Western Regional Climate Center


September in the West

September 2015

Well-above normal temperatures were present across southern California, the southern Great Basin, and the central and southern Rocky Mountain states. Temperatures were slightly cooler than normal in the Pacific Northwest. Remnants of tropical storms interacting with upper level low-pressure systems produced above normal precipitation in scattered areas of the West.

In the beginning of the month, a low-pressure system moved across the Northwest, bringing below normal temperatures and welcome precipitation for areas that saw onset of drought this past summer. Mid-month, the remnants of Hurricane Linda interacting with an upper level low brought precipitation to several areas. Near record precipitation was observed in Southern California, where Los Angeles had its 3rd wettest September at 2.39 in (61 mm); normal September precipitation at this location is 0.24 in (6 mm). All rain fell on the 15th, which became the second wettest September day since records began in 1877. Moisture was dragged northeastward, bringing precipitation to areas of Utah, Idaho, and western Montana in association with this moisture plume. Butte, Montana observed 2.36 in (60 mm) for the month, 236% of normal. Further west, Stanley, Idaho recorded 2.09 in (53 mm) for September, 227% of normal and the 4th wettest September since records began in 1916. Towards the end of the month, remnants of Tropical Depression 16E moved across the southwest boosting monthly precipitation totals across southeastern Arizona and New Mexico. Nogales, along the southern Arizona border, recorded 5.47 in (139 mm) this month, 360% of normal. This precipitation helped to erase drought conditions along the southern Arizona and New Mexico borders.

Some areas of the West received little precipitation, which is not unusual for September. Sacramento, California received no precipitation this month (normal is 0.35 in /9 mm), though this has occurred in 38 other years in Sacramento’s 139-year record. Several areas east of the Rockies saw a drier than normal September as well. Buffalo, in north-central Wyoming recorded 0.11 in (3 mm) for the month, 8% of normal. Areas of eastern Colorado and Wyoming moved into the US Drought Monitor’s “abnormally dry” category. No other areas of drought degradation were observed in the West this month.

After a three-month streak of above normal temperatures, a few low-pressure systems brought cooler air to the Pacific Northwest this month. Temperatures averaged to slightly cooler than normal for this region, with departures of 0-3 F (0-2 C) below normal. Elsewhere in the West, warmer temperatures dominated. In coastal southern California, temperatures at Santa Maria averaged to 71.2 F (21.8 C), 8 F (4.4 C) above normal and the second warmest month of all months in a 68-year record. The Great Basin also saw above normal temperatures. In eastern Nevada, Ely observed its warmest September on record at an average 62.3 F (16.8 C), 5.5 F (3 C) above normal. Temperatures were above normal in the central and southern Rocky Mountain states as well, especially east of the Rockies. Temperatures at Pueblo, Colorado averaged to 72.0 F (22.2 C), 7.3 F (4 C) above normal and the warmest September in a 62-year record. Every day this September had above normal average temperatures in Pueblo.

An active hurricane season in the central and eastern Pacific supported a wetter than normal September for much of the Hawaiian Islands. Record or near record September precipitation was observed at several locations. Honolulu had its wettest September on record at 4.48 in (114 mm), 2.08 in (53 mm) greater than the previous record set in 1947. Records for Honolulu began in 1940. Further north, temperatures across Alaska were slightly cooler than normal this month with the greatest departures in the Interior of 2-4 F (1-2 C) below normal.. Fairbanks reported an average 42.4 F (5.7 C), 2.5 F (1.4 C) below normal. Much of South Central and Interior Alaska were wetter than normal. Fairbanks and Anchorage both recorded their wettest Septembers on record at 3.74 in (95 mm), 340% of normal and 7.71 in (196 mm), 258% of normal, respectively. Records for Fairbanks began in 1929 and for Anchorage in 1952.

Significant Events for September 2015

September 14: Flooding in Southern Utah: Flash flooding associated with remnants of Hurricane Linda killed 13 in Hildale Utah when their cars were washed away by fast moving water. In nearby Zion National Park, 7 hikers trapped during flash flooding died in a narrow canyon resulting in a total 20 deaths from this event.

September (all month): Early snow for parts of Alaska: Unusually early snow fell in Fairbanks on the 13th and in Anchorage on the 18th, blanketing elevated terrain in these areas with several inches of snow. The last early occurrence of snow was 1992 in Fairbanks and 1993 in Anchorage.

September 30: End of Southwest Monsoon Season: The monsoon season, which began on June 15, was average this year with most of Arizona and New Mexico receiving 75-125% of normal precipitation. Much of the precipitation did not result from a traditional Southwest Monsoon pattern setting up in the atmosphere and bringing moisture from the Gulf of Mexico, but from the availability of moisture from the remnants of tropical storms.

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