Friday, May 11, 2012

The nonsupercell tornado outbreak of 2012 May 09 along the Gulf Coast

One or two reported waterspouts in a day along the Gulf Coast is a fairly common occurrence during the warm season.  But to see two reports of multiple simultaneously occurring waterspouts is quite unusual.  That's what happened on May 9 when reports started to come in of six waterspouts southwest to southeast of Bayou La Batre, AL around 1145 CDT, and then two more waterspouts were reported northwest of Grand-Isle, LA by 1430 CDT (figure 1).  The two by Grand Isle apparently passed across the island together according to Tim Osborne, a NOAA employee.  

Figure 1.  A map showing the location of the waterspouts near Bayou La Batre, AL (upper right) and Grand Isle, LA (lower left).  The picture fro Grand Isle was taken by Capt. Danny Wray and the Bayou La Batre image was taken off a Twitter feed @tbJowers.

A news story from caught my attention mainly because they classified these vortices as 'fair-weather waterspouts', especially referring to the waterspouts by Bayou La Batre.  See this quote "FOX10 News Meteorologist Michael White says these waterspouts are considered non-tornadic, or fair weather".  Really, nontornadic?  They got their terminology from the National Ocean Service, a NOAA site.  So I went to their site with the term "fair-weather waterspout" and I found this.

Fair weather waterspouts usually form along the dark flat base of a line of developing cumulus clouds. This type of waterspout is generally not associated with thunderstorms.
Okay, they determined that fair-weather waterspouts are nontornadic because they are not associated with thunderstorms.  Perhaps on the surface that may seem okay since the definition of a tornado is a violently rotating column of air in contact with the surface and pendant from a cumuliform cloud (see AMS, 2012).  Oh wait, the definition didn't specifically say 'thunderstorm' but instead a cumuliform cloud.  But I'll let that flaw ride for now and discuss whether or not these vortices came from cumulonimbus clouds. These clouds represent deep, moist convection, the most common cloud associated with thunderstorms. Although the presence of lightning is not a necessary condition for defining a cumulonimbus.  Some cumulonimbus updrafts are too weak to split enough electric charge to generate lightning.  

So is FOX10tv correct in saying the waterspouts offshore of Bayou La Batre were nontornadic, pendant from cumulus clouds?  To test this idea, I show you the radar data from Mobile (figure 2).  I'm to assume that cumulus clouds should not generate heavy precipitation (>40 dBZ) and certainly not lightning.

Figure 2.  A four panel radar image of the area in the vicinity of the waterspouts offshore of Bayou La Batre from 1640 to 1649 UTC.  The waterspouts occurred around 1645 UTC.  The top left panel represents the lowest scan reflectivity from KMOB.  The top right panel shows base velocity at the lowest scan.  The lower left panel shows the maximum expected hail size (blues are > 0.25", yellow is > 0.75"), The lower right panel shows spectrum width of velocity.  Sometimes isolated peaks in spectrum width may indicate the presence of a unresolved vortex.
Clearly this loop shows a line of very heavy convective rain showers developing along an axis that most certainly spans the region where the waterspouts formed.  The maximum expected hail size panel (MEHS) indicates that these heavy precipitation cores extend upward into subfreezing temperatures and that the inferred presence of hail indicates an inferred presence of lightning.  Lightning was reported as a matter of fact west of KJKA around 1645 UTC.  The visual manifestations of one of the precipitation cores can certainly be seen left of the waterspouts in the photograph in figure 3.

Figure 3.  A photograph of four waterspouts offshore of Bayou La Batre, AL somewhere around 1645 UTC.  This picture was taken from the twitter feed @tbJowers.

These waterspouts were certainly pendant from a cumulonimbus cloud actively generating lightning - hardly fair weather.  Judging by the considerable spray lofted by some of these vortices, they're certainly intense, enough to pose danger to small craft that may wander too close.  These are tornadoes.

What about the vortices that crossed Grand Isle, LA?  A larger view of the picture in figure 1 shows two vortices made visible by well defined funnels (figure 4).  Each one was embedded in a somewhat larger circulation manifesting themselves as collars.  They're both adjacent to a pretty dense precipitation core.  In fact the vortices appear to be embedded along an outflow boundary forming from the precipitation.  

Figure 4.  Photograph of the Grand Isle tornadoes somewhere near 1930 UTC.  This picture was taken from the southeast of the tornadoes by Tim Osborne of the New Orleans National Weather Service.

The nearest NWS Doppler radar to these vortices shows a developing thunderstorm northwest of Grand Isle with heavy precipitation (figure 5).  The location and timing of these vortices is much more precise allowing for a more direct comparison to the features viewed by the radar. Two notable features include two specific bulls eyes of high spectrum width at 1931 UTC just to the northwest of Grand Isle.  The vortices are quite likely colocated with these spectrum width bulls eyes.  Both of these local peaks also happen to be located along a convergence zone that is likely marking the onset of the outflow boundary.  The maximum expected hail size shows values somewhere between 0.25 and 0.5" and that indicates heavy precipitation extends high enough into the atmosphere that this storm is a cumulonimbus cloud, and quite likely an active thunderstorm.  Clearly these vortices are tornadoes too, and one of them blew the roof off of a house.

Figure 5.  Same as figure 2 except for the Grand Isle, LA thunderstorm viewed by the KLIX Doppler radar.  
Having settled comfortably with the idea that both of these events were tornadic, what kind were they?  That could be a difficult question to answer since the atmosphere is a complex dynamic system with many soft boundaries between phenomena of one type or another.  Nevertheless in the world of classifying tornadoes we like to associate them with common mechanisms involved in their formation.  

The most common involves the tilting of horizontal vorticity into the vertical through a complex interaction updraft and downdraft within a supercell.  The implication is that a mesocyclone is typically present through a deep layer starting below 2000' above ground and should be detectable by Doppler radar.  Supercells also exhibit significant hook echoes, inflow notches in the radar reflectivity, long-lived strong echoes overhanging the low-level notches much like the conceptual model in figure 6.

None of these storms had any detectable mesocyclones aloft during the tornadoes (figs 4 and 5).  Nor did the storms exhibit any of the reflectivity-based signatures of supercells.  That is not to say that some of these processes were entirely absent on this day.  After all, there was actually some vertical wind shear on this day, perhaps not quite enough to strongly suggest supercell environments with low-level mesocyclones.

Figure 6.  A conceptual model of a supercell as viewed by S-band radar.  The cloud boundaries were superimposed on the reflectivity as well as areas of cold pool (blue shading) and large scale wind flow (broad arrows).  

Another common tornado production method involves superimposing a developing thunderstorm on top of a pool or axis of air with a lot of low-level vorticity.  The thunderstorm updraft then stretches that vorticity into a tornado as the conceptual model in figure 7 shows.
Right away there is some merit to this model.  Both sets of tornadoes formed as the thunderstorms were forming.  Supercell induced tornadoes typically occur only during the storm's mature phase.  

Figure 7.  A conceptual model of nonmesocyclonic tornado formation.  Low-level strong horizontal shear rolls up into small circulations that could grow into tornadoes should a deep convective updraft pass overhead.
Figure 8.  A surface map from 2012 May 09, 1643 UTC with a cold front analyzed with the blue curved line.  The tornadoes at Bayou La Batre occurred just after this plot while the Grand Isle tornadoes occurred 90 minutes later.
But the question is, was there a pool of strong horizontal wind shear available before storm formation?  The surface plot in figure 8 suggests that was possible.  Both areas of tornado formation occurred in close proximity to a weak cold front.  Winds across the front showed adequate convergence and some indication of horizontal shear.  In fact, an analysis of vertical vorticity showed areas of high values in the vicinity of Bayou La Batre and Grand Isle.  Both of these areas exhibited high values of CAPE (Convective Available Potential Energy) in the 0-3 km layer (figure 9).  These high values of low-level CAPE are important indicators of good low-level buoyancy for which allows convective updrafts to quickly pull up and stretch the low-level horizontal shear.  Whether or not there were pre-existing circulations on the front is one question I cannot answer given the lack of a nearby radar.  But at least the background ingredients appeared to be favorable for nonmesocyclonic tornadoes.  This environment was similar to that found in many studies (e.g., Brady and Szoke, 1989; Wakimoto and Wilson, 1989).  That we had not one but a small outbreak of simultaneous tornadoes plays very well with the concept of a boundary with strong horizontal shear rolling up into separate small circulations (called misocyclones) then stretched into tornadoes as Lee and Wilhelmson, 1997 demonstrated in a two part publication.    Figure 10 shows a three-dimensional visualization of their numerical simulation of nonmesocyclonic tornadogenesis.

Figure 9.  An analysis of near surface vertical vorticity (blue contours), instability in the low-levels (0-3 km CAPE red contours) and low-level winds (blue barbs) on 2012 May 09 17 UTC.  The star indicates the location of the Bayou La Batre tornadoes.

Figure 10.  A numerical model visualization of vortex sheet rollup leading to multiple nonmesocyclonic tornadoes from Lee and Wilhelmson, 1997 part II.

Once an environment is quite favorable for generating tornadoes, we often see more than one simultaneously.  This is true whether or not the origin of vorticity in a tornado draws from strong enough pre-storm horizontal or vertical wind shear.  Here are some examples of simultaneous tornadoes coming from supercells as a result of strong vertical shear (14 April 2012 near Cherokee, OK,  far back in 10 May 1991 near Lazbuddie, TX13 March 1990 Hesston-Goessel, KS tornado merger, 07 November 2011 Tipton, OK cyclonic-anticyclonic tornado pair). Simultaneous multiple tornadoes originating from sheets of strong horizontal shear appear in multiple links such as from oil platforms in the Gulf of Mexico (here), and off the coast of Australia (here).

Now that we established that the two outbreaks of tornadoes were likely nonmesocyclonic (nonsupercell) in nature, why do I not call them waterspouts over water and tornadoes over land?  Simply because the nature of the underlying surface has no immediate impact on how the vortices formed. Perhaps there is a secondary role in the form of a smoother surface allowing the vortices to intensify.  But that doesn't prevent the same mechanism from producing nonmesocyclonic tornadoes on land.  After all, land has a huge variety of smoothness. But if we see one of these tornadoes over forest vs prairie, we still call them tornadoes and not prairie-spouts or forest-spouts.  There is the term landspout that was unfortunately coined to describe a nonmesocyclonic tornado.  I say unfortunately because it served to obscure the fact that now we have two names to identify the same kind of tornado.  In fact, we have three:  landspout, nonsupercell tornado, nonmesocyclonic tornado.  Incidentially the visualization in figure 10 came from an overland simulation with a generic surface because technically the authors weren't concerned about whether it was water or not.    So, please, if an intense vortex forms from a cumulonimbus cloud, call it a tornado whether or not it's over land or water!

I mentioned before about returning to the distinction between fair weather vs tornadic waterspout.  I noticed a waterspout informational page from the National Ocean Service describing the differences in vortex formation between so called fair weather waterspouts and tornadic waterspouts.  The quote is below:

Fair weather waterspouts usually form along the dark flat base of a line of developing cumulus clouds. This type of waterspout is generally not associated with thunderstorms. While tornadic waterspouts develop downward in a thunderstorm, a fair weather waterspout develops on the surface of the water and works its way upward. By the time the funnel is visible, a fair weather waterspout is near maturity. Fair weather waterspouts form in light wind conditions so they normally move very little.
This statement is misleading in one way and incorrect in another.  

First there is the misleading part.  There is nothing in the glossary of meteorology that defines a fair weather waterspout.  According to the definition of a waterspout in AMS 2012, it is a tornado over water and a tornado can be pendant from any cumuliform cloud.   But given those conditions, what would prevent me from identifying that tornado?  It would be the strength of the vortex.  If a vortex strengthens enough to generate at least EF0 winds  and is connected to a cumulus, or cumulonimbus cloud, it is a tornado no matter what kind of surface lies underneath! If a boater sees a vortex lofting spray consider it a tornado.
Second, there is the incorrect statement that fair weather waterspouts develop from the bottom up and tornadic waterspouts build downward. The reality is nonmesocyclonic tornadoes develop upward from the near surface more often than not (e.g., Brady and Szoke, 1989; Wakimoto and Wilson, 1989).  That means both the Bayou La Batre and Grand Isle tornadic events likely developed from the near surface upward.  In fact 50% of mesocyclonic or supercell tornadoes develop upward with time as Trapp et al. 1999 showed.  More recent analysis from VORTEX2 shows that upward growth of mesocyclonic tornadoes may be more common than earlier thought.  


I resident of Grand Isle just shared an incredible video of the tornado as it approached the island and then actually hit their house while the video was rolling.  


AMS, 2012: Glossary of meteorology. Available online at []Brady, R. H., and E. J. Szoke, 1989: A case study of non-mesocyclone tornado development in northeast Colorado: Similarities to waterspout formation. Mon. Wea. Rev.,117, 843–856.

Lee, Bruce D., Robert B. Wilhelmson, 1997: The Numerical Simulation of Non-Supercell Tornadogenesis. Part I: Initiation and Evolution of Pretornadic Misocyclone Circulations along a Dry Outflow Boundary. J. Atmos. Sci., 54, 32–60.
_____, Robert B. Wilhelmson, 1997: The Numerical Simulation of Nonsupercell Tornadogenesis. Part II: Evolution of a Family of Tornadoes along a Weak Outflow Boundary. J. Atmos. Sci., 54, 2387–2415.      
Trapp, R. J., E. D. Mitchell, G. A. Tipton, D. W. Effertz, A. I. Watson, D. L. Andra, M. A. Magsig, 1999: Descending and Nondescending Tornadic Vortex Signatures Detected by WSR-88Ds. Wea. Forecasting, 14, 625–639.          

Wakimoto, R., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev.,117, 1113–1140.  

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