Tornado Forecasting

An overview of how I do forecasting before a storm chase.

A Source of Lift

Usually, thunderstorms won't initiate without some type of lift. Lift is usually produced by jet stream dynamics, surface boundaries, and sometimes topography (orographic lift). When the flow aloft is diffluent, it tends to produce lift underneath it. Frontal boundaries, drylines and outflow boundaries are sources of lift as airmasses with different densities converge. Lift can also be produced by upslope flow.

Convective Available Potential Energy (CAPE)

A measure of instability. When air near the Earth's surface is lifted, it expands and cools. Eventually condensation can occur, which releases latent heat and further increases buoyancy. This is what drives thunderstorm development. CAPE measures how much buoyancy is available. Usually I'm looking for CAPE of least 1,000 J/KG.

Convective Inhibition (CINH)

Convective Inhibition measures how much warm air aloft is inhibiting thunderstorm initiation (negative buoyancy). Usually this needs to be less than 50 J/KG to allow thunderstorms to initiate, but it varies depending on the strength of the lifting.

Deep Layer Shear

In order to sustain thunderstorm updrafts, precipitation needs to moved away from the updraft by sufficient vertical wind shear (change in wind speed or direction with height). Usually I look at the effective bulk wind difference (EBWD). It should be 25 knots at a minimum, and preferably higher than 40 knots.

Low Level Shear

Shear in the lowest 1 KM is important for tornado formation. 0-1 KM storm relative helicity (SRH) should be at least 100 M2/S2, and preferably higher then 200 M2/S2.

Veering With Height

Discrete supercells are the most favorable storm mode for tornadoes. Supercells are more favored when the wind veers with height (turns clockwise). Ideally, winds at the surface will be from the east, and as you go up from the south, then from the west.

Lifted Condensation Level (LCL)

The lifted condensation level determines the height of the cloud base. When rain falls below the cloud base, evaporative cooling occurs. If the cloud base is too high, storms tend to produce too much cold outflow for tornadoes to form. Generally, I want LCL heights to be below 1.5 KM.

Storm Motion Vectors

Discrete supercells are most likely when the storm motion takes it away from the boundary that initiated the convection. If storms are moving parallel to a boundary, they tend to merge into a line. If storm motion is perpendicular to the boundary, supercells will be favored. Storms should also be moving faster than the boundary or they may be forced into a line or even be undercut by the boundary. For storm chasing purposes, a storm motion to the east if preferred because it is more aligned with the road network. Slower moving storms are also preferred. It can be difficult to keep up with a storm moving to the northeast at 40+ mph if the roads are all east-west and north-south.