September in the West
September showed characteristics typical of an autumn transition, with several low-pressure systems sweeping across the area bringing cooler temperatures and the first snow of the season for many locations. Temperatures were generally near or slightly cooler than normal west of the Rocky Mountains, and above normal precipitation was observed in a broad swath of the West stretching from San Diego, California, northeast to eastern Montana.
During the first week of the month, a cold low-pressure system brought precipitation, over an inch in some locations, and several inches of early season snowfall to some mountainous areas of the Inland Northwest. Further south, remnant moisture from Hurricane Newton impacted southeast Arizona and southwestern New Mexico on the 6th -8th. Tucson received 1.25 in (32 mm) on the 7th and a monthly total of 1.60 in (41 mm), 124% of normal. Much of the West’s above normal precipitation this month was associated with Hurricane Paine interacting with an upper level disturbance September 19th-23rd. San Diego observed 0.32 in (8 mm) for the month, all on the 19th-21st. This was 188% of normal and 13th wettest September since records began in 1939. In northern Arizona, Pipe Springs National Monument logged 2.87 in (73 mm), nearly all of which fell on the 21st-23rd. This was 295% of normal and the 2nd wettest September in a 54-year record. In the eastern Great Basin, Burley, Idaho, recorded 3.31 in (84 mm) precipitation for the month, 2.03 in (52 mm) of which fell on the 21st-23rd. This was 602% of normal and the wettest September since records began in 1948. Lewistown, Montana, observed 0.97 in (25 mm) related to Hurricane Paine remnants and a month total of 2.41 in (61 mm). This was 178% of normal and the 21st wettest September in a 121-year record.
Drier than normal conditions were observed in California, southern Oregon, eastern Washington, northern Idaho, and the western Great Basin, not atypical for this time of year. Wenatchee, Washington, recorded 0.05 in (1 mm), 15% of normal. Drier than normal conditions were also observed in Colorado and scattered areas of the Southwest, where Colorado Springs logged 0.16 in (4 mm), 13% of normal and tied for 5th driest September since records began in 1948. Areas of abnormally dry conditions or increasing drought severity were indicated this month in the southeastern Great Basin, central Utah, southern Idaho, along the Idaho-Montana-Wyoming border, and in portions of northern Colorado. Improvements in drought conditions this month were made across large areas of Arizona and New Mexico, eastern Utah, and southeastern Montana.
Much of the West observed slightly cooler than normal average temperatures for September with some pockets of temperatures more than 2 F (1.1 C) below normal. In southern Arizona, Ajo reported an average temperature of 83.3 F (28.5 C), 3.2 F (1.8 C) below normal. In western Nevada, Lovelock observed an average temperature of 61.6 F (16.4 C), 3.6 F (2 C) below normal and tied for the 10th coolest September on record. In the Pacific Northwest, temperatures at John Day, Oregon, averaged to 55.1 F (12.8 C), 4.7 F (2.6 C) below normal and the 7th coolest September since records began in 1903.
Areas east of the Rocky Mountains and portions of central and northern California saw slightly to well above normal temperatures. Mt. Shasta City in northern California recorded an average temperature of 62.4 F (16.9 C), 1.9 F (1.1 C) above normal and the 19th warmest since records began in 1948. In eastern Colorado, Pueblo observed an average temperature of 69.8 F (21 C), 5.1 F (2.8 C) above normal and the 5th warmest September in a 63-year record.
Many areas of Hawaii observed above normal precipitation this month, mostly related to weakening Hurricane Lester passing to the north of the Islands in the first week of the month as well as an upper level disturbance mid-month. Honolulu, Oahu, observed its second wettest September in a 77-year record. Honolulu received 2.92 in (74 mm) of rainfall, 417% of normal. On Maui, Kahului recorded 1.24 in (31 mm) for the month, 326% of normal and the 6th wettest September since records began in 1905. To the north, every major station in Alaska reported above normal temperatures for September. Along the Pacific Coast, Cold Bay observed its warmest September on record at 51.3 F (10.7 C), 3.2 F (1.8 C) above normal. In the Bering Sea, St. Paul Island also had its warmest September on record at 49.9 F (9.9 C), 4.6 F (2.6 C) above normal. Records for Cold Bay began in 1950 and St. Paul Island in 1892. Precipitation was near normal across most of the state, though in the interior of the state, Bettles observed its wettest September in a 66-year record with 5.34 in (136 mm), 280% of normal.
Significant Events for September 2016
September 25-27: Coastal California heat wave: During the last week of September, synoptic conditions favored significant heating over coastal central and southern California producing average temperatures 10-15 F (5-8 C) above normal. Peak high temperatures on the 26th reached 101 F (38.3 C) in San Diego, 104 F (40 C) in Los Angeles, 101 F (38.3 C) in Santa Barbara, and 96 F (35.6 C) in Monterey. While these high temperatures are not unprecedented, they are uncommon for the coastal area.
September 12-13: Flooding on windward side of Maui, Hawaii: Tropical moisture interacting with an upper level low-pressure system produced heavy rainfall. Some areas of windward Maui observed 5+ in (127+ mm) of rainfall during the event resulting in damaging floods, mudslides, and rockfalls that blocked roads and inundated homes.