Tornado Watch 133 – April 3, 2012
On April 3, 2012, 17 confirmed tornadoes struck the Dallas, TX, area. No deaths were caused but significant property damage occurred. This storm, having hit a major metropolitan area, also resulted in some remarkable video footage of the tornadoes.
This storm system was well forecast, and at 1210 pm on April 3, the Storm Prediction Center (SPC) issued tornado watch 133 (TW133), covering a large portion of east central Texas including Dallas.
TW133 provides an excellent case study of mesoscale weather. In this analysis, I will first look to the ‘big picture’ synoptic-scale setup for the outbreak of severe weather. I will then turn to a mesoscale analysis of the storms that were the cause of this system by examining 100mb MLCAPE/MLCIN, 0-6 km shear, and 0-1km SR helicity. I will then apply the Rasmussen Technique to discuss storm motion.
The Big Picture – Synoptic Setup for TW133
We will first look to the synoptic-scale set up on April 3, which favored deep moist convection over eastern TX. On the morning of April 3, 2012, this area saw an approaching surface cold front, with the low passing north of the Dallas area. The approaching cold front would provide much of the lifting force necessary to encourage deep moist convection (DMC).
At 500mb, there was a closed low centered over New Mexico, with a trough extending further south, putting Texas in the vorticity-rich region of that trough.
Winds at 500mb also favored convection, with a mid-level jet putting eastern TX in the right-exit region, also favoring DMC.
As was noted by the SPC in its Mesoscale Discussion 430, an outflow boundary running generally west-east was approaching the area from the north. It is subtle but can be seen on the following visible satellite image as a thickening band in the clouds, oriented generally on an east-west axis.
Finally, turning to 850mb, we find a relatively strong wind drawing moisture northwards towards Texas from the Gulf of Mexico.
This low-level jet stream also served to enhance wind shear; 850mb winds were southerly, 500mb winds generally westerly, and surface winds, as shown below, light and variable.
Thus around lunchtime on April 3, we had a synoptic scale setup that strongly favored DMC. We had a cold front approaching and interacting with an outflow boundary approaching at a right angle to the cold front, in a region rich in vorticity at the mid-tropospheric levels. Next, we will turn to an examination of the mesoscale.
Having established that the big picture favored an outbreak of severe weather, we will examine the mesoscale.
First, we need to look to see if there was sufficient instability in the atmosphere to favor DMC. One tool to do this is 100mb MLCAPE/MLCIN. CAPE is a measure of the potential for strong updrafts, one key factor favoring DMC. High levels of CAPE equate to high potential for upward motion. CIN (Convective Inhibition) is a measure of the amount of energy needed to initiate convection – in essence, CIN must be largely overcome in order for convection to initiate (though CIN does NOT need to fall to 0). “100 mb ML” CAPE and CIN refers to CAPE and CIN measured in the mixed layer – it is intended to account for the mixing of surface air with dryer air higher in the atmosphere via convective eddies. MLCAPE and MLCIN more realistically represent the atmosphere than does surface based CAPE and CIN because it takes into account this convective mixing.
Now let’s turn to MLCAPE/MLCIN on April 3 at 17Z, approximately the time our watch was issued.
For our subject area (basically the green ellipsis), we have moderately high levels of MLCAPE in the range of 1000-3000 J/kg, and modest CIN. It’s important to remember that CIN does not need to equal zero for convection to develop; these moderate levels of CIN would not prevent convection.
Our second step (once we know that MLCAPE/MLCIN is consistent with development of storms) is to look at 0-6 KM shear. This tool is helpful for determining the mode of thunderstorms that will develop in an area, with high levels of shear (>35kts) being associated with the development of supercells. Vertical wind shear measures the change in wind speed and direction with height. 0-6 km shear measures this change through a layer of the atmosphere from the surface to 6km.
Bulk shear at the time the watch was issued in central and eastern Texas was in the range of 30-60kts. This level of vertical wind shear suggests the development of supercellular storms in areas with sufficient CAPE and minimal CIN. It is important to note that the guidelines for shear in relation to storm mode are not firm guidelines, so the fact that part of our area is seeing shear of only in the 30kt range does not preclude development of supercells. The atmosphere near Dallas was primed at a mesoscale for the development of supercells.
The third mesoscale concept we will examine is 0-1km storm relative (SR) helicity. SR helicity is a tool we can use to identify areas of likely tornadogenesis. SR helicity is a measure of the rate at which the updraft of a supercell ingests streamwise vorticity. Values of greater than 100 m2 s-2 equate to a heightened risk of supercellular tornadogenesis. SR helicity is very valuable to identify areas where we may find tornadoes.
Looking at 0-1 km SR Helicity at 17Z, approximately when the watch was issued, we can see that our area of interest had pockets of heightened SR helicity, including a large pocket >100 indicating areas primed for tornado development.
When we examine the image from two hours later, we can see SR Helicity has increase all over eastern Texas, with the “100+ area” covering virtually all the watch box.
These heightened areas of SRH indicate that supercells in this area are more likely to generate tornadoes.
Finally, on the mesoscale, we can turn to the Rasmussen Technique to forecast the direction of supercellular movement in our storm. This technique is one available method for predicting supercellular motion is superior to using simple mean cloud layer winds as a proxy, as it factors in the effect of vertical wind shear on storm motion.
To do this, we must first obtain a representative sounding. The 12Z sounding for Ft. Worth (FWD) is the best proxy we have.
The first step in the Rasmussen technique is to determine the mean wind between the ground and 500m, with surface wind serving as a proxy. We then must look at the wind at 4km (using 600mb as a proxy).
At the surface, winds were 5 kts from the southeast, while at 600mb winds were blowing at 40kts from the southwest.
Next, we plot this on a hodograph. The surface and 4km winds are showin in green. Next, we draw the shear vector between the surface and 4 km winds, shown in green. Our next step is to draw a vector whose length is 17 kts at a right angle to the green line, 60% of the distance from the origin to the 4km measure. This is shown in red.
Finally, we draw a vector from the origin to the end of the red vector. The result is our storm motion vector, in this case 250 degrees at approximately 28 kts. This compares rather favorably with the storm motion vector in TW 133 of 220 at 25 kts. The difference may possibly be attributed to the time difference between the 12Z sounding and the watch issued closer to 17Z.
Conclusions – Making Connections
There is also an obvious connection between the synoptic scale information we had and vertical wind shear. Our surface, 850mb and 500mb wind directions and speeds as shown above were quite variable, causing our 0 – 6 km wind shear vector to be relatively high. The low-level shear, shown by the dramatic increase in speed and change in direction from the surface to 850mb of the wind, also favors heightened SR Helicity; a magnitude of shear this high at this low level will increase SR Helicity as it is one of the factors that drive the calculation of SR Helicity. The 850mb winds also caused warm, moist air from the Gulf to be drawn into the area of interest, resulting in reducing CIN through low-level moistening and heating.
TW133 provided timely and accurate warning to the residents of Dallas and surrounding areas of impending tornados. Indeed, the watch proved prescient, with over 20 tornadoes reported.
In this case, we had synoptic conditions favoring the development of thunderstorms, including an approaching cold front interacting with an outflow boundary, coupled with strong divergence in the upper atmosphere. The resulting cluster of tornadoes is clear to see on the Storm Report map.
Tornadoes formed as TW133 predicted including the many seen by local residents in Dallas. The warning likely saved lives prevented injury.