( Some case studies : 10 April 2005 in northwest Kansas 7 July 2004 in northern Kansas 18 April 2003 in central Nebraska )
Ingredients supporting tornado events associated with closed 500
mb cold core lows (summary by
Jon Davies)
( Some case studies : 10 April 2005 in
northwest Kansas
7 July 2004 in northern
Kansas 18 April 2003 in central
Nebraska )
Weather settings with potential for tornadoes near closed 500 mb cold core lows are often difficult to diagnose using "accepted" supercell tornado parameters, which is why they will be summarized separately here. Total CAPE is often small in these situations (400-800 J/kg) but "squeezed" down low in the atmosphere due to the lower tropopause (see example skewT diagram) associated with the 500mb cold core circulation aloft. As a result, vertical accelerations relative to the ground are probably stronger than would be expected from looking at the total CAPE alone, even though the storms are usually small, low-topped, and hard to detect on radar (tops < 30,000-35,000 ft). Because of the small total CAPE involved, it is important to realize that resulting SRH/CAPE combinations will tend to appear much smaller than "typical" supercell tornado situations, so that composite parameters sush as EHI and STP (see supercell tornado parameters ) usually do not work well in these settings. Many low-topped tornadic storms near 500 mb lows are supercells ("mini-supercells" as in the photo above), and occur in environments with considerable vertical wind shear. But other documented tornadoes spawned by low-topped storms in some 500 mb low situations appear to occur in environments with relatively weak shear (see example skewT diagram), suggesting that some of these events may involve processes that have more in common with non-mesocyclone tornadoes. More research is needed. While most tornadoes associated with closed 500mb cold core lows tend to be weak, some have been documented to reach F2 or greater intensity, so these events aren't something to ignore.
Regardless of the detectable shear environment, many tornado settings from mini-supercells or low-topped storms near closed 500 mb lows have at least three ingredients in common:
Recognition of a certain surface pattern and set of ingredients (see recent SLS conference paper by Davies
and Guyer 2004) with minimal surface moisture requirements within 200 statute miles of
the midlevel low can help provide a "heads up" to forecasters and warning
meteorologists in 500 mb cold core low situations with potential for tornadoes. As
noted above, surface moisture and parameters typically associated with supercell tornadoes
ofen appear lacking with such events, making the tornado potential in such situations
harder to detect. The following composite of a typical tornadic 500mb cold core low
setting in the plains may help::
<(composite
example by Jon Davies)
Surface dew points in the low to mid 50's (oF) in a narrow band along the warm
front east of the surface low may suggest enough moisture combined with the cold air aloft
and heating pushing in from a cloud-free "slot" behind the Pacific front/dryline
to generate CAPE over a localized area for mini-supercells/low-topped storms near the
surface "focus" or boundary intersection. Don't let unimpressive total
CAPE amounts mislead in such cases!
Here are a few tornadic cases associated with closed 500mb cold core lows that are useful for study:
- 18 April 2003
in central Nebraska
- 7 July 2004
in northern Kansas
- 10 April
2005 in northwest Kansas