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The upper-level high-pressure system centered over southern Colorado and northern New Mexico during August, continued to keep monsoon moisture out of northern New Mexico during the first week in September.
Then, beginning on Sept. 9, an upper-level low-pressure system from the Pacific Northwest began to sink into the Great Basin states, moving the upper-level high-pressure system to the east of New Mexico and Colorado.
This combination of stationary low pressure to our west and high pressure to our east produced a deep layer of southerly winds, bringing moisture from both the Pacific Ocean and the Gulf of Mexico, into New Mexico, according to LANL meteorologist Jean DeWart.
From Sept. 10-15, Los Alamos received over 7 inches of rain, greater than most cities in Oregon received for the entire month of September. (Oregon has also experienced a record September rainfall total, but due to early winter storms, not monsoon storms.)
Sept. 13 was the wettest calendar day in Los Alamos records with 3.52 inches of rain.
The previous record was 3.48 inches, received on October 5, 1911. For the 24-hour period from noon on September 12 through noon on September 13, Los Alamos received 5.08 inches. For the 4-day period of Sept. 10 to 14, Los Alamos received 6.69 inches of rain, a return period of rainfall of over 1000 years.