March 5, 2011 Southern Louisiana Severe Wx

This past Saturday, March 5, 2011, southern Louisiana was rocked with another round of severe weather, with the primary storm modes being wind and tornadoes.  Unfortunately, this episode of severe weather proved to be both damaging and deadly, as a fairly short lived tornado produced EF2 damage in the town of Rayne, LA killing one person and injuring twelve others in its wake.  This was one of nine tornadoes that were produced across Southern Louisiana last Saturday, with four occurring in WFO LCH's CWA and five occurring in WFO LIX's CWA (County Warning Area).


What caused this event?
Through the analysis of upper air soundings and maps, I was able to identify several important features that were responsible for the initiation of this severe weather event.  In order for severe thunderstorms to occur, several conditions must be in place in conjunction with several atmospheric processes.  A moist layer of sufficient depth must be present in the lower to mid troposphere (including the boundary layer), instability, and source of lift (e.g. forcing from a front, upper level shortwave).  A fourth ingredient, wind shear, is crucial for the formation of supercell thunderstorms and tornadoes. 

This event initiated when several subtle shortwaves downstream of a well defined vort max passed over SE Texas and SW Louisiana, providing the lift needed for thunderstorms to develop.

 

 A series of subtle shortwaves are denoted by the red circle.  A well defined vort max (red area) can be identified over west Texas.  The "X" in this area denotes an area of maximum vorticity while the N to the north of this area denotes an area of minimum vorticity.  With the area of interest being downstream of this vort max, positive vorticity was advected into the area with time thus enhancing vertical lift over SE Texas and SW LA 

The 12Z sounding from WFO LCH profiled the atmosphere about an hour before the initiation of this event.  Multiple features in this sounding support the potential of severe weather.  The black boxes denote the depth of two significant dry layers in the atmosphere.  The wind profiler shows backed surface winds and veering winds with height.  Winds increase with height, thus indicating the presence of speed and directional wind shear.  The red box in this area of the sounding shows slight backing of the winds at upper levels, thus indicating slight cold air advective tendancies.  The "inferred temp advection" graph supports this observation, denoting temperatures decreasing at rates of 0.6 and 0.7 degrees Celsius per hour.  The hodograph in the top right area of the picture supports the observations made from the wind profile.  This tool shows how wind direction and speed change with height.  0-1 km shear (denoted by the black box) indicated that the atmosphere was conducive for rotating updrafts.  Despite these features, a weak capping inversion in the lower levels was inhibiting convection at the time the atmosphere was profiled (SB CIN values of -46 and ML CIN values of -149).  This cap quickly eroded in the hour following this sounding and allowed thunderstorms to develop.  

The Event

The image above is a base reflectivity overview of the entire MCS.  The red "L" just to the south of the Louisiana coast (Cameron Parish) denotes the location of the subsynoptic low that influenced storm motion.  The black arrows show the differences in storm motion with respect to distance from and location around the low.  Convection well ahead of the main thunderstorms featured a northerly component of motion whereas activity behind and north of the low featured a southerly component of motion (air rotates counterclockwise around low pressure systems in the northern hemisphere).  The main convection associated with this system moved to the eastnortheast as the entire MCS propogated to the east. 

Surface observations around the time of initiation (~13Z).

Jefferson Davis EF0's

This is a base reflectivity (BR) and storm relative velocity (SRV) image the supercell thunderstorm that produced two EF0 tornadoes in Jefferson Davis Parish.  At the the screen shot was take, the second EF0 tornado was in progress.  The base reflectivity product features an impressive signature that supports the strong velocity couplet that is featured in the SRV product.  The BR image shows areas of higher reflectivity associated with heavier rainfall "spiraling" toward the area of lower pressure where the mesocyclone and tornado were located.  The behavior of the rainbands is a direct result of the strong inbound/outbound velocities that are shown by the SRV product.  The red arrow in the BR image denotes modified inflow air (modified by wetbulb cooling at the surface due to rainfall) being advected into the inflow notch to the northeast of the mesocyclone and tornado.  This particular storm eventually gusted out as a rear inflow jet built into the area, leading to several more tornadoes, including the Rayne EF2.

Crowley & Rayne Tornadoes

Both the Crowley tornado (EF0) and the Rayne tornado (EF2) were the result of bookend vorticies, as a rear inflow jet built into the area.  This inflow jet may have been the result of wet-bulb cooling occuring in the mid-levels of the thunderstorm that was located just to the west of the HP supercell that produced the Jefferson Davis Parish tornadoes.

In this series of photos, I utilized the base reflectivity (BR), storm relative velocity (SRV), echo top (ET), and normalized rotation (NROT).  The echo top product shows cloud top heights.  The NROT product is an algorithm that was developed by the makers of GR Analyst.  This product features a range of -5 to +5. Anything above 1.0 is significant and values above 2.5 are extreme.   A detailed explanation of this product can be found here: http://grlevelxstuff.com/forum/showthread.php?t=314.

The BR product in the photos above show streaks of lighter reflectivity in the trailing stratiform precipitation behind the tornado producing thunderstorm.  This signature suggests the presence of a rear inflow jet.  This occurs as strong winds change the direction of rain droplets from vertical to horizontal, thus preventing the radar beam from intercepting some these droplets and leading to lighter reflectivity returns.  The series of photos also shows the evolution of the velocity couplet, with it being the strongest and tightest over the city of Rayne.  Gate to gate velocity (inbound/outbound velocity combined) was 85 knots and NROT values were 1.53 at the time.  Echo tops associated with this storm peaked ~41,000 feet.

All together, 683 homes were damage with 42 being completely destroyed and 48 receiving major damage in Rayne.  Unfortunately, these two tornadoes injured a total of 15 people and killed one woman when a tree fell on the house she was in. 

How does a bookend vortex form?

Bookend vorticies are the result of shear vorticity (a form of relative vorticity).  The photo above illustrates this process very clearly.  As strong winds move through an area, it creates areas of vorticity on both sides of the area of strongest wind.  This is the same principal as dragging your hand in bathwater.  The water that is not being pushed by your hand has a tendancy to rotate away from the area of forcing, thus creating vorticity on a small scale.  Although the image above is a model of this principle at the 300 hPa level, this can occur at any level in the atmosphere.


Southeastern Louisiana Tornadoes
The tornadoes that were produced in southeastern Louisiana were also the result of a shear vorticity/bookend vortex situation.  Of the five tornadoes in this region, four were EF0's and one was rated EF1.

These two cases from SE Louisiana explicitly show the same radar signatures that the Crowley and Rayne storms exhibited.  Bookened vorticies and areas of rotation are depicted by the black circles in each image.


What lessons were learned?
In conclusion, March 5, 2011 is another classic example of the kinds or tornadoes that we are faced with in Southern Louisiana and throughout the southeastern US.  Unlike many cases from the Great Plains, tornado producing storms in the south are often messy storm structure wise, making them much more difficult to detect and dangerous to chase.