Ingredients necessary for large hail : 

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The primary ingredients necessary for the production of large hail are often also the ones necessary for the production of  "organized" convection. Those are : instability, moisture, lift and vertical wind shear. Since supercell storms are known to be prolific hail producers, identifying pre-convective environments that specifically favor supercell formation is usually very useful. This is often attributed to the fact that the presence of a mesocyclonic circulation within a thunderstorm cell increases updraft speeds via storm-scale pressure perturbations. Updraft speeds can sometimes increase by as much as a factor of 2.

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Furthermore, the following specific atmospheric ingredients are known to favor large hail in severe convective storms including supercells :

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• High elevation : This limits the time hailstones have to melt before reaching the ground which increases the probability of receiving larger hail (the Jura and northern/southern Alpine foothills are notorious for large hail)

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• Low freezing levels : This limits the time hailstones have to melt before reaching the ground as well and also increases the vertical depth within the storm cloud where hailstone formation is possible.

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• Dry mid-level air : This favors evaporational cooling of environmental air into a thunderstorm and will result in a lower wet-bulb zero level, thereby also limiting the time hailstones have to melt before reaching the ground. This same dry entrainment can also favor strong downdraft-driven straight-line winds at the surface.

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• High CAPE : This is probably the most important factor in determining hail size. The higher the CAPE, the higher the upward vertical velocities inside the updraft which directly aids in suspending heavier/larger hailstones and accumulating ice layers on them within the thunderstorm. 

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• High vertical wind shear : Strong mid/upper-level winds tilt the updraft which allows a separation between the updraft and the downdraft. This in turn allows both the updraft and downdraft to strenghten and for CAPE to be maximized to its fullest potential since precipitation loading becomes less of a problem.

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• Low precipitable water (PW) : Since the weight of the precipitation will influence the strength of the updraft to some extent even in tilted storms, airmasses with higher moisture values will result in more precipitation loading. This will tend to reduce the CAPE since the force of gravity pushes down on the hydrometeors. Therefore low PW values when coupled with high CAPE can produce large hailstones. Low precipitation supercells are notorious for producing very large hail.

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So to summarize :

• Hailstone size is maximized by : high elevation, low freezing levels, low PW, dry mid-level air, high CAPE and large wind shear. Supercell thunderstorms usually thrive in these types of environments, especially low-precipitation supercells, since they thrive in low PW environments.

• Hailstone size is minimized by : low elevation, high freezing levels, high PW, moist  mid-levels, low CAPE and weak wind shear. These environments favor weak maritime tropical airmass convection which is not conducive for hail.

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In a nowcasting environment, additional clues that can alert forecasters on the presence of large hail in on-going convection via radar, include :

 

• Reflectivity values above 55 dBZ : generally agreed-upon reflectivity threshold for the outer periphery of thunderstorm hail cores

• Vertically Integrated Liquid (VIL) Density values above 3.5 (severe threshold with dime/nickel (1.8 cm / 2.1 cm size), above 4.0 (1 inch or 2.5 cm hail) and above 4.3 (golfball 5 cm size)

• The presence of hail spikes : reflectivity extensions behind thunderstorm reflectivity echoes in the shape of a spike in the down-radial direction of the radar. This signature is sometimes not visible in filtered radar data but always visible in raw data (Refl-U product in RadarLive PBS intranet).

• The presence of lightning jumps : lightning jumps (especially intra-cloud)  have been shown to be often associated with increased hail production in thunderstorms and show some predictive value in terms of nowcasting imminent hail falls at the ground (0 to 45 minute leadtimes, Wapler 2017).

• Probability of Hail (POH) and Maximum Expected Severe Hail Size (MESHS) radar algorithms : these are helpful but sometimes tend to overestimate hail probabilities and hail size diameters.