Severe Weather

Forecasting severe convective weather remains one of the most challenging tasks facing operational meteorology today, especially in the mid-latitudes, where severe convective storms occur most frequently and with the greatest impact. The forecast difficulties reflect, in part, the many different atmospheric processes of which severe thunderstorms are a by-product. These processes occur over a wide range of spatial and temporal scales, some of which are poorly understood and/or are inadequately sampled by observational networks. Therefore, anticipating the development and evolution of severe thunderstorms will likely remain an integral part of national and local forecasting efforts well into the future.

Modern severe weather forecasting began in the 1940s, primarily employing the pattern recognition approach throughout the 1950s and 1960s. Substantial changes in forecast approaches did not come until much later, however, beginning in the 1980s. By the start of the new millennium, significant advances in the understanding of the physical mechanisms responsible for severe weather enabled forecasts of greater spatial and temporal detail. At the same time, technological advances made available model thermodynamic and wind profiles that supported probabilistic forecasts of severe weather threats.

This article provides an updated overview of operational severe local storm forecasting, with emphasis on present-day understanding of the mesoscale processes responsible for severe convective storms, and the application of recent technological developments that have revolutionized some aspects of severe weather forecasting. The presentation, nevertheless, notes that increased understanding and enhanced computer sophistication are not a substitute for careful diagnosis of the current meteorological environment and an ingredients-based approach to anticipating changes in that environment; these techniques remain foundational to successful forecasts of tornadoes, large hail, damaging wind, and flash flooding.

On the evening of 5 April 2003, three supercells traversed northern portions of Dallas-Fort Worth, Texas causing nearly one billion dollars in hail damage, making the event the sixth costliest storm in Texas history to date (ICT 2003). The storms produced a swath of damaging hail that extended about 400 mi. (645 km) across the state from near Snyder to near Texarkana, Texas. A hail damage survey team spent several days examining roofs and other items as well as interviewing eyewitnesses in an effort to map the hail sizes and direction of hailfall. Indirect measurements of hail size were derived from examining roof coverings, vents, wooden fences, and air conditioners. Indirect measurements were then correlated with ice impacts of similar items conducted in the laboratory. We also measured the sizes of hailstones that persons had kept in their freezers and compared them to our findings.

This paper will discuss: 1) the meteorological history of the event, 2) our hail damage survey results, 3) the correlation between hail sizes and
various indirect measurements, and 4) the correlation between hail sizes and radar derived estimates.

A protocol has been developed by the authors for assessing hail-impact damage to steep-sloped roofing systems.  The protocol includes a definition of hail-caused damage to roofing, a detailed field inspection procedure, and a calculation method for determining repair or replacement of hail-damaged roofing based on economics.

Although the Fujita Scale has been in use for 30
years, the limitations of the scale are well known to its
users. The primary limitations include a lack of
damage indicators, no account of construction quality
and variability, and no definitive correlation between
damage and wind speed. These limitations have led
to inconsistent ratings of tornado damage and, in
some cases, overestimates of tornado wind speeds.
Thus, there is a need to revisit the concept of the
Fujita Scale and to improve and eliminate some of the
limitations.  This paper summarizes our work in developing the EF (Enhanced Fujita) scale which involves determining the degrees of damage (DoDs) to various damage indicators (DIs).

On 22 May 2011, a large and destructive tornado traveled through southern portions of Joplin, MO, a city of approximately 50,000 people. According to the Storm Data, the tornado killed 158 people and injured more than 1000. Total property damage exceeded two billion dollars.  This paper summarizes the results of our damage survey.  The EF-scale was utilized on several building types.

Severe convective storms occasionally result in loss of life and property in Turkey, a country not known for its severe convective weather. However, relatively little is known about the characteristics of Turkish severe weather environments. This paper documents these characteristics using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data on tornado and severe hail days in Turkey from 1979 to 2013. Severe storm environments are characterized by larger convective available potential energy (CAPE) in Turkey compared to the rest of Europe, but the CAPE values are less than those in typical U.S. severe storm environments. Severe hail is associated with large CAPE and vertical wind shear. Nonmesocyclonic tornadoes are associated with less CAPE compared with the other forms of severe weather. Deep-layer vertical wind shear is slightly weaker in Turkish supercell environments than in U.S. supercell environments, and Turkish tornadic supercell environments are chara...

On May 22, 2011, a large and violent tornado struck southern portions of Joplin, MO. The tornado killed 158 people and injured more than 1000 people. The tornado was rated EF-5 on the Enhanced Fujita Scale, with maximum winds exceeding 89 ms-1 (200 mph). During the week following the tornado, the author conducted aerial and ground surveys of the damage. Surveying the damage was a challenge due to the large numbers of damaged or destroyed buildings (about 8,500 structures) as well as the rapid clean-up that was ongoing. Certain buildings were being demolished with heavy equipment within days after the tornado. Therefore, the author decided to select a sample of 14 buildings constructed with concrete masonry unit (CMU) walls.

During the evening of 27 April 2014, an EF4 tornado struck the communities of Mayflower and Vilonia, Arkansas. The tornado traveled 66 km, passing just northwest of the Little Rock Metropolitan area, and had a maximum path width of about 1200 m. During its 56 minute lifespan, the tornado killed 16 people, including one person who sought refuge in an above ground storm shelter. The fatality occurred when a hard object struck the storm shelter door causing it to deform excessively into the shelter.  This was the first known fatality in an above ground storm shelter.

There has been a dearth of systematic comparative studies of environmental and natural events during the Early and High Middle Ages. Using new analytic concepts and methods, this study examines multiple and highly diverse sources in relation to extreme natural events. It represents the first precise study of the reliability of contemporary perception concerning such events, and also considers their instrumentalization.
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Die Umwelt wird zunehmend als vierte Grundkategorie der Geschichtswissenschaft neben Politik, Wirtschaft und Kultur wahrgenommen, aber gerade zum Früh- und Hochmittelalter fehlten bislang systematisch vergleichende Untersuchungen der Naturereignisse. Da die bisher angewandten Arbeitsweisen am frühmittelalterlichen Material versagen, mussten neue methodische Konzepte der Auswertung entwickelt werden, bei denen die überlieferten extremen Naturereignisse im Fokus stehen. Diese wurden aus historiographischen Quellen gefiltert, kategorisiert und analysiert. Dabei stellt sich einerseits heraus, dass mittelalterliche Autoren teilweise über unerwartet genaue Beobachtungsmöglichkeiten verfügten, sie aber andererseits ihre Beobachtungen nicht unreflektiert niederschrieben und keiner empirischen Genauigkeit verpflichtet waren. Insgesamt konnten anhand von Parallelüberlieferung oder Vergleichen etwa 1175 Ereignisse bezüglich ihrer intentionalen Nutzung analysiert werden. So bietet die Arbeit einen genauen Einblick in die Zuverlässigkeit der Wahrnehmung und die mögliche Instrumentalisierung der Darstellung extremer Naturereignisse und ihrer Folgen in den Quellen des 6. bis 11. Jahrhunderts.

The Super Outbreak of tornadoes over the central and eastern United States on 3-4 April 1974 remains the most outstanding severe convective weather episode on record in the continental United States. The outbreak far surpassed previous and succeeding events in severity, longevity, and extent. In this paper, surface, upper-air, radar, and satellite data are used to provide an updated synoptic and subsynoptic overview of the event. Emphasis is placed on identifying the major factors that contributed to the development of the three main convective bands associated with the outbreak, and on identifying the conditions that may have contributed to the outstanding number of intense and long-lasting tornadoes. Selected output from a 29-km, 50-layer version of the Eta forecast model, a version similar to that available operationally in the mid-1990s, also is presented to help depict the evolution of thermodynamic stability during the event.

Over the western United States, the hazards posed to aviation operations by convective storm-generated downbursts have been extensively documented. Other significant hazards posed by convective downbursts over the intermountain western U.S. include the rapid intensification and propagation of wildfires and the sudden generation of visibility-reducing dust storms (haboobs).  The existing suite of GOES downburst prediction algorithms employs the GOES sounder to calculate potential of occurrence based on conceptual models of favorable environmental thermodynamic profiles for downburst generation. Previous research has demonstrated the effectiveness of the Dry Microburst Index (DMI) as a prediction tool for convectively generated high winds. A more recently-developed diagnostic nowcasting product, the Microburst Windspeed Potential Index (MWPI) is designed to diagnose attributes of a favorable downburst environment: 1) the presence of convective available potential energy (CAPE), and 2) the presence of a deep surface-based or elevated mixed layer with a large temperature lapse rate. This paper presents an updated assessment of the MWPI algorithm, case studies demonstrating effective operational use of the MWPI product, and recent validation results.  MWPI data were collected for downburst events that occurred during the 2014 convective season and were validated against surface observations of convective wind gusts as recorded by wind sensors in high-quality mesonetworks. Favorable validation results include a statistically significant correlation (r > 0.6) and low mean error (< 1 kt) between MWPI values and confirmed downburst wind speeds measured in situ.

As the climate warms, stressors are developing that challenge the adaptive capabilities of Arctic peoples. In Nunavut, one of Canada’s Arctic territories, increased weather variability and changes in physical and climatic conditions are having profound
effects on residents. One problem is that while these changes have magnified risks associated with travel and land-based activities, individuals lack sufficiently reliable and useful
information on which to base decisions. In this paper, we argue that weather-related risk assessment can be improved by integrating local and scientific weather knowledge and
making this information accessible to residents through the creation of weather hazards impact advisory groups. We present a qualitative case study of Iqaluit, the capital of Nunavut, using data from participant observation and semi-structured interviews conducted with indigenous and non-indigenous long-term residents in summer 2009. We examine how long-term residents of Iqaluit acquire, perceive, and use both local and scientific weather knowledge. We find that various barriers, such as a lack of land-based experience, cultural and linguistic differences, and an absence of social networks, prevent most people from
obtaining all the information required to make fully informed decisions about the risks associated with land-based activities at different locations. Experienced hunters are perceived
to be a reliable source of weather-related information, while scientific weather knowledge is not as accessible or informative as it could be. Increasing the potential use of traditional and scientific hazardous weather knowledge, by making both more universally accessible, can enhance strategies for adapting to climate change in the Arctic.

On 31 May 2013, an intense, multiple-vortex tornado meandered through sparsely populated areas near El Reno, Oklahoma. This exceptionally wide tornado had a complex path, changing rapidly in both speed and direction. The tornado killed eight people, all in vehicles, including three severe storm researchers. This paper summarizes our findings from conducting a damage survey using the EF (Enhanced Fujita) scale developed by the Wind Science and Engineering Center. Survey teams also documented damage to 344 non-standard DIs which included 112 oil tank batteries. The maximum damage rating was determined to be EF3. Total path length was 26.2 km.

On 4 May 1999, the Wind Science and Engineering Research Center at Texas Tech University dispatched three survey teams to the Oklahoma City area to conduct a tornado damage survey. The author was the leader of one of the teams whose purpose was to survey tornado damage in and around the suburb of Moore, Oklahoma. The survey team was given five tasks: 1) to map out the damage path and assign F-scale numbers to damaged buildings, 2) to document the performance of housing, 3) to interview witnesses, 4) to document projectiles, and 5) to assess the performance of any above-or belowground shelters within the damage path. This paper will present the methodology utilized for conducting the tornado damage survey and will summarize the observations and findings of the survey team. Wind speeds necessary to cause the observed damage to residences were found to be significantly lower than the established F-scale wind speeds. The author returned to the disaster area three months later and discovered that, in general, the quality of new home construction had not improved.

Hail is considered to be among the most complex extreme weather phenomena of the atmosphere. Every year, notably in the southern Brazilian States, destructive hailstorms result in serious economic losses and cause a great social impact destroying crops, homes, medical facilities and schools. The aim of this study is to document the spatial, annual, and diurnal variation in destructive hailstorm frequency during a 22 year period from1991 to 2012 in Brazil. The analysis is based on a collection of reports released by the Brazilian National Civil Protection Secretariat – SEDEC. Based on reports of emergency assistance given to the population affected by a disaster, the information discussed in this work is assumed as representative only of destructive hailstorms. The analysis reveals a large spatial variability, with the majority of hailstorm occurrences distributed in the three southernmost Brazilian States. Within those states, the number of hail reports was observed to increase with increasing population density in rural areas. Hailstorms were reported most often in the late afternoon and evening of thewinter/spring transition, in agreement with a fewother areas in the subtropics with available studies, but different from the majority of studies for temperate zones, which suggest spring/summer as the hail season. Although the results show some discrepancies compared to satellite hail signatures, the findings of this work confirm that southern Brazil is a region prone to the development of strong convective storms,with high annual numbers of destructive hail events.

Twenty-eight predecessor rain events (PREs) that occurred over the United States east of the Rockies during 1995–2008 are examined from a synoptic climatology and case study perspective. PREs are coherent mesoscale regions of heavy rainfall, with rainfall rates ≥100 mm (24 h)−1, that can occur approximately 1000 km poleward of recurving tropical cyclones (TCs). PREs occur most commonly in August and September, and approximately 36 h prior to the arrival of the main rain shield associated with the TC. A distinguishing feature of PREs is that they are sustained by deep tropical moisture that is transported poleward directly from the TC. PREs are high-impact weather events that can often result in significant inland flooding, either from the PRE itself or from the subsequent arrival of the main rain shield associated with the TC that falls onto soils already saturated by the PRE. The composite analysis shows that on the synoptic-scale, PREs form in the equatorward jet-entrance region o...

On 23-25 March 1998, Norman, Oklahoma, and
nearby Tinker Air Force Base hosted a three-day celebration
to commemorate the 50th anniversary of the
first successful tornado forecast and the advances that
have occurred in tornado forecasting and warning in
the past half-century. The event was a cooperative effort
of the National Oceanic and Atmospheric Administration
(NOAA), the United States Air Force
(USAF), and the University of Oklahoma. Participation
also included the Norman Chamber of Commerce, as
well as the chambers of commerce from surrounding
municipalities in central Oklahoma.

A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyoming, Idaho, and Colorado on 31 May 1994. The event was unusual in that it occurred not only in an environment of relatively limited moisture, but also one with a thermodynamic profile favorable for dry microbursts in the presence of moderate midtropospheric flow. The development and evolution of the severe wind-producing convective system is described, with emphasis on the synoptic and mesoscale features that may have contributed to its strength and maintenance. A very similar derecho that affected much the same region on 1 June 2002 is more briefly introduced. Questions are raised regarding the unique nature of these events and their potential utility in achieving an increased understanding of the mechanics of derecho-producing convective systems in more moisture-rich environments.

On October 11, 1846, the island of Key West, Florida was struck by a very powerful hurricane. This paper examines the available historical material to gain an understanding of the storm’s broad-reaching effects on the island community and the physical alterations it wrought on the landscape. Many of the experiences of the 1846 hurricane hold lessons for what might be expected in a modern-era storm.

A suite of products has been developed and evaluated to assess hazards presented by convective storm downbursts
derived from the current generation of Geostationary Operational Environmental Satellite (GOES) (13-15). The existing
suite of GOES downburst prediction products employs the GOES sounder to calculate risk based on conceptual models
of favorable environmental profiles for convective downburst generation. A diagnostic nowcasting product, the
Microburst Windspeed Potential Index (MWPI), is designed to infer attributes of a favorable downburst environment: 1)
the presence of large convective available potential energy (CAPE), and 2) the presence of a surface-based or elevated
mixed layer with a steep temperature lapse rate and vertical relative humidity gradient. These conditions foster intense
convective downdrafts upon the interaction of sub-saturated air in the elevated or sub-cloud mixed layer with the storm
precipitation core. This paper provides an updated assessment of the MWPI algorithm, presents recent case studies
demonstrating effective operational use of the MWPI product over the Atlantic coastal region, and presents validation
results for the United States Great Plains and Mid-Atlantic coastal region. In addition, an application of the brightness
temperature difference (BTD) between GOES imager water vapor (6.5μm) and thermal infrared (11μm) channels that
identifies regions where downbursts are likely to develop, due to mid-tropospheric dry air entrainment, will be outlined.

A climatology of tornadoes in Turkey is presented using records from a wide variety of sources (e.g., the Turkish State Meteorological Service, European Severe Weather Database, newspaper archives, Internet searches, etc.). The climatology includes the annual, diurnal, geographical, and intensity distributions of both mesocyclonic and nonmesocyclonic tornadoes. From 1818 to 2013, 385 tornado cases were obtained. The tornadoes range from F0 to F3, with F1 being the most frequently reported or inferred intensity. Mesocyclonic tornadoes are most likely in May and June, and a secondary maximum in frequency is present in October and November. Nonmesocyclonic tornadoes (waterspouts) are most common in the winter along the (southern) Mediterranean coast and in the fall along the Black Sea (northern) coast. Tornadoes (both mesocyclonic and nonmesocyclonic) are most likely in the afternoon and early evening hours.

The National Environmental Satellite, Data, and Information Service (NESDIS) Center for Satellite Applications and Research (STAR) has developed and evaluated a suite of products that assess convective storm–generated downburst potential derived from Geostationary Operational Environmental Satellite-13–15 (GOES-13–15). The existing suite of downburst prediction algorithms employs the GOES sounder to calculate risk based on conceptual models of favorable environmental thermodynamic profiles for downburst occurrence. A diagnostic nowcasting product, the Microburst Windspeed Potential Index (MWPI), is designed to identify attributes of a favorable downburst environment: 1) the presence of large CAPE and 2) the presence of a surface-based or elevated mixed layer with a large temperature lapse rate. This paper provides an updated assessment of the MWPI algorithm, presents case studies demonstrating effective operational use of the MWPI product, and presents validation results for the Great Plains and mid-Atlantic coastal region of the United States. MWPI data were collected for downburst events that occurred during the convective seasons of 2007–13 and were validated against surface observations of convective wind gusts as recorded by wind sensors in high quality mesonetworks over the southern Great Plains and the Chesapeake Bay region. Favorable validation results include a correlation greater than 0.6 and low mean error [<0.1 knot (kt; where 1 kt = 0.51 m s−1)] between MWPI values and measured confirmed downburst wind speeds over contrasting climate regions of the continental United States. Case studies over the mid-Atlantic region and northern Florida highlight the adaptability of the MWPI algorithm to severe convective storm forecasting and warning operations.