Forecasting The Heat
While not as dramatic as other kinds of severe weather, extreme heat can be a life-threatening condition. Because heat waves can often affect large numbers of people and wide geographical areas, special assistance is often needed.
Factors that contribute to heat waves are just as complex as those that contribute to severe thunderstorms, tornadoes or snowstorms. Like these phenomena, extreme heat is caused by different conditions in different parts of the country at different times of the year.
Forecasters often look for higher than average pressures in upper levels of the atmosphere. This often corresponds with a warm mass of air associated with an upper-level ridge where winds aloft flow in an anticyclonic (clockwise) direction.
When such a pattern becomes very pronounced, or "amplified," and continues for several days in the same general region, surface temperatures will likely rise to above average levels underneath the upper-level ridge.
Geography plays an important part in temperature forecasting. As air flows down a slope, it compresses as its altitude decreases and atmospheric pressure increases. This compression can act to increase the temperature of a parcel of air as the average speed of the air molecules increases.
Forecasters call this "downsloping," and heat waves can accompany a flow of wind that descends from higher elevation.
Heat waves in the Northeast often occur under a generally westerly flow of air, as extra heat is gained when air descends the eastern slopes of the Appalachians.
Along the West Coast, an easterly flow of air favors heat waves in normally cooler coastal locations. In California, this land breeze, or "offshore flow," carries hot, dry air from the Sacramento and San Joaquin Valleys and the California deserts into coastal regions.
This is "fire weather," and residents in this part of the country know to be particularly alert when gusty, dry, hot winds blow from the east.