Thunderstorms Spring 2016 Kyle Imhoff

Atmospheric Stability A Matter of Stability bubbles of air vs. the atmospheric environment bubbles change temperature by either dry adiabatic lapse rate (9.8C/1km) (5.5F/1,000ft) moist adiabatic lapse rate (5C/1km) (3.5F/1,000ft) Environment lapse rate varies Stability - a measure of whether an air bubble moves when prodded (heated or forced lift) unstable - moves and never returns stable - may move, but returns to same place

Atmospheric Stability Stable Air Characteristics clear or stratiform clouds generally light surface winds smoke, haze, smog accumulate Inversions are common (morning fog) Unstable Air Characteristics clear or cumuliform clouds generally gusty winds lack of haze Thunderstorms can develop if enough moisture is available

Thunderstorms Thunderstorms always form in an unstable air mass - and are associated with Cumulonimbus Clouds (Cb) Most thunderstorms occur in the spring and summer (with a maximum in frequency around the solstice - June 22) The majority of thunderstorms occur between 2-8pm due to daytime heating

Thunderstorm Evolution Initial Stage Cumulus to Mature Strongest Vertical Motions (25 m/sec) Cloud grows from 15,000-35,000 ft Electrification occurs Downdraft starts Heaviest rain and hail Life span: 20 minutes Mature Stage Mature to Dissipating Weaker Vertical Motions (5-10 m/sec) Cloud reaches maximum height - spreads out into Anvil Downdraft spreads far Lighter rain Less Electrification

Life Cycle of a Thunderstorm

Where Storms Form Formation Zones Differences in Temperature and Moisture Clear/Cloudy regions Sea/Lake Breeze Mountain vs Valley Edge of Fog areas Dry line - West U.S. Large River Valley

T-storm Types Types of T-Storms Air mass, single cell Clusters Supercell (HP or LP) Squall Lines Derechoes Mesoscale Convective Complexes (MCC’s)

Squall Line

Squall Line/Air Mass Thunderstorms

Effects The Effects of Thunderstorms: Gust Front Tornadoes Straight-line winds Derecho Microbursts Tornadoes Mini and Maxi Fujita Scale Flash Flooding Hail pea to softball sized

Severe Thunderstorms Four main ingredients: Instability Moisture Lifting Mechanism Wind speed changes direction/speed with height (called wind shear)

Forecasting Thunderstorms Storm Prediction Center Issues watches, outlooks, discussions regarding potential severe thunderstorms and tornadoes NWS Forecast Offices Issue warnings on severe thunderstorms and tornadoes

Summary Thunderstorms come in many shapes and sizes – all produced by convection Key ingredients are instability, moisture, and a lifting mechanism Very difficult to predict timing and location of thunderstorms Important due to their potential socioeconomic impacts