PROSSER, Wash.– Although the calendar said September, the stormy weather at the end of last month looked more like November or December.
“The September 28 to 30 superstorm was impressive in its own right, and the fact that it happened so early in the autumn is extremely rare and quite remarkable,” said AgWeatherNet meteorologist Nic Loyd. “Luckily, this storm occurred at a time of low stream flows. Had these same rainfall totals been observed during the late winter or spring, much more widespread flooding would have ensued.”
Nearly three inches of rain fell at east Olympia during the first storm on Sept. 28, while winds gusted as high as 71 mph when the storm arrived in eastern Washington. As a second powerful storm front crossed the state on Sept. 29, more heavy rain and wind struck western Washington, while heavy snow blanketed the Cascade high country. Total storm rainfall of more than ten inches were recorded in the mountains, while East Olympia received a three-day total of nearly five inches.
The superstorm was only the final exclamation point on an active and dynamic month of September. An unusual severe weather outbreak on the evening of Sept. 15 had also produced heavy rainfall, high winds, and damaging hail in central Washington.
“Unfortunately, some corn fields in Franklin County were flattened and apples were blown off trees by the powerful 60-mile-per-hour winds,” said AgWeatherNet Director Gerrit Hoogenboom. “Luckily, however, the state’s apple crop exhibited good quality and was still expected to be one of the largest ever.”
Some AgWeatherNet stations in central Washington reported all-time-record 15-minute rainfall values of around one inch during the severe storms. Interestingly, the storms were preceded by unusually warm weather for mid-September. Earlier that day, College Place (near Walla Walla) recorded a high temperature of 101 degrees, which marked the latest date that any AgWeatherNet station has ever recorded a 100+ degree reading. Hot weather had dominated for much of the first half of September, although heavy rain associated with a cool upper low caused numerous debris flows and road closures of Highway 410 and the North Cascades Highway on Sept. 5.
Generally, the trend was warm weather early in the month, and then cloudy with cooler daytime temperatures in the second half of the month. In fact, the combination of the early heat with the cloudy and unsettled weather later in the month led to record warm low temperatures for September. Prosser’s monthly mean low temperature of 52.4 degrees was 4.5 degrees above average, and surpassed the old 1990 record by nearly one degree. It was the first time since 1998 that the average September low had been above 50 degrees. September also marked the second consecutive month of record warm nighttime temperatures. Prior to last month, three and a half years had passed since Prosser had experienced such a warm monthly low, relative to average.
Other meteorological highlights for September include the 1.86-inch rainfall at Puyallup on Sept. 6, the 1.4-inch rainfall at East Olympia on Sept. 22, and the 95-degree temperature recorded at Lynden (north of Bellingham) on Sept. 11. Overall, September was a warm, wet, and changeable weather month in Washington.