Western Regional Climate Center


September In The West

Continuing this summer’s theme of record heat and dry conditions, September 2012 saw above normal temperatures and below normal precipitation for much of the West. Late season monsoon activity over the southern Great Basin and Mojave Desert and a duo of storms hitting south-central Alaska brought locally heavy precipitation, while other locations remained parched.

Following its warmest August on record, Reno, Nevada experienced its warmest September as well with a monthly average temperature of 70.7 F (21.5 C). This value narrowly eclipses the previous record of 70.6 F (21.4 C) set in 2011. Reno’s record began in 1888. In California’s Central Valley, Fresno also saw consecutive August-September record average monthly temperatures. Fresno’s temperature this month averaged to 81.3 F (27.4 C), the highest September mean on a record began in 1878. Further south, Needles and Death Valley, California also logged their warmest average September temperatures at 91.3 F (32.9 C) and 96.3 F (35.7 C), respectively.

In southern Alaska, cool and wet conditions dominated the month. Two strong storms brought heavy precipitation, cool temperatures, and high winds to the region. An impressive 26.16 in (664.5 mm) of rain fell in Valdez this month, 16.54 in (420.1 mm) above average. This total sets both the September record and the record for wettest calendar month in Valdez. The previous wettest month was 20.59 in (523 mm) in November, 1976; records at Valdez began in 1949. The two storms resulted in widespread flooding throughout south-central Alaska. The Nenana river reached its highest level on record, reducing the Parks Highway to one lane. Sustained winds of 40-50 mph (64-80 kph) were observed around Anchorage, with gusts up to 70-80 mph (112-129 kph). Due to full foliage and moist soil, the strong winds downed trees throughout the region causing power outages, closed roads, and structure damage.

The North American monsoon season (June 15-Sept 30) wrapped up this month, with many locations receiving near or above their normal monsoon precipitation totals. Las Vegas, Nevada received 1.18 in (30 mm) on September 11th, the location’s wettest September day on record. The downpours resulted in extensive flooding in the Las Vegas area. The month ranks as the 5th wettest in a record that began in 1937. In northern Arizona, Bagdad received 10 in (254 mm) for the monsoon season, 187% of normal for the station. Elsewhere in the region, Prescott and Flagstaff received 76% and 100% of their average Monsoon precipitation. Albuquerque, New Mexico finished the season at 72% of normal. September 2012 was the 5th wettest at Denver, Colorado, with a total of 2.95 in (74.9 mm) of rainfall. It was also the 12th consecutive September with no snowfall in Denver. The other 12-year stretch of no September snowfall occurred from 1914-1926.

Precipitation was lacking elsewhere in the West, with many Pacific Northwest locations noting their driest Septembers on record and extended periods with no measurable precipitation. Both Billings and Missoula, Montana received only trace precipitation, their driest Septembers on records beginning in 1934 and 1893, respectively. At Missoula, it was only the second time in the station’s history that any month in the year received no measurable precipitation. Sheridan, Wyoming also recorded only trace precipitation in September tying the driest on record. Beginning the 11th of August, 51 consecutive days passed at Sheridan without measurable precipitation, the longest dry period in a record beginning in 1920. In Washington, Spokane, Bellingham, and Olympia all set or tied their driest years on record. Dry conditions also dominated Hawaii, where Lihue, Kauai, received only 35% of its normal September rainfall. Precipitation at Lihue has been below average nine of eleven months of the current water year.

Significant Events for September 2012

Northwest Fires: With dry fuels, high temperatures, and low relative humidity prevailing throughout the month, many large wildfires ignited or grew throughout September. Smoke from the fires lead to poor air quality in eastern Washington and Oregon, northern Idaho, western Montana, and Northern California. One of the largest, at 339,110 acres (137,233 hectares) by month’s end, was the Mustang Fire on the Idaho-Montana border. The North Pass Fire, 10 miles northeast of Covelo, CA has burned 41,983 acres (16,990 hectares) and consumed 26 structures. The national average of large fire events for 2012 remains at 77% of the 10-year average, while acreage burned this year is at 129% of the recent average.

September 11: Las Vegas area flooding: A band of thunderstorms brought heavy precipitation to the Las Vegas Valley and surrounding areas. Flooding closed roads, swamped cars, damaged homes and businesses, and resulted in one fatality. Flood damages in Las Vegas are estimated at $75 million. Southwest Utah, northeastern Arizona, and southeastern California also experienced heavy precipitation and damaging flash floods.

September 2012 Departure from Normal Temperature and Percent of Normal Precipitation for Western United States

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