October in the West
Temperatures were well above normal throughout the West this month. Widespread areas had temperatures 4-6 F (2-3 C) above normal, with even greater departures across the Intermountain West and coastal California. Precipitation was above normal in much of the Great Basin and Southwest, while coastal areas generally saw below normal rainfall.
An “inside slider” low-pressure system brought precipitation to the Northwest, Great Basin, Sierra Nevada, and the Southwest over the first week of October. The low then cut off from the large-scale flow, traversed southwest across Mexico, and returned to southern California on October 15-16, bringing moist air and another round of precipitation. During this and two other storm systems, Las Vegas, Nevada recorded 1.16 in (29 mm) of rain for its 3rd wettest October 1958-present. Northwest of Las Vegas, Pahrump, Nevada observed 2.48 in (63 mm) rainfall for the month, 958% of normal, their wettest October since consistent records began in 1959. Death Valley, California picked up 1.14 in (29 mm) for the month, its wettest October on record, 1628% of normal and roughly half of their typical annual precipitation. Rome, Oregon reported 2.08 in (53 mm) of rain, 346% of normal, the 5th wettest October in the station’s 66-year record. In southeastern New Mexico, Roswell received 4.63 in (118 mm) for the month, 346% of normal. Flagstaff, Arizona received 224% of normal October precipitation (3.72 in/94 mm). To the north, in Northeastern Montana, Glasgow received 1.67 in (42 mm) this month. This was the 4th wettest October in Glasgow’s 67-year record.
Many coastal locations remained dry or saw below average precipitation, not uncommon in this transition to the wet season. San Francisco, California saw no precipitation this month, which occurred in 12 other years in the station’s 167-year record. Drought conditions held steady for California this month and improvements were seen in Arizona, New Mexico, the southern portions of Utah and Idaho, northern Montana, and northwest Washington, generally in association with October’s areas of above normal rainfall. Small areas of deteriorating drought conditions were observed along the western Colorado-Wyoming border, where precipitation was 50% of normal or less this month.
Temperatures were exceptionally warm throughout the West this month. Though a few low-pressure systems traversed the area, anomalously strong high pressure dominated in between storm events. In southern California, many coastal locations from Monterey Bay to San Diego had a record warm October, likely in association with the anomalously very warm coastal waters. Temperatures at Santa Barbara averaged to 70.5 F (21.4 C), 8.9 F (4.9 C) above normal and 4 F (2 C) above the previous record set in October 1976 for the warmest in a 75-year record. In San Diego, the average October temperature was 74.4 F (23.5 C), 7.7 F (4.3 C) above normal and 2.2 F (1.2 C) above the previous record set in 1983. Records for San Diego began in 1939. To the north, temperatures at Boise, Idaho averaged to 59.7 F (15.3 C), beating out October 2014 for the second warmest on record and just shy of the 1988 record of 59.8 F (15.4 C). Boise saw its latest 90 F (32.2 C) reading on October 10; the previous record was October 3, 1963; records there began in 1940. Further east, in southeastern Wyoming, Rawlins reported an average 50.8 F (10.4 C) for the month, 7.5 F (4.2 C) above normal, the warmest October in a 65-year record.
Precipitation was highly variable across Alaska, with stations along the North Slope observing below normal precipitation and stations in the Interior and Southcentral regions ranging from <50% of normal to >150% of normal. In the Southeast, remnants of Tropical Storm Oho were entrained into a low-pressure system tracking towards Alaska, bringing copious rainfall to Ketchikan on October 7-9. On the 8th, 7.21 in (183 mm) of precipitation was observed, the 6th wettest day since records at Ketchikan began in 1910. Ketchikan recorded 30.64 in (778 mm) for the month, 159% of normal. Above normal temperatures were observed throughout Alaska, with the greatest departures in the Interior, Western, and Southcentral regions. King Salmon reported an average 42.8 F (6 C) for the month, 9.3 F (5.2 C) above normal and the 3rd warmest October since records began in 1917. To the southwest, above normal temperatures continued for some areas of Hawaii. Hilo recorded its warmest October on record at and average 78.7 F (25.9 C) for the month, 3.1 F (1.7 C) above normal. This makes October 2015 the 5th warmest month of all time at Hilo. September, August, and July 2015 hold the 1st, 2nd, and 4th place spots. Precipitation was variable across the state, with Lihue, Kauai in the far northwest observing its 2nd driest October on record at 0.36 in (9 mm), 9% of normal. Hilo, in the southeast, had its 6th wettest October on record with 15.84 in (402 mm), 162% of normal. Records for Lihue began in 1950 and records for Hilo in 1949.
Significant Events for October 2015
October 5th, 18th: Flooding in southern Nevada: Heavy precipitation caused localized flooding in downtown Las Vegas on the 5th. Later in the month, another storm brought high precipitation rates resulting in closure of a portion on Interstate 95, a major route between Reno and Las Vegas. Flooding in this area is more typical of the June-September monsoon season. October 15: Debris-laden flooding on Interstate 5 (Grapevine), California Highway 58 in southern California: Rain rates over 2 inches per hour were observed in association with a low-pressure system. Rain fell on steep areas of the Tehachapi and San Emigdio mountains where it was channeled by terrain and collected debris. The water and debris then inundated the steep alluvial fans and roadways in the area. October 15: California’s Folsom Lake reaches lowest level in 20+ years: This major reservoir was at just 17% of capacity and 31% of average level this month, the lowest since the 1987-1992 drought. The record low occurred in 1977.