Worst Tornado Outbreak?


     The April 27, 2011 tornado outbreak was one for the record books.  The Storm Prediction Center (SPC) issued a “High Risk” of severe storms for areas of eastern Mississippi, central and northern Alabama, extreme northwest Georgia (GA), and parts of southeastern Tennessee (TN) the morning of the tornado outbreak.  A large “Moderate Risk” area was also issued for much of the South and Tennessee Valley.  Below was their Day 1 Convective Outlook valid 12Z 4/27 through 12Z 4/28. (Links above show descriptions of each risk)

Figure 1. SPC's Day 1 Convective Outlook valid 12Z 4/27/2011 through 12Z 4/28/2011.

        Because it’s so easy to concentrate on the High Risk area, it is very easy to ignore the Moderate and Slight risks. Though SPC did a great job forecasting this event, this is not always the case.  There have been  occasions in which the majority of the severe weather reports were outside the High Risk area, when a High Risk was issued.

Figure 2. SPC reports from 4/27/2011 as of 4:30 CDT 4/28/2011. Red dots are tornado reports, Green dots are hail reports, Blue dots are wind reports. Black triangles are "Large" hail reports and Black squares are "High" wind reports. (**NOTE: This map of reports is not up to date**)

     As this blog is being written, there has been a total of 170 tornado reports (shown in Figure 2 above) reported to the SPC from the April 27, 2011 severe weather outbreak.  This DOES NOT mean there were 170 tornadoes; “reports” may contain multiple reports of the SAME tornado.  All reports are PRELIMINARY.  I did an overlay on how accurate SPC’s outlook was compared to the reports, it is shown below.

Figure 3. Overlay of SPC's convective outlook from 4/27 AND SPC's strom reports from 4/27.(**NOTE: reports are not up to date**)


     What caused all of this severe weather to happen and why are there so many casualties?

     The following maps are upper-air maps taken from Storm Prediction Center’s Upper-Air Maps at 00Z 4/28/2011 (7:00 pm CDT).  The 12Z 4/27  maps were also quite impressive, but for lack of space and time, I will just show you the 00Z maps. *Please click on each map to enlarge.*

At 300 mb:

Figure 4. 300mb map from 00Z 4/28.

        As you can see in figure 4, above, an intense negatively tilted trough is visible with a nice jet (highest winds speeds) near the base.  The yellow lines are lines of divergence.  At 300mb (~30,000 ft), this indicates rising motion at or near the surface.  Rising motion is needed to produce clouds, rain, and storms.  So, instability is already seen at the upper-levels with this powerful trough and upper-level divergence.

At 500 mb:   

Figure 5. 500mb map from 00Z 4/28.

     Instability is also present at 500 mb (~18,000 ft).  Notice the enclosed line (circle) over Minnesota (MN) and Wisconsin (WI).  This is a closed low, which usually indicates a strong weather system (it is also slightly visible at 300 mb).  The severe weather tends to, but not always, occur on the east side of the trough axis because of warm, moist advection that makes the lapse rates just right for severe weather.  There is a jet max (highest wind speeds) over Louisiana, Mississippi, and Alabama, wind shear needed for severe storms.

At 850 mb:

Figure 6. 850 mb map from 00Z 4/28.

     As we get closer to the surface, you can tell the upper-level/mid-level low gets stronger over the Great Lakes region.  Once again, if you notice the trough axis: Mississippi, Alabama (AL), Tennessee (TN), and Georgia (GA) all fall east of the axis and at this point; the Low Level Jet (LLJ) is strongest over AL.  Also, the green lines indicate dewpoint temperatures >= 8 degrees C, or moisture from the Gulf of Mexico.  Low level moisture (at or around 850 mb) is something needed for severe weather.  This moistens the atmosphere and makes the troposphere very unstable, affecting the lapse rates.  Warm air under cold air supports convective mixing for thunderstorms.

At The Surface:

Figure 7. Surface fronts and pressures from HPC from 00Z 4/28

          As we look at the surface, you first notice the cold front extending from Canada through the Midwest region and down through MS and LA.  Warm, moist air converges with the cold front and rises above the front, causing lifting.  And as I mentioned before, lifting at the surface creates clouds, which leads to rain, then thunderstorms.  With warm, Gulf moisture lifting over cooler, drier air,  AND the upper-level support we just saw above, this is why there was a huge tornado outbreak.

     The reason why so many people were affected by this massive storm system was because a lot of the tornadoes occurred in heavily populated areas, because there were numerous tornadoes, and the environment was rich with energy to support large, long-lasting tornadoes.


      The last update I received there were 272 total fatalities (subject to change) for 4/27/2011, alone.  I will continue to update the statistics below.  Unfortunately,  this number is expected to rise as many people are still missing.  Hundreds of thousands, if not millions of people lost power in the state of Alabama, alone.   Hundreds, if not thousands of people have been injured by the storms, some still missing.  As I said before, this day will be one for the record books.  The National Weather Service will be conducting many surveys where tornadoes have struck to determine the magnitude of the tornado.  There have already been preliminary reports of EF-3s and EF-4s around MS and AL.

    Going back to my blog title question: Is this the worst tornado outbreak the U.S has ever seen?  It is hard to say right now.  It’s especially hard to compare to the “Super Outbreak” that occurred on April 3, 1974, in which 315 people lost their lives and more than 5,000 people were injured.  This was considered to be one of the deadliest tornado outbreaks in the U.S since 1950.  There still needs to be a lot of surveys done and more data collection and it may take weeks or even months to sort everything out. (Link shows more stats)


STATS FOR 4/27 TORNADO OUTBREAK (as of 3:00 PM CDT 5/07):

Deaths from this Event342 (at least 250 in Alabama)

Tornado Deaths from the Month of April : ~395

SPC Tornado Reports on 4/27 : 292*

SPC Tornado Reports the Week Of**: 514 

SPC Tornado Reports for the Month of April: ~1,091

Preliminary Number of Tornadoes (CONUS) for the Month of April: 875

Preliminary Number of Tornadoes that Occurred from 8 AM 4/25 – 8 AM 4/28:  305

*NOTES: “Reports” may contain multiple reports of the same tornado.
**NOTES: “Week Of” refers to the week of 4/24 to 4/30 2011.

All sources are provided in links above. 

**More info. found at The WeatherMatrix’s blog found here***


Rank: 2nd deadliest of all time in U.S. history.

1.) March 18, 1925 – 747 deaths

2.) April 27, 2011 – 342 deaths so far

3.) March 21, 1932 – 332 deaths

4.) May 17, 1840 – 317 deaths

5.) April 3, 1974 – 310 deaths

6.) May 27, 1896 – 305 deaths

Source: Harold Brooks, NSSL


Number of Tornadoes Sorted By EF-Scale, Surveyed & Confirmed by the NWS:

EF-0: 26
EF-1: 38
EF-2: 31
EF-3: 19
EF-4: 14
EF-5: 3




I will end the blog with a few devastating pictures posted on Boston.com:

Overnight tornadoes left part of Pratt City, Alabama in ruins April 28. (Marvin Gentry/Reuters)

Pleasant Grove, Ala., sits devastated after an April 27 tornado. (Butch Dill/AP)

Michael Dunn is hugged by his mother Patricia Dunn as they stand in the road that lead to his house which was destroyed after a tornado touched down April 27 in Concord, Ala. (Jeff Roberts/Birmingham News/AP)

Residents survey the destruction after a tornado hit Pratt City, Ala. April 27. (Butch Dill/AP)

A car lies overturned and buildings destroyed in Tuscaloosa April 27. (Dusty Compton/Tuscaloosa News/AP)

 Comments and Questions ALWAYS welcome.

About Brian

University of Oklahoma graduate with a degree in Meteorology. Follow me on Twitter: @WeatherInformer
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1 Response to Worst Tornado Outbreak?

  1. Pingback: Help! I’m Having a HAARP Attack! (Pt. I) « The Unexamined Life

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