A Statistical Analysis and Synoptic Climatology of U.S. Heat Waves

1. A Statistical Analysis and Synoptic Climatology of U.S. Heat Waves Scott C. Runyon and Lance F. Bosart Department of Earth and Atmospheric Sciences, University at Albany, State University of New York

2. Introduction: Why study heat waves? • Heat waves are a major contributor to weather–related fatalities • Understanding the characteristics of heat waves would lead to improved forecasts • These forecasts may become more critical given the possibility of an increase in the frequency and intensity of heat waves

3. Introduction (cont.): • Previous work has largely focused on specific heat wave events (e.g. July 1995) or extended “heat wave-droughts” (e.g. 1980 & 1988) • Published synoptic climatologies have been limited in scope to the Midwest or Great Plains (e.g. Chang & Wallace, 1984)

4. Purpose: • Document differences in heat waves as a function of both season and region • Understand both the dynamical and thermo-dynamical contributions to regional heat waves • Resolve annual and decadal trends in heat wave frequency

5. Overview: • Methodology/Definitions • Results • Skewed datasets • Northeast heat wave statistics • Case Study: 7-11 June 1984 • Summary • Conclusions • Future Work

6. Methodology: • Temperature data was extracted from the National Climatic Data Center’s (NCDC) high resolution surface dataset • Database contains daily high temperatures for 54 stations over a 54-year period (1948-2001) • Surface stations were selected on the basis of dataset continuity and geographical coverage

7. Methodology (cont.): Stations in Database

8. Methodology (cont.): • Definitions: • An anomalously hot day was initially defined as a day having a high temperature at least 2 standard deviations (σ) above the normal high temperature for the date • This definition was later changed to a day having a high temperature above the climatological 97.5 percentile threshold for the date • A heat wave was defined as three or more consecutive anomalously hot days – no matter the season

9. Methodology (cont.) • Regions mirror the NCDC Standard Regions for Temperature and Precipitation • Heat waves were considered regional when two or more stations within that region had overlapping warm days • All anomalously hot days, heat waves, and regional heat waves were identified for each station and each region across the country

10. Results “Skewness” of Daily High Temperature Data • Initially using a 2σ high temperature anomaly to define a hot day  a surprising amount of variability in number of heat waves identified from one city to the next • A serendipitous discovery: a discrepancy between number of anomalously cold days and the number of anomalously hot days for most stations in the dataset

11. Results: Top Ten “Skewed” Cities Positively Skewed Negatively Skewed

12. Most Negatively Skewed City: Denver • In 54-year dataset, only 49 days met old criteria as anomalously hot days (T≥2σ) • Only 2 heat waves were identified in the entire dataset

13. Denver, CO Daily High Temperatures – Spring Mean + 2σ Mean Mean - 2σ March 1 – May 31

14. Denver, CO Daily High Temperatures – Summer June 1 – August 31

15. Denver, CO Daily High Temperatures – Fall September 1 – November 30

16. Denver, CO Daily High Temperatures – Winter December 1 – February 29

17. Most Positively Skewed City: Los Angeles • In 54-year dataset, 989 days met old criteria as anomalously hot days (T≥2σ) • Hence, over 130 heat waves were identified in the dataset

18. Los Angeles, CA Daily High Temperatures – Spring March 1 – May 31

19. Los Angeles Daily High Temperatures – Summer June 1 – August 31

20. Los Angeles, CA Daily High Temperatures – Fall September 1 – November 30

21. Los Angeles, CA Daily High Temperatures – Winter December 1 – February 29

22. New Method: • An anomalously hot day is now defined as a day having a high temperature above the daily climatological 97.5 percentile threshold • Heat wave definitions remained unchanged • Updated Figures: • DNR: 736 Days, 41 Heat Waves • LAX: 669 Days, 66 Heat Waves

23. Stat. Analysis of Northeast Heat Waves

24. Stat. Analysis of Northeast Heat Waves

25. Stat. Analysis of Northeast Heat Waves

26. Stat. Analysis of Northeast Heat Waves

27. Stat. Analysis of Northeast Heat Waves

28. Stat. Analysis of Northeast Heat Waves

29. Stat. Analysis of Northeast Heat Waves

30. Stat. Analysis of Northeast Heat Waves

31. Stat. Analysis of Northeast Heat Waves

32. Stat. Analysis of Northeast Heat Waves Results: • A trend toward more Winter and Spring heat waves with time • A trend toward fewer Summer and Autumn heat waves with time • Decades of the 1950’s, 1980’s and 1990’s had the highest frequency of heat waves

33. A Case Study: 7-11 June 1984 • A long-lasting, region-wide Northeast heat wave • Both thermodynamic and dynamic signatures can be seen

34. June 4, 1984 ALB: 80° F LGA: 85° F BOS: 85° F

35. June 5, 1984 ALB: 84° F LGA: 88° F BOS: 89° F

36. June 6, 1984 ALB: 81° F LGA: 88° F BOS: 70° F

37. June 7, 1984 ALB: 89° F LGA: 93° F BOS: 89° F

38. June 8, 1984 ALB: 93° F LGA: 96° F BOS: 97° F

39. June 9, 1984 ALB: 93° F LGA: 95° F BOS: 96° F

40. June 10, 1984 ALB: 91° F LGA: 95° F BOS: 95° F

41. June 11, 1984 ALB: 90° F LGA: 96° F BOS: 98° F

42. June 12, 1984 ALB: 82° F LGA: 84° F BOS: 85° F

43. June 4, 1984 ALB: 80° F LGA: 85° F BOS: 85° F

44. June 5, 1984 ALB: 84° F LGA: 88° F BOS: 89° F

45. June 6, 1984 ALB: 81° F LGA: 88° F BOS: 70° F

46. June 7, 1984 ALB: 89° F LGA: 93° F BOS: 89° F

47. June 8, 1984 ALB: 93° F LGA: 96° F BOS: 97° F

48. June 9, 1984 ALB: 93° F LGA: 95° F BOS: 96° F

49. June 10, 1984 ALB: 91° F LGA: 95° F BOS: 95° F

50. June 11, 1984 ALB: 90° F LGA: 96° F BOS: 98° F