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ARKANSAS RIVER BASIN, COLORADO. KANSAS, NEW MEXICO D11 Rainfall in the amounts and intensities that occurred * . . on June 16 usually requires some persistence of several conditions. There must be (1) large amounts of low.level mois'ture and a strong influx of this moisture to supply the rain-producing mechanism continuously, (2) un- stable atmospheric conditions, particularly at upper levels, and (3) one or more mechanisms to lift the air. All of these conditions were present on June 16 (U.S. Weather Bureau, written corn- mun" 1966). The air movement near the surface was one of the contributing factors. There were moderate winds from the southeast June 14-18 bringing moist air from the Gulf of Mexico, and surface dewpoints were in the low 60's and upper 50's (OF), which are unusually high for eastern Colorado. The influx of moisture was rapid, and the moist air was in a rather deep layer. The low-level southeasterly flow was particularly strong on June 16; wind speeds of over 40 knots near 2,000 feet above the ground were reported at Amarillo, Tex., and Dodge City, Kans. The surface wind at Amarillo began gusting during the afternoon of June 16. The low-level flow had the characteristics of the low-level jet in this region, but the relatively high winds at the ground produced the unusual condition of a low-level jet in depth, A trough over the Western United States was retarded and intensified at the 500 mb (millibar) level at approximately 18,000 feet, or 5,500 meters, By June 16, a quasi-stationary cold low had been created at 500 rob over southern Nevada. "'.. '" This situation brought in cold air aloft which reduced the atmospheric stability to moderately low levels. The air circulation about the low produced some lifting. This was not a prime factor in caus- ing rain, but it was significant. The general upslope from east to west of the High Plains caused a major uplift because the low-level wind had an easterly, or upslope, component. The orographic effect of small-scale features'" '" .. caused shower activity" '" .. early in the storm, Showers and thunderstorms started to develop over the area east of the'" III '" mountains dur- ing the morning of June 16. By early afternoon the storms were located along a north-south line roughly from Denver to Pueblo. .. '" .. The upper level steering winds had only a slight westerly component; therefore the thunderstorms were not carried away from the mountains, as generally happens, but remained over the high-rainfall areas for more than an hour" " III. More than 14 inches of rain fell near Palmer Lake and near Larkspur in about 4 hours * '" *. By 1800 hours the westerly component of the upper level steering winds increased and moved the storm line eastward. - RAINFALL ON JUNE 17 Activity increased in southeastern Colorado and northeastern New Mex- ico on June 17. The U.S. Weather Bureau aI Amarillo, Tex., reported moderate thunderstorms with tops at 30,000 or 40,000 feet over north- eastern New Mexico and the Texas and Oklahoma Panhandles; one cell near Dalhart, Tex., had a top at 50,000 feet. Steering-level winds were from the southwest, so there was a more westerly componenI than on June 16. The storm cells that developed on the afIernoon of June 17 caused heavy rains in some areas for the second consecutive day. Already swollen rivers were pushed to destructive stages, and more long-term rainfall and dis- charge records were exceeded. Rainfall amounts occurred which had never before been recorded in some areas. Fourteen inches of rain fell June 17 northeast of Colorado Springs, Colo. The 2-day total rainfall of more than 10 inches on June 16-17 at weather staIion Two Buttes 1NW (pI. 2) exceeded the maximum recorded for any month since recordkeeping began in 1890, and the monthly total for Il1n"'" ""'y.....p""n""'n th"", nrf",v;nm: mnnthlv rprorei hv more than 5 inches: 7.86