Tornado Forecasting Checklist

Tornado Forecasting Checklist:

  1. Analyze the synoptic-scale environment:
  • Identify the location and strength of the jet stream:
    • Strong upper-level jet stream (>75 knots) with divergence aloft
    • Jet streak positioning favorable for upper-level support
  • Determine the position and intensity of upper-level troughs and ridges:
    • Negatively tilted trough or closed low approaching the region
    • Difluence in the upper-level wind field
  • Assess the presence and strength of low-level and upper-level wind shear:
    • 0-6 km bulk shear >40 knots
    • 0-1 km storm-relative helicity (SRH) >150 m²/s²
    • 0-3 km SRH >250 m²/s²
  • Evaluate the potential for moisture advection and instability:
    • Moisture pooling with dew points >60°F (15°C)
    • Steep mid-level lapse rates (>6.5°C/km in the 700-500 mb layer)
  1. Assess thermodynamic instability:
  • Analyze soundings for the presence of a “loaded gun” profile:
    • Steep lapse rates in the lowest 3 km of the atmosphere
    • High moisture content in the lower levels (dew points >60°F or 15°C)
    • Strong vertical wind shear (0-6 km bulk shear >40 knots)
  • Calculate convective available potential energy (CAPE) and convective inhibition (CIN):
    • Surface-based CAPE (SBCAPE) >1500 J/kg
    • Mixed-layer CAPE (MLCAPE) >1000 J/kg
    • CIN <50 J/kg
  • Determine the lifted condensation level (LCL) and the level of free convection (LFC):
    • LCL heights <1500 meters AGL
    • LFC heights <2500 meters AGL
  • Evaluate the potential for convective initiation:
    • Presence of a lifting mechanism (frontal boundary, dryline, outflow boundary, etc.)
    • Daytime heating and destabilization of the boundary layer
  1. Identify mesoscale features:
  • Locate frontal boundaries, drylines, and outflow boundaries:
    • Strong moisture gradients and wind shifts along boundaries
    • Confluence and convergence along boundaries
  • Assess the strength and orientation of low-level convergence and upper-level divergence:
    • Surface convergence >10⁻⁴ s⁻¹
    • Upper-level divergence >10⁻⁵ s⁻¹
  • Determine the presence and strength of low-level jets and their role in moisture transport and wind shear:
    • Low-level jet (LLJ) with winds >30 knots at 850 mb
    • LLJ oriented perpendicular to the frontal boundary or dryline
  • Evaluate the potential for mesoscale convective systems (MCSs) and supercell thunderstorms:
    • Presence of a “tail-end Charlie” or “pendant” echo on radar
    • Discrete supercell thunderstorms with strong rotation and hook echoes
  1. Consider composite parameters:
  • Significant Tornado Parameter (STP) >1
  • Supercell Composite Parameter (SCP) >4
  • Energy Helicity Index (EHI) >2
  • Vorticity Generation Parameter (VGP) >0.2

When analyzing soundings for the presence of a “loaded gun” profile, look for the following characteristics:

  1. Steep lapse rates in the lowest 3 km of the atmosphere:
  • Lapse rates should be close to or exceed dry adiabatic (9.8°C/km)
  • Steep lapse rates indicate a highly unstable lower atmosphere
  • This instability allows for rapid vertical acceleration of air parcels, which is crucial for the development of strong updrafts in thunderstorms
  1. High moisture content in the lower levels:
  • Look for dew points greater than 60°F (15°C) in the lowest 1-2 km of the atmosphere
  • High dew points indicate an abundance of moisture, which fuels thunderstorm development and can lead to greater instability
  • Moist air in the lower levels also contributes to lower cloud bases and a greater potential for tornadoes to reach the ground
  1. Strong vertical wind shear:
  • 0-6 km bulk shear should exceed 40 knots
  • Strong vertical wind shear is essential for the development of rotating updrafts (mesocyclones) in supercell thunderstorms
  • The change in wind speed and direction with height helps to create a horizontally rotating column of air, which can be tilted vertically by the updraft, leading to the formation of a mesocyclone
  1. Capping inversion:
  • Look for the presence of a capping inversion, typically characterized by a layer of warm, dry air aloft
  • The capping inversion acts as a “lid” on the atmosphere, preventing the premature release of instability
  • This allows for the buildup of energy (CAPE) and moisture in the lower levels, which can be explosively released when the cap is broken, leading to rapid thunderstorm development
  1. Dry air intrusion in the mid-levels:
  • Look for evidence of dry air in the mid-levels of the atmosphere (typically around 700-500 mb)
  • Dry air in the mid-levels can enhance the potential for strong downdrafts and the development of rear-flank downdrafts (RFDs) in supercell thunderstorms
  • RFDs play a crucial role in the formation and intensification of tornadoes by providing additional rotation and convergence near the surface