Destruction and Tragedy in Dixie: The April 27th Outbreak of 2011

I've just completed my second class at Penn State in their Weather Forecasting program. This class focused on severe weather and mesoscale forecasting. The main theme of the class was to associate the role of the synoptic scale with the mesoscale. For the class, we had to create blog entries on various subjects assigned to us by our instructor. I will post those that I made during the class, with some reflections and deeper insights for each. We had a fairly restrictive word count, so I had to aggressively edit my work.

This first entry was actually my last assignment. I chose to examine Tornado Watch #235 (PDS) issued on April 27th of this year. This was the WW issued across the areas hardest hit by the severe weather in the southeast. The Storm Prediction Center and the many NWS WFOs deserve much credit for the extremely accurate forecasts many days in advance of this storm system.

INTRODUCTION

On April 27^th, 2011 at 1845Z, the Storm Prediction Center (SPC) issued Tornado Watch #235 for much of Alabama and parts of Georgia, Mississippi and Tennessee until 03Z. The watch was labeled a Particularly Dangerous Situation (PDS), emphasizing the high confidence of the SPC forecasters for destructive tornadoes in the Tennessee River Valley. The severe weather event that transpired was of historic and unprecedented proportions.

Figure 1: Tornado Watch #235 at 1845Z on April 27th, 2011 (SOURCE: Storm Prediction Center).

This post analyzes the synoptic scale conditions in place that primed the region for this outbreak. We also review key mesoscale features that impacted the storm mode, longevity, and direction. To this end, we first review conditions at 500mb, 300mb, 850mb, and the surface. Then we review the ML CAPE, 0-6 km shear, and 0-1 km helicity. Because the watch was issued at 1845Z, much of the analysis begins at 18Z to mimic what the forecasters at the SPC were reviewing just prior to the issuance of the watch.

THE BIG PICTURE

Synoptic scale features set the stage for weather features at the mesoscale. This event is no different, as SPC forecasters were predicting this event in the experimental 6-day outlook the week prior using progs of synoptic features.

500 mb Analysis

The first analysis is at 500 mb, which at 18Z shows a deep trough across the middle portion of the country. This trough has a tight gradient of height lines nudging into the region, encouraging cooling aloft. The trough encourages divergence aloft downstream, which is directly over our area of concern.

Figure 2: 500mb Analysis at 18Z. Note the deep trough over the central US, with the area of concern downstream. This trough encouraged steepening lapse rates by cooling aloft. (SOURCE: Storm Prediction Center)

As the trough approaches, upper level pockets of cold air steepen the environmental lapse rates, thereby enhancing CAPE. This will be apparent as we analyze the mesoscale.

A loop of 500 mb shows the evolution of the trough through the forecast period, advancing through the area of concern.

Figure 3: 500 mb loop from 16Z to 05Z (SOURCE: Storm Prediction Center)

300 mb Analysis

At 300 mb, we can see a well-defined jet streak present over the southern plains and curving up slightly towards our area of concern. Jet streaks at 300 mb, especially curved ones, can prime areas in the right exit region for deep moist convection by amplifying divergence aloft. Note in the image below, that the right exit region is directly over the area of concern (dotted area).

Figure 4: 300 mb Analysis at 18Z. Note the curved nature of the jet streak and the right exit region over the area of concern. (SOURCE: Storm Prediction Center)

This jet streak certainly intensifies as the event unfolds. By 23Z, during the height of the outbreak, the jet streak is sharply curved and promoting upward motion with high levels of divergence.

Figure 5: 300 mb analysis at 23Z. Note the sharp curve and the divergence present downstream.

850 mb Analysis

At 850 mb we look for low level jet streams conveying warm, rich air inland from the Gulf of Mexico. At 18Z, a low level jet stream is well organized over the Southern Mississippi Valley, helping to warm and moisten the boundary layer. The timing also intersects with key diurnal heating periods. Winds are out of the SSW, which contrast starkly with the winds at 500 mb, which were out of the W and WSW.

Figure 6: 850 mb Analysis at 18Z. Note the conveyer belt of warm moist air from the Gulf, priming the environment for DMC. (SOURCE: Storm Prediction Center)

Surface Analysis

At the surface, we can see a low centered over Arkansas with a trailing cold front and a dry line along a pre-frontal trough. Initially, the dry line will provide the synoptic lifting over the region, although later the cold front over takes the dry line.

Figure 7: Surface Analysis at 18Z. Note the presence of the dry line along the pre-frontal trough. (SOURCE: Hydrometeorological Prediction Center)

Surface Temperatures & Dewpoints

At 18Z, notice the tongue of warm, moist air from the Gulf, with dewpoint temperatures over 70°F in our area of concern. This is a broad area that could support long-lasting convection, meaning storms could develop and potentially have a steady supply of warm, moist air. Also of note is the dew point gradient representing the dry line through Louisiana.

Figure 8: Surface Temperatures and Dewpoints at 18Z (SOURCE: Storm Prediction Center)

MESOSCALE ANALYSIS

Now that we have set the table with the synoptic scale, we can evaluate the features at the mesoscale. For this analysis, we review four elements: ML CAPE, 0-6 km shear, 0-1 km helicity, and storm motion as determined by the Rasmussen Technique.

100 mb CAPE & CIN

CAPE is Convective Available Potential Energy and measures the positive area between the level of free convection (LFC) and the equilibrium level (EL), which translates into energy measured in J/kg. What this essentially tells forecasters is the potential for strong updrafts if parcels reach the LFC. Mixed Layer CAPE, or 100 mb CAPE, is determine by using the mean potential temperature and the mean mixing ratio of the lowest 100 mb of the troposphere. This considers the effect of mixing eddies in the lowest levels that affect the LFC height. ML CIN, on the other hand, is convective inhibition, and measures the energy required to lift a parcel to the ML LFC. Generally, forecasters look for areas with high ML CAPE and low ML CIN to forecast strong updrafts and potentially severe weather.

The image below shows the ML CAPE and ML CIN at 18Z. Note the broad area of ML CAPE values >2000 J/kg and absence of ML CIN over our area of concern. Parcels of air would require minimal forcing to reach the LFC and when they did, they would erupt in strong updrafts to the EL.

Figure 9: ML CAPE/CIN Analysis at 18Z showing a broad area of high CAPE (>200 J/kg) and minimal CIN. This area is prime for strong updrafts. (SOURCE: Storm Prediction Center)

0-6 km Shear

Vertical wind shear is the change in direction and speed of wind with height. The 0-6 km analysis measures this change through the layer from 0 to 6 km AGL. For forecasters, wind shear helps define storm mode, longevity and evolution. Wind shear values in excess of 35 kts are capable of supporting supercells with rotating updrafts. Wind shear helps maintain discrete updraft and downdrafts so that they do not interfere and rob the storm of warm moist air. The forward flank downdraft is typically found NE of the updraft region, as the upper level winds carry the precipitation downstream. This allows the updraft to maintain a source of warm air.

Figure 10: 0-6 km Shear at 18Z. Shear values greater than 35 kts will support supercells. The area of concern has values over 60 kts and upwards of 80 kts. (SOURCE: Storm Prediction Center)

0-1km Helicity

Helicity is a combination of low level shear and storm relative winds. This measures how fast an updraft intakes streamwise vorticity at the lowest levels (measured in meters² seconds^-2), which in this case is the lowest 1000m. Forecasters look at values greater than 300m²s^-2 for an increased threat of tornadoes. You can see in the analysis below that at 18Z, values of helicity are well above 300m²s^-2 for much of the area of concern. Certainly, storms in this region had the potential to inject streamwise vorticity at exceptional rates.

Figure 11: 0-1km Storm Relative Helicity. This shows how fast storms inject streamwise vorticity, which forecasters use to predict supercells and tornadoes.

Determining Storm Motion: Rasmussen Technique

Using the Rasmussen Technique, storm motion was determined to be out of the WSW (260°) at 35 kts. This is a bit off from the motion the SPC forecasters listed in the WW, which was 250° at 40 kts. The calculations of the Rasmussen Technique and sounding from the Birmingham WFO at 18Z are shown below.

Figure 12: Rasmussen Technique used to calculate storm motion (direction and speed). This method was slightly off that listed by the SPC in the WW. (SOURCE: William Smith)

Figure 13: 18Z sounding from BMX shows a curved hodograph and high low level shear. (SOURCE: Storm Prediction Center)

Radar Reflectivity from BMX from 18Z - 23Z

The Birmingham radar shows the impressive tornadic supercells racing across the state. Note the key features of the echoes:

1. Well-defined hook echoes in the SW quadrant of the storm

2. Well-defined v-notches indicating strong updrafts that divert the upper level winds left and right downstream

3. Forward flank downdrafts in the NE region, well away from the updraft region

Figure 14: BMX Reflectivity from 18Z to 23Z shows well-defined supercell thunderstorms.

Figure 15: The regional composite reflectivity shows a broader view, and you can get a sense of the longevity of these supercells. The supercell that struck Tuscaloosa and Birmingham continued through Georgia and Tennessee before dissipating. (SOURCE: UCAR)

CONCLUSION

The outbreak of April 27^th, 2011 is one of historic proportions. As mentioned earlier, this event came as no surprise to forecasters, as it was included in every Convective Outlook for a week prior. Those forecasters were looking specifically at synoptic scale features that would prime the environment for this event. The deep trough over the middle of the country would increase divergence aloft, increase cold air aloft, and bring strong mid level winds that led to a highly sheared environment. An upper level jet at 300 mb curved and placed its right exit region directly over an area primed for convection. That convection was primed due to an active 850 mb low level jet stream that conveyed warm humid air into the Lower Mississippi Valley. At the surface, the large scale forcing from the cold front arrived at the height of the instability and easily forced parcels to the LFC. The highly sheared environment, a result of the contrasting winds at 500 mb and 850 mb, determined the storm mode and sustained long-track tornadic supercells.

As this post is written, damage assessments and field surveys are ongoing. This will likely be one of the worst modern day tornado outbreaks in US history. Over 300 people died, and thousands more were injured. Early field surveys indicate multiple EF-5 tornadoes, and many at EF-4 and 3. The image below shows the tornado tracks from the entire event. Overlooking the number of tornadoes, the most impressive feature is the length of unbroken paths.

Figure 16: Preliminary Tornado Paths from April 27th, 2011 (SOURCE: NWS)