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By the morning of the 5th, the high pressure center from the Great Lakes had begun to drift eastward. This resulted in reduced flow into the Penrose storm. The easterly component of the flow over the western part of the Great Plains weakened, and a more southerly component began to dominate. This reduced the lifting effect of the first upslopes of the Rockies; however, the moisture inflow was still sufficient to produce scattered rains in Colorado, New Mexico, Texas, and Oklahoma. The heaviest rains were occurring farther south in New Mexico and Texas, and were associated with a cold front that was moving into the region on the 5th and 6th. These rainfalls were not nearly as intense as those that had occurred in Colorado on the evening of the 3rd an~ the morning of the 4th. The High, which had been centered near the Great Lakes, continued to drift farther to the east, resulting in diminished strength of the moist airflow from the Gulf of Mexico northward. As the cold front moved through New Mexi co, Texas, and Oklahoma, it pushed out the final remnants of the moist easterly flow. The isohyetal pattern (fig. 2.5) shows rainfall centers in four states that exceeded 6 in. The centers are located at Penrose, CO (12 in.); Hope, NM (6.4 in.); Shattuck, OK (7.3 in.); and Plainview, TX (6.3 in.). A fifth center of 5.9 in. was located at Cimmaron, KS. Mass curves of rainfall for representative stations in the centers at Penrose, Hope, and Shattuck (fig. 2.5) indicate the differing natures of the precipitation in the different centers. The rainfall at the Penrose center, and other large amounts in Colorado, generally occurred over a relatively short duration (less than or equal to 24 hr). At Hope, Shattuck, and Plainview (mass curve not shown), the precipitation occurred over a longer time period, generally in excess of 48 hr. At Penrose, 87 percent of the total storm rainfall occurred in the maximum 6-hr period, while at other locations in Colorado with large precipitation amounts, the greatest 6-hr amount accounted for 60 to 85 percent of the total storm amount. The average of the greatest 6-hr amounts for Colorado stations was approximately 78 percent of the total storm rainfall. By contrast, in the other three centers of the storm, the ratios of the greatest 6-hr amounts to the total storm precipitation amounts are significantly less, being 29 percent at the Plainview, TX center, 31 percent at Hope, NM, and 47 percent at Shattuck, OK. Other reports of heavy rainfall outside of Colorado show 6-hr to total storm ratios ranging from approximately 20 to 74 percent. An average of these ratios outside of Colorado was approximately 46 percent. 2.4.1.3 Spriogbrook. Montana - June 17-21. 1921 (32). This was a large area extra tropical cyclone that occurred over eastern Montana and western North Dakota. The primary rainfall center occurred at Springbrook, MT where 15.1 in. of precipitation fell in approximately 100 hr. Over 85 percent of the total storm rainfall fell in a period of about 18 hr. The precipitation centers in North Dakota were considerably smaller; 5.3 in. at Powers Lake, ND and 4.9 in. at Beach, ND. At 0600 on June 17, a slow-moving cold front extended from eastern Montana southwestward through Arizona (fig. 2.6). Warm moist air from the Gulf of Mexico was being pumped northward by a high pressure system centered over Mississippi. A wave, which was forming on the front, was positioned in northeastern Arizona. The wave moved quickly northeastward along the front, and, by 0600 June 18, was situated in southeastern Wyoming with a warm front extending eastward along the South Dakota-Nebraska border. The moist unstable air from the Gulf of Mexico was lifted over the warm front and deflected around the Low in Wyoming. Convective activity was occurring in the vicinity of both the warm and cold fronts. 21