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stretched from the Great Lakes through Kansas and Colorado northward across centml Montana, The leading li.out was characterized hy wind shifts and a pressure trough, whereas the tmiJing front was marked hy 'I less pronounced pressure trough, relatively strong tempemture gra- dients, and an increase in wind speed, Thermal packing was most pronounced along and to the rear of the tmiling li'ont across Kansas and Nehraska, Dewpoiut temperatures were high with values of 600F (1.5,50C) extending northwestward from Kansas into Colorado and Nehraska. A band of very moist air lay just to the rear of the trailing front where dewpoints of 20 650F (180C) had moved into southwestern Nehraska, Early morning shower and thundershower activity was oc- curring from Missouri to western South Dakota and also over much of the inter- mountain "Vest. Upper-air features were dominated hy a large, negatively tilted or "hent h'lck" ridge (the ridgeline sloped from NNW to SSE), which extended from southern Texas to western Canada, A closed high was present over the Centml Plains. Moist conditions were present over the inter- mountain \Vest and eastern slopes of the Rockies from 700 through 300 miL A weak short-wave trough at 700 mh (Fig. 5) arced from near Salt Lake City, Utah, to EI Paso, Texas, Light southeast- erly winds were present over southwest- ern Nebraska, northwestern Kansas, and much of eastern Colorado, Hot, dry condi- tions at 700 mh over the southern plains indicated that deep convection would probably be suppressed south of the polar air lTIass. Two weak 500 mh (Fig, 6) short-wave troughs-one over Mexico and another over Arizona and New Mexico--were im- hedded in the southerly flow west of the ridgeline, A broad area of falling heights with a weak fall center over the Four Cor- ners 'lI:ea was associated with these troughs, A small, closed anticyclonic circu- lation over the Colorado mountains pro- duced westerlv winds at 500 mh over Den- ver while win~ls were southerly at Grand ]unctiOlL At 300 mh (Fig, 7) winds were light southerly over Colorado while stronger (20 to 45 kt-1O to 23 m s-') south to south-southeasterlv flow extended from Baja California north~ard to Utah. The LFM 500 mh vorticity analysis (Fig. 8a) defined a weak vorticity maximum over New Mexico, The 12 h forecast (Fig, 8b) indicated that the "hent hack" ridge- line would move slightly eastward during the day, allowing weak south-southeasterly flow to hecome estahlished over the Front R.lI1ge, The two weak short waves merged into a single trough that was forecast to extend from southeastern Idaho to west Texas, "Veak positive vorticity advection was forecast to occur over most of the Rocky Mountain region during the day, which would coutrihute to further de- stahilization of the air mass, Analvses of the Totals and Lifted Indi- ces (Fig, 9) depicted a potential for moder- ate to heavy thunderstorm activity over northern Arizona, much of Utah and Nevada, and from western Kansas into northeastern Colorado, The Totals Index is defined as 2(TR,0 - T,oo) - DR"" where T B50 is the 850 mh temperature, T,oo is the 500 mh temperature, and DB,o is the 850 mh dewpoint depression, all expressed in oC. Values of this index 20 46 reflect filVorahle conditions for thunderstorm development, and values 2050 indicate a potential for moderate to heavy storm activity, (See Mill- er, 1972, for a more complete discussion of this index,) The Lifted Index (LL) was computed hy lifting a parcel possessing the mean thermodynamic characteristics of the lowest 100 mh layer adiahatically to 500 mh, and then suhtracting its temperature from the environmental temperature at that leveL The 1200 GMT Denver sounding (Fig, 10) was very moist (average vapor mixing ratio for the lowest 100 mh layer was 12,0 g kg-') helow a temperature inversion at 670 mh, \Vinds ahove the inversion were light and variahle, and winds in the cool air mass .~. 5