quantifying the significance of the 26-27 april 2011 severe weather outbreak

Quantifying the Significance of the 26-27 April 2011

Severe Weather Outbreak

Matthew S. Stalley, Chad M. Gravelle, Charles E. Graves

Saint Louis University

John P. GaganNOAA/NWS Springfield, MO

2011 National Weather Association Annual MeetingBirmingham, Alabama

17 October 2011

Motivation

• Question: When severe weather is expected, how likely are major severe weather outbreaks?

• More specifically: How likely is a significant tornado outbreak when the environment is similar to 27 April 2011?

• The answer may provide some (more appropriate?) measure of tornado outbreak significance and situational awareness.

• When the synoptic-scale pattern is similar, does an outbreak occur 1 of 4 times or 1 of 50 times (i.e., probabilistic awareness)?

Defining an Outbreak

• 48-h tornado reports within the domain (having at least 1 severe report) were ranked between 1 January 1979 and 31 December 2010.

• For each category (i.e., tornado, long-track tornado (≥10km), and significant tornado), outbreaks were defined as being in the top 2%.

• Outbreak Thresholds:Tornado: 23Long-track Tornado: 6Significant Tornado: 6

• 26-28 April 2011*Tornado: 217Long-track Tornado: 76Significant Tornado: 73

* Statistics are preliminary SPC reports within the domain shown above.

48-h Tornado reports ending 1200 UTC 28 April 2011

Assessing the Similarity Between Environments - Example

1800 UTC 27 April 2011 1800 UTC 11 April 2008

• The similarity was assessed between the April 2011 case and historical data using the North American Reanalysis from 1 January 1979 to 31 December 2010 (46,752 possible historical events).

• 500-hPa Height Correlation: 0.985• 500-hPa Height Mean Absolute Error: 23.322 m• 500-hPa Height Field Score: 0.961

Tornado Outbreak Likelihood: 500-hPa Heights 1800 UTC 27 April 2011

• Top 20 matches ranked using the 500-hPa score.

• Troughs are located in similar positions.

• 13 of 20 have closed circulations in the northern Plains.

Tornado Outbreak Likelihood: 500-hPa Heights 1800 UTC 27 April 2011

Top 117 (0.25%):Tornado: 14 (12.0%)Long-Track: 15 (12.8%)Significant: 15 (12.8%)

Top 20:Tornado: 3 (15.0%)Long-Track: 3 (15.0%)Significant: 2 (10.0%)

Tornado Outbreak

Sig. Tornado Outbreak

Long-Track Tornado Outbreak

Tornado Outbreak Likelihood: 500- and 850-hPa Heights 1800 UTC 27 April 2011

• Top 20 matches ranked using the sum of the 500-hPa and 850-hPa height scores.

• 16 of 20 have closed circulations.

• 850-hPa south-southwest wind maximums are evident over the area of interest.

Tornado Outbreak Likelihood: 500- and 850-hPa Heights 1800 UTC 27 April 2011

Top 117 (0.25%): Tornado: 17 (14.5%)Long-Track: 18 (15.4%)Significant: 19 (16.2%)


Tornado Outbreak



Tornado Outbreak Likelihood: 500-, 850-hPa Heights , and PWTR

1800 UTC 27 April 2011

• Top 20 matches ranked using the sum of the 500-hPa height, 850-hPa height, and precipitable water scores.

• High atmospheric moisture content is evident over the area of interest.

Tornado Outbreak Likelihood: 500-, 850-hPa Heights , and PWTR

1800 UTC 27 April 2011



Tornado Outbreak



Tornado Outbreak Likelihood: Supplemental Fields

Mass and Wind:300-hPa Heights 500-hPa Heights500-hPa Isotachs850-hPa HeightsSea-level Pressure

Moisture:2-m Dewpoint850-hPa Mstr. Trans.Precipitable Water600-hPa Dew. Dep.

Temperature:2-m

Temperature

Instability:Lifted Index

• It has been shown that adding atmospheric fields in the assessment methodology identifies environments more representative for tornado outbreaks.

• The atmospheric fields chosen to supplement the previous assessments were based on previous research and the Storm Prediction Center’s Convective Outlooks for the 26-28 April 2011 event.

Tornado Outbreak Likelihood: Supplemental Fields1800 UTC 27 April 2011

• Top 20 matches ranked by total score using the sum of the 11 atmospheric fields.


Top 20:Tornado: 10 (50%)Long-Track: 9 (45%)Significant: 10 (50%)


Median = 13

Super Tuesday Tornado

Outbreak

• Examining the distribution of tornado reports for each event in the top 117 similar historical environments provides a probabilistic expectation of tornadoes.

• Similar environments are conducive to multiple tornadoes, however extreme outbreaks are still very rare.

Outbreak Criteria (23)

26-28 April 201121-22 January 1999


Median = 2


Outbreak

• Although 28% of similar environments did not have a long-track tornado, 25% had 6 or more long-track tornadoes.

• Given a severe weather 48-h period (7155), the likelihood of at least 1 long-track tornado is about 6%. However, similar environments have at least 1 long-track tornado 72% of the time.


26-28 April 201121-22 January 1999


Median = 2


Outbreak

• Similar to the long-track tornado distribution, the probability of at least one significant tornado is 73%.

• Consequently, not only is the April 2011 environment associated with tornadoes, but also with long-track and significant tornadoes.


26-28 April 201128 February and 1 March

1997

1974 Super Outbreak

• But what about the comparison to the 1974 “Super Outbreak”?

• 148 tornadoes• 97 significant tornadoes• 80 long-track tornadoes

• Compared to 26-28 April 2011:• 277 tornadoes• 79 significant tornadoes• 78 long-track tornadoes

Fujita 1975

1974 Super Outbreak

• Using the Global Reanalysis Dataset, the similarity was computed using the 11 atmospheric fields between the April 2011 case and…1. the 1974 Super Outbreak.

2. the top 20 NARR similar environments.

Scores for Top 20 Historical Environments

8.458

6.549

(#1)

(#20)

1974 Super Outbreak

Scores for Top 20 Historical Environments

8.458

6.549

1974 Super Outbreak8.176 (#6)

(#1)

(#20)

• Using the Global Reanalysis Dataset, the similarity was computed using the 11 atmospheric fields between the April 2011 case and…1. the 1974 Super Outbreak.

2. the top 20 NARR similar environments.

Conclusions

• With an environment similar to the April 2011 event, tornado, long-track tornado, and significant tornado outbreaks are anticipated.

• The potential is even higher when analyzing the most similar environments (top 20).

• However, it is difficult to anticipate the magnitude and extent of the April 28 outbreak because the likelihood, even under tornado outbreak conditions, of this event is extremely rare.

• When the number of long-track and significant tornadoes are compared to the 1974 Super Outbreak, results are similar.

• Furthermore, these environments are statistically similar. This leads to speculation that there may be signals in the large-scale data that suggest the potential to anticipate catastrophic tornado outbreaks.

Questions and [email protected]

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quantifying the significance of the 26-27 april 2011 severe weather outbreak

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