Low CAPE tornadoes?

[Sarah Berling]

I'm fairly new to the meteorology scene, as far as academics go, though I've been interested in it for several years. My question, though, is: Is it possible to have a tornado on a weak CAPE day? On April 16 or 17, 2005, Albuquerque, NM had its first tornado in 20 years, an actual, bona fide tornado that touched down. But checking on the Skew-t for that day, the CAPE is minuscule: at max 200 J/kg. It might be me; I might have read the graph wrong, but in the case that I'm not, is there an explanation for this?

Sarah

 

[Andy Wehrle]

It is certainly possible to have a tornado (even many, significant tornadoes-4/20/04) on a day with un-impressive overall CAPE values. The explanation seems to lie in vertical stretching of CAPE concentrated in low levels, but I am no expert. Jon Davies has a good case study of that outbreak that might help answer your question. http://members.cox.net/jondavies3/042004il.../042004ilin.htm

 

[Joe Nield]

I was in ABQ shortly after the event visiting my brother. The Sandia mountains are nestled up against the city to the east. According to him, the storm came over the mountain and produced the tornado near the airport shortly afterwards. This would suggest to me that in addition to orographic lifting aiding a very weak CAPE environment, topographic effects may have contributed by locally enhancing the storm-relative helicity, providing just enough turn in the low levels to spin up a brief tornado.

I have to wonder if CAPE really was that weak. Was this a 12Z sounding? If so, daytime heating would have had plenty of time to warm the boundary layer, most likely dramatically increasing CAPE values by the afternoon.

 

[Glen Romine]

I think this is a common misconception that a sounding tells you what the 'true' convective environment is like in the general area and time. Perhaps the spacing between sites reinforces this ideal that differences are small and subtle. However, this is often not the case - especially in the vicinity of active convection. Considerable circulations can be generated by active storms, both horizontal and vertically oriented, and these can lead to marked changes in the local environment that a sounding might sample. Further, the surface inflow air thermodynamics must be well known - as small changes in the inflow parcel characteristics can have marked impacts on the magnitude of the index values derived from it. So, an individual sounding gives you a crude idea of the general characteristics of an environment - but could be only roughly related to the actual environment that the storm updraft 'senses'.

Back more to the point - can a 200 J/Kg CAPE be adequate to support tornadic storms? The answer is yes - but it needs a lot of help from other factors - so that the vertical accelerations within the updraft are aided by additional factors beyond just parcel buoyancy. The more the vertical acceleration is compacted in the lower levels - the better.

Glen