7. SUPERCELLS AND TORNADOES

A. Supercells

The common factor in all supercells is the deep, persistent mesocyclone, of the storms precipitation characteristics. Supercells occur in environments with an extremely wide range of CAPE. Johns and Doswell (1992) came up with the following conclusions on supercells:

1. Characteristics of a Supercell Hodograph (Brown 1990)

  • Marked directional shear in the lowest 3 km (9800 ft)
  • Marked speed shear from 3 to 12 km (9800- 39,000ft)
  • 2 to 3 km deep layer shear at mid altitudes where the wind shear is a minimum (less than 1 m/s per km on average).

2. LP Supercells (Bluestein and Parks 1983)

  • Found in the surface dry line environment over the western Great Plains
  • Characterized by low moisture values in low levels (Inverted V sounding)
  • Relatively high LFCs
  • Lifted Index is not representative due to a lack of low level moisture
  • Large hail is the main threat, but narrow rope like tornadoes can occur

3. HP Supercells (Moller et al. 1990)

  • Found in central, southern, and eastern United States
  • Significant instabilities, but with helicity which is only marginal for classic supercells
  • High moisture content lowers LFC, thus less lift is needed to sustain convection
  • Significant low level warm air advection across a pre-existing thermal boundary
  • Produce tornadoes, heavy rain and flash flooding, large hail, and damaging winds

4. Low Topped Supercells

  • Typically occur during cool seasons with Type D Miller (1972) patterns.
  • Cold core upper low (mid level temperatures -20°C) positioned above and slightly upwind of a surface low.
  • Low equilibrium level and low tropopause (less than 30,000ft).
  • Often drying in the mid levels allows diurnal surface heating to play a large role in a development of severe thunderstorms near the leading edge of the upper level cold air pool.
  • Low values of CAPE, generally 1000 J/kg, however, airmass is likely more unstable when you consider vertical distribution of CAPE.
  • Steep 850 mb to 600 mb lapse rates (Davies 1993).
  • Moderate to strong wind fields while exhibiting significant low level vertical wind shear (Foster et al. 1995).
  • Davies (1993) found magnitudes of EHI (>3.0), 0-2 km AGL storm relative inflow (>20 kts), and 3-6 km AGL winds ( >30 kts) significant and suggestive of tornadic supercells.
  • Tornadoes typically occur within the warm sector southeast of the upper cold low, in relative close proximity of the cold air aloft, yet also close to the mid level jet.
  • Additional information can be found in Boyne (1995) and Murphy and Woods (1992).

5. Mid Level Storm Relative Wind Speed With Supercells (Thompson 1998).

  • Observed or forecast mid level (500mb) storm relative (SR) wind speeds can also help discriminate between tornadic and non-tornadic supercells.
  • 500mb SR winds of 7 to 10 m/s seem to be where supercells transition from non-tornadic to tornadic in many cases.
  • 500mb SR wind speed forecasts using PCGRIDDS analyses of Eta Model output shows that a threshold of 8 m/s can be effective at determining where supercell tornadic storms may occur.
  • Forecasters should be especially aware of supercell tornado potential when 500 mb SR wind speed is forecast, and storm inflow can be enhanced by mesoscale processes (boundaries, mesolows).

6. Other Supercell Notes

  • Typical 0-3 km SRH threshold of 150 m/s for mesocyclone formation (Davies Jones et al.1990).
  • SRH can be significantly influenced by smoothing of data, vertical resolution, and small (1-2 m/s or less hodograph changes (Markowski et al. 1998).
  • Weisman (1996) considers the 0-6 km shear vector a better indicator of supercell potential than SRH, with a typical threshold of 20 m/s.
  • Due to variations in structure, supercells occurring in the same thermodynamic environments may differ in size, amount and distribution of hail (Sturtevant 1995).

Additional information can be found in Przybylinski (1996).

B. Tornadoes

The tornado arguably may be the most difficult weather feature to anticipate. Hales (1996), a forecaster at SPC for more than 25 years, has stated that the three primary forecasting keys for tornadoes are:

Tornadoes form in many different types of air masses, some of which are understood and some which are not. Miller classified upper air soundings associated with tornadoes into four types (Great Plains, Gulf Coast, Pacific Coast, and High Plains). This section will only include the Great Plains variety as it is the most common and severe. For additional information on tornado types (Miller 1972). This paper will not address non-supercell tornadoes, but additional information can be found in R. Smith (1996).

1. Type I - Great Plains Type

  • Optimum airmass structure for severe weather and tornadoes
  • Continental tropical air overruns maritime tropical air at 8000 - 10000 ft
  • Subsidence inversion, conditionally unstable above and below it, and stable through it
  • Wind increase in speed and veer with height
  • Winds increase in altitude in the dry air above the inversion, having a component of > or = 30 kts perpendicular to the flow in the warm moist air
  • Dew point >55°F, Lifted Indices < or = -6 and Total Totals of >54
  • Severe weather occurs most often in late afternoon due to strong surface heating

2. Shear Versus Instability

a. Energy Helicity Index (EHI) (Hart and Korotky 1991)

The EHI is an index that incorporates vertical shear and instability for the purpose of forecasting supercell thunderstorms. It is related directly to SRH in the lowest 2 km and CAPE (J/kg) by the following equation:

Higher values indicate unstable conditions and/or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather.

b. SRH vs CAPE (Johns et al. 1993) Study

Figure 6 shows the plot of the 0-2 km AGL SRH verus CAPE for 242 strong and violent tornado cases. For a given range of CAPE, there appears to a range of helicity that is most favorable for strong or violent tornado formation, with the values decreasing as CAPE increases.

3. Bulk Richardson Number (BRN)

The BRN measures the relative importance to CAPE and vertical wind shear and correlates well to observed storm types.

Here, u = vertical wind shear and is calculated by taking the difference between the density weighted mean wind over the lowest 6 km of the profile and a representative surface layer wind (500 m mean wind).

  • Weisman and Klemp (1986) state that the dimensionless BRN is a better indicator of storm type than of storm severity and works best with CAPE values from 1500 to 3500 J/kg.
  • Davies and Johns (1993) state that the BRN correlates well with observed storm type. However, it is a poor predictor of storm rotation in low levels because it does not account for low level curvature shear.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6. Scatter diagram showing combinations of CAPE in J/kg and 0-2 km AGL positive wind shear for 242 tornado cases during 1980-1990 (Johns et al. 1993). Figure reproduced from Johns and Doswell (1992).

 

BRN Values

Expected Convection

< 10

Strong vertical wind shear/weak CAPE. Shear may be too strong given the weak buoyancy to develop sustained convective updrafts. With sufficient forcing, thunderstorms may still develop and rotating supercells could develop in the high shear environment.

11-49

Severe weather potential, some supercells

>50

Multi-cells likely

4. Shear Magnitudes of Hodographs in Tornado Forecasting (Davies and Johns 1993) and (Davies 1989)

  • Magnitude of the vertical wind shear in the 0-2 km layer (0-6600ft) may have the most direct impact on enhancing updraft rotation in tornadic supercells.
  • Mean shear S = hodograph length (m/s)/depth of layer (m).

Davies and Johns gave the following shear and helicity values for strong and violent tornadoes:

TABLE 4
Shear Magnitudes for Strong and Violent Tornadoes



Layer

Average Positive Shear For Strong Tornadoes
(F2/F3)

Average Positive Shear For Violent Tornadoes
(F4/F5)

0-2 km 13.4 14.7
0-3 km 10.5 11.7
0-4 km 9.0 10.0

Research suggests that the vertical wind shear structure is the most crucial element in supercells (Doswell et al. 1993). Additionally, the combination of vertical wind structure and storm motion produce enough storm relative helicity to allow the mesocyclone to reach the surface.

TABLE 5
Helicity Magnitudes for Strong and Violent Tornadoes



Layer

Helicity Observed/Assumed For Strong Tornadoes
(F2/F3)

Helicity Observed/Assumed For Violent Tornadoes 
(F4/F5)
0-2 km 359/317 460/415
0-3 km 369/339 519/452
0-4 km 378/357 539/478

Observed Helicity - No storm motion used
Assumed Helicity - Has storm motion 20 degrees to the right at 85% of the mean 0-6 km wind

Note that helicity is subject to rapid temporal and spatial changes.

5. Violent Tornado Outbreaks and Pattern Recognition

Johns and Sammler (1989) defined violent tornado outbreaks as: 1) 10 tornado events with one F4 tornado having a path of 30 miles or more, and 2) six or more tornado events with one or more F4 tornadoes having a combined length of 60 miles or more. The following forecasting conclusions came from their study of 77 outbreaks:

  • Temperatures at 850 mb rise, and the 850 mb dew points rise, frequently by > or = 5°C over 12 hours.
  • In all 77 cases, the low level moisture extends above 850 mb level.
  • Most outbreaks are associated with a double jet structure with the center-point usually between the jets.
  • When only one jet is evident at 500 mb, the outbreak center-point is usually beneath the axis of the jet.
  • Often associated with a rapidly moving 500 mb shortwave trough.
  • Average winds from 850 mb to 500 mb are 10 to 20 kts stronger in weak instability cases (0 to -3 Showalter Stability Index (SSI)) than in strong instability cases (-7 to -10 SSI).
  • The 850 mb to 500 mb directional shear values are smallest with weak instability cases and largest with strong instability cases.
  • Directional shear appears to be a major contributor to the shear magnitude associated with violent tornado outbreaks in the plains states during the warm season.
  • Speed shear appears to be a primary contributor to shear magnitude in the area east of the plains in the cool season.

SPC uses classic pattern recognition as severe weather forecasting tool (Imy 1998). Additional information can be found in by contacting SPC or reading Kriehn (1993) and Kleyla (1991).

6. Boundaries

Miller (1967) was one of the earliest papers documenting the importance of identifying supercell and boundary interactions.

a. Maddox et al. (1980) came to the following conclusions:

  • Boundaries are a source of enhanced CAPE, convergence, and positive relative vorticity.
  • Therefore, they are conducive to the development of tornadic storms, even in cases when the environment was only marginally favorable for severe convection.

b. Markowski et al. (1998) found that nearly 70% of all significant tornadoes during VORTEX 95 occurred along or behind boundaries.

7. Tropical Cyclone Tornadoes

Tropical cyclones, especially hurricanes, that make landfall in the United States frequently produce tornadoes, especially in the right front quadrant. McCaul Jr. has written numerous papers on landfalling tornadic tropical cyclones, most notably 1991. Infrequently, tropical cyclones produce more tornadoes during the post landfall stage than during landfall (Weiss and Otstby 1993).

8. ADDITIONAL SEVERE WEATHER TOPICS

 


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