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i TABLE XX--Upper level meteorological parameters associated with excessive two day snowfalls oniWolf Creek Pass. Storm events have produced 4.0 inches water equivalent per 48 hours at Wolf Creek West ancj/ or Wolf Creek Summit. ,", '--- 1w. C. West W. C. Summit 700 mb 500 mb 1700 mb 500 mb 700 mb 700 mb , Mean I I 2 Day 2 Day Wind Wind Wind Wind Mixing Equiv. 500 mb T~mp.l Convectiv, Date I Precip. Precip. Direction ~irection Speed Speed Ratio IFot. TemI Temp. Ad~eCtio Instability I (Inches) (Inches) (Degrees) (Degrees (MPS) (MPS) (GM/KGM) (0 K) (" K) (OC/12hr) (0 C) (Unseeded) ! 12-19-67 1. 97 1. 65 220 240 18 28 2.43 304.8 -22 7.; 1 3.9 12-20-67 2.11 I 2.10 I 220 220 17 28 3.00 306.4 ,-21 0 I 3.5 Total 4.08 3.75 , I : i Average 220 230 17.5 I 28 2.72 305.6 -21. 5 3.6 I 3.7 , ..-- (Seeded) I , 1-7-65 3.05 2.39 230 230 9 32 3.60 312.6 -13 0, 10.4 1-8-65 1. 52 .98 230 230 17 32 2. 99 311.9 1-18 0, 1.5 Total 4.57 3.37 I 12-6-66 2.43 3.03 260 280 22 29 4.42 317. 1 -12 8.'9 7.4 2-7-66 2.40 2.13 240 250 16 27 4.61 318.8 -14 3.' 1 3. 3 Total 4.83 5.16 ! ------ Average 240 248 16 30 3. 90 315. 1 -14.3 The mixing ratio at 700 mb is generally near 3.0 gm/kgm or higher during these storms. The combination of the relatively warm and moist condi- tions results in 700 mb eq uivalent potential tempera- tures of 3050 K to 3190 K during these storms. It is interesting that the unseeded two day storm contained cloud top temperatures (500 mb) that were several degrees colder than the two seeded storm events. The difference in the mean cloud top temperatures during the unseeded and seeded episodes is over 7 degrees. It is, of course, impossible to statistically test this result with the sample size available. The four winter seasons of 1964-68 were surveyed for continuous storm periods result- ing in a total of 3.5 inches water equivalent or more at Wolf Creek West and/or Wolf Creek Summit. Nine stormy periods met this criteria during the four years resulting in a frequency of a little more than two a winter season. Table XXI indicates the date and totals for these stormy episodes. From Table XIX it is seen that over half of the stormy regimes have occurred during December irl this particular four year study. However, with a sample taken over such a short time interval, it is probably unwise to place too much credence on the monthly distribution of these stormy episodes. It does appear that prolonged . snowstorms resulting in 3 to 8 feet of snow over a relatively few days can be expected about two times a year on the average at Wolf Creek Pass. , Table XXI also brir\.gs out another interesting point. During two-thiI'-ds of these stormy periods recording precipitation gages ceased to function correctly at one of the observing sites. It is of irlterest that the heaviest storm period was not seeded. Th~s pre-Christmas storm of 1967 was the result of the formation of a large, quasi-stationary upper level vortex centered generally over the area of Arizona and Southern California for several days. Because of its quasi- stationary nature abundant moistu~e was advected to the San Juan Barrier from off the Pacific coast of Southern California and Lower California. Favorable orographic flow persisted for several days resulting in 7. 5 to 8.0 incheg of water equivalent on the Pass. I E. Weather Modification Hydrology 1. Introduction Most of the stable f,low of streams in the Upper Colorado River Basin comes from snow melt. Flows during the snow melt period, normally April through August, account for 75 to 85% of total annual flow in Colorado River tributaries. If the flow that is delayed by passage through ground water is included, possibly 90% of the streamflow results from precipitation that fell as snow. Therefore, most of the effective precipitation for producing runoff falls in the winter and early spring months. Average runoff per'unit area varies from less than one inch annually in the lower elevation semi-':desert area up to over twenty inches 62 -I I , I I