Once convection occurs in your warning area, and if the environment remains conducive to wet microbursts, the forecaster now relies on numerous nowcasting tools to decide which storms require warnings. These tools help the forecaster discern a more complete picture of storm morphology so that correct warning decisions can be made.

Cell trends are one such tool. Prior to a downburst reaching the surface and causing damage, there are typically specific indicators in the cell trend data. Cell trends that indicate the intensity or height of a storm (i.e., Vertically Integrated Liquid, or VIL, and Height of Maximum Reflectivity, or HMR) often decrease rapidly prior to a downburst being observed at the surface. The specific cell trend indicators are listed in Table 3. An example of this trend is shown in Fig. 5. Unfortunately, these cell trend indicators may only give a five- to twenty-minute lead time for a convective wind warning (Wheeler, 1998). Verification statistics for the cell trends in Table 3 are provided in Table 4.

Table 3. Cell Trend Indicators of an Impending Downburst

VIL	Decreases by 10 kg m-2 and
HMR	Decreases by 8 kft

 
 

Besides radar cell trends, there are numerous radar characteristics that can help with wet microburst nowcasting. One prominent trait of wet-microburst-producing storms is a weak-echo trench (Fig. 6; Mackey, 1998), an area of low reflectivity between two cells in close proximity. A weak-echo trench is a particularly strong indicator if one of the cells is relatively smaller, weaker, and new; and the other cell is relatively larger, stronger, and developed rapidly. These weak-echo trenches indicate dry, ambient air is in close vicinity to the storms that can be entrained into the storm core at the beginning of downburst formation. Another radar characteristic that is indicative of dry, ambient near a storm is a low reflectivity notch (Fig. 7; Mackey, 1998). The key difference between these two phenomena is that a low reflectivity notch indicates dry air entering the storm core, not just in vicinity of the storm core. Both of these traits may be visible in a vertical cross-section, if the cross-section axis is chosen properly.

There are many other downburst characteristics visible in base and derived radar data. These characteristics include:

• Precipitation cores (area >55 dBZ) extending to the midlevel minimum in
• A rapidly descending precipitation core
• Secondary convection (related to weak echo trench)
• Cell mergers/complexes
• Storms with collapsing updrafts/overshooting tops (or other signs of storm decay)
• Other indicators of severe convection (squall line, bow-echo, etc.)
• Weak radial convergence at midlevels (around the level of )
• Rotation, but at rotational velocities less than user-defined mesocyclones.

There are some instances that a forecaster will be able to make visual observations of a storm that has the potential to produce a downburst. In some cases, these visual cues may be the only significant clue that a downburst is imminent. A detailed discussion on the visual observation of downbursts is available from the NSSL: "Microbursts: A Handbook for Visual Identification". We highly recommend forecasters to read this document.