CHAPTER 12

TORNADOES

CHAPTER 12

TORNADOES

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

Tornado days per yearSignificant tornado days per century

Fig. 12.1, p. 334

Tornado “days”

Fig. 12.4, p. 337

NSSL Animation: Tornado probability climatologyhttp://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

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

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

Before a tornado forms, a low cloud called the “wall cloud” will appear Before a tornado forms, a low cloud called the “wall cloud” will appear from under the main updraft and mesocyclone from under the main updraft and mesocyclone

• Dust-whirl stage

• Mature stage• Shrinking• Decay stage• NWS

Jetstream

Fig. 12.3, p. 336

Wedge tornado

Windsor, CO

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

Example from bookExample from book

Surface map

Fig. 12.9, p. 340

Asymmetric Winds

Fig. 12.10, p. 340

Fig. 12.11, p. 341

1-3 days in advance – convective 1-3 days in advance – convective outlookoutlook

A few hours in advance – severe A few hours in advance – severe thunderstorm or tornado watch – this thunderstorm or tornado watch – this means conditions are favorable – keep means conditions are favorable – keep alertalert

www.spc.noaa.govwww.spc.noaa.gov

Warnings:Warnings:Issued for one or more counties when Issued for one or more counties when a severe thunderstorm or tornado has a severe thunderstorm or tornado has been spotted or observed by radar been spotted or observed by radar

Basement or small, interior room on ground floor◦ Do NOT open windows

Mobile Home: Must leave. Period. Auto: See previous slide

◦ NEVER under an overpass

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

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

Greensburg, KS, May 4, 2007

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:

Super outbreak

Table 12.4, p. 345

Table 12.5, p. 346

Super Outbreak, 27 April 2011◦ http://www.srh.noaa.gov/bmx/?

n=event_04272011◦ http://www.wunderground.com/blog/JeffMasters/

comment.html?entrynum=1796

Joplin EF-5, 22 May 2011, ~160 Fatalities◦ http://www.crh.noaa.gov/sgf/?

n=event_2011may22_summary

Fig. 12.15, p. 346

Fig. 12.15, p. 346

Fig. 12.16, p. 347

Fig. 12.17, p. 347

NWS Photo

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

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

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

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