Diagnosis

Once you have confidently identified a Forward Flank Notch signature, this section will help you estimate the storm severity associated with it. Generally, the spatial and temporal scales of a signature are loosely related to the updraft strength. In other words, the larger and/or more long–lived the signature, the stronger the updraft that produced it. In velocity–based signatures, updraft severity can usually also be gauged by the magnitude of the measured radial velocities. Examining a storm’s overall temporal evolution will suggest whether the storm is becoming more or less severe. Radar signatures and associated storm developments can also be time–shifted relative to each other, as is the case in supercell tornadoes that occur during the collapse of the parent storm.

When comparing signatures to diagnose relative severity, keep in mind that it is assumed that signatures are sampled at equal ranges from the radar. Otherwise, a storm sampled at greater range (with a wider beam) can appear to be weak and/or weakening, while a storm sampled at a closer range (with a narrower beam) can appear to be strong and/or strengthening.

Degree of Severity

  • Longevity of the Forward Flank Notch – persistence of the Forward Flank Notch indicates that the attendant updraft is continually strong enough to maintain the signature. The FFN is therefore a proxy for the steadiness of the updraft strength.

Most Likely Convective Hazards

If a thunderstorm has been determined to be severe and possesses a Forward Flank Notch signature of significance, the following severe weather should be considered to be included in the severe thunderstorm warning:

  • Damaging winds – a Forward Flank Notch is a representation of a strong updraft, with potential to produce a strong downdraft.
  • Large hail – a strong updraft has the potential to produce large hail, providing the updraft extends past the freezing layer, to –10 to –30 °C (hail growth) layer. Some consideration of the temperature of the environment between the thunderstorm base and the ground is useful to identify the potential for hail melting on its descent to the ground.

If the FFN signature is indicative of a supercell, also consider flash flood potential for slower–moving or very large storms, and tornado potential in environments of high boundary layer relative humidity in strong 0–1 km shear.

See Conceptual Models for more details on why particular severe weather should be included.