Additional information and maps can be found at the Hydrometeorological Prediction Center's Web Page. About three percent of the country was much warmer than normal while less than one percent of the country was much cooler than normal. The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the temperature for each of 344 climate divisions in the U.S. by using their 1931-90 mean and standard deviation, then weighting these divisional values by area. These area-weighted values are then normalized over the period of record.
Positive values indicate warmer than the mean and negative values indicate cooler than the mean.
The preliminary June 1999 national standardized temperature index also averaged near the long-term mean. This warmth stretched to the landmass surrounding the Hudson Bay. A storm track and transient trough near the center of the continent brought unusually cool temperatures. A persistent trough over western Canada and the U.S. Pacific Northwest kept this region cooler than usual. Clouds and rain kept the southern plains and northern Mexico cooler than average and this rain was much needed in Mexico.