Monday, June 13, 2011

Forecasting Severe Hail

This blog post will attempt to describe thermodynamic and dynamic characteristics I look for when forecasting severe hail. Some factors, such as storm microphysics and kinematics, are currently impossible to analyze/forecast but play an equally large role in governing the threat for severe hail.


Buoyancy: The environment was moderately unstable with a 2,435 J/kg of SBCAPE and very little SBCIN (-2 J/kg). Although the CAPE profile is fairly deep, it is rather thin. Within the hail growth zone, there appears to be about 820 J/kg of CAPE.

Deep layer shear: Despite relatively weak winds in the lowest 4 km, the degree of veering and the stronger winds near 6 km more than make up for this. 0-6 km shear of 37 kt is more than adequate. Even stronger winds are noted at 7 and 9 km.

Mid-level lapse rates: A 700-500 mb lapse rate of 6.2 C/km is respectable for a tropical environment. However, the lapse rate within the hail growth zone is closer to moist adiabatic.

Freezing level: 14,381 feet is rather high and likely allowed for more melting. The deep moist layer from the surface through ~650 mb also increased the melting potential of hail stones.

Overall threat: The threat for severe hail is pretty high based on this sounding. The buoyancy alone should allow for at least some threat for severe hail. However, the decent deep layer shear and large CAPE in the hail growth zone certainly increase the hail threat. Some negative factors for significant hail (2" or larger in diameter) are the weaker lapse rates in the hail growth zone, the high freezing level, and the moist profile from the surface to about 650 mb.

Summary: A few reports of hail up the golf ball size were reported in Broward County. This is extremely rare for the middle of June, but the thermodynamic and dynamic environment was supportive. Additionally, storm microphysics and kinematics were likely very favorable for severe hail production.

It should also be mentioned that this sounding profile was also quite favorable for wet microburst production. Indeed, several severe wind gusts were reported, including a 64 mph gust measured at Weston, FL.


Severe Thunderstorm Rocks Western Broward Metro -- WFO Miami

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