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

Overview/Motivation Heat waves are a major contributor to weather-related fatalitiesHeat waves are a major contributor to weather-related fatalities From 1986 to 2004:From 1986 to 2004: Source: Severe weather event # of fatalities Hurricanes*323 Tornadoes960 Lightning1081 Floods1575 Extreme heat 2596

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

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)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)Published synoptic climatologies have been limited in scope to “Midwest” or Great Plains (e.g., Namias 1955, 1983; Chang & Wallace 1984)

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

Overview MethodologyMethodology –Initial –Revised ResultsResults –Northeast vs. regional heat wave statistics –Northeast summer heat wave composites Antecedent conditionsAntecedent conditions Evolution of heat wavesEvolution of heat waves ConclusionsConclusions

Methodology Temperature data were gathered from the National Climatic Data Center’s (NCDC) high resolution surface datasetTemperature 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)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 precipitationStations selected on basis of both dataset continuity and coverage within the NCDC’s Standard Regions for temperature and precipitation

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

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

Methodology (cont.) Temperature data is widely assumed to be normally distributedTemperature data is widely assumed to be normally distributed Los Angeles, CA and Denver, CO: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

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

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

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

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

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

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

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

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

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

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

June 1 – August 31, 1948 – 2001 Mean Mean +2σ Mean −2σ Denver, CO: Daily High Temperatures

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

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

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

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

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

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

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

Los Angeles:Los Angeles: –Large temperature anomalies more likely positive and associated with Santa Ana (offshore) events –Typically, marine layer moderates daily high temperatures Denver: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 explanationsNon-normal temperature distributions are common and have distinct physical explanations Methodology (cont.)

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