/40 weather blog prior to these storms. (I've put in bold the most important phrases as they relate to this discussion).
April 27, 2011
Dr. Tim Coleman 9:31 AM
Sunshine will warm temperatures quickly this morning through the 70s, and it will reach the lower 80s by afternoon. However, the morning balloon data from NWS Birmingham showed very cold air aloft. And, even though dewpoints dropped to the upper 50s as the thunderstorms mixed up the atmosphere, they are already rising, and will rise throughout the day. As the sun continues to heat the atmosphere up, the 70 mph winds at 3,000 feet will start to get mixed with the surface layer winds, so gradient winds will get very gusty by late morning, probably 30 mph at times. And those winds will be out of the south, bringing in Gulf moisture (dewpoints in SW Alabama are in the 70s). Once we get warm, humid air back in place at the surface by noon or so, with cold air aloft, the air will be very unstable. CAPE values (atmospheric potential energy) will reach 3,000 to 5,000 J/kg, supporting strong updrafts and intense storms this afternoon.
The other problem is the wind shear. The main upper-level system will approach by late afternoon. Winds will increase in speed and change direction with height, creating spin about a horizontal axis. Storm updrafts will tilt this rotation into the vertical (like a spiraling football getting tipped at the line of scrimmage), producing rotating storms. The helicity, or a measure of this wind shear, will also be very high, between 300 and 600 m2/s2. It is rare to see this much wind shear in an almost summerlike environment with warm, humid air like this.
The combination of extreme instability (like we see on summer afternoons) and extreme wind shear (like we see often in winter) will create a very dangerous setup over Alabama this afternoon. The energy-helicity index (a combo of CAPE and wind shear) measures the overall tornado potential well. Anything over 2 means tornadoes are possible, and over 5 tornadoes are likely. On April 8, 1998, the EHI was around 6. Today, as you can see on the maps below for 3, 5, and 7 pm, we expect EHI values of 8-10 over much of north and central Alabama, some of the highest I’ve ever seen.
Bill Murray 11:00 am
I don’t think I have ever heard a tornado watch referred to as a long lead time, long duration, particularly dangerous situation tornado watch before. But I have now in the latest mesoscale discussion from SPC.
That sounds bad…and it is bad. It will be issued for much of North and Central Mississippi, possibly into West Central Alabama.
James Spann 1:29 pm
A major severe weather outbreak is likely anytime from now until midnight. See the high STP (Significant Tornado Parameter) values from the RUC at 1:00… remember, anything over 2 is very significant. These are about as high as they get.
So let's look at the possible reasons for this severe weather outbreak. Remember, I am one who does not like to use anecdotal evidence to prove a position, but I will use it to support one.
God did it. Nope. God does not micromanage our lives or the world around us. But God probably does like to see how we respond to crisis, and there is opportunity for good to come out of any situation, including this one.
Random event. For weather or any other measurable thing there is a range of values for normal occurrences and sometimes results fall outside of that range. Records will be broken. This could just be a huge event, an outlier, that will now be the record that someday will be broken.
Result of global warming. Hmmm. With climate change, experts agree that there will be more extreme weather events. This could mean more frequent snowstorms (oddly enough) and more powerful hurricanes (Katrina). But tornadoes?
According to a 2007 report NASA scientists have developed a weather model that predicts that really violent storms may become more common due to global warming. Read it.
Their model simulates the strength of updrafts in storms, and estimates how the strength will increase with a warming climate. They explain this in the article, then conclude with:
So there is evidence that climate change, aka global warming, makes this type of monster storm more possible because it creates an atmosphere where updrafts are stronger and horizontal winds are stronger. Together that provides the recipe for disaster.
The central and eastern areas of the United States are especially prone to severe storms and thunderstorms that arise when strong updrafts combine with horizontal winds that become stronger at higher altitudes. This combination produces damaging horizontal and vertical winds and is a major source of weather-related casualties. In the warmer climate simulation there is a small class of the most extreme storms with both strong updrafts and strong horizontal winds at higher levels that occur more often, and thus the model suggests that the most violent severe storms and tornadoes may become more common with warming.
The prediction of stronger continental storms and more lightning in a warmer climate is a natural consequence of the tendency of land surfaces to warm more than oceans and for the freezing level to rise with warming to an altitude where lightning-producing updrafts are stronger. These features of global warming are common to all models, but this is the first climate model to explore the ramifications of the warming for thunderstorms.