Good Friday 2009 may seem like any other day to persons outside the borders of Rutherford County, but for folks inside those borders, that day will forever be known as the day of the Murfreesboro tornado. The Good Friday tornado outbreak spawned 85 tornadoes across the Deep South over the course of two days, ten of which impacted Middle Tennessee. The most powerful tornado of the ten struck Murfreesboro around lunchtime on April 10. Rated an EF-4, the tornado hit with winds exceeding 170mph, was nearly a 1/2 mile wide, and a damage path length of 23 miles. Nearly 900 structures were damaged or destroyed. This tornado touched down and continued unimpeded through sections of the city that were heavily populated. While we mourn and remember the two individuals we lost, there easily could’ve been dozens more.
The Murfreesboro tornado was only one of ten that impacted the Middle Tennessee area, but was by far the strongest. The other nine tornadoes that struck the area were rated EF-0s and EF-1s. Clearly, the atmospheric ingredients were supportive of multiple tornadoes, and the possibilities of strong, long-lived tornadoes were tangible, but the question we keep asking ourselves is, “Why was this tornado such an anomaly…why was this one so much stronger than the others?”. As we go along through this retrospective, we’ll investigate this notion, form some ideas, render a hypothesis, and hopefully come to a plausible conclusion.
April 8, 2009
Weather models were beginning to hint upon a fairly significant weather event for the Deep South a few days before Good Friday. All of the indicators forecasters – and amateur meteorologists – look for during severe weather episodes were prevalent; instability, temperature, atmospheric moisture, wind shear. Seeing one or two of these indicators progged via numerical models isn’t alarming, but seeing all four factors present in the same geographical region is eye-opening. The Storm Prediction Center (SPC) began bringing notice to the potential of the event via their Day 3 Convective Outlook:
"...OH/TN VLYS AND THE DEEP SOUTH... AT LEAST ISOLD SVR TSTMS WILL BE POSSIBLE EARLY IN THE PD ACROSS PARTS OF THE MID-SOUTH AND LWR MS VLY...ALONG/AHEAD OF THE SFC LOW/CDFNT. THIS ACTIVITY SHOULD BEGIN TO WEAKEN AND MOVE ENE DURING THE MORNING. PRE-FRONTAL PCPN-INDUCED MOISTENING AND STRONG LLVL MOISTURE TRANSPORT WILL LIKELY DESTABILIZE A LARGE REGION AHEAD OF THE ADVANCING UPR SYSTEM DURING THE AFTN. SFC-BASED TSTMS SHOULD INITIATE BY MID-AFTN...ESPECIALLY ALONG THE TRACK OF THE STRONGEST HEIGHT FALLS...NAMELY FROM ALONG THE OH RVR SWD INTO THE TN VLY. THOUGH THE MID-LVL SYSTEM WILL BE WEAKENING WITH TIME...PRESENCE OF MODEST WLY MID-LEVEL FLOW ATOP 30 KTS OF WSW H85 FLOW WILL PRODUCE SUFFICIENT SHEAR FOR ORGANIZED STORMS WITH LARGE HAIL...DMGG WINDS AND PERHAPS TORNADOES. THESE STORMS WILL MOVE QUICKLY ENE TO THE CNTRL APLCNS OVERNIGHT...POSSIBLY AS SMALL-SCALE LEWPS/BOWS GIVING AT LEAST ISOLD SVR WIND PROSPECTS TO THE E OF THE MOUNTAINS INTO SRN VA."
Word began to spread through the interwebs that a potentially significant weather event was starting to look possible later in the week. The various news markets weren’t sounding the alarm bells too loudly, nor should they have. The scheduled event was more than 60 hours out. However, the model data on Thursday looked even more concerning and all forms of media began to take notice.
April 9, 2009
The situation was becoming a bit more ominous as the latest data was being reviewed concerning the 10th. The severe weather parameters were increasing and it was becoming quite clear that a potentially damaging severe weather event was going to happen Friday. The only question that remained was who was going to get hit the hardest. The SPC highlighted Middle Tennessee as “ground zero” for the event. Here is the SPC’s Day 2 Convective Outlook:
"...OH/TN VALLEYS INTO THE CNTRL/ERN GULF STATES AND EWD TO THE CAROLINAS... EML/STEEP LAPSE RATE PLUME OBSERVED OVER THE SRN PLAINS THIS MORNING WILL BE ADVECTED EWD INTO THE REGION WITH WSWLY AIRFLOW REGIME...ALONG SRN THROUGH ERN PERIPHERIES OF MIDLEVEL TROUGH. WHILE WARM SECTOR BOUNDARY LAYER WILL NOT BE OVERLY MOIST /I.E. DEWPOINTS IN 50S OVER KY/TN TO LOWER/MID 60S OVER THE GULF STATES/...THESE STEEP LAPSE RATES COUPLED WITH GENERALLY COOL MIDLEVEL THERMODYNAMIC PROFILES WILL YIELD MLCAPE APPROACHING 1000-1500 J/KG OVER THE TN VALLEY/CUMBERLAND PLATEAU...TO 1500-2000 J/KG OVER THE CNTRL/ERN GULF STATES. TSTMS /SOME SEVERE/ ARE EXPECTED TO BE ONGOING FRIDAY MORNING WITHIN ZONE OF DEEP ASCENT AHEAD OF SURFACE LOW OVER PARTS OF THE LOWER OH AND LOWER MS VALLEYS...AS WELL AS WITHIN WAA REGIME FARTHER E ACROSS PARTS OF ERN TN/AL/NRN GA. THE FORMER ACTIVITY WILL LIKELY INCREASE IN BOTH COVERAGE AND INTENSITY BY LATE MORNING/EARLY AFTERNOON ALONG/AHEAD OF COLD FRONT FROM CNTRL KY/MIDDLE TN SWD INTO AL AS STRONGER FORCING/HEIGHT FALLS ASSOCIATED WITH SECONDARY VORTICITY MAXIMUM ACT ON DESTABILIZING AIR MASS. GIVEN THE STEEP LAPSE RATES/MODERATE INSTABILITY AND DEEP-LAYER SHEAR RANGING FROM 30-40 KT INVOF SURFACE LOW OVER KY TO 50-65 KT INTO MS/AL/GA...ENVIRONMENT WILL BE QUITE SUPPORTIVE OF EMBEDDED SUPERCELL STRUCTURES CAPABLE OF VERY LARGE HAIL AND DAMAGING WINDS. CURRENTLY...IT APPEARS THAT THE GREATEST TORNADO THREAT WILL EXIST ACROSS MIDDLE/ERN TN INTO NRN PARTS OF AL/GA AND PERHAPS AS FAR E AS WRN SC FRIDAY AFTERNOON INTO EVENING. HERE...REINTENSIFICATION OF A SWLY LLJ WILL OCCUR IN RESPONSE TO ABOVE-MENTIONED...SECONDARY VORTICITY MAXIMUM RESULTING IN NOTABLY STRONGER LOW-LEVEL SHEAR /I.E. 0-1 KM SHEAR APPROACHING 30-35 KT/. SHOULD FUTURE NUMERICAL GUIDANCE AND OBSERVATIONAL DATA CONTINUE TO SUGGEST A SIMILAR THREAT...A MODERATE RISK MAY BE REQUIRED IN SUBSEQUENT DAY ONE OUTLOOKS."
That’s particularly strong wording coming from the SPC, even more so when the “event” is not scheduled to begin in earnest for another 24-36 hours. Quite often, the “MODERATE” risks the SPC alluded to are relegated to events that potentially pose a significant danger to life and property. Unfortunately, that became all too real on Friday.
April 10, 2009
Good Friday began rather inauspicously. Temperatures were rather cool, but the air mass was really moist. Here’s the observed surface analysis, valid for Good Friday at 8:30am:
Instrumentation at Nashville International read 53*F, with a dew point of 51*F. Not out of the norm at all for an early spring morning. However, was the presence of a surface low over the southern Ozarks of Missouri was cause for concern. The counter-clockwise flow around that low pressure area did two things: 1) it provided the focus for enhanced lift and storm initiation, and 2) it sent gobs of warm, moist air surging into Tennessee, as evidenced here by the surface observations three hours later.
Notice the readings at Nashville International at 11:27am. In just three hours, temperatures have risen 6 degrees and the dew point has risen 3. That’s a tremendous airmass modification for such a short duration of time, especially when the sunshine is at a minimum. Here’s the water vapor imagery taken at 11:32am:
There was very little – if any – sunlight warming the atmosphere. The airmass was indeed modifying at a frightening rate and the SPC noticed it. At 10:00am, they issued a Mesoscale Discussion. These are discussions highlighting changing atmospheric conditions and the potential for strengthening weather. This discussion detailed the threat for tornadoes and damaging hail was increasing for across Middle Tennessee for the next few hours.
"THE THREAT FOR TORNADOES...LARGE HAIL AND DAMAGING WIND CONTINUES ACROSS A PORTION OF WRN TN...WRN KY AND NRN MS. THE THREAT WILL GRADUALLY SHIFT EWD INTO MIDDLE TN...S CNTRL KY DURING THE NEXT COUPLE HOURS. ACTIVITY WILL ALSO BEGIN TO AFFECT PARTS OF NRN AL WHERE A NEW WW WILL LIKELY BE NEEDED SOON. STORMS THAT ORIGINALLY DEVELOPED ALONG THE COLD FRONT HAVE MOSTLY DISSIPATED. HOWEVER...NEW STORMS HAVE DEVELOPED IN OVER WRN TN AND NRN MS ALONG CLOUD STREETS WITHIN WARM SECTOR JUST EAST OF THIS BOUNDARY. DESPITE MORE LIMITED MOISTURE WITH NWD EXTENT INTO KY...COLD TEMPERATURE ALOFT WILL MAINTAIN ADEQUATE INSTABILITY IN THIS REGION. LATEST OBJECTIVE ANALYSIS INDICATES AN AXIS OF MODERATE INSTABILITY FROM MS NEWD THROUGH WRN AL INTO WRN AND MIDDLE TN WHERE HIGHER DEWPOINTS EXIST. FORCING FOR ASCENT ASSOCIATED WITH EWD ADVANCING VORT MAX...NWD ADVECTION OF LOW LEVEL MOISTURE AND BOUNDARY LAYER WARMING WILL CONTINUE TO PROMOTE THUNDERSTORM DEVELOPMENT IN THESE AREAS NEXT FEW HOURS. BULK SHEAR AND LOW LEVEL HODOGRAPHS WILL REMAIN MORE THAN SUFFICIENT FOR SUPERCELLS AND BOWING SEGMENTS WITH A THREAT OF TORNADOES...LARGE HAIL AND DAMAGING WIND.
At 11:23am, the SPC issued their midday Day 1 Convective Outlook. As promised in their previous discussions, a MODERATE risk area had been issued for Middle Tennessee (the MODERATE risk was issued in the previous early morning outlook, and was carried over to the afternoon outlook):
"...OH/TN RIVER VALLEYS ACROSS CENTRAL GULF COAST STATES/SRN APPALACHIANS AND PORTIONS OF THE SOUTHEAST... OVERALL SETUP CONTINUES TO FAVOR A POSSIBLE OUTBREAK OF SEVERE THUNDERSTORMS ACROSS MUCH OF THE TN RIVER VALLEY/SRN APPALACHIANS AND ADJACENT AREAS THROUGH THE EVENING. MORNING MODELS AND OBSERVED DATA TRENDS INDICATE LITTLE CHANGES ARE NECESSARY TO THE OUTLOOK. 15Z SURFACE MESOSCALE ANALYSIS AND VISIBLE IMAGERY INDICATE RESIDUAL OUTFLOW BOUNDARY FROM FAR UPSTATE SC/NRN GA NWWD INTO N-CENTRAL AL AND MIDDLE TN. THIS BOUNDARY HAS CREATED AN AXIS OF GREATER MOISTURE /I.E. 60+F SFC DEW POINTS/ AND RESULTANT MLCAPE NOSING INTO MIDDLE TN LATE THIS MORNING. IN ADDITION...MOSTLY CLEAR SKIES ARE SUPPORTING INCREASED MIXING AND VEERED SURFACE WINDS WITHIN WARM SECTOR ACROSS MUCH OF MS INTO FAR NWRN AL...AHEAD OF MAIN COLD FRONT NOW MOVING ACROSS THE MID SOUTH. INTENSE/DEEP ASCENT IMMEDIATELY AHEAD OF UPPER LOW IS FOCUSING MOST ROBUST SEVERE THUNDERSTORM DEVELOPMENT LATE THIS MORNING FROM WRN KY INTO FAR NRN MS. GREATEST BACKING OF SURFACE WINDS/SHEAR WILL LIKELY CONTINUE FROM THE TN RIVER VALLEY NWD TO THE OH RIVER VALLEY AS SURFACE LOW SHIFTS INTO CENTRAL KY. ALTHOUGH QUESTIONS REMAIN ABOUT NWD EXTENT OF RICHER LOW LEVEL MOISTURE WHICH MAY BE TRAPPED FROM FAR S-CENTRAL KY SWD AS TRIPLE POINT SHIFTS ACROSS MIDDLE TN. REGARDLESS...DEEP LAYER SHEAR AND SBCAPE WILL INCREASE THROUGH THE DAY AND SUPPORT AT LEAST A THREAT OF ISOLATED TORNADOES NWD TOWARDS THE OH RIVER WITH VERY COLD MID LEVEL TEMPERATURES OFFSETTING MARGINAL SURFACE MOISTURE. THREAT WILL BE ENHANCED NEAR THE TRIPLE POINT OF OUTFLOW BOUNDARY AND NE-SW ORIENTED BROKEN LINE OF CONVECTION AS IT SHIFTS ACROSS SRN/MIDDLE TN THIS AFTERNOON AND INTO THE SRN APPALACHIANS THIS EVENING...INCLUDING POTENTIAL FOR A FEW STRONG AND/OR LONGER-LIVED TORNADIC SUPERCELLS. ATTM...APPEARS SRN-MIDDLE/SERN TN AND NRN AL/NWRN GA WILL SEE GREATEST RISK OF DAMAGING TORNADOES."
At 11:34am, a Tornado Watch was issued for Rutherford County, and not 45 minutes later, the first tornado warning of the day for Rutherford County was issued at 12:13pm for a reported funnel cloud just to the east of Franklin. That warning didn’t expire until 12:45pm, but at 12:38pm, the National Weather Service in Nashville issued a Tornado Emergency for Rutherford County:
“…TORNADO EMERGENCY FOR MURFREESBORO AND NEARBY LOCATIONS…
...A TORNADO WARNING REMAINS IN EFFECT UNTIL 115 PM CDT FOR SOUTH CENTRAL WILSON...RUTHERFORD AND NORTHWESTERN CANNON COUNTIES... AT 1236 PM CDT...TRAINED WEATHER SPOTTERS REPORTED A TORNADO. THIS TORNADO WAS LOCATED 5 MILES WEST OF MURFREESBORO...MOVING NORTHEAST AT 30 MPH. THE TORNADO WILL BE NEAR... MURFREESBORO BY 1245 PM CDT... WALTERHILL BY 1255 PM CDT..."
According to the NWS in Des Moines, IA, a tornado emergency “generally means that significant, widespread damage is expected to continue and a high likelihood of numerous fatalities is expected with a large, strong to violent tornado.” Unfortunately, that is exactly what we had on the ground. The supercell that prompted the warning had been strengthening as it propogated across the midstate, and it finally spawned a tornado across far western Rutherford County. Storm spotters reported seeing the tornado over mainly rural sections of the area, but it wasn’t until the tornado was caught on the TDOT TrafficCam at I-24 and I-840, that we knew how powerful and dangerous it was going to be.
Radar Doesn’t Lie
The Rutherford County storm began as an embedded thunderstorm with supercellular-like structures, but quickly transformed into a legitimate supercell, evidenced by the radar presentation. Using GR2Analyst, a specialized radar program, we were able to download and “load” old radar data of the storm for reanalysis. Up first is the base reflectivity spanning the life of the tornado.
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Notice on the first screengrab, you see a faint hook echo, which is the “calling card” of the supercell. The storm loses the hook echo, but soon regains it with a vengeance, featuring a debris ball. During the most recent Graduate Spotter Class put together by the good folks at NWS Nashville, Scott Unger, the meteorologist guiding the class, said that radar presentations of debris balls are reserved for the most powerful of tornadoes (EF-4s and EF-5s). I think this image lends credence to that.
The storm relative velocity data of the storm is equally as impressive as the base reflectivity. Storm relative velocity, or SRV, is base velocity minus the storm relative motion. Put simply, what we’re really seeing is the true winds blowing towards and away from the radar site without the storm’s forward motion. The majority of storm damage was done as the storm crossed over Murfreesboro Road just N of Thompson Lane and continued northeast through populated subdivisons, finally lifting well east of Highway 231 and just south of Compton Road, near Lascassas Pike.
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Here’s a Google Earth image of the damage path via Facebook from NWS Nashville:
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Final storm survey information from NWS Nashville:
Rutherford County Tornado: Location / Time of event: Rutherford County, 04/10/09 12:19 PM Beginning Point: Kelley Road and Hwy 41A - SW of Murfreesboro Ending Point: Eight (8) Miles NE of Murfreesboro Rating: EF4 - Maximum estimated wind speed around 170 MPH Path Length: 23.25 Miles Maximum Width: Almost 1/2 Mile Fatalities: 2 Injuries: 42 KML Data File: Good Friday Tornado Outbreak 04/10/09 Summary of Damages: ...MONDAY UPDATE... ...APRIL 10TH TORNADO IN MURFREESBORO REACHED EF-4 INTENSITY... THE NATIONAL WEATHER SERVICE DAMAGE ASSESSMENT TEAM HAS DETERMINED THAT THE TORNADO WHICH STRUCK MURFREESBORO ON FRIDAY AFTERNOON REACHED EF-4 IN INTENSITY. EF-4 INTENSITY...WITH WINDS NEAR 170 MPH...WAS REACHED IN THE HIGHLAND PARK DRIVE NEIGHBORHOOD AS WELL AS THE TOMAHAWK TRACE AREA. SEVERAL WELL CONSTRUCTED HOMES WERE DESTROYED IN THESE AREAS...HARDWOOD TREES WERE DEBARKED...AND VEHICLES WERE TOSSED CONSIDERABLE DISTANCE. THE TORNADO WHICH STRUCK FRIDAY WAS THE 28TH TORNADO TO HIT RUTHERFORD COUNTY SINCE 1950. IT WAS THE FIRST KILLER TORNADO DURING THAT PERIOD.
Damage was extensive around the Tomahawk Drive area of Murfreesboro. Several well-constructed homes were either flattened or blown off of their foundations. The two fatalities sustained by the tornado also occured in this area. This storm affected a great many of our community and a select few decided to share their experiences with us:
“Well 5 years ago my mom, my sisters and I went to Murfreesboro to help my grandmother around the house. While we were helping her she decided to go take a shower. We were doing our own little things with the lights on and the tv running and all of a sudden, BOOM,everything off. We look outside the backdoor and there was a tornado at the maximum of 1/2 mile away, not knowing if It was going to turn towards us or anything. When all this was over my grandmother walks out and has no clue about what had just happened.”
Thought I’d share my parents story of the tornado. My mother & two younger brothers were at their home when my Dad tried to call panicking because as he sat in line at the bank nearby he saw a massive tornado touch down right in the direction of their house. He was sure it leveled them. My two younger brothers crawled into the crawl space of the converted garage but my Mother braved it (stupid really) by the front door. She watched it touch down across the way on Tomahawk and mow blocks of houses and trees down as it then moved up onto Sulphur Springs were it destroyed more homes and lives. It took a while before my got thru to find out they were ok.
My husband and I came that evening to help them clean up their yard and got to survey the damage. I will never forget it. The normal drive down Broad street looked completely different. The precious lives that were lost have not been forgotten either.
If I can dig up the amazing picture my family captured I will send it along as well.”
“I was working in Franklin that day. My wife and daughter were out shopping at Target in Murfreesboro unbeknownst to me. The storm was bad in Franklin but not tornado. As I tracked it, I saw it was heading right through the Blackman area, which is near our house. I tried calling my family and had no answer. Really started to panic when it was confirmed that it touched down and was fairy prolonged track. Called frantically trying to reach wife, no answer. Calling friends to see if they had heard from my family. Nothing. About 30 minutes later as I was about to leave work and dash home, my wife called me from the parking lot of Target. They had been hunkered down in the back of the store during the store and had no signal. Hence no phone calls to make or receive. My daughter was 4 at the time and the storm/tornado really made an impression on her. She has been fascinated with tornadoes ever since. So much so, we watched Storm Chasers on Discovery, a lot. She read any book on tornadoes we brought home. Last year we drove up to Louisville Science Center twice within 5 days so she could meet Dr. Josh Wurman and see the DOW and then to meet Sean Casey and sit in TIV2 and also to watch his recently released IMAX film (I have pics/video of her meeting them). She wants to be a storm chaser to this day. Also, a family at church had their garage and deck wiped out over in the Haynes Rd area of Murfreesboro. Our church and many other volunteers came out in droves to help clean up and bring food and water to those affected. Even my 4 yr daughter helped. Powerful to see the destruction. More powerful to see people come together and help. “
“The day of Apil 10, 2009 to me was an amazing Day. I was having lunch with a friend at Burger King on Memorial when I received a call from another friend who works at the Murfreesboro Airport. He said he was looking at the radar at the airport and said that it looked as if a nasty storm was headed our way. When the first tornado warning was issued I decided, with no experience (DO NOT TRY THIS) to chase this tornado warned storm. I heard from the radio that a funnel cloud was spotted at the Almaville exit off of 840, so i got into my car and got on I-24 and went towards the 840 junction. Once i got towards Medical Center Parkway the cars in front of me disappeared because the hail and rain was so heavy and the wind was so strong that i was only going about 10mph on the interstate. I went to the almaville exit on 24 and took murfreesboro road back from smyrna and that is when i realized that a tornado has been through the town. I immediately tried to get to the areas to help, but the cops had everything blocked off and I was not able to help the way i wanted.”
Jennifer’s personal pics:
“I’ll forever remember this day as “the one that got away”. Back in 2009, I was only a year into storm chasing and loving every second of it. My chase partner and I met up at the Wendy’s in Smyrna and decided to head south to catch some convection that was moving NE out of Mississippi. My meteorological prowess today is quite meager, but was light years behind that five years ago, and we really didn’t use meteorology when we chased…only our mobile radar. We got through Franklin around lunchtime and was on I-65 S when we heard the tornado warning/tornado emergency issued for Rutherford Co. Cursing loudly, we quickly exited the interstate N of Columbia and started making our way back E through rural Marshall Co, planning to hit Hwy 231 in Bedford Co and head N. We knew we wouldn’t make it to Murfreesboro to catch that storm, but we realized that every storm in the midstate was rotating and getting tornado-warned. We made it to Shelbyville, and parked in the Lowes parking lot off of Hwy 231 and waited for a tornado-warned storm coming out of Lewisburg. We were able to get good video of a rotating wall cloud, but it never produced a tornado. Getting pelted with hail, we drove N and tried to get into downtown Murfreesboro to survey damage, but they weren’t letting ANYBODY in. Dejected and aggravated, we grudgingly headed home and surveyed the damage through the television news stories. We learned a valuable lesson that day, and that carried over to our future chases…if you’re unsure which direction to head, the best advice is to let the storms come to you.”
(non user-submitted damage pics courtesy of OHX – http://www.srh.noaa.gov/ohx/?n=20090410)
Revisiting the Anomalous Claim
Earlier, we unofficially coined the Murfreesboro tornado as an “anomaly”. Merriam-Webster Online Dictionary defines “anomaly” as “something different, abnormal, peculiar, or not easily classified”. This tornado was not anomalous in the literal sense, but more in the general sense, as it was several categories stronger than any of the other tornadoes that touched down on Good Friday. Clearly, the storm was “abnormally strong”, but, why? Here is where we try to answer that question, but be forewarned. The following few sections will be packed with weather maps, weather models, and full-on weather geekery. Hopefully, we can make a case as to why this storm was so anomalous, and can fashion a hypothesis that passes the meteorological litmus test.
The first map shown is the 0z run of the NAM, valid for 7pm on April 9, 2009.
The top left map is the synoptic look at 850mb, roughly 5000′ above ground level. The surface low pressure is ~990mb with almost a due east trajectory towards Tennessee. The counter-clockwise flow helped to funnel in moisture-laden air that helped fuel the storms. The top right map depicts the upper level jet stream structure at 300mb, 30,000′ above ground level. Take a look at the area of greens rounding the base of the upper level low pressure area. That area of green is what we call a jet max, or jet streak. By definition, a jet max/jet streak is simply an area or pocket of faster winds contained inside the jet stream. These maxes/streaks help to provide upper level divergence for any lower-level convergence. Think of it like this…the surface low pressure is a vacuum, sucking up all the air particles around it. These particles get picked straight up and flow into the upper levels. Once these particles reach the upper levels, if they’re not “pushed away”, they’ll clog the system. These jet maxes and jet streaks “push away” these particles and that helps to promote lift for thunderstorm development in the atmosphere.
The next map is the 12z run of the NAM, valid for 7am, April 10.
We can see the surface low pressure is much closer to Tennessee, and the upper level wind fields are becoming very supportive of convective development later in the day. The atmosphere was changing, too, as evidenced by the weather balloons sent up by NWS Nashville. The 12z balloon sent back some rather benign numbers:
temperature = 51.8F
dew point = 46F
lifted index = +3.95 (air is very stable)
CAPE = 0
But the 18z balloon told a very different story:
temperature = 64F
dew point = 60F
lifted index = -4.45 (moderately unstable)
CAPE = 1112 j/kg
The 18z balloon went up before the Murfreesboro tornado, and the data gives credence to the type of atmosphere we were dealing with that day. Temperatures were warm, we had 60*F dew points, plenty of instability, and plenty of wind shear (upper-level support given by the jet stream and lower-level support by the surface low pressure).
The atmosphere was primed, and you could feel it. Everyone who has spent time in Tennessee knows what that “feel” means. The storms were coming…the only question that remained was what the severity would be. Even as lunchtime approached, the atmosphere was still changing. Not large, wholesale changes, but minute, microscale changes that quite often define severity and storm mode (QLCS, supercells, or nothing). Listed below are some data values gleaned from the SPC MesoAnalysis archives. These values are valid for 11am:
0-1km EHI (energy helicity index). The EHI is a computation involving instability and storm relative helicity. The value was 5-7. Anything over 1-2, is grounds for concern.
Bulk shear values averaged 50kts.
Lapse rates (the difference in temperature with elevation) were equally as impressive. Typically, the steeper the lapse rate, the greater the instability. The 500mb-700mb lapse rate was ~7.5C/km. Anything higher than 6C/km is cause for concern.
Lifted indices were -6 to -7. Severe thunderstorms are likely with values lower than -6.
SBCAPE (surface-based instability) values were around 1500 j/kg. That’s plenty of instability for big-time thunderstorms, and are quite indicative of values seen during major tornado outbreaks.
0-1km SRH (storm relative helicity) was between 300-400 m2s2 and 0-3km SRH was around 400 m2s2.
As we hit the 12p-1p timeframe, the atmosphere played a $1 slot machine and hit triple cherries. Every convective element present across Middle Tennessee began to rotate. I can distinctly remember my radar screen being covered by 17 different tornado warnings at the same time. But what happened? Sure, the atmosphere was clearly supportive of tornadoes, but what caused previously sub-tornadic storms to suddenly go full-tornadic? We submit for your perusal, our own hypothesis.
The evidence will be stated via bullet points, and the hypothesis proposed via paragraphical structure:
1) The dew point temperature had a 4 degree increase between 9am and noon;
2) The 0-3km EHI values increased from 5-7 to a solid 7 for much of southeastern Middle Tennessee between 11a and 12p;
3) The MUCAPE values (most unstable instability) increased by 1,000 j/kg between 9am and 12pm;
4) SBCAPE values increased 600-1,000 j/kg from 9a-12pm;
5) 0-1km SRH increased to 400 ms2s over much of Middle Tennessee
6) 0-3km SRH increased from 400 m2s2 to a 500 m2s2 bullseye over Rutherford County;
7) An amateur meteorologist once told me about something he noticed during years of studying severe weather events. He said to take note of where the 1004 and 1006mb isobars are on the surface weather map as the chances of a significant tornado are heightened in that general geographic area, as the differential between pressure allows for stronger, isolated lower level winds. Here’s the surface weather map for April 10, 2009;
Rutherford County just happens to lie squarely between the 1004 and 1006mb isobars. Clearly, this fact does nothing to prove or disprove his supposition or even if it makes meteorological sense, but I’ve known and talked with this amateur meteorologist for years and I have no reason to discount his opinion.
In summation, all the indicators of a potentially significant severe weather event were present and increased as we got closer to lunchtime. The big player was the weakening surface low pressure. As it tracked over southern Kentucky, it provided enough of a bump in wind shear and dew point temperature to raise the storm relative helicity and instability, and in doing so, lit the proverbial fuse. All of these factors explains why every storm began to rotate, but doesn’t really demonstrate why Murfreesboro’s tornado was so much stronger than the rest. In our opinion, we conclude, that it was the increasing 0-3km SRH values. That’s a tremendous amount of “tappable” wind shear that extended deep into the atmosphere. There was enough ambient moisture and enough ambient instability across all levels of the atmosphere, that once the updraft began to rotate, it tapped into all of the aforementioned ingredients and soon became a killer tornado. Can this hypothesis be proven? Probably not, but as we digested the available information, it became apparent that the factors laid out have merit.
EDIT: In the interests of full disclosure, I must detail a conversation I had with then-OHX Warning Coordination Meteorologist Tom Johnstone. I asked his opinion on what the impetus of the Murfreesboro tornado was and he admitted, while not 100% sure, that remnant outflow boundaries leftover from earlier convective elements led to the storms of April 10, 2009 to rapidly intensify.
On April 10, 2009, Murfreesboro lost it’s innocence. It was brutally ravaged by one of Mother Nature’s most powerful forces. Sadly, Kori Bryant and her daughter, the sweet Olivia Bryant were ripped from our lives, never to be forgotten. Our community lost homes; lost cherished keepsakes; lost our security. Five years ago, the geographic center of the state became ground zero for an event that most of us did not see coming. Has anything changed? Absolutely. The advent of social media has helped generate a new-found awareness across the meteorological community, and has saved countless lives during this country’s most recent tornado outbreaks. With the success of Twitter and various other outlets, as a community, we’re now able to gather information and see for ourselves what the forecast holds. Our community may have lost our innocence on that day those five short years ago, but since then we’ve gained our strength, increased our resolve, and heightened our awareness. We can all say that a tornado will never hit us again, but the odds are stacked squarely against it. However, when it does happen – and it will – we’ll be better prepared and will forge ahead with our collective heads held high and our collective hands held out for our neighbors.
Informational sources and special thanks to:
NWS Nashville (www.weather.gov/ohx)
Storm Prediction Center (spc.noaa.gov)
National Climatic Data Center (ncdc.noaa.gov)
IEM Data Archive (mesonet.agron.iastate.edu/archive/)
Merriam-Webster Online Dictionary (www.merriam-webster.com)