Rainy September falls short of a record

Local meteorologists were hoping for the rainiest September on record this year, but — to their chagrin — the total rainfall was about an inch short.

"We break dailies all the time, but if you break a monthly record, that's pretty good," said Cliff Mass, a University of Washington meteorologist.

Still, last month's 4.8 inches of rainfall were enough to give it the third-highest September total since 1945, according to Jeff Michalski, a meteorologist with the National Weather Service.

Where did that rain come from?

At the end of the month, a typhoon in Asia was followed by a large storm in the Pacific Ocean — which soaked British Columbia and Forks in Clallam County — and some of that moisture likely worked its way inward, Michalski said. He also said atmospheric rivers — large bands of clouds and moisture — stretched offshore and aimed right at Seattle.

In addition to that, Michalski said slow-moving storms came off the Oregon coast, lingering over the Seattle area long enough to drench it with precipitation — particularly on Sept. 17. That day was by far the rainiest of the month, with a recorded 1.49 inches at Sea-Tac Airport, the official reporting station for Seattle.

"It was hit or miss, and Sea-Tac got hit," Michalski said.

Mass chalked up September's rain to a front that was parked offshore and an airflow from the southwest that created what he called "a very significant anomaly."

Septembers usually have more sunny days, Mass said, but last month there was a pattern of persistent moisture.

"It's a chaotic, complex system that can get stuck," he said.

Michalski said the most rainfall for September is 5.95 inches, recorded in 1978. The second highest was 5.57 inches in 1969.

Michalski said it should be cooler and mainly dry on Saturday, but there's a chance of isolated showers on Sunday and Monday.

Carly Flandro: 206-464-2108 or cflandro@seattletimes.com