FYI: National Weather Service Assessment Report from the April 27, 2011 tornado outbreak
Service Assessment
The Historic Tornadoes of April 2011
U.S. DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
National Weather Service
Silver Spring, Maryland
NWS Service Assessment Report from the April 27, 2011 tornado outbreak.
Following is the Executive Summary, but for the report can be found online at http://www.nws.noaa.gov/os/assessments/pdfs/historic_tornadoes.pdf
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On April 27, 2011, a series of devastating tornadoes struck the southeastern United States.
This tornado event was the third deadliest in the country since systematic tornado record keeping
began in 1950. With 316 fatalities (31 in Mississippi, 234 in Alabama, 32 in Tennessee, 15 in
Georgia, and 4 in Virginia), it follows only the 1974 Super Tornado Outbreak (368 deaths) and
the 1965 Palm Sunday Tornado Outbreak (337 deaths). Damages from this tornado episode
resulted in over $4.2 billion in property damage and more than 2,400 injuries.
The National Weather Service (NWS) formed a Service Assessment Team to evaluate its
performance. To strengthen NWS relationships with other federal agencies involved with
disaster work, for the first time this assessment had a co-leader from the Federal Emergency
Management Agency. The team interviewed staff and reviewed products from the Storm
Prediction Center (SPC), Weather Forecast Offices at Memphis and Morristown, TN, Jackson,
MS, Huntsville and Birmingham, AL, and Peachtree City, GA. The team gathered feedback
from partners and users of NWS products including media outlets, Emergency Managers at the
state and local level, first responders, and the public. One
of the team’s tasks was to assess
societal impacts of this event.
This tornado outbreak was anticipated and forecast days in advance. The SPC began
focusing on the affected area in its convective outlook products 5 days prior to the event. It
continued emphasizing, refining, and enhancing the threat leading up to the event, ultimately
issuing a high risk convective outlook on the morning of April 27 for a large portion of the
impacted area.
The Weather Forecast Offices (WFO) in the affected area prepared for severe weather
operations and indicated the risk of severe weather and tornadoes as much as 5 days in advance.
Hazardous Weather Outlooks, Web images, pre-recorded multimedia briefings, and webinars
discussed the potential impacts. Emergency Managers and media staff interviewed indicated
they were well prepared for the severe weather that occurred.
All tornado fatalities occurred within the boundaries of tornado watches and were preceded
by tornado warnings. There were several tornado watches issued in the afternoon before severe
weather began. The lead time, the time from the initial three watch issuances to the first
tornadoes in those watches, averaged 2.4 hours. Watch lead times to the first significant tornado
in each area ranged from 3-6 hours. The mean lead time for tornado warnings in the assessment
area was 22.1 minutes. The probability of detection was 89 percent and the false alarm ratio was
49 percent.
Despite the excellent performance of the SPC and WFO staff, the tornadoes resulted in a
substantial death toll. Contributing factors to the high number of casualties included:
A large number of rare, long-track, violent tornadoes
Tornado tracks intersecting densely populated areas
Damage to warning dissemination sources
Individuals in the affected areas who did not respond to warnings until confirmed by
more than one communication source
People in the paths of the storms who waited for visual confirmation before taking
protective action
The rapid pace of the storms, which moved at 45-70 mph, giving people who waited for
secondary confirmation a smaller window of time in which to take shelter
Residences that did not have adequate storm shelters
So many deaths resulting from an event in which the NWS performed well suggested that
societal and sociological factors regarding warning response played a role. To address this, the
NWS included four social scientists on the assessment team: one each from NWS, the Centers
for Disease Control and Prevention, the University of Northern Alabama, and the University of
Delaware. This team had the largest number of social scientists ever included on a Service
Assessment, reflecting the depth of sociological information in this report. Appendix C
describes procedures for data collection and assessment methodologies employed by the social
scientists.
The team determined that WFOs and state and local emergency management had developed
excellent relationships. The team also found opportunities for improved use of NWS products
and services in planning and training for natural hazards.
There were several success stories during this event. The relationships fostered by WFOs
with their media and emergency management partners allowed for efficient exchange of
information before and during the tornadoes. WFOs in impacted areas used webinars,
multimedia presentations, Web graphics, and social media services to disseminate information to
key partners and the public. Emergency management and media partners unanimously praised
these decision support services. Members of the public who heeded the warnings and took cover
in underground storm shelters survived the most violent tornadoes.
The team submitted 24 recommendations to address NWS performance, safety, and outreach
programs. In addition, the team identified 14 best practices. Appendix B offers definitions of
facts, findings, recommendations, and best practices followed by a complete listing of findings,
recommendations, and best practices found in the main body of the report. Appendices D and E
contain summaries of the tornado events that impacted the St. Louis metropolitan area and
North Carolina/South Carolina/Virginia, respectively.
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