Meteo 3: Chapter 14
Spawning severe weather
Synoptically-forced storms
Read Chapter 14
Thunderstorms arise from “deep convection”
 INGREDIENTS FOR SEVERE WX
 1) Warm, moist air in lower troposphere (sunshine
usually precedes severe wx)
 2) Instability (aided by cold air aloft)
 3) Strong triggering mechanism (low pressure,
fronts, upper-level trough)
Watches- http://www.spc.noaa.gov/products/watch/
 Severe Thunderstorm
Watch: Issued by Storm
Prediction Center when
conditions are favorable for
severe weather
 Tornado Watch: Issued by
SPC if conditions favor
tornadoes
 Warnings issued by
local NWS offices if
severe wx/tornadoes
are observed or
indicated by radar
Vertical Wind Shear
 Vertical Wind Shear: A change in the speed and/or
direction of the wind with height
 If wind in direction of t-storm motion increases with
height, the updraft tilts forward
 Updraft and downdraft separate, prolonging t-storm
life & allowing it to strengthen
Squall Line
 Narrow, linear line of
storms that develop to
east of cold front in warm
sector
 Look for moisture
convergence
(fuel)…upper-level trough
is the “spark”
 Lasts 6-8 hours…weaken
as they move in more
stable environment
“State College Effect”
Derecho – damaging wind storm
 Clusters of downbursts that produce a widespread,
damaging wind storm
– Associated with squall lines
– Much larger area of wind damage than with microbursts
 Bow echo on radar…intense wind at “surge region”
Derecho - St Louis July 19th, 2006
 90 mph wind gusts knocked out power to
half a million St. Louis metro area residents
 Radar loop showing gust front
 Heat indices of 100-110 degrees next day
(what temperature feels like when moisture
content is accounted for)
More on derechos
 What makes a squall line w/ bow echoes a derecho?
– 1) Many reports of severe wind damage over an area
whose major axis is >= 250 mi
– 2) Some damage as bad as seen with a weak tornado or
reported gusts > 75 mph
 Most triggered by mid-latitude cyclones in warm
season…usually form along stationary front
 Fast moving
Meteo 3: Chapter 15
Tornadoes
Some material pulled from Zack
Byko’s website
Tornadoes!
 Tornado: A violently rotating column of air
extending from a cloud to the ground.
 Sometimes there’s a classic condensation
funnel, other times only rotating dirt/debris
on ground
 Appearance depends on moisture & ground
surface
 Powerful tornadoes usually have
condensation funnel
Importance of capping inversion
Synoptic setup for tornadic thunderstorms
Tornado
Alley
example
Tornado formation
 Typically form from Supercells - large, solitary,
long-lasting thunderstorms with rotating updrafts
(mesocyclone)
Tornado formation
 Strong vertical wind shear creates
‘horizontal rolls’
 Strong updrafts at rear of storm cell tilt
horizontal roll vertically
 Roll is tilted into mesocyclone, a rotating
column of air sometimes found in severe
storms
Tornado formation
 Roll stretches vertically, spinning faster and
faster (think conservation of angular
momentum again!)
Tornado formation
 If rotating column of air reaches ground, a
tornado is born
 Photos courtesy of the National Severe Storms
Center
Cycloid damage path caused by suction vortices:
how a tornado can destroy one house and spare the
neighbors altogether
Suction Vortices
Fujita Scale
 A scale measuring intensity of tornadoes
 F0 – weakest -> F5 - strongest
 http://en.wikipedia.org/wiki/Fujita_scale
OK, REAL Tornado Safety
 http://www.srh.noaa.gov/oun/severewx/safety.html#tornado
 Final note: Always take heed of watches and
warning, even though severe weather
usually impacts a very small fraction of a
watch area.
Whirlwinds that are not tornadoes
 Dust Devils
▪ Waterspouts
Now, some video
 http://www.youtube.com/watch?v=otZR3VTh
4Vo
 http://www.youtube.com/watch?v=s7uW4Sb
tHEM
 http://www.youtube.com/watch?v=2rKctpFBz8
 http://www.youtube.com/watch?v=2veLsNZrl
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