Meteo 3: Chapter 14Meteo 3: Chapter 14

Spawning severe weatherSpawning severe weatherSynoptically-forced stormsSynoptically-forced storms

Read Chapter 14Read Chapter 14

Thunderstorms arise from “deep convection”Thunderstorms arise from “deep convection”

INGREDIENTS FOR SEVERE WXINGREDIENTS FOR SEVERE WX

1) Warm, moist air in lower troposphere (sunshine 1) Warm, moist air in lower troposphere (sunshine usually precedes severe wx)usually precedes severe wx)

2) Instability (aided by cold air aloft)2) Instability (aided by cold air aloft) 3) Strong triggering mechanism (low pressure, 3) Strong triggering mechanism (low pressure,

fronts, upper-level trough)fronts, upper-level trough)

Watches- http://www.spc.noaa.gov/products/watch/Watches- http://www.spc.noaa.gov/products/watch/

Severe Thunderstorm Severe Thunderstorm Watch:Watch: Issued by Storm Issued by Storm Prediction Center when Prediction Center when conditions are favorable for conditions are favorable for severe weathersevere weather

Tornado Watch:Tornado Watch: Issued by Issued by SPC if conditions favor SPC if conditions favor tornadoestornadoes

WarningsWarnings issued by issued by local NWS offices if local NWS offices if severe wx/tornadoes are severe wx/tornadoes are observed or indicated by observed or indicated by radarradar

Vertical Wind ShearVertical Wind Shear

Vertical Wind Shear:Vertical Wind Shear: A change in the speed and/or A change in the speed and/or direction of the wind with heightdirection of the wind with height

If wind in direction of t-storm motion increases with If wind in direction of t-storm motion increases with height, the updraft tilts forwardheight, the updraft tilts forward

Updraft and downdraft separate, prolonging t-storm Updraft and downdraft separate, prolonging t-storm life & allowing it to strengthenlife & allowing it to strengthen

Squall LineSquall Line

Narrow, linear line of Narrow, linear line of storms that develop to storms that develop to east of cold front in warm east of cold front in warm sector sector

Look for moisture Look for moisture convergence (fuel)…convergence (fuel)…upper-level trough is the upper-level trough is the “spark”“spark”

Lasts 6-8 hours…weaken Lasts 6-8 hours…weaken as they move in more as they move in more stable environmentstable environment

“State College Effect”

Derecho – damaging wind stormDerecho – damaging wind storm Clusters of downbursts that produce a widespread, Clusters of downbursts that produce a widespread,

damaging wind stormdamaging wind storm– Associated with squall linesAssociated with squall lines– Much larger area of wind damage than with microburstsMuch larger area of wind damage than with microbursts

Bow echo on radar…intense wind at “surge region”Bow echo on radar…intense wind at “surge region”

Derecho - St Louis July 19Derecho - St Louis July 19thth, 2006, 2006

90 mph wind gusts knocked out power to 90 mph wind gusts knocked out power to half a million St. Louis metro area residentshalf a million St. Louis metro area residents

Radar loop showing gust frontRadar loop showing gust front Heat indices of 100-110 degrees next day Heat indices of 100-110 degrees next day

(what temperature feels like when moisture (what temperature feels like when moisture content is accounted for)content is accounted for)

More on derechosMore on derechos

What makes a squall line w/ bow echoes a derecho?What makes a squall line w/ bow echoes a derecho?– 1) Many reports of severe wind damage over an area 1) Many reports of severe wind damage over an area

whose major axis is >= 250 miwhose major axis is >= 250 mi– 2) Some damage as bad as seen with a weak tornado or 2) Some damage as bad as seen with a weak tornado or

reported gusts > 75 mphreported gusts > 75 mph

Most triggered by mid-latitude cyclones in warm Most triggered by mid-latitude cyclones in warm season…usually form along stationary frontseason…usually form along stationary front

Fast movingFast moving

Meteo 3: Chapter 15Meteo 3: Chapter 15

TornadoesTornadoes

Some material pulled from Zack Some material pulled from Zack Byko’s Byko’s websitewebsite

Tornadoes!Tornadoes!

Tornado: Tornado: A violently rotating column of air A violently rotating column of air extending from a cloud to the ground. extending from a cloud to the ground. 

Sometimes there’s a classic condensation Sometimes there’s a classic condensation funnel, other times only rotating dirt/debris funnel, other times only rotating dirt/debris on groundon ground

Appearance depends on moisture & ground Appearance depends on moisture & ground surfacesurface

Powerful tornadoes usually have Powerful tornadoes usually have condensation funnelcondensation funnel

Importance of capping inversionImportance of capping inversion

Synoptic setup for tornadic thunderstormsSynoptic setup for tornadic thunderstorms

Tornado Tornado Alley Alley exampleexample

Tornado formationTornado formation Typically form from Typically form from SupercellsSupercells - large, solitary, - large, solitary,

long-lasting thunderstorms with rotating updrafts long-lasting thunderstorms with rotating updrafts (mesocyclone) (mesocyclone)

Tornado formationTornado formation

Strong vertical wind shear creates Strong vertical wind shear creates ‘horizontal rolls’‘horizontal rolls’

Strong updrafts at rear of storm cell tilt Strong updrafts at rear of storm cell tilt horizontal roll verticallyhorizontal roll vertically

Roll is tilted into Roll is tilted into mesocyclonemesocyclone, a rotating , a rotating column of air sometimes found in severe column of air sometimes found in severe stormsstorms

Tornado formationTornado formation Roll stretches vertically, spinning faster and Roll stretches vertically, spinning faster and

faster (think conservation of angular faster (think conservation of angular momentum again!)momentum again!)

Tornado formationTornado formation If rotating column of air reaches ground, a If rotating column of air reaches ground, a

tornado is borntornado is born Photos courtesy of the National Severe StormsPhotos courtesy of the National Severe Storms

Center Center

Cycloid damage path caused by suction vortices:Cycloid damage path caused by suction vortices:how a tornado can destroy one house and spare the how a tornado can destroy one house and spare the

neighbors altogetherneighbors altogether

Suction VorticesSuction Vortices

Fujita ScaleFujita Scale

A scale measuring intensity of tornadoesA scale measuring intensity of tornadoes F0 – weakest -> F5 - strongestF0 – weakest -> F5 - strongest http://en.wikipedia.org/wiki/Fujita_scalehttp://en.wikipedia.org/wiki/Fujita_scale

OK, REAL Tornado SafetyOK, REAL Tornado Safety

http://www.srh.noaa.gov/oun/severewx/safety.html#tornadohttp://www.srh.noaa.gov/oun/severewx/safety.html#tornado

Final note: Always take heed of watches Final note: Always take heed of watches and warning, even though severe weather and warning, even though severe weather usually impacts a very small fraction of a usually impacts a very small fraction of a watch area.watch area.

Whirlwinds that are not tornadoesWhirlwinds that are not tornadoes

Dust Devils Dust Devils ▪ Waterspouts▪ Waterspouts

Now, some videoNow, some video

http://www.youtube.com/watch?v=otZR3VThttp://www.youtube.com/watch?v=otZR3VTh4Voh4Vo

http://www.youtube.com/watch?v=s7uW4Sbhttp://www.youtube.com/watch?v=s7uW4SbtHEMtHEM

http://www.youtube.com/watch?v=2rK-http://www.youtube.com/watch?v=2rK-ctpFBz8ctpFBz8

http://www.youtube.com/watch?http://www.youtube.com/watch?v=2veLsNZrlSwv=2veLsNZrlSw