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shower activity, lay generally to the rear of the trailing front over the plains. This cloudiness persisted through the after- noon, and the reduction in insolation helped to maintain the thermal contrast across the trailing front. The 1920 GMT Sterling sounding (Fig. 13) was taken about 40 km to the rear of the trailing front and exhibited impor- tant differences from the Sterling and Den- ver morning soundings. The L.I. had de- creased from + 1 to -4, while the LFC had lowered 160 mb to 640 mb. Precipitable water contents of 0.78 in (1. 97 cm) in the lowest 150 mb layer and of 1.31 in (3.34 cm) in the surface-to-500 mb layer were almost double the July means for Denver. Winds above a temperature inversion (considered to be offronta] origin) at 720 mb were west- erlyat 10 to 20 kt (5-10 m S-l), indicating that Sterling was very near the upper ridge- line, Easterly low level flow had increased to 10 to 15 kt (5-8 m S-l). The Sterling sonde had sampled the air mass just behind the trailing front and found it to be condi- tionally very unstable with an unusually high moisture content. This air mass re- quired lifting of approximately 140 mb to release its instability, and was moving westward and southwestward toward the Colorado Front Range at is to 20 kt (8-10 m S-l). The 2000 GMT regional analysis (Fig. 14a) showed that the leading front had re- mained nearly stationary except over southwestern Kansas and southeastern Colorado. Veering winds and decreasing dewpoints at Garden City and Dodge City, Kansas, indicated that the leading front had retreated northward in this region. The trailing front had moved southward and westward and was about to overtake the leclding front across southern Kansas. Radar and satellite data (Figs. 14b and c) showed that convective cloud and thun- derstorm activity had continued to in- crease. The Utah squall line had intensified and was moving north-northeastward. Large thunderstorms had developed over 16 northwestern New Mexico and southwest- ern Colorado. An intense thunderstorm had formed along the leading frontal boundarv in southeastern Colorado. Small cumulus' clouds dotted most of the Plains and a line of small cumulonimbus and tow'- ering cumulus clouds had be!-,'l.lll to de- velop along the secondary frontal surge as it moved onto the Palmer Ridge. By 2200 GMT (see Fig. 15a) the trail- ing front had overtaken and reinforced the leading front across Kansas. It had become better defined with higher dewpoints and stronger easterly winds behind it. High temperatures in eastern Colorado m;d Kansas had helped trigger a line of thun- derstorms along and to the rear of the front. The large storm in eastern Colorado had a top indicated by radar (Fig. 15b) of 56,000 ft MSL (17 km). The storms in this line appeared nearly circular on the satellite photograph (Fig. 15c), which indicated that they had developed in a low wind shear environment. Surface pressure had continued to fall west of the Continental Divide, and the surface low pressure area was centered north of Grand Junction. The pressure at Rawlins, Wyoming, only 150 n mi (278 km) to the northeast, was holding 10 mb higher than that at Grand Junction. Thunderstorm activity was widespread over the \Vest with large, apparently intense storms indicated over northwestern New Ylexico, south- western Colorado, and central Utah. Some thunderstorms had developed over the north-central mountains in Colorado; how- ever, they remained well west of the Big Thompson drainage. A final sounding was taken at Sterling at 2202 GMT (Fig. 16) when the trailing front was located approximately 120 km t~ the west. During the 2 hand 40 min that had elapsed since the 1920 GMT sounding several important changes in the air mass had occurred. The mean vapor mixing ratio in the lowest 100 mb layer had decreased 1.3 g kg-! to 12.5 g kg-I. The L.I. had increased to a value of -2 (indicating more