CHAPTER 12
TORNADOES
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Some supercells create enough rotation that the
circulation will contract and reach the ground – a
tornado
Defined as a violently rotating column of air, in
contact with the ground, and below a cumuliform
cloud
Significant tornado days per century
Tornado days per year
Fig. 12.1, p. 334
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Tornado “days”
Fig. 12.4, p. 337
NSSL Animation: Tornado probability climatology
http://www.nssl.noaa.gov/hazard/tanim8094/sigtanim2195.html
http://www.nssl.noaa.gov/hazard/tanim8094/viotanim2195.html
Fig. 12.6, p. 338
Fig. 12.7, p. 338
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Funnel Cloud until circulation reaches ground
Shapes
Mostly < 135 mph, but can exceed 250 mph
Diameters:
◦ Most: 300 to 2000 ft
◦ Small as 20 feet, big as 1+ mi!
Mostly CCW (because most tornadic supercells
rotate CCW)
Cyclostrophic balance, HPGF and Centrifugal
Mostly move SW to NE, 25 to 50 mph
Can last a few mintues, or hours (long track,
tornado “family”)
Fig. 12.2, p. 335
Before a tornado forms, a low cloud called the “wall cloud” will appear
from under the main updraft and mesocyclone
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Non-supercell tornadoes also occur – they often form along
boundaries in unstable environments, e.g. a squall line MCS. (like
bookend vortex of a bow echo).: landspouts & waterspouts
• Dust-whirl
stage
• Mature stage
• Shrinking
• Decay stage
• NWS
Jetstream
Fig. 12.3, p. 336
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Wedge
tornado
Windsor,
CO
Example from book
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In addition to moisture,
instability, and lifting,
we need strong wind
shear
At low levels, southerly
winds bringing warm,
moist air into the area
Aloft, advection of dry
air adds to instability
Upper-level divergence
leads to low-level
upward motion
Surface map
Asymmetric Winds
Fig. 12.9, p. 340
Fig. 12.10, p. 340
Fig. 12.11, p. 341
1-3 days in advance – convective
outlook
A few hours in advance – severe
thunderstorm or tornado watch – this
means conditions are favorable – keep
alert
www.spc.noaa.gov
Warnings:
Issued for one or more counties when
a severe thunderstorm or tornado has
been spotted or observed by radar
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Basement or small, interior room on ground
floor
◦ Do NOT open windows
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Mobile Home: Must leave. Period.
Auto: See previous slide
◦ NEVER under an overpass
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http://vimeo.com/22221449
Radar
Radar (Velocity)
Satellite
032 WFUS53 KOAX 100008 TOROAX IAC133-100045- /O.NEW.KOAX.TO.W.0010.110410T0008Z-110410T0045Z/
BULLETIN - EAS ACTIVATION REQUESTED TORNADO WARNING NATIONAL WEATHER SERVICE OMAHA/VALLEY NEBRASKA 708
PM CDT SAT APR 9 2011 THE NATIONAL WEATHER SERVICE IN OMAHA HAS ISSUED A
* TORNADO WARNING FOR... NORTHEASTERN MONONA COUNTY IN WEST CENTRAL IOWA...
* UNTIL 745 PM CDT * AT 707 PM CDT...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A SEVERE
THUNDERSTORM CAPABLE OF PRODUCING A TORNADO. THIS DANGEROUS STORM WAS LOCATED 11 MILES EAST OF
WHITING...OR 34 MILES SOUTHEAST OF SIOUX CITY...AND MOVING EAST AT 25 MPH.
* LOCATIONS IMPACTED INCLUDE... MAPLETON.
PRECAUTIONARY/PREPAREDNESS ACTIONS... THIS TORNADO WARNING REPLACES THE SEVERE THUNDERSTORM WARNING
THAT WAS IN EFFECT FOR THE SAME AREA. GO TO A BASEMENT OR SMALL INTERIOR ROOM ON THE LOWEST FLOOR! TAKE
COVER NOW. MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A STURDY BUILDING. AVOID WINDOWS. IF IN A
MOBILE HOME...A VEHICLE OR OUTDOORS...MOVE TO THE CLOSEST SUBSTANTIAL SHELTER AND PROTECT YOURSELF FROM
FLYING DEBRIS. &&\
Newspaper
SPC
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Based on damage – rating is assessed after the tornado (different from
hurricanes, when the category is assigned while the storm is
happening)
Originally developed by Dr. Ted Fujita of the University of Chicago in
the 1970s
Updated in 2007 to the “Enhanced Fujita” scale
EF4 and EF5 tornadoes are very rare, but most deaths are caused by
them
Fig. 12.12, p. 344
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Greensburg, KS, May 4, 2007
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Outbreak: Typically, conditions will be favorable
over a large area for supercells, and we may have
many tornadoes for a synoptic event.
Tornado Families
◦ Single supercell, multiple tornado “drops”
◦ Typical pattern:
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Super outbreak
Table 12.4, p. 345
Table 12.5, p. 346
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Super Outbreak, 27 April 2011
◦ http://www.srh.noaa.gov/bmx/?n=event_0427201
1
◦ http://www.wunderground.com/blog/JeffMasters/c
omment.html?entrynum=1796
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Joplin EF-5, 22 May 2011, ~160 Fatalities
◦ http://www.crh.noaa.gov/sgf/?n=event_2011may2
2_summary
Fig. 12.15, p. 346
Fig. 12.15, p. 346
Fig. 12.16, p. 347
NWS Photo
Fig. 12.17, p. 347
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Basic requirements are the usual for a
thunderstorm (M , L, I ), and strong vertical
wind shear.
Supercell Tornadoes
◦ Wind sheer causes spinning vortex tube that is
pulled into thunderstorm by the updraft
◦ Terms: Mesocyclone, BWER, rear flank downdraft,
vertical stretching, funnel cloud, rotating cloud, wall
cloud, RFD, FFD
Fig. 12.18, p. 348
Fig. 12.20, p. 349
Fig. 12.19, p. 348
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Nonsupercell Tornadoes
◦ Boundaries give rise to low level rotation
◦ Convection stretches rotation
◦ Also bookends of bow echoes
 Remember the Florida bow echoes?
Fig. 12.24, p. 353
Fig. 12.23, p. 352
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Doppler radar measures the speed of
precipitation toward and away radar unit
Two Doppler radars can provide a 3D view
Couplet and TVS (Tornado Vortex Signature)
NEXRAD is the national Doppler system
◦ Due for an upgrade which will be able to tell hail
from drops better
NWS Photo
Fig. 12.26, p. 354
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Rotating column of air that is connected to a
cumuliform cloud over a large body of water
Tornadic waterspout vs. fair weather
Dust Devils are cousins, same balance
◦ Cyclostrophic
Fig. 12.28, p. 356