Western Regional Climate Center


September In The West

September was generally dry in the West with isolated pockets of high precipitation in the southern part of the region due to a strong mid-month monsoon surge. Temperatures ranged from near normal and slightly below normal on the coast and in the Southwest to record breaking in the Northwest.

Temperatures along the California and southern Oregon coast remained below normal as has been the case all summer. The Santa Barbara, California airport had one of the greatest negative temperature departures from normal this month in the West. Average temperature was 62.9 F (17.2 C), the 8th coolest September temperature in the record beginning in 1941.

Many high temperature records were set throughout the Northwest, especially along the Oregon-Washington border, Northern California, Idaho and Wyoming. Northern Nevada and Montana also saw several high temperature ties and records. Seattle-Tacoma Airport experienced a streak of nine consecutive 80 F (26.6 C) and greater days ending September 11, breaking a record of eight consecutive days in 1989. The average temperature for the month at Seattle-Tacoma was 64.0 F (17.8 C), the 5th warmest September on record since 1948. Pullman, Washington had a monthly average of 62.7 F (17 C), the 4th warmest September on a record beginning in 1941. Nearby Lewiston, Idaho experienced its 3rd warmest September on record with a monthly average of 69.4 F (20.7 C). Phoenix saw several high temperatures tied, as did Las Vegas; though near and slightly above normal temperatures prevailed in Arizona, New Mexico, southern Nevada, Utah and Colorado.

The Southwest monsoon season is typically taken as ending September 30, allowing rainfall totals for the season to be compared to previous years. The monsoon activity was slow for most of the season (since June 15). A final burst in mid-September brought record rainfall to Tucson, Arizona as well as significant precipitation to the Four Corners region into central Colorado. September 15 saw 2.83 in (71.9 mm) at the Tucson Airport, breaking a daily record of 1.18 in (29.9 mm) set in 1944. The same station received 5.6 in (142.2 mm) of precipitation for the month, the wettest September on record. The monsoon season total for Tucson, 8.62 in (218.9 mm), ranks as the 10th wettest in a record dating back to 1895.

Colorado Springs also experienced intense precipitation due to the monsoon surge. On September 14, Colorado Springs received a daily record 4.5 in (114.3 mm), shattering the previous record of 0.46 in (11.68 mm) set in 1967. Their September was 5.91 in (150.1 mm), surpassing the previous 1984 record of 5.01 in (127.2 mm) in a record dating from 1948.

Other Southwest locations did not fare as well as Tucson and saw lower than average monsoon precipitation for the month and season. Albuquerque Airport received 0.4 in (10.2 mm) for the month and 1.72 in (43.7 mm) total for the season, making it the 5th driest monsoon season on record in Albuquerque since 1892. Phoenix totaled 0.02 in (0.5 mm) for September, far below the 0.75 in (19 mm) average for the month. The season total of 1.6 in (40.6 mm) is also well below the average of 2.77 in (70.3 mm). The scattered nature of September’s precipitation in the Southwest allowed for drought conditions to persist in Arizona, New Mexico, and southern Colorado, with only slight improvements in southern Arizona and Colorado.

The Northwest, northern Nevada, and California remained generally dry and at below average precipitation values due to persistent upper level high pressure throughout the month that held precipitation to the north. Salem, Oregon received 0.35 in (8.9 mm) of total precipitation, the 9th driest September since records began at that location in 1893. Very few locations in the Northwest received above average precipitation. One such station, Quillayute on Washington’s Olympic Peninsula, received a total of 7.68 in (195 mm) for the month, the 9th wettest September at this location and 167% of the average rainfall of 4.59 in (116.6 mm).

Significant Events for August 2011

September (all month) Northwest Wildfires: Large acreages of lightning caused fires burned throughout the month in Montana, Oregon, and Idaho. In some areas, especially Missoula, MT, extensive smoke and haze to populated areas and reducing air quality and visibility.

September (all month), Severe drought: The Southwest experienced moderate to exceptional drought during the month of September, with only little relief provided by monsoon rains in the first half of the month. The severity of the drought was reduced in parts of Arizona and southern Colorado by the end of the month; severe drought conditions persist in New Mexico.

September 2011 Departure from Normal Temperature and Percent of Normal Precipitation for Western United States

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