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Northern Illinois University

Department of Geography

Walker Ashley
washley@niu.edu

Courses - Research

Department of Geography > Faculty Staff Directory > Walker Ashley > Research

Walker Ashley

Research, Grants and Presentations

Publications
Hall, S.G., and W.S. Ashley, 2008: The effects of urban sprawl on the vulnerability to a significant tornado impact in northeastern Illinois. Natural Hazards Review, 9, 209-219. Abstract

Ashley, W.S., A.J. Krmenec, and R. Schwantes, 2008: Vulnerability due to nocturnal tornadoes. Weather and Forecasting, 23, 795-807. Abstract

Ashley, S.T., and W.S. Ashley, 2008: Flood fatalities in the United States. Journal of Applied Meteorology and Climatology, 47, 805-818. Abstract

Ashley, S.T., and W.S. Ashley, 2008: The storm morphology of deadly flooding events in the United States. International Journal of Climatology, 28, 493-503. Abstract

Ashley, W.S., and A. Black, 2008: Fatalities associated with nonconvective high-wind events in the United States. Journal of Applied Meteorology and Climatology, 47, 717-725. Abstract

Ashley, W.S., 2007: Spatial and temporal analysis of tornado fatalities in the United States: 1880-2005. Weather and Forecasting, 22, 1214-1228. Abstract

Ashley, W.S, T.L. Mote, and M.L. Bentley, 2007: An extensive episode of derecho-producing convective systems in the United States during May-June 1998: A multi-scale analysis and review. Meteorological Applications, 14, 227-244. Abstract

Suckling, P. W., and W. S. Ashley, 2006: Spatial and temporal characteristics of tornado path direction. The Professional Geographer, 58, 20-38. Abstract

Ashley, W.S., and T.L. Mote, 2005: Derecho Hazards in the United States. Bulletin of the American Meteorological Society, 86, 1577-1592. Abstract

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Research Grants:

NIU Undergraduate Research and Apprenticeship Program. Contribution of Severe Weather 'Outbreak' Events to the Overall U.S. Hazard Climatology. Fall 2008-Spring 2009.

NIU Research and Artistry Grant. Lightning Fatalities in the United States: An Assessment of Risks and Vulnerabilities. Summer 2008.

NIU Undergraduate Research and Apprenticeship Program. The Development of an Annotated Bibliography of the Climatology of Severe Weather. Fall 2007-Spring 2008, co-PI with M. L. Bentley.

NIU Undergraduate Research and Apprenticeship Program. Spatial Analysis of Weather-related Fatalities in the U.S. Fall 2007-Spring 2008, co-PI with M. L. Bentley.

National Science Foundation, Geography & Regional Science Program and Physical & Dynamic Meteorology Program, Climatological and Event-based Radar Delineation of UHI Convection for Urban Corridors within the Southeastern U.S., co-PI with M. L. Bentley (NIU) and J. A. Stallins (FSU), 2007-2010.

NIU Undergraduate Research and Apprenticeship Program. Spatial and Temporal Analysis of U.S. Tornado Casualties. Fall 2006-Spring 2007.

NIU Research and Artistry Grant. Windstorm-induced Casualties in the United States: 1960-2005. Summer 2006.

NIU Undergraduate Research and Apprenticeship Program. Windstorm-induced Casualties in the United States: 1960-2005. Spring 2006.
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Publications:

Hall, S.G., and W.S. Ashley, 2008: The effects of urban sprawl on the vulnerability to a significant tornado impact in northeastern Illinois. Natural Hazards Review, 9, 209-219.

Abstract: A sprawling U.S. population continues to spread into the fringes of urban development placing both populations and property in areas that were once largely unoccupied. Population tallies, housing unit totals, and housing values for 1990 and 2000 are examined to determine the extent to which this growth has affected the tornado hazard in northeastern Illinois. The growing town of Plainfield, Ill., located southwest of Chicago, is examined to determine how vulnerability to a tornado impact has changed in the town since an F5 tornado stuck the community in 1990. The population and housing data indicate an increase of 8,629 persons and 3,058 housing units affected if the tornado were to have occurred in 2000 rather than 1990. Estimations of housing value affected by the Plainfield tornado indicate a 50% increase in 2000 compared with 1990 values. In addition to studying the impacts on Plainfield, four other scenarios are examined in suburban Chicago counties using the 1990 Plainfield tornado as a model for a potentially devastating strike. The large increase in total value of homes affected for each scenario highlights the overall increase in wealth throughout the study area, specifically along the urban fringe of development. The physical vulnerability throughout the study area has increased with the rise in population, but the most socially vulnerable areas appear to remain in the older urban centers.

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Ashley, W.S., A.J. Krmenec, and R. Schwantes, 2008: Vulnerability due to nocturnal tornadoes. Weather and Forecasting, 23, 795-807.

This study investigates the human vulnerability caused by tornadoes that occurred between sunset and sunrise from 1880 to 2007. Nocturnal tornadoes are theorized to enhance vulnerability because they are difficult to spot and occur when the public tends to be asleep and in weak building structures. Results illustrate that the nocturnal tornado death rate over the past century has not shared the same pace of decline as those events transpiring during the daytime. From 1950 to 2005, a mere 27.3% of tornadoes were nocturnal, yet 39.3% of tornado fatalities and 42.1% of killer tornado events occurred at night. Tornadoes during the overnight period (local midnight to sunrise) are 2.5 times as likely to kill as those occurring during the daytime hours. It is argued that a core reason why the national tornado fatality toll has not continued to decrease in the past few decades is due to the vulnerability of these nocturnal events. This vulnerability to nocturnal tornadoes is magnified when other factors such as escalating mobile (or “manufactured”) home stock and an increasing and spreading population are realized. Unlike other structure types that show no robust demarcation between nocturnal and daytime fatalities, nearly 61% of fatalities in mobile homes take place at night revealing this housing stock’s distinct nocturnal tornado vulnerability. Further, spatial analysis illustrates that the American South’s high nocturnal tornado risk is an important factor leading to the region’s high fatality rate. The investigation emphasizes a potential break in the tornado warning dissemination system utilized currently in the United States.

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Ashley, S.T., and W.S. Ashley, 2008: Flood fatalities in the United States. Journal of Applied Meteorology and Climatology, 47, 805-818.

This study compiles a nationwide database of flood fatalities for the contiguous United States from 1959 to 2005. Assembled data include the location of fatalities, age and gender of victims, activity and/or setting of fatalities, and the type of flood events responsible for each fatality report. Because of uncertainties in the number of flood deaths in Louisiana from Hurricane Katrina, these data are not included in the study. Analysis of these data reveals that a majority of fatalities are caused by flash floods. People between the ages of 10 and 29 and 60 yr of age are found to be more vulnerable to floods. Findings reveal that human behavior contributes to flood fatality occurrences. These results also suggest that future structural modifications of flood control designs (e.g., culverts and bridges) may not reduce the number of fatalitiesnationwide. Spatially, flood fatalities are distributed across the United States, with high-fatality regions observed along the northeast Interstate-95 corridor, the Ohio River valley, and near the Balcones Escarpment in south-central Texas. The unique distributions found are likely driven by both physical vulnerabilities for flooding as well as the social vulnerabilities.

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Ashley, S.T., and W.S. Ashley, 2008: The storm morphology of deadly flooding events in the United States. International Journal of Climatology, 28, 493-503.

This study investigates the synoptic and mesoscale environments associated with deadly flooding events in the United States from 1996 to 2005. A manual environment classification scheme, which includes analyses of surface charts, 500 hPa maps, and composite radar data (where available), is utilized to ascertain the primary ascent mechanisms and storm types producing these fatal flood events. Of the ten classifications in the scheme, the two most dominant ascent mechanisms associated with deadly floods include frontal boundaries (45%) and tropical systems (22%). Findings illustrate that mesoscale convective systems were responsible for 36% of the total number of flood fatalities over the period. The ten classifications are spatially and temporally analysed in order to assess region-specific risks associated with deadly flooding events. Copyright  2007 Royal Meteorological Society

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Ashley, W.S., and A. Black, 2008: Fatalities associated with nonconvective high-wind events in the United States. Journal of Applied Meteorology and Climatology, 47, 717-725.

A database was compiled for the period 1980–2005 to assess the threat to life in the conterminous United States from nonconvective high-wind events. This study reveals the number of fatalities from these wind storms, their cause, and their unique spatial distributions. While tornadoes continue to cause the most wind-related fatalities per year, nonconvective high winds (defined as phenomena such as downslope and gap winds, gradient winds, dust storms, and winds associated with midlatitude cyclones) have the potential to fatally injure more people than thunderstorm or hurricane winds. Nonconvective wind fatalities occur more frequently in vehicles or while boating. Fatalities are most common along the West Coast and Northeast in association with passing extratropical cyclones, with fewer fatalities observed in the central United States despite this region’s susceptibility for high-wind gusts. A combination of physical and social vulnerabilities is suggested as the cause for the unique fatality distribution found. More than 83% of all nonconvective wind fatalities are associated with the passage of extratropical cyclones.

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Ashley, W.S., 2007: Spatial and temporal analysis of tornado fatalities in the United States: 1880-2005. Weather and Forecasting, 22, 1214-1228.

A dataset of killer tornadoes is compiled and analyzed spatially in order to assess region-specific vulnerabilities in the U.S. from 1880-2005. Results reveal that most tornado fatalities occur in the lower-Arkansas, Tennessee, and lower-Mississippi River Valleys of the Southeastern U.S. – a region outside of traditional “Tornado Alley.” Analysis of variables including tornado frequency, land cover, mobile home density, population density, and nocturnal tornado probabilities demonstrate that the relative maximum of fatalities in the Deep South and minimum in the Great Plains may be due to the unique juxtaposition of both physical and social vulnerabilities. The spatial distribution of these killer tornadoes suggests that the above national average mobile home density in the Southeast may be a key reason for the fatality maximum found in this area. A demographic analysis of fatalities during the latter part of the database record illustrates that the middle aged and elderly are at a much greater risk than younger people during these events. Data issues discovered during this investigation reveal the need for a concerted effort to obtain critical information about how and where all casualties occur during future tornado and hazardous weather events. These new, enhanced data, combined with results of spatially explicit studies exploring the human sociology and psychology of these hazardous events, could be utilized to improve future warning dissemination and mitigation techniques.
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A multi-scale analysis is presented on widespread and long-lived convectively generated windstorms, known as derechos. Analyses of the derecho-producing environments during 15 May–30 June 1998 indicate that this exceptional episode of derechos and derecho groupings (or series) was supported by ingredients (i.e. moisture, instability, and wind shear) that were supplied by the large-scale setting. In particular, the semi-stagnant subtropical ridge and associated capping inversion across the southern tier of the U.S. were important in supplying amplified moisture and instability to derecho-genesis regions through an underrunning process. Regions of preferred derecho formation appeared to correspond to shifts in the overall strength and position of the ridge, illustrating the importance of the ridge in focusing successive organized convection. Initiating mechanisms varied widely and were not restricted to warm-air advection regimes along quasi-stationary boundaries that forecasters often associate with warm-season derecho environments. In several cases, derecho-producing convective systems were generated by tropospheric features not consistent with common conceptual models of derecho environments such as closed lows and strong vorticity maxima. Further, three distinct series types were identified and classified based on their initiating mechanisms.
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Suckling, P. W., and W. S. Ashley, 2006: Spatial and temporal characteristics of tornado path direction. The Professional Geographer, 58, 20-38.
Common perception is that tornadoes travel in paths from the southwest quadrant of directions toward the northeast. This study examines path directions for 6,194 tornadoes that occurred in the eastern two-thirds of the United States during the twenty-three-year period 1980-2002. At the national scale, nearly 70 percent of
tornadoes included in the study propagated from the west, west-southwest, and southwest, with west-southwest being the highest frequency origin direction. Nevertheless, distinct seasonal and regional variations were found. In central and northern areas of the country, a more westerly or northwesterly path origin prevails during late spring and summer. The midtropospheric flow, convective typology, and synoptic patterns of tornado outbreaks are thought to contribute to the distributions observed in the climatology.
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Ashley, W.S., T.L. Mote, and M.L. Bentley, 2005: On the episodic nature of derecho-producing convective systems in the United States. International Journal of Climatology, 25, 1915-1932
Convectively generated windstorms occur over broad temporal and spatial scales; however, one of the larger-scale and most intense of these windstorms has been given the name 'derecho'. This study illustrates the tendency for derecho-producing mesoscale convective systems to group together across the United States - forming a derecho series. The derecho series is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 h separating individual events. A derecho dataset for the period 1994-2003 was assembled to investigate the groupings of these extremely damaging convective wind events. Results indicate that over 62% of the derechos in the dataset were members of a derecho series. On average, nearly six series affected the United States annually. Most derecho series consisted of two or three events; though, 14 series during the period of record contained four or more events. Two separate series involved nine derechos within a period of nine days. Analyses reveal that derecho series largely frequent regions of the Midwest, Ohio Valley, and the south-central Great Plains during May, June, and July. Results suggest that once a derecho occurred during May, June, or July, there was a 58% chance that this event was the first of a series of two or more, and about a 46% chance that this was the first of a derecho series consisting of three or more events. The derecho series climatology reveals that forecasters in regions frequented by derechos should be prepared for the probable regeneration of a derecho-producing convective system after an initial event occurs.
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Ashley, W.S., and T.L. Mote, 2005: Derecho Hazards in the United States. Bulletin of the American Meteorological Society, 86, 1577-1592.
Convectively generated wind-storms occur over broad temporal and spatial scales; however, the more widespread and longer lived of these windstorms have been given the name “derecho.” Utilizing an integrated derecho database, including 377 events from 1986 to 2003, this investigation reveals the amount of insured property losses, fatalities, and injuries associated with these windstorms in the United States. Individual derechos have been responsible for up to 8 fatalities, 204 injuries, forest blow-downs affecting over 3,000 km2 of timber, and estimated insured losses of nearly a $500 million. Findings illustrate that derecho fatalities occur more frequently in vehicles or while boating, while injuries are more likely to happen in vehicles or mobile homes. Both fatalities and injuries are most common outside the region with the highest derecho frequency. An underlying synthesis of both physical and social vulnerabilities is suggested as the cause of the unexpected casualty distribution. In addition, casualty statistics and damage estimates from hurricanes and tornadoes are contrasted with those from derechos to emphasize that derechos can be as hazardous as many tornadoes and hurricanes.
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Ashley, W.S., T.L. Mote, P.G. Dixon, S.L. Trotter, J.D. Durkee, E.J. Powell, and A.J. Grundstein, 2003: Distribution of mesoscale convective complex rainfall in the United States. Monthly Weather Review, 131, 12, 3003-3017.
Several annual mesoscale convective complex (MCC) summaries have been compiled since Maddox strictly defined their criteria in 1980. These previous studies have largely been independent of each other and therefore have not established the extended spatial and temporal patterns associated with these large, quasi-circular, and, typically, severe convective systems. This deficiency is primarily due to the difficulty of archiving enough satellite imagery to accurately record each MCC based on Maddox's criteria. Consequently, this study utilizes results from each of the MCC summaries compiled between 1978 and 1999 for the United States in order to develop a more complete climatology, or description of long-term means and interannual variation, of these storms. Within the 22-yr period, MCC summaries were compiled for a total of 15 yr. These 15 yr of MCC data are employed to establish estimated tracks for all MCCs documented and, thereafter, are utilized to determine MCC populations on a monthly, seasonal, annual, and multiyear basis. Subsequent to developing an extended climatology of MCCs, the study ascertains the spatial and temporal patterns of MCC rainfall and determines the precipitation contributions made by MCCs over the central and eastern United States. Results indicate that during the warm season, significant portions of the Great Plains receive, on average, between 8% and 18% of their total precipitation from MCC rainfall. However, there is large yearly and even monthly variability in the location and frequency of MCC events that leads to highly variable
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Presentations : Invited Talks - Papers Presented at Professional Meetings

Invited Talks

Ashley, W. S., 2008: Tornadoes: Fatalities associated with nature’s most intense windstorm. Invited talk for NIU Notables, DeKalb, IL.

Ashley, W. S., and A. Black, 2007: Tornadoes: Fatalities associated with nature’s most intense windstorm (with a little nonconvective spice thrown in). Invited talk at Joint Chicago-NIU AMS Chapter Meeting, DeKalb, IL.

Ashley, W. S., 2007: Tornadoes: Fatalities associated with nature’s most intense windstorm. Invited talk at Moraine Valley Community College, Palos Hills, IL.

Ashley, W. S., 2007: Derechos and tornadoes: Fatalities associated with Nature’s most intense windstorms. Invited talk to College of DuPage, Glen Ellyn, IL.

Ashley, W. S., 2006: Disaster preparedness: From the national to the local scale. NIU Dept. of Geography Brown Bag Seminar, DeKalb, IL.

Ashley, W. S., 2006: Derechos and tornadoes: Fatalities associated with Nature’s most intense windstorms. Invited talk to California University-Pennsylvania, California, PA.

Ashley, W. S., D. Changnon, R. Greene, and M. Bentley, 2005: Hurricane Katrina and the human migration aftermath. NIU Dept. of Geography Brown Bag Seminar, DeKalb, IL.

Ashley, W. S., 2005: Hazards of derecho-producing convective systems in the United States. Joint Chicago-NIU AMS Meeting, DeKalb, IL.

Ashley, W. S., 2005: Climatology and hazards of derecho-producing convective systems in the United States. University of West Florida, Pensacola, FL.

Ashley, W. S., 2005: Climatology and hazards of derecho-producing convective systems in the United States. Southern Illinois University, Carbondale, IL.

Ashley, W. S., 2005: Hazards of derecho-producing convective systems in the United States. Northern Illinois University, DeKalb, IL.

Ashley, W. S., 2004: Climatology and hazards of derecho-producing convective systems in the United States. University of Wyoming, Laramie, WY.