Overview of Tornadoes in the United States

Including comparison of Super Outbreak, April 3 – 4, 1974 with

Tornado Outbreak, April 25 -28, 2011

Jeffrey Permuy | May 26, 2011

Section I

What you need to know about tornadoes

Tornado: Definition (specific to the U.S.)

• Violently rotating column of air aided by:1. Warm, moist air advancing northward from the Gulf of

Mexico at low levels of the atmosphere.• Near the surface (about 1000mb of atmospheric pressure).

2. Dry air advecting east of the Rocky Mountains at mid levels of the atmosphere.• Near 10,000 feet (about 700mb of atmospheric pressure).

3. Colder air channeling southward from the Arctic at higher levels of the atmosphere.• Near 18,000 feet (about 500mb of atmospheric pressure).• Location of the Jet Stream: ribbon of strong moving air

dictating steering currents for weather at the surface.

Tornadoes: Ingredients and Recipe

• How it comes together:– Geography and topography of North America provides an

ideal breeding ground for the development of severe and supercell thunderstorms and tornadoes, particularly in the flat basin of the Midwest, and the Southeast where the confluence and clash of different air masses adds to atmospheric instability and vertical wind shear.

– Rocky Mountains along the western terrain of the continent and the Appalachian chain in the east, forms an upside triangle (like a wedge) allowing cold air to funnel southward, mixing with dry air from the leeward side of the Rockies and with warm, moist air from the Gulf of Mexico.

Tornadoes: Seasonal shifts

• Frequency of tornadoes by season and region: – Early spring: Tornadoes tend to occur in the Southeast,

and Gulf states from February to April. – Late spring: Tornadoes spread farther north into Kansas,

Nebraska and the Tennessee Valley region. – Mid-summer: Most of Tornado Alley is active and

tornadoes may occur throughout the U.S. – Late summer: Tends to bring some of the stronger

tornadoes into the upper Midwest and Ohio valleys.– Autumn: Pattern shifts southward. – Winter: Fewest tornadoes occur.

Tornadoes: Other characteristics• Did you know…

– Vast majority of tornadoes develop within a supercell thunderstorm: a thunderstorm with a deep rotating updraft known as a mesocyclone.

– Supercells are the most powerful thunderstorms; hence, they spawn proportionally more dangerous tornadoes.

– The highest incidence of tornadoes are in late spring and early summer.

– Tornadoes typically move in a northeast trajectory with an average forward speed of about 30 mph.• Twisters have been tracked at slower and much faster speeds.

– 95% of twisters rotate counterclockwise (cyclonic) because air moves west to east as it ascends the mesocyclone. • The rare anticyclonic tornadoes, clockwise rotation, are the

norm in the Southern Hemisphere.

Tornado from a supercell thunderstorm(illustration)

Tornado Detection: Aided by Observation

• Inflow bands– Ragged bands of low cumulus clouds extending from

the main storm tower to the southeast or south. • The presence of inflow bands suggests that the storm is

gathering low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.

Tornado Detection: Aided by Observation

• Beaver’s Tail– Smooth, flat cloud band extending from the eastern

edge of the rain-free base to the east or northeast. • Beaver’s Tail usually skirts around the southern edge of the

precipitation area, suggesting presence of rotation.

Tornado Detection: Aided by Observation

• Wall Cloud– Isolated cloud lowering attached to the rain-free base

of the thunderstorm, usually to the rear of the visible precipitation area. • A wall cloud is about two miles in diameter and marks the

area of strongest updraft in the storm.• A wall cloud does not always produce a tornado. When it

does, it exists for 10 - 20 minutes before a tornado appears. • A wall cloud may rotate with strong surface winds flowing into

it, and have rapid vertical motion indicated by small cloud elements rising into the rain-free base.

Tornado Detection: Aided by Observation

• Rear Flank Downdraft (RFD)– Downward rush of air on the back side of the storm

that descends along with the tornado. • RFD looks like a "clear slot" or "bright slot" just to the rear

(southwest) of the wall cloud. It can also look like curtains of rain wrapping around the cloud base circulation. Eventually, the tornado and RFD will reach the ground within a few minutes of each other.

• RFD causes gusty surface winds that occasionally have embedded downbursts. It is the motion in the storm that causes the hook echo feature on radar.

Tornado Detection: Aided by Observation

• Condensation Cloud– Water droplets that extend downward from the base

of the thunderstorm. • If it is in contact with the ground it is a tornado; otherwise, it

is a funnel cloud. Dust and debris beneath the condensation funnel confirm a tornado's presence.

• Condensation cloud may or may not be connected with a wall cloud.

Tornado Detection: Aided by Radar

• Tornadic Vortex Signature (TVS)• Mescocyclone• Hook Echo

– These distinguishing features are picked up by Doppler radar data and analyzed by computer programs, making it easier to identify severe weather. • Forecasters are trained to recognize precise radar signatures

produced by sophisticated algorithms. • Today's weather radars typically provide on average 11

minutes lead-time, pinpointing locations in harm's way with a high degree of accuracy.

Tornado Prediction: Aided by Integration

• Advanced Weather Interactive Processing System (AWIPS)– Interactive computer system that integrates all

meteorological, hydrological, satellite and radar data, enabling forecasters to prepare and issue more accurate and timely forecasts and warnings. • This telecommunications system, with its advanced

information processing, is the cornerstone of the National Weather Service (NWS) modernization and restructure.

Tornado from a supercell thunderstorm(illustration with descriptors)

Tornado from a violent supercell thunderstorm(radar signature with descriptors)

Radar reflectivity image from May 3, 1999 indicating powerful (F5*) tornado that devastated Moore, Oklahoma (NE of twister at time of image). Mescocyclone and tornado make up the classic "hook" shape of the echo, a common feature with strong and violent tornadoes.

*Tornado rating by wind speed and strength (F-scale; EF-scale) are covered in the next three slides.

Tornadoes: Rating by wind speed

• Fujita (F) Scale– Developed by Dr. Tetsuya Theodore Fujita to estimate

tornado wind speeds based on residual damage.• Its ratings have been used since 1971 and was instrumental

in the analysis of the Super Outbreak, April 3 – 4, 1974.• Enhanced Fujita (EF) Scale

– Developed by a forum of meteorologists and wind engineers as an enhancement (more accurate measure) to the F scale.  • It has been approved by the National Weather Service

(NWS) and replaced the original F scale in February 2007.

Tornadoes: Fujita Scale

*Based on survey of 18,545 tornadoes occurring from 1950 to 1977, for which intensity and path could be determined.

Tornadoes: Enhanced Fujita Scale

• On average, 77% of tornadoes are considered weak (EF0 or EF1).• The other 23% of tornadoes are significant (EF2 to EF5).• About 5% of tornadoes are intense (EF3 or greater). • Only 0.1% of tornadoes are the most violent (EF5).

Section II

Data illustrations and trends

Tornadoes: From 1950 – 2010

• 2004 was the most prodigious year in twisters (1817) since the NWS began documenting and archiving tornadoes in 1950.

• The 10 most prolific years in tornadoes (1200+) have occurred since 1992. The bar graph illustrates an undulating pattern with an upward slope.

Tornadoes: From 1950 – 2011*• Additional information:

– Much has changed since NWS began forecasting tornadoes in 1950: • Population increase: More tornadoes are observed and reported.• Better technology: More tornadoes are detected by meteorologists.• Greater knowledge: Fewer tornadoes are mistaken for straight line wind

damage; downbursts; gustnadoes, short-lived whirling gust front.• Confirmation: Fewer tornadoes are double counted by separate eye

witnesses, reporting the same twister.– From 1950 to 1989, only two of the yearly totals exceeded 1,000

tornadoes (1973 and 1982).– From 1990 to 2010 all but one year yielded more than 1,000

tornadoes (2002); annual average during this trend is 1,255. • 1,228* tornadoes have already been documented in 2011, pending

confirmation from the NWS (as of May 26th).• No country or continent comes close to the frequency of tornadoes in the

U.S. Canada is a distant second with 80 – 100 twisters per year.

Tornadoes: By State, 1953 - 2004

• TX (139), OK (57), FL (55), KS (55), and NE (45) have averaged the most tornadoes in the 52-year span. Overall total is 908.– IA (37), IL (35) and MO (30) round out the top 8 with 30+ tornadoes; 11

other states have averaged 20+ twisters.– Highest incidence of tornadoes are Central and Southeastern regions.

Tornadoes: Per 10k2 Miles by State, 1953 - 2004

• FL (9.4), OK (8.2), KS (6.7), IA (6.6), IL (6.2) and IN (6.1) have the highest average of tornadoes per 10k2 miles in the 52-year span.– NE (5.8), MS (5.7), MD (5.7), LA (5.6), and TX (5.2) round out the top 11

with 5+ tornadoes per 10k2 miles. MD only averages 6 twisters annually.– Highest concentration of tornadoes are Central and Eastern regions.

Tornadoes: Strong to Violent by State

• TX (29), OK (17), KS (11) and IA (11) have the highest average of strong to violent tornadoes in the 46-year span. Overall total is 202.– 13 other states produced an average of 5+ strong to violent tornadoes.– Highest incidence of significant tornadoes are Central and Southern regions.

Tornadoes: By Month plus Hail (2000 - 2010)

• April through July is most prolific span with 100+ tornadoes occurring in each month. The frequency of tornadoes peaks in May.– The occurrence of hail also peaks in May; however, the ratio of hail to

tornadoes increases during the summer months. • Strong thunderstorms often produce hail whether there is a mesocyclone present

or not. Atmospheric conditions must be more volatile to spawn tornadoes.

Tornado Monthly Distribution and Hail:Tornado Ratio (2000 - 2010)

24 3586

172

295

238

11885 89

67 6838

10.00

11.53

9.89

11.66

14.36

14.29

2.932.04

1.783.80

5.40

6.53

0

50

100

150

200

250

300

350

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Hail:Tornado RatioTornado

Tornadoes: Probability of F2+ by Month (1954 – 1997)

• The more rugged, accurate Gaussian curve (std dev = 3 days) illustrates two peaks, coinciding with late May and early June.– Data shows rise in F2+ twisters, esp. from mid-March to mid-June, followed by

a decline; except for smaller peaks in mid-August and mid-November. • The Gaussian curve is fairly consistent with the monthly distribution of all reported

tornadoes in the previous slide based on raw data from 2000 - 2010.

Probability of significant tornado is the likelihood that at least one tornado, F2 or greater, will touch down on given days of the year. The two Gaussian curves, standard deviation of 3 days and 15 days, are an exponential representation of the raw data.

Tornadoes: Very Strong to Violent, 1950 – 2005(by Year)

• 1974 had the most intense tornadoes (118) during the 56-year span, followed by 1965 (75) and 1957 (64). Six other years reported 50+ intense twisters.

Tornadoes: Very Strong to Violent, 1950 – 1998(by Region)

Tornadoes: May from 1950 – 2010 (On average, more tornadoes are reported in May than any other month)

• 2003 produced the most tornadoes in May (543) during the 61-year span. The bar graph illustrates an upward trend in the cyclical peaks.

Tornadoes: April from 1950 – 2011*

• 1974 produced the most tornadoes in April (267) during the 61-year span until the record-breaking April in 2011 with a preliminary report of 875* twisters.

Tornadoes: 875 reported in April 2011(Most on record for any month)

Section III

Comparing two historic outbreaks: April 3 – 4, 1974 v. April 25 – 28, 2011

Super Outbreak: April 3 – 4, 1974

Timeline 16 – 18 hour period

# of tornadoes 148

Weak (F0, F1) 53 (35.8%)

Strong (F2, F3) 66 (44.6%)

Violent (F4, F5) 29 (19.6%)

Intense (F3, F4, F5) 64 (43.2%)

Total coverage path 2,500 – 2,600 miles

Region most affected Eastern U.S.

Super Outbreak: April 3 – 4, 1974

Final map provided by Dr. T. Theodore Fujita after thorough on-site and aerial analysis of tornado tracks

(Map includes cities)

Super Outbreak: April 3 – 4, 1974

Final map provided by Dr. T. Theodore Fujita after thorough on-site and aerial analysis of tornado tracks

(Map includes legend, city and topography)

Super Outbreak: April 3 – 4, 1974

• Additional information:– 147 tornadoes covered 13 states plus one in Canada.– Hardest hit states were Alabama, Kentucky and Ohio.– Xenia, OH was the most devastated city.– The longest path from one of the deadly twisters, a

F4, cut 121 miles diagonally through northern Indiana.– Lowest barometric pressure during the event was

28.98 in.– Later it was determined La Niña conditions (cold

phase of the Southern Oscillation in the Pacific Ocean) was prevalent during the outbreak.

Tornado Outbreak: April 25 – 28, 2011

Timeline Over four days

# of tornadoes (unofficial) 326

Weak (EF0, EF1) 238 (73.0%)

Strong (EF2, EF3) 73 (22.4%)

Violent (EF4, EF5) 15 (4.6%)

Intense (EF3, EF4, EF5) 37 (11.3%)

Total coverage path Not yet determined

Most coverage path (one day) 1,500 miles (April 27th)

Region most affected Southeast U.S.

Tornado Outbreak: April 25 – 28, 2011

Preliminary map of tornado tracks based on NWS forecast office damage surveys.

(Map includes legend and topography)

Tornado Outbreak: April 25 – 28, 2011 (*tornado counts pending confirmation from the NWS)

• Additional information:– 326* tornadoes were concentrated in the Southeast, and

scattered as far north as upstate New York.– 266* twisters touched down during a 24-hour period from

8:00 AM EDT, April 27th to 8:00 AM EDT, April 28th.– Hardest hit states were Alabama, Mississippi and Georgia.– Tuscaloosa, AL was the most devastated city.– The continuous path of a EF5 tornado crossed northern

Alabama into southern Tennessee, covering 132 miles.– Lowest barometric pressure during the event was 29.15 in.– Lingering influence of La Niña conditions (cold phase of

the Southern Oscillation in the Pacific Ocean) was prevalent during the outbreak.

Tornado Outbreak: April 25 – 28, 2011

• Tornado filmed on April 27th in Empire, Alabama around 5:00PM EDT, traversing hilly terrain.

– http://www.youtube.com/watch?v=zEr2lCnQcow&feature=share

Tornadoes: April 3 - 4, 1974 v. April 25 - 28, 2011Afternoon satellite image: Apex of Tornado Outbreak, April 27, 2011

Afternoon satellite image: Apex of Super Outbreak, April 3, 1974

In a 24-hour period from the morning of April 27th to the morning of April 28th, a record-breaking 266 tornadoes were spawned, covering 1,500 miles. 13 of those twisters were violent (EF4 and EF5).

In an 18-hour period from the afternoon of April 3rd to the morning of April 4th, three squall lines simultaneously produced 148 tornadoes, covering 2,500+ miles. 29 of those twisters were violent (F4 and F5).

Tornadoes: Sources and Research

• Linked sites to National Oceanic & Atmospheric Administration (NOAA):– National Climatic Data Center (NCDC): Archives for weather data.– Storm Prediction Center (SPC): Prediction and information on

severe weather.– National Severe Storms Laboratory (NSSL): Understand causes

of severe weather to assist NWS. – Earth System Research Laboratory (ESRL): Physical Science

division – Observe and understand earth systems.– National Weather Service (NWS): Regional government weather

forecasting offices.• Other articles and data published online.• David M. Ludlum The American Weather Book (1982).