Forecasting Heat Waves Using Climatic Anomalies

The synoptic scale conditions associated with heat waves over the United States are characterized by a strong subtropical anticyclone. Studies in Europe have also revealed the importance of a strong and persistent anticyclone in association with summer heat waves. It is believed that the anticyclone produces subsidence leading to relatively cloud free conditions and a subsidence inversion which may help build and maintain the low-level heat. A recent study has revealed that heat waves produce more deaths than severe thunderstorms, tornadoes, and lightning. This implies the need to better forecast these more synoptically driven events.

A comparison of the 1995 and 1999 heat waves over the United States revealed the importance of the apparent temperature. During the July 1999 event, the apparent temperatures were lower than in July 1995 event. Not surprisingly, the 1995 event produced more fatalities. The apparent temperature is an accurate measure of how hot it really feels when mositure is added to the actual air temperature. The apparent temperature emphasizes the need to know the moisture content of the air associated with a heat wave.

Using reanalysis data, the characteristics of heat waves in the eastern United States were examined from 1948-2003. It was found that positive 850 hPa and 700 hPa temperature anomalies along with positive precipitable water anomalies were well correlated to heat waves. The latter is related to the apparent temperature. It is also understood that antecedent dry conditions may also be a necessary component of persistent heat, but this study addresses the actual synoptic patterns common to heat waves. Positive anomalies in the 500 hPa heights, 1000-500 hPa thickness, and 1000-850 hPa thickness values were also found to be good indicators of heat waves. Over the Mid-Atlantic region, low-level westerly wind anomalies were also shown to be associated with heat waves. These data suggest a synoptic scale thumb print can be obtained to define potential heat wave events. European heat waves shared similar characteristics although the precipitable water anomalies appeared not to be as significant in European events.

Ensemble forecasts from the National Centers for Environmental Prediction (NCEP) medium-range ensemble forecast system (MREF), are presented to show how the climatic anomalies can be used to forecast heat waves. The Great European heat wave of August 2003 is used as an example. During this event, which killed thousands of people in France, Germany, Spain, and Italy, the MREF forecast the thermal anomalies over Great Britain and western Europe with lead-times on the order of 5-7 days. This implies that the synoptic scale "thumb prints" associated with heat-waves are predictable with relatively long lead-times. This suggests that an extreme event forecast index should be developed to help better anticipate future heat waves.