Review of March 2nd, 2012 Tornado Outbreak

On Friday, March 2nd, 2012, a multitude of severe weather occurred across the County Warning Area (CWA) including wind, hail, and tornadoes. A total of 12 tornadoes were confirmed with the majority being low-end tornadoes, however, 4 tornadoes were rated EF-2 or greater including one EF-3. The Chattanooga EF-3 spanned through Hamilton, Polk, McMinn, and Bradley counties with a final path length of 41mi and a width of 400yds. 44 injuries and no fatalities were reported with this tornado.

The aerial photo to the right is from Island Cove Marina in Hamilton county after the EF-3 tornado.

Prior to the event, the Storm Prediction Center's Day 2 Convective Outlook issued on March 1st had the majority of the CWA under a slight risk with far western portions under a moderate risk of severe weather. By the release of the Day 1 Outlook on the morning of March 2nd, the previous moderate risk area was expanded and covered roughly 95% of the CWA with an area of high risk added over central Kentucky and north central Tennessee. The Probabilistic Tornado outlook for the area ranged from 5-15% with the western portions of the CWA in a hatched zone(10% or greater chance of EF2-EF5 tornadoes).

Strong synoptic dynamics were in place prior to and during this severe weather outbreak. On March 2nd, 2012, the entire continental United States was under the influence of an upper level trough and the Morristown CWA under an area of significant divergence aloft. Upper level wind speeds ranged from 110 knots with southwest flow over Tennessee to 140 knots over the Mid-West where two separate jet streaks were present. The northern most jet streak spanned from northeast Oklahoma to west Indiana while the southern most jet streak spanned from northeast Texas to northwest Mississippi. These two jet streaks played a significant role in the entire synoptic setup due the jet streak coupling that aided in enhancing vertical motions and increasing lift.

Similarly, there were strong jet dynamics at the 500mb level. An intense jet streak with wind speeds up to 130 knots and southwest flow was located underneath the northern jet streak found at the 300mb level. The strength of this jet streak was influenced by the jet aloft and also assisted with increasing low level wind speeds. Over the CWA the 500mb jet speeds reached upwards of 80 knots with southwest flow.

At 850mb there is evidence of a significant low-level jet. Wind speeds across the southern Appalachian were substantially greater than their surrounding areas. Over Tennessee and beneath the previously mentioned upper-level jets, wind speeds ranged from 45-55 knots with southwesterly flow. This flow played a key role in both warm and moist air advection into the CWA. It is also important to note the vast differences in wind speeds between the multiple layers as wind shear is a key component to the formation and sustainability of severe storms.

In the early morning hours a surface low pressure system was centered in Missouri with a warm front that extended into southeast Tennessee. As the day progressed the system traveled northeast and gradually strengthened due to the presence of strong upper level divergence. By the late evening, the system began to occlude near the Michigan/Indiana border as the warm front exited the region to the northeast. Surface winds were primarily coming from the south with a small eastern component present in northeast Tennessee and southwest Virginia. This flow not only increased the warm and moist air advection into the area but the directional shear between differing atmospheric levels is also essential for severe weather. Lastly, research has shown that surface boundaries play a key role in the generation of severe weather. Surface boundaries, such as the warm front in this scenario, increase moisture convergence and vorticity, thus, enhancing lift.

As previously mentioned, wind shear is an important component for the production of severe weather. Furthermore, the seasonal timing of this event emphasizes the need for significant wind shear as thermodynamics and instability in the southern Appalachian region during early March are going to be relatively weak. 0-6km bulk shear has proven to be effective in determining the potential for having storms with a rotating updraft. By the evening on March 2nd, local values of 0-6km shear ranged from 60 to 80 knots while values of 40 knots and greater are considered optimal.

Another aspect of shear that is often utilized by forecasters is the 0-1km shear. This level of shear has proven to be useful in distinguishing the capability of a storm to produce significant tornadoes. Values of 20 knots are considered optimal values. In the afternoon and early evening hours values in the CWA were double the optimal value at around 40 knots. Later in the evening, small areas of 50 knots were observed in southwest Virginia while values in Tennessee degraded down to 30.

Directional shear is also important in determining if a storm is capable of producing a significant tornado. For this, forecasters will often look at a parameter known as Storm Relative Helicity (SRH), of which the typical minimum required value for the occurrence of tornadoes is 150 m2/s2. During this event, directional shear was just as abundant as our other shear parameters with values ranging from 300-600 m2/s2. Combining all of these shear parameters together suggests that the environment was highly favorable for rotating updrafts and the potential for tornadoes was serious.

Instability is another key component of severe weather and there are many different ways meteorologists assess instability. One of the most common ways is through Convective Available Potential Energy (CAPE). Research shows that low level CAPE is necessary for the generation of tornadoes and 0-3km MLCAPE has proven to be effective at determining if a supercell will be able to produce a significant tornado. The warm/moist air being advected into the CWA in the low levels aided the atmosphere in destabilization and 0-3km MLCAPE values maximized in a small portion of southern Tennessee at roughly 100 J/Kg. Optimal values of 0-3km MLCAPE are considered to be 70 J/Kg.

Lastly, the height of the cloud base also plays an important role of tornadogenesis. The Lifting Condensation Level (LCL) is the parameter forecasters use to determine these heights. If LCL heights are too high, rotation will have a difficult time reaching the surface and evaporational cooling will cause low level instability to decrease. It is suggested that LCL Heights 800m and lower are favorable for the generation of tornadoes. LCL heights in the CWA were as low as 500m in southwest Tennessee in the early morning and afternoon before increasing to 1000m or higher in the evening.

We will perform a radar review for 3 of the significant tornadoes that occurred on this day.

The animated image to the right contains various radar products often used to monitor severe weather. The top left panel is a reflectivity loop which is a measurement of the amount of energy reflected from an object in the atmosphere back to the radar. Heavier rainfall and larger objects will have higher reflectivity values.

The top right panel is storm velocity. This radar product gives forecasters an idea of the varying motions in a storm as red colors represent outbound motions and green colors represent inbound. Strong rotation is suggested at the location where both inbound and outbound velocities are coupled together and appear to get brighter or more intense. This radar indicated rotation is referred to as a velocity couplet and is often associated with a tornado. A velocity couplet can be paired with other radar products such as correlation coefficient (CC) to confirm such.

CC is located in the bottom right panel. It is useful for detecting non-meteorological signals such as shingles, large branches, or other debris that can be lofted into the air by a strong tornado. Notice the area of low values signaled by deep blue colors in the final few frames. This is referred to as a tornado debris signature (TDS) and is represented by the low CC values. When co-located with the velocity couplet, it is often indicative of a reasonably strong tornado.

Lastly, the bottom left panel is Normalized Rotation (NROT). This is a fairly complex derived product that tries to find areas of rotation. NROT has a value range maximum value range of -5 to +5. Values above 1.0 are considered to be significant while values 2.5 and greater are considered to be extreme.

The supercell that produced the multi-county EF-3 tornado had been tornado warned since 1106AM EST and even produced a short-lived EF-1 tornado in Marion county 10 minutes after the warning was posted.

The storm survey team determined that the start path of the EF-3 tornado was just a short distance northwest of Harrison, at approximately 1241PM EST. The looping image on the right displays the tornado during its peak intensity.

Just prior to touchdown, a Bounded Weak Echo Region(BWER) can be seen. The BWER is indicative of a strong updraft and is important to forecasters as it can be one of the first signs of potential severe weather. Furthermore, rotation acts to enhance updrafts and the BWER can actually be aligned with a velocity couplet with a gate-to-gate shear value of approximately 120 knots and an NROT value of 2. Additionally, these areas of rotation are also co-located with an area of relatively lower CC values near the last frames. This is known as a tornadic debris signature and can be used as a confirmation of a tornado. Sometimes, a general idea on the strength of the tornado can be determined by the size of the debris signature and how high it reaches. With this tornado, a debris signature could be seen upwards of 18,000 feet which aligns well with the EF-3 rating.

This damage path of this tornado ended shortly off frame, near Etowah, where the velocity couplet began to weaken and a debris signature was no longer visible above 8000 feet. 44 injuries were reported with this tornado.

Shortly after the long-track EF-3 tornado had lifted off the ground, that same supercell spawned another tornado in Monroe county. This tornado ended up being rated an EF-2 and had a path length of 14 miles.

On reflectivity it can be seen that there were actually two separate supercells. As the cell further to the west strengthened, the gust front associated with the forward flank downdraft played a key role in amplifying the rotation of the leading storm. The Monroe county EF-2 touched down at 147PM EST near Tellico Plains. A good idea of the location can be gathered by looking at the velocity and NROT panels. Co-located with an NROT value that ranges from 1.5 - 2 is a velocity couplet with a gate-to-gate shear of roughly 103 knots. Additionally, a debris signature can be seen following the path of the two aforementioned areas of interest. This debris signature could be seen just shy of 10,000 feet.

This tornado would go on to dissipate just west of the Appalachians at approximately 209PM EST. There were 3 injuries reported with this tornado.

The last tornado of this event that we will look at is the Cherokee county EF-2. This tornado developed later in the evening at roughly 744PM EST just near Hiwassee Village, NC. This tornado had the second longest track of the tornadoes to occur in the Morristown CWA at 21 miles, ending just east of Jones Mill.

The animated radar loop begins at 730PM EST where a BWER can be seen as well as reflectivity values approaching near 70 dBZ suggesting this storm had a pretty intense updraft. CC values are also low in association with the BWER which is fairly common with these very intense updrafts. Looking at the velocity product, it can also be seen that this storm has a velocity couplet, although less defined than that of the previous storms. NROT values for this storm maxed out with a value of 1.5 at 805PM EST. Lastly, there is also a weak tornadic debris signature co-located with rotation throughout the loop most noticeable just north of Murphy at 800PM EST . Thankfully this tornado went through a relatively lower populated area and no injuries were reported.

This interactive map contains the Morristown CWA outlined in a thick white line. Included within the border are the tornado tracks determined by NWS Storm Survey teams. The colors of the paths correspond to the maximum intensity a tornado experienced during its lifespan and are placed at the center of the path. The length of the lines depict the true path length, however, the thickness of a line does not necessarily denote the actual width of the damage path. Feel free to click on a path to pull up some brief details on that tornado. As mentioned before, the majority of the tornadoes experienced during this severe weather outbreak were low-end tornadoes with relatively shorter path lengths and little to no injuries. However, 4 EF-2 or greater tornadoes did occur, including a long track EF-3 that spanned through 4 unique counties.