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

2. Source: www.nws.noaa.gov/om/severe_weather/65yrstats.pdf
Overview/Motivation
Heat waves are a major contributor to weather-related fatalities
From 1986 to 2004:
Severe weather event
# of fatalities
Hurricanes*
323
Tornadoes
960
Lightning
1081
Floods
1575
Extreme heat
2596
Source:

3. Overview/Motivation (cont.)
Improved forecasting may result from a better understanding of conditions antecedent to and during heat waves
These improved forecasts may become critical given the possibility of an increase in frequency & intensity of heat waves

4. Overview/Motivation (cont.)
Previous work has largely focused only on individual events (e.g., McQueen et al. 1956, 1957; Livezey and Tinker 1996) or on extended “heat wave–droughts” (e.g., Namias 1982, 1991)
Published synoptic climatologies have been limited in scope to “Midwest” or Great Plains (e.g., Namias 1955, 1983; Chang & Wallace 1984)

5. Goals
Resolve annual and decadal trends in heat wave frequency nationwide
Investigate conditions that lead to onset of Northeast heat waves
Understand synoptic evolution of and dynamic contributions to Northeast heat waves

6. Overview Methodology Initial Revised Results
Northeast vs. regional heat wave statistics
Northeast summer heat wave composites
Antecedent conditions
Evolution of heat waves
Conclusions

7. Methodology
Temperature data were gathered from the National Climatic Data Center’s (NCDC) high resolution surface dataset
Daily high temperatures were extracted for 54 surface stations over a 54-year period (1948–2001)
Stations selected on basis of both dataset continuity and coverage within the NCDC’s Standard Regions for temperature and precipitation

9. Methodology (cont.)
An anomalously hot day was initially defined as a day having a high temperature ≥ 2 standard deviations (σ) above the normal* high temperature
Result:
Large variability in number of anomalous warm events between stations
Extreme ratios of anomalously hot days to anomalously cold days at many stations

10. Methodology (cont.) This initial method led to a discovery:
High temperatures at most stations are not normally distributed (“skewed”)
Negatively Skewed
Positively Skewed
DEN
94.2
5.8
LAX
5.7
94.3

11. Temperature data is widely assumed to be normally distributed
Methodology (cont.)
Temperature data is widely assumed to be normally distributed
Los Angeles, CA and Denver, CO:
Most positively and negatively skewed datasets, respectively
Using 2σ method 989 (49) anomalously hot days were found in Los Angeles (Denver) for all seasons

12. Most Positively Skewed Station: Los Angeles, CA
Methodology (cont.)
Most Positively Skewed Station: Los Angeles, CA

13. Los Angeles, CA: Daily High Temperatures
Methodology (cont.)
Los Angeles, CA: Daily High Temperatures
Mean +2σ
Mean
Mean −2σ
June 1 – August 31, 1948 – 2001

14. H L
Los Angeles, CA: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Warm Summer Days

15. Los Angeles, CA: Daily High Temperatures
Methodology (cont.)
Los Angeles, CA: Daily High Temperatures
September 1 – November 30, 1948 – 2001

16. H L
Los Angeles, CA: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Warm Autumn Days

17. Los Angeles, CA: Daily High Temperatures
Methodology (cont.)
Los Angeles, CA: Daily High Temperatures
December 1 – February 29, 1948 – 2001

18. H L
Los Angeles, CA: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Warm Winter Days

19. Los Angeles, CA: Daily High Temperatures
Methodology (cont.)
Los Angeles, CA: Daily High Temperatures
March 1 – May 31, 1948 – 2001

20. H L
Los Angeles, CA: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Warm Spring Days

21. Most Negatively Skewed Station: Denver, CO
Methodology (cont.)
Most Negatively Skewed Station: Denver, CO

22. Denver, CO: Daily High Temperatures
Methodology (cont.)
Denver, CO: Daily High Temperatures
Mean +2σ
Mean
Mean −2σ
June 1 – August 31, 1948 – 2001

23. H L
Denver, CO: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Cold Summer Days

24. Denver, CO: Daily High Temperatures
Methodology (cont.)
Denver, CO: Daily High Temperatures
September 1 – November 30, 1948 – 2001

25. H L
Denver, CO: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Cold Autumn Days

26. Denver, CO: Daily High Temperatures
Methodology (cont.)
Denver, CO: Daily High Temperatures
December 1 – February 29, 1948 – 2001

27. H L
Denver, CO: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Cold Winter Days

28. Denver, CO: Daily High Temperatures
Methodology (cont.)
Denver, CO: Daily High Temperatures
March 1 – May 31, 1948 – 2001

29. H L
Denver, CO: Composite Mean Sea Level Pressure (hPa) – Ten Most Anomalous Cold Spring Days

30. Methodology (cont.) Los Angeles: Denver:
Large temperature anomalies more likely positive and associated with Santa Ana (offshore) events
Typically, marine layer moderates daily high temperatures
Denver:
Large temperature anomalies more likely negative and associated with upslope events
Typically, mean westerly, downsloping flow influences daily high temperatures
Non-normal temperature distributions are common and have distinct physical explanations

31. Methodology (cont.)
Anomalously hot day: A day having a high temperature greater than the daily climatological 97.5 percentile threshold
New method eased distribution problems [Los Angeles 669, Denver 736]
Heat wave: Three or more consecutive anomalously hot days
Regional heat wave: When heat waves occurring at two or more stations within a region had at least one overlapping day