Diagnosis
Once you have confidently identified a Bounded Weak Echo Region signature, this section will help you estimate the storm severity associated with it. Generally, the spatial and temporal scales of a signature are 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 BWER
- A longer-lived BWER indicates a steady, strong updraft core and is pointing towards (but is not caused by) steady storm-scale rotation.
Average dBZ of BWER
- The higher the average dBZ echo outlining the BWER, the stronger the associated updraft as it is able to create very large radar targets (most likely hail). For an updraft to be deemed strong, the suspended reflectivity values need to be at least 50 dBZ.
Vertical Depth of the BWER
- The larger the height, H, the distance between the ground and the lower level of the overhang echo aloft, the more likely the updraft resides in the ideal hail growth layer (-10ºC to -30ºC) with supercooled liquid water droplets required for the production of large hail. This should be confirmed against the -10°C to -30°C layer of a proximity sounding. A "taller" BWER also indicates that echo development or descent is prevented in a taller column of air, a property related to updraft intensity.
Vertical height (H) of a BWER signature.
Considering all these aspects will help to determine overall whether you are dealing with a significant signature. In the context of a severe thunderstorm warning decision, a significant BWER could single-handedly provide sufficient evidence for a severe thunderstorm, based on its tight connection to storm-scale rotation. Generally, radar information should never be used in isolation and should always be used 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 BWER of significance, the following convective hazards should be considered to be included in the severe thunderstorm warning:
- Damaging winds – A BWER is a representation of an intense updraft, with potential to produce a strong downdraft as well as intense inflow to the updraft. Destructive winds should be expected given some additional evidence confirming that you are dealing with a supercell (unless the storm is elevated above a strong inversion and above a cold air mass).
- Large hail – A strong updraft has the potential to produce large hail, providing the updraft extends into the hail growth layer, -10º to -30ºC. Warnings for giant hail may be warranted due to the supercellular classification of the thunderstorm.
- Heavy rainfall resulting in flash flooding – A particularly strong updraft has the potential to produce large amounts of precipitation accumulating and falling as heavy rainfall resulting in flash flooding. Melting hail could also contribute to heavy rainfall. However, even when dealing with a supercell, flash flooding is not an automatic hazard, in particular for smaller or faster-moving storms. Supercells tend to have a comparatively low precipitation efficiency, but also process inordinate amounts of water vapour. The result of these two opposing drivers for heavy rainfall is that flash flooding is more likely with larger and/or slower-moving supercells.
- Tornado – Tornadoes are possible and therefore should be at least considered, subject to evidence that the near-storm environment supports tornadoes (strong 0- to 1-km AGL shear, and a low LCL) and the presence of low-level rotation or at least strong low-level convergence in the radial velocities.
See Conceptual Models for more details on why particular severe weather should be included.