( Some case studies : 18 June 2001 Minnesota/Wisconsin 18 April 2002 in Kansas (nontornadic) 12 June 2004 in south central Kansas )
Ingredients supporting significant supercell (mesocyclone)
tornadoes (summary by
Jon Davies)
( Some case studies : 18 June 2001
Minnesota/Wisconsin 18 April 2002 in Kansas
(nontornadic) 12 June 2004 in south
central Kansas )
From the VORTEX field project in the mid 1990s, and findings and work by Rasmussen, Markowski, and other researchers (see Erik Rasmussen's discussion ), the following ingredients appear necessary to support significant supercell tornadoes (tornadoes associated with low-level mesocyclones, which comprise the majority of significant tornadoes):
These ingredients are often enhanced near boundaries (typically warm
fronts or old outflow boundaries), if air on the cool side of the boundary isn't
immediately too cold or too dense (see Doswell case study, and "elevated" environment example).
As a result, many notable supercell tornado occurrences tend to be localized near
boundaries:
Occasionally, ingredients are strong enough over a large area without an inhibiting
capping inversion that tornadoes can occur with several storms over a large area and may
not be so "boundary-dependent":
Days like 4/3/74 (Super Outbreak), 3/13/90 (Hesston KS), 4/26/91 (Andover KS), and 5/3/99
(Moore OK) are extreme examples of this.
But for most supercell tornado events, diagnosis and evolution of surface boundaries from surface map analysis, satellite, and radar information is important, along with associated parameter assessment in the vicinity of boundaries.
Regarding environment parameters, it is important to monitor how numerical model profiles and parameter fields "fit" with the evolving surface pattern and any boundaries that might suggest additional enhancement of ingredients relevant to tornadoes. Of most importance is comparison of model-derived data to hourly surface maps and other observed data (such as soundings and profilers) to see if the model estimations have a reasonable handle on temperatures, dew points, winds, and possible boundaries. If not, one can adjust parameter assessments accordingly. Relying on parameter field "bull-eyes" without reference to current surface map observations, features, and boundary locations will encourage a poor or wrong assessment of the situation.
When using parameter fields and values (such as, for
example, SPC's mesoscale
analysis page, or from Earl Barker's
page), the table and explanation below (click to enlarge) is one subjective method of
assessing relative support for significant supercell tornadoes. Don't treat the
table values as "thresholds", but look instead for parameter values that are
more favorable in value without one or more parameters falling sharply into the
"poor" category:
subjective table of relative values... not to be used as strict "thresholds"!
Another reference tool is SPC's Significant Tornado Parameter (STP) which incorporates some of the parameters above. But use it only as a a guideline to start with (not a stand-alone parameter!), proceding to look more carefully over the area of interest at the individual parameters and ingredients that go into the composite STP.
The above table (using lowest 100 mb mixed-layer lifted parcels) serves only as a starting point for a forecaster to build an experience base with supercell tornado environments. It is important not to treat the parameter values like "magic numbers" or the parameters themselves like "black box" parameters. Instead, it is best to try to relate the parameters to processes (as noted in the explanation below the table above) that relate to tornado development from recent research. Look for areas where convection is expected, often along boundaries, and assess whether the estimated values for most or all parameters appear generally favorable for processes supporting supercell tornadoes in that area. Here are some tornadic and nontornadic supercell case studies using SPC mesoanalysis graphics and other images that may help:
- 18 June 2001
in eastern Minnesota and northwest Wisconsin
- 17 April
2002 in Oklahoma, Kansas, and Iowa (including a nighttime tornado, and other
nontornadic supercells that were "elevated")
- 18 April
2002 in northern Kansas (nontornadic "elevated" supercell)
- 23 June 2002
in northern South Dakota
- 27 March 2004
in Kansas and Oklahoma
- 12 June
2004 in south central Kansas
Some caveats about using the paramaters in the table above:
****** Be careful in cases where total CAPE is small (e.g., < 800-1000 J kg-1).
If SRH is large (e.g., >200-250 m2s-2) and deep
shear is strong (e.g., > 45-50 kts) and the storms are not strongly
"elevated" (e.g., there is CAPE from near-surface parcels and CIN <
50-75 J kg-1), tornadoes may be possible even though composite parameters such
as EHI and STP look unfavorable. Small CAPE situations, particularly those along the
Gulf Coast and Southern states of the U.S. in the cool season, are a problem for EHI and
STP because large SRH and strong deep shear can sometimes help to generate supercells
capable of supporting tornadoes in environments with relatively small total CAPE (e.g.,
400-1000 J kg-1).
****** Also be careful in cases where total CAPE is quite large (e.g., > 4000 J
kg-1). SRH and deep shear can be rather marginal in these situations
(e.g., 50-75 m2s-2 and 30-35 kts, respectively) for tornadoes to
occur because the large CAPE generates strong updrafts with only marginal shear required
to keep a storm organized as a possible supercell.
Again, the above information can be used as a starting
point to build an experience base, getting a feel for parameter environments and fields
combined with surface and upper air patterns that support notable supercell tornadoes.
