Brief Case Studies of Non-mesocyclone "Landspout" Tornadoes        by Jon Davies
(Important ingredients: a well-defined boundary, steep low-level lapse rates, significant CAPE, and little or no CIN))

An informal study I did in 2004 and 2005 about non-mesocyclone "landspout" tornado environments suggested several important ingredients.  These featured steep low-level lapse rates (e.g., 0-2 km or 0-3 km above ground) overlapping significant CAPE along well-defined windshift boundaries with little if any CIN in the local environment, and a well-mixed moisture depth.  In theory, such environments would have potential for enhancement of  low-level stretching through rapid parcel ascent (steep low-level laspe rates) with reduced mixing and entrainment for parcels below cloud base (notable moisture depth with no significant temperature inversion present to retard rising parcels).  As an example of these characteristics, see this RUC sounding profile associated with the first case below. 

An awareness of such characteristics may be useful in the short term (usually only 1-3 hrs) to highlight some environments where tornadoes from non-mesocyclone processes may be possible.  To increase the odds of such a "mesoscale accident", the combination of low-level lapse rates, CAPE, and very small CIN would probably need to be "maximized" along a well-defined windshift boundary (vertical vorticity source; e.g., Wakimoto and Wilson 1989) where thunderstorms were expected to development (see this composite).  Often, the presence of low-level CAPE (e.g., CAPE below 3 km AGL) suggests significant moisture depth and a lack of CIN, although in high plains environments with very high LCL heights (near 3000 m AGL) seen in locations like eastern Colorado, low-level CAPE does not always need to be present as long as there is some moisture depth and CIN is small.   Stretching of vorticity by thunderstorm updrafts along such boundaries could then result in non-mesocyclone "landspout" tornadoes.  (If some notable deep shear were present along with at least some SRH, it is even possible that some storms might even develop marginal supercell characteristics.)

The following 3 cases are brief examples highlighting storms developing along boundaries in environments having steep low-level lapse rates with significant CAPE or low-level CAPE.  It may be notable that the one case where CIN was sizable (19 May 2003) did not produce tornadoes along the boundary. 

Case 1:  27 August 2004 (south-central KS)

from 23z RUC analysis:
<Sig Tor Parameter    <0-3 km lapse rate
<0-3 km MLCAPE      <tornado reports
<0033z Base Refl.     <F2 tornado S of Wellington KS

In this case, the F2 tornado had no pre-existing mesocyclone on radar, and developed rapidly from a new updraft at a boundary intersection that was probably a focus of increased vertical vorticity.  It is possible that the low-level thermodynamic environment combining steep low-level lapse rates and low-level CAPE enhanced low-level stretching at this intersection.  It can be seen that the tornado had full condensation all the way to ground from a high cloud base, not the visual appearance of a weak "landspout".  A severe thunderstorm watch was in effect over the area at the time.

Case 2:  18 April 2004 (Minnesota, Iowa, Nebraska, South Dakota)

from 22z SPC mesoanalysis:
<21z surface                 <0-1 km SRH
<MLLCL height             <0-3 km lapse rate (LR3)
<MLCAPE/MLCIN        <best overlap LR3 and MLCAPE
<2230z refl. mosaic       <tornado reports

Most of the tornadoes for this event appeared to be nonsupercell/non-mesocyclone in nature along the boundary southwest of the surface low where cloud bases were high, but low-level laspe rates were steep and there was low-level CAPE present just above the high LCL heights.  A severe thunderstorm watch was originally in effect in the area where most of the tornadoes occurred.

Case 3:  19 May 2003 (west central and northwest Texas)   nontornadic case

from 22z SPC mesoanalysis:
<21z surface                <0-3 km lapse rate
<MLCAPE/MLCIN        <2146z base reflectivity

This nontornadic case had separate areas of steep low-level lapse rates and low-level CAPE with no overlap, possibly reducing potenital for enhanced low-level stretching along the boundary.  This  may possibly have impacted the lack of tornado reports from storms that developed directly on the surface boundary that was apparently rich in vorticity.

Some comments about assessing settings with short-term potential for non-mesocyclone tornadoes:
1) Sharp well-defined boundaries that are near-stationary or slow-moving with little temperature change across them (like a weak cool front or trough) seem to work best for producing non-mesocyclone tornadoes.  The most typical wind shift is a sharp one from south or south-southwest to west or northwest across the boundary.
2)  Storms on or near boundaries where low-level lapse rates are steep and CAPE significant can produce tornadoes from storms with high cloud bases in high LCL environments.
3)  The above ingredients will be of little use over most of the western U.S. where mountainous terrain results in highly variable and inconsistent lapse rate and CAPE fields in low-levels.
4)  Boundaries oriented northeast to southwest seem to be the most common producers of non-supercell/non-mesocylone tornadoes when steep low-level lapse rates and low-level CAPE overlap those boundaries where thunderstorms develop (see composite).

My hope is that some of the above material will prove useful to forecasters regarding short-term situational awareness of non-supercell/non-mesocyclone tornadoes, which are clearly difficult to forecast.

Jon Davies -- updated 8/25/08                                       

back to Case Studies/Briefs page
back to "landspout" tornado overview