A Statistical Analysis and Synoptic Climatology of Heat Waves over the Northeast United States

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. Overview/Motivation • Heat waves are a major contributor to weather-related fatalities • From 1986 to 2004 across the U.S.: Source: www.nws.noaa.gov/om/severe_weather/65yrstats.pdf

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* * 31-day running mean • 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 ERI 28.8 71.2 BOS 30.7 69.3 LGA 37.7 62.3 CAR 40.8 59.2 ALB 44.5 55.5

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

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

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

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

18. Methodology (cont.) • Los Angeles: • 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

19. Methodology (cont.) • Anomalous 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

20. Statistical Analysis

21. Statistical Analysis (cont.)

22. Statistical Analysis (cont.)

23. 40 stations

24. Northeast Heat WaveComposites Created using NCEP/NCAR Reanalysis Dataset (0000 & 1200 UTC)

25. Loop 1: Antecedent Conditions Composites are three-day averages centered on 7, 5, 3, & 1 day(s) prior to heat wave, 2nd day into heat wave

26. Antecedent Conditions(7 Days Prior) n=17 200 hPa heights & anomalies (dam) 700 hPa heights & anomalies (dam)

27. Antecedent Conditions(5 Days Prior) n=17 200 hPa heights & anomalies (dam) 700 hPa heights & anomalies (dam)

28. Antecedent Conditions(3 Days Prior) n=17 200 hPa heights & anomalies (dam) 700 hPa heights & anomalies (dam)

29. Antecedent Conditions(1 Day Prior) n=17 200 hPa heights & anomalies (dam) 700 hPa heights & anomalies (dam)

30. Antecedent Conditions(Onset) n=17 200 hPa heights & anomalies (dam) 700 hPa heights & anomalies (dam)

31. + - + 700 hPa height anomaly correlations (from Namias 1955) 700 hPa heights & anomalies (dam) 1 day before onset of heat wave

32. Loop 2: Synoptic Evolution of Heat Waves

33. Synoptic Evolution of Heat Waves(4 Days Prior) n=17 700 hPa heights (dam), Temp. (°C, red), & winds (m s-1) 200 hPa heights (dam), & winds (m s-1)

34. Synoptic Evolution of Heat Waves(2 Days Prior) n=17 700 hPa heights (dam), Temp. (°C, red), & winds (m s-1) 200 hPa heights (dam), & winds (m s-1)

35. Synoptic Evolution of Heat Waves (First Day) n=17 700 hPa heights (dam), Temp. (°C, red), & winds (m s-1) 200 hPa heights (dam), & winds (m s-1)

36. Synoptic Evolution of Heat Waves (Midpoint) n=17 700 hPa heights (dam), Temp. (°C, red), & winds (m s-1) 200 hPa heights (dam), & winds (m s-1)

37. Synoptic Evolution of Heat Waves(Last Day) n=17 700 hPa heights (dam), Temp. (°C, red), & winds (m s-1) 200 hPa heights (dam), & winds (m s-1)

38. Loop 3: Air Mass Transport

39. Synoptic Evolution of Heat Waves(4 Days Prior) n=17 850 hPa: 18°C isotherm

40. Synoptic Evolution of Heat Waves(2 Days Prior) n=17 850 hPa: 18°C isotherm

41. Synoptic Evolution of Heat Waves(First Day) n=17 850 hPa: 18°C isotherm

42. Synoptic Evolution of Heat Waves(Midpoint) n=17 850 hPa: 18°C isotherm

43. Synoptic Evolution of Heat Waves(Last Day) n=17 850 hPa: 18°C isotherm

44. Conclusions

45. Conclusions – Statistical Analysis • Negative trend in Northeast & Southeast hot days and heat waves turns positive • 1950’s, 1980’s, and 1990’s are peak in heat wave frequency • Although weaker, trends seen in other eastern regions except Southern • Trends east of Rockies match those found by DeGaetano and Allen (2002); Gaffen and Ross (1999)

46. Conclusions – Antecedent Conditions • Correlation between strong 700 hPa anticyclone in eastern Pacific and positive height anomalies over Northeast U.S. in agreement with past teleconnection research • Enhancement and eastward progression of ridge over North America coupled to amplification of ridge (trough) over Eastern Pacific (West Coast) • Origin of wave energy appears to be equatorial Pacific

47. Conclusions – Heat Wave Evolution • “Kicking trough” builds ridge in the East, displacing warm air mass eastward • Northeast located underneath both anticyclonic shear side of jet exit and downstream of ridge axis (favored area for subsidence) • Westward extension and intensification of Bermuda High allows for west-northwesterly (downsloping) flow into Northeast

49. Statistical Analysis (cont.)

50. 14 stations