April 27 th was the most active day of the "Super Outbreak of 2011" which took place between April 25 and 28, 2011.

April 27th was the most active day of the "Super Outbreak of 2011" which took place between April 25 and 28, 2011. During those four days, over 300 tornadoes were confirmed in the Eastern US, killing over 300 people. Most of the storm-related deaths occurred on April 27, when several violent tornadoes (EF-4 and EF-5) developed in the southeast US, with the state of Alabama being hardest hit.

After reviewing the background concepts above, complete the story below by applying the concepts of supercell thunderstorm development in analyzing the linked weather charts and imagery from this case study weather event, which will focus on the atmospheric conditions over Central Alabama on the afternoon and evening of 27 April 2011.

To complete the story, either circle one of the bolded options, or fill in the blank, as indicated, to make a correct statement.

Analysis of Radar and Satellite Imagery

In the visible satellite image loop valid 1615 Z (11:15 am CDT) to 2315 Z (8:15 pm CDT), several convective cells with overshooting cloud tops are moving from southwest to northeast / from east to west over Central Alabama, showing the location and movement of the supercell / air mass thunderstorms that produced the large tornadoes.

In the radar reflectivity loop (which visualizes precipitation intensity) for the afternoon of 27 April 2011, the signature _hook echo_ feature is visible within the precipitation cells (as discussed in Chapter 15, Observing Tornadoes and Severe Weather), indicating storm rotation and the presence of the mesocyclone.

Analysis of Surface Conditions

On the surface maps for 2100 Z (4 pm CDT) 27 Apr 2011 and 0000 Z 28 Apr 2011 (7 pm CDT 27 Apr 2011):

A surface low pressure system and warm / cold front, associated with a developing Mid-Latitude cyclone, are present.
Central Alabama is located within the region of likely supercell thunderstorm development - just ahead of / behind the cold front, within the warm / cold sector of the low pressure system.
Strong convergence / divergence of air into the center of the low (indicated by the observed surface wind direction in the station models), along with mechanical lifting of air along the cold front, act as trigger mechanisms to propel air parcels upward from the surface to the point of saturation, forming the convective clouds.
Surface observations of relatively low / high temperatures and low / high dew point temperatures indicate the presence of a mT / cP air mass in the southeast US ahead of the cold front. This warm, moist surface air is providing the fuel for the developing thunderstorms. _____________ are also being observed at some locations ahead of the cold front, indicated by the pink ""-like symbol in the station model.

Analysis of Upper-Level Conditions

850 mb Map Analysis

On the 850 mb map valid at 00Z on 28 April 2011 (7 pm CDT 27 April 2011):

The presence of a ____ - ____ ____ over Central Alabama is indicated by the 850 mb station model observation over Central Alabama, which shows a 60 kt southwesterly wind and a dew point depression, T - Td, of 17° C - 16° C = 1 °C.
This "tongue" of warm and high / low relative humidity air enhances the conditional instability in the lower levels of the atmosphere and provides additional energy for the developing thunderstorms.

700 mb Map Analysis

On the 700 mb map valid at 00Z on 28 April 2011 (7 pm CDT 27 April 2011):

Using 700 mb station model observation over Central Alabama, the dew point depression, T - Td,

at 700 mb is 9° C - (-3)° C = 12 °C.

The relatively lower dew point temperature and larger dew point depression at the mid-levels, compared to that of the air below it, indicates the presence of convective stability / instability, which means air parcels in the lower layer will become unstable, and thus very buoyant, when lifted.

500 mb Map Analysis

On the 500 mb map valid at 00Z on 28 April 2011 (7 pm CDT 27 April 2011):

The ridge / trough located just to the west of Central Alabama, indicates the presence of warm / cold air aloft over Central Alabama, which, coupled with the warm surface air, produces a large environmental lapse rate, and thus stable / unstable conditions, over Central Alabama.

300 mb Map Analysis

On the 300 mb map valid at 00Z on 28 April 2011 (7 pm CDT 27 April 2011):

We observe a core of strong / weak winds, indicating the presence of the polar jet stream over Central Alabama.
To the east of the 300 mb ridge / trough, there is significant spreading of the wind, field, indicating strong divergence / convergence of air in the upper levels.
The strong winds and the spreading in the wind field aloft over Central Alabama work together to generate large updrafts / downdrafts, propelling warm, moist air parcels within building cumulonimbus clouds to great heights, as seen in the visible satellite image loop for the afternoon of April 27, 2011.

Sounding Analysis

The BMX - 110428/0000 Observed Sounding was generated using radiosonde data collected at the Birmingham, Alabama (in Central Alabama) National Weather Service location (station ID: BMX) 00Z on April 28, 2011 (7 pm CDT April 27, 2011).

The sounding data also indicates the presence of the necessary conditions for supercell thunderstorm development in Central Alabama:

Analyzing the data listed on the lower right side of the sounding diagram:
The surface to 3 km environmental lapse rate (Sfc - 3 km AGL LR) and 3 km to 6 km environmental lapse rate (3 km - 6 km AGL LR) is 6.7 °C/km and 7.6 °C/km, respectively, indicating that the atmosphere is absolutely stable / conditionally unstable from the surface to 6 km the above ground.
The convective available potential energy (CAPE) values calculated from the BMX sounding data are 3458 J/kg and 4635 J/kg. Using Table 1 above, these CAPE values correlate to a probability of moderate / very strong convection (updrafts) and intense thunderstorms / severe thunderstorms possibly with tornadoes, accurately predicting the weather that took place over Central Alabama on this day.
Analyzing the BMX sounding diagram:
The surface temperature (red plot) is 26 °C (79 °F) and the surface dew point temperature (green plot) is 23 °C (73.5 °F), indicate very warm and very dry / humid surface air.
Below 850 mb, the dew point depression (difference between the temperature and dew point temperature) is large / small, indicating high relative humidity in the lower atmosphere.
Around the 850 mb level, there is a temperature inversion creating a stable / unstable layer that "caps" moisture and heat, allowing convective potential energy to build in the lower atmosphere.
Between 850 mb and 700 mb, the dew point temperature (green plot) decreases / increases sharply, representing the convectively unstable layer of cooler and drier / more humid air which allows air parcels to become even more buoyant and rise even faster.

Looking at the wind profile on the right side of the sounding diagram, there is strong vertical _____ _____, indicated by the changing wind direction with height, and the increasing wind speed with height. This wind pattern with height promotes:

the formation of the _________, the core of rotation for supercell thunderstorms, and
increased / decreased lifespan of thunderstorms by vertically tilting the storm, preventing the warm, moist updraft from being cut of the downdraft.