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THUNDERSTORMSTHUNDERSTORMS

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Ingredients for a Ingredients for a thunderstormthunderstorm

Lift fromLift from Cold/warm frontCold/warm front Gust frontGust front Daytime heatingDaytime heating

Low-level moistureLow-level moisture InstabilityInstability Wind shearWind shear

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Severe thunderstormsSevere thunderstorms

One or more of the following:One or more of the following: Winds stronger than 58 mphWinds stronger than 58 mph Hailstones larger than 0.75 inHailstones larger than 0.75 in TornadoTornado

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Air mass thunderstormsAir mass thunderstorms

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Single Cell or “Pulse”Single Cell or “Pulse” Single-Cell StormsSingle-Cell Storms Moves with mean Moves with mean

environmental wind over 5-environmental wind over 5-7 km.7 km.

30- 60 minute duration.30- 60 minute duration. Rainfall can be moderate to Rainfall can be moderate to

heavy, small hail is heavy, small hail is possible.possible.

Vertical wind shear is small.Vertical wind shear is small. Form in very weak shear Form in very weak shear

environments with new environments with new storms forming in an storms forming in an unorganized mannerunorganized manner

Can be severe at dissipationCan be severe at dissipation

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Multi Cell ClustersMulti Cell Clusters

Cluster of 2-4 Cluster of 2-4 stormsstorms

Feed off gust frontFeed off gust front Moderate Moderate

unidirectional unidirectional shear shear

Moderate Moderate instabilityinstability

Follow mean 700 – Follow mean 700 – 500 mb flow500 mb flow

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Mesoscale Convective Mesoscale Convective ComplexComplex

May – Aug (86%)May – Aug (86%) Multi celled storms Multi celled storms

“gone wild”“gone wild” Oval shaped Oval shaped Cloud shield Cloud shield

< -32°C = 100,000km2 < -32°C = 100,000km2 < -52°C = 50,000 km2< -52°C = 50,000 km2

Last at least 6 hoursLast at least 6 hours Usually night eventsUsually night events Flash floodsFlash floods HailHail

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Squall LinesSquall Lines Linear multi cell Linear multi cell

stormsstorms Frontal or pre-Frontal or pre-

frontalfrontal Vertical tilt Vertical tilt

increases severityincreases severity Moist low level S Moist low level S

or SE inflowor SE inflow Mid level flow >25 Mid level flow >25

knots and ┴ to low knots and ┴ to low level flowlevel flow

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Squall Line and gust Squall Line and gust frontfront

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Squall Line LifespanSquall Line Lifespan

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SupercellSupercell

Balanced updraft Balanced updraft and downdraftand downdraft

Low level Low level mesocyclogenesis mesocyclogenesis more likelymore likely

Moderate hook Moderate hook echoecho

Tornadoes are Tornadoes are commoncommon

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SupercellSupercell

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Hook Echo of SupercellHook Echo of Supercell

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SupercellSupercell

Photo by: Steven Maciejewski

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Supercell reflectivitySupercell reflectivity

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Lightning and thunderLightning and thunder

Lightning = flashes of light caused Lightning = flashes of light caused by enormous electrical dischargesby enormous electrical discharges

Superheats the air abruptly resulting Superheats the air abruptly resulting in enormous shock wavesin enormous shock waves thunderthunder

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Source: Environment Canada

Lightning processLightning process

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Types of lightningTypes of lightning

Within cloudWithin cloud Cloud-to-cloudCloud-to-cloud Cloud-to-airCloud-to-air Cloud-to-groundCloud-to-ground

Negative lightningNegative lightning Positive lightningPositive lightning

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Positive lightning Positive lightning (~10%)(~10%)

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Lightning strike densityLightning strike density

FlashesDays with flashes

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HailHail

Ice pelletsIce pellets Only occur within Only occur within

cumulonimbus cumulonimbus cloudsclouds

The longer a frozen The longer a frozen pellet stays aloft in pellet stays aloft in a cloud, the larger a cloud, the larger the hail can bethe hail can be

““Wet-bulb zero” Wet-bulb zero” height importantheight important

Courtesy of: http://www.chaseday.com/hailstones.htm

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Hail frequency in the USHail frequency in the US

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TornadoesTornadoes

As cumulonimbus clouds rise high As cumulonimbus clouds rise high into the troposphere, the bottom into the troposphere, the bottom winds move slow due to surface winds move slow due to surface friction, but the upper winds move friction, but the upper winds move fastfast

Results in rotation of air parallel to Results in rotation of air parallel to the groundthe ground

Updrafts or downdrafts can cause Updrafts or downdrafts can cause the vortex to become verticalthe vortex to become vertical

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TornadoesTornadoes

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Some keys to tornado Some keys to tornado formationformation

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Fujita scaleFujita scale F0-F1: F0-F1: 69% of 69% of

tornadoetornadoes; 5% of s; 5% of deathsdeaths

F4-F5: F4-F5: 3% of 3% of

tornadoetornadoes; 70% of s; 70% of deathsdeaths

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Tornado frequency in the Tornado frequency in the USUS

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Severe wind frequency in Severe wind frequency in the USthe US