Diagnosis

Once you have confidently identified a Damaging Low-Level Winds 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

  • Magnitude of the measured radial velocities – The stronger the radial velocities along the radar beam, the higher the likelihood of stronger flow on the surface.
  • Proximity of radar-observed damaging winds to the surface – The closer a radial wind measurement is to the surface, the higher the chance that most of this wind magnitude will also be present at the surface. The lower the static environmental stability below the radar beam, the higher the likelihood that vertical momentum can be transferred from the beam height to the ground.
  • Horizontal extent of the damaging winds – The larger the area of damaging winds the more destruction is likely.
  • Longevity of the damaging winds – The longer the damaging wind signature exists, the longer-lived the storm and more damage likely.

Considering these aspects of Low Level Damaging Winds will help to determine overall the severity of the storm. The presence of Damaging Low-level Winds is one of the few signatures that can influence you directly into issuing a severe thunderstorm warning. Generally, radar information should never be used in isolation and should always be considered in conjunction with the near storm environment and any reports.

Most Likely Convective Hazards

If a thunderstorm has been determined to be severe and possesses a Damaging Low-level Winds signature, the following convective hazards should be considered to be included in the severe thunderstorm warning:

  • damaging winds, with some consideration to upgrading the warning to include destructive winds, especially with observed winds of above 67 knots close to the surface.

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