A Statistical analysis and synoptic climatology of heat waves over the United States

Heat waves remain a relatively understudied weather phenomenon within the meteorological research community. This is surprising given the large number of heat wave related deaths that occur annually. With the goal of improving the forecasts of these high-impact weather events, this study presents a synoptic climatology of regional heat waves in the United States. A statistical analysis detailing the frequency and duration of heat waves will also be presented.

Heat waves were identified using a 54-station dataset of daily high temperatures over a 54-year period from January 1948 to December 2001. These data were retrieved from the National Climatic Data Center (NCDC) Daily Surface Dataset. Stations were distributed equally across the conterminous United States and separated for analysis purposes into the nine standard climate regions defined by NCDC. A persistent high temperature anomaly was considered to be a heat wave if, for at least three consecutive days, high temperatures at a station exceeded the climatological 97.5 percentile temperature (approximating a plus two standard deviation anomaly). Percentiles were used in place of standard deviation anomalies because it was found that high temperatures in most cities are not normally distributed. Heat waves were defined as regional when they occurred simultaneously at two or more cities within one of the NCDC regions. In order to identify temporal trends in heat wave frequency, annual and decadal time series of both high temperature events and all days exceeding the 97.5 percentile were also prepared.

Statistical analysis shows that, for most cities, high temperatures more than two standard deviations below normal are more frequent than daily highs more than two standard deviations above normal. This result reflects the well-known tendency for cold spells to be shorter but stronger than warm spells and is nicely seen at Denver where severe cold tends to be associated with upslope conditions. An exception is Los Angeles where abnormally high temperatures are associated with downslope offshore flow.

Preliminary research also indicates that heat waves are most frequent in the western, mid-western and southeastern regions of the country, with the fewest heat waves occurring in the Northwest and Northeast. A time series analysis indicates a decreased (increased) frequency of summer heat waves in the Northeast and Mid-West (Southeast and Southwest). Composite analyses will be used to show regional differences in the synoptic-scale structure and evolution of heat waves.