Diagnosis

Once you have confidently identified a Bow Echo 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

The probability of damaging or destructive surface winds increases with:

  • The magnitude of the radial velocity within the RIJ
  • A decrease in height above ground level of the RIJ – as discussed in the Detection section, the lower the height of the damaging radial winds, the more likely it is that most of that momentum will be transferred to the surface increasing the likelihood of damaging surface winds.
  • A decrease in static stability between the ground and the elevated RIJ – strong, pre-existing cold pools near the surface make it less likely that the RIJ can transport strong winds to the surface.
  • Significant individual storm-scale severe signatures within the Bow Echo – as the Bow Echo is a Mesoscale signature it can possess any of the storm scale signatures, such as:
    • WER
    • BWER
    • Mesocyclones
    • 50 dBz Echo Top Height
    • Storm Top Divergence
    • TBSS
    • Forward Flank Notch
    • Tight Low Level Reflectivity Gradient
    • Hook Echo
    • Low Level Convergence
    • Low Level Divergence
    • Very High Reflectivities

A significant presence of any of the above signatures would support a case for severity of the Bow Echo.

Considering these aspects of the Bow Echo and strong Rear Inflow Jet will help to determine the overall severity of the storm. Even stronger Bow Echo signatures should still be accompanied by some additional investigation that firm up system severity, but with the presence of a strong Rear Inflow Jet, this is one of the few signatures that can influence you directly into issuing a severe thunderstorm warning, in particular for damaging winds. 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 line of thunderstorms has been determined to be severe and contains a Bow Echo signature with a strong Rear Inflow Jet, the following convective hazards should primarily be considered for inclusion into a severe thunderstorm warning:

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

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