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O1APTER 3 METEOROLOGICAL CONDITIONS AND FORECASTS During the past 25 years meteorologists have come to recognize that important atmospheric processes occur over a wide spectrum of interacting scales. The conditions contributing to the Big Thompson Canyon flash flood were no exception--large or synoptic scale conditions for substantial convective activity were present, but all data from radars, satellites, and surface observations indicate that small or meso-scale features, several hundred kilometers or less in size, were significant factors in triggering the very heavy rainfall. Unfortunately, there are substantial gaps in our knowledge and understanding of meso-scale meteorology. Synoptic Scale Features A nearly stationary upper atmospheric high pressure ridge with very light winds had persisted over Arizona, Utah, New Mexico, and Colorado for several days prior to the flood. Moisture at the 500 mb (50 kPa) level, about 6 kilometers above sea level, had been high over Colorado since 6:00 a.m. on July 29. Relative humidities from the surface to 500 mb had remained well below 50% over Colorado until 6:00 a.m. on July 31, when Grand Junction and Denver reported values of 68% and 82% respectively. Precipitable water values (another measure of atmospheric moisture) gradually doubled from just above one-half inch at 6:00 a.m. on July 29, to slightly more than one inch at 6:00 a.m. on July 31. This increase was unusual but not unprecedented for summer months. Lifted index values, showing atmospheric stability, had remained near zero throughout the period, indicating that the air mass was conditionally unstable and that thunder- storms would occur if deep convection were initiated by afternoon heating or orographic lifting. During the 36-hour period preceding the flood, low level moisture began to concentrate in a zone about 200 miles (320 km) wide on the north side of the cold front and advect westward from the central plains. Surface dew points in central Colorado were generally in the 40's at 6:00 a,m. on July 30. These near-normal values had risen into the 50's 24 hours later. Dew points in northeastern Colorado rose from the 50's into the 60's during the same period. The rise in dew point values was greatest in Colorado, western Kansas, southeastern Wyoming, and eastern Utah. By 6:00 a.m. on July 31, surface dew point values in eastern Colorado were 10 to 150F above July- August normals. Surface pressures rose in the northern plains, moving a weak cold front slowly southward through the central plains. By 6:00 a.m. on July 31, the front extended from north-central Illinois across central Missouri into western Kansas and eastern Colorado. Easterly winds in the lower levels on the north side of the front continued to blow upslope toward the Front Range of the Ro~ky Mountains. 27