Hansen, E. Marshall

Three cases of extreme local precipitation within the Intermountain summer season were studied to determine the source of moisture for these events. The rains occurred at Phoenix, Ariz. on June 22, 1972 (133 mm in 2 hours), at Elko, Nev. on August 27, 1970 (105 mm in about 2 hours), and at Morgan, Utah on August 16, 1958 (about 150 rnm in 1 hour). Synoptic data were used to analyze surface and upper levelmoisture changes in time and space. In each case the study showed that a tongue of high moisture at low levels approached the vicinity of or encompassed the storm area prior to onset of the rain. The tongue of moisture was very narrow in reaching toward the Elko and Morgan storms and could be traced, through continuity of changes in pattern with time, back to the Gulf of California. The moisture is believed to be conveyed through the natural channel provided by the Gulf and the paralleling ridges. The low-level moisture followed a path controlled to some extent by major mountain barriers. A general conclusion is that greater emphasis should be given to tropical Pacific moisture in evaluating extreme summer precipitation values for the Intermountain region.

National Weather Service (NWS)