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Large Hail

Atmospheric ingredients and forecast parameters/thresholds defining environments favorable for large hail

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Atmospheric Ingredients

Ingredients necessary for large hail : 

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The ingredients necessary for large hail most often include the ones necessary to produce organized convection which 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 also usually quite useful. 

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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 become stronger 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 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. 

  • Hailstone size is minimized by low elevation, high freezing levels, high PW, moist  mid-levels, low CAPE and weak wind shear.

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In a nowcasting environment, additional clues regarding the presence of large hail in on-going thunderstorm cells using radar :

 

dBZ values above 55 dBZ

Vertically Integrated Liquid (VIL) values above specific thresholds

Hail spikes 

Forecast Parameters

Forecast ingredient parameters useful in determining whether pre-convective environments are favorable for large hail :

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  • For High Instability :

    • CAPE [J/kg] :​

      • MLCAPE (Pivotal Weather - ICON)​

      • MUCAPE (Météociel - AROME) , MUCAPE (Pivotal Weather - GFS)

    • Lifted Index [°C] :

      • SBLI (Pivotal Weather - GFS)

      • MULI (WRF-NMM 2km)​

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  • For Supercell Thunderstorm Probability :

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Forecast Parameter Thresholds

Moisture

Mixing Ratios

< 3 g/kg : very dry

​3 - 5 g/kg : dry 

5-7 g/kg : slightly humid/moist

7-12 g/kg :  moderately humid/moist

12-15 g/kg : very humid/moist

> 15 g/kg : extremely humid/moist

Dew Points

< -3 °C : very dry

​-3 to +5 °C : dry 

5 to 10 °C : slightly humid/moist

10 to 17 °C :  moderately humid/moist

17 to 20 °C : very humid/moist

> 20 °C : extremely humid/moist

Equivalent Potential Temperatures (850 hPa)

< 5 °C : very dry

​5 - 18 °C : dry 

18 - 30 °C : slightly humid/moist

30 - 45 °C :  moderately humid/moist

45 - 60 °C : very hot & humid (très lourd)

> 60 °C : extremely hot & humid (ext. lourd)

Instability

CAPE

​0 : stable

0-700 J/kg : weakly unstable

700-1500 J/kg :  moderately unstable

1500-3000 J/kg : very unstable

> 3000 J/kg : extremely unstable

Lifted Indices

>+2°C : stable

​+2 to 0 °C : stable/neutral

0 to -2 °C : weakly unstable

 -2 to -4 °C :  moderately unstable

-4 to -6 °C : very unstable

< -6 °C : extremely unstable

Temperature Lapse Rates

​<  5.5 - 6.0 °C/km : stable

6.0 - 7.0 °C/km : slightly unstable/steep

7.0 - 8.0 °C/km  :  moderately unstable/steep

8.0 - 9.0 °C/km : very unstable/steep

> 9.0 °C/km : extremely unstable/steep

Lift

Level of Free Convection (LFC)

​< 1000 m : very low (convective initiation very easy) 

1000 - 1500 m : moderately low (conv. initiation easy)

1500 - 2000 m :  average height (conv. initiation probable if low-level lift or convective temperature reached)

2000-3000 m : moderately high (conv. initiation more difficult and isolated unless strong lift)

> 3000 m : very high (conv. initiation very difficult and unlikely unless strong lift)

Convective Inhibition (CIN)

​0 to -50 J/kg  : weak CIN (cap easily broken)

-50 to - 100 J/kg : moderate CIN (cap regionally broken if moderate lift) 

-100 to -150 J/kg :  strong CIN (cap locally broken if moderate lift)

< -150  J/kg : very strong CIN (strong large-scale lift needed to erode cap)

Temperature Advection (850 & 700 hPa)

​> -5 °C/hr : strong CAA / strong subsidence

-5 to -2 °C/hr : moderate CAA / moderate subsidence

-2 to 0 °C/hr :  weak CAA / weak subsidence

0 to +2 °C/hr : weak WAA / weak ascent

+2 to +5 °C/hr : moderate WAA / moderate ascent

> +5 °C/hr : strong WAA / strong ascent

CAA = Cold Air Advection

WAA = Warm Air Advection

Also use square method : the smaller the geopotential/isotherm square, the stronger the temperature advection. 

Vorticity Advection (500 hPa)

​> -30 x10-5/s : strong NVA / strong subsidence

-30 to -15 x10-5/s : moderate NVA / moderate subsidence

-15 to 0 x10-5/s :  weak NVA / weak subsidence

0 to 15 x10-5/s : weak PVA / weak ascent

15 to 30 x10-5/s : moderate PVA / moderate ascent

> 30 x10-5/s : strong PVA / strong ascent

NVA = Negative Vorticity Advection

PVA = Positive Vorticity Advection

Also use square method : the smaller the geopotential/vorticity isopleth square, the stronger the vorticity advection.

Q-Vectors

Divergence of Q-vectors : sinking motion (the stronger the Q-vector divergence (blue contours) , the stronger the sinking motion)​

Convergence of Q-vectors : rising motion (the stronger the Q-vector convergence (red contours) , the stronger the rising motion)

Potential Vorticity (PV)

Advection of High PV : airmass ascent along isentropes

Advection of Low PV : airmass subsidence along isentropes

Convection can produce high PV areas due to diabatic heating

Dynamic Tropopause

Advection of low tropopause Theta air : airmass ascent

Advection of high tropopause Theta air : airmass subsidence

Advection of low tropopause pressure : airmass ascent

Advection of high tropopause pressure : airmass subsidence

Vertical Wind Shear

0-6 km Bulk Shear

0 - 15 kts : weak shear (favors ordinary convection - airmass/pulse thunderstorms)

15 - 35 kts : moderate shear (favors multicellular convection - multicell clusters/squall lines)

> 35 kts : strong shear (favors organized convection - isolated supercells, supercells embedded in lines, bow-echoes)

0-3 km Bulk Shear

0 - 10 kts : weak shear (favors gust fronts clearly outrunning the convection)

10 - 30 kts : moderate shear (favors gust-fronts closer to the convection)

> 30 kts : strong shear (favors deep cold pools with gust fronts on leading edge of convection)

Contact

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