A Review of November’s Windstorm

On November 13, the first windstorm of the 2017-2018 storm season rolled through the area. While this was not a major windstorm by any means, it delivered quite a blow to the coast, Northern Interior and Puget Sound lowlands.

This storm was unusual for several reasons. First, there were hundreds and hundreds of lightning strikes off the coast with this storm due to an intense squall line along its cold front. This squall line rolled through Seattle a little past 2 AM on the morning of 11/13, delivering a brief burst of strong winds and heavy rain that woke up many across the region. While having embedded squall lines in Pacific Northwest windstorms is not rare (the Hanukkah Eve Windstorm of 2006 is a prime example—during this storm the Leschi neighborhood saw approximately an inch of rain from 4-5pm and severe urban flooding took the life of a woman in Madison Valley), having so much lightning from them is.

Second, this storm actually had two areas of low pressure embedded in it that were circling around each other. Low-pressure systems that have this feature are often called “double barrel lows,” and the existence of two low-pressure centers typically hinders storm development and can dramatically alter its track. The November 13 windstorm was originally headed north of Vancouver Island from the SSW, but it stalled a little over 300 miles off the mouth of the Columbia River and subsequently began tracking to the WNW into Southern Vancouver Island.

The highest winds from this storm came from the storm’s “bent-back occlusion,” which rolled into the Seattle area around 5pm. An occluded front is a front formed when the storm’s cold front overtakes its warm front, and a bent-back occlusion is an occluded front that has been wrapped around the center of the storm. Over 60,000 people from Seattle City Light lost power, and some of the more exposed locations recorded 50+ mph winds. The 520 bridge gusted to 52 mph, and West Point near Magnolia, which commonly experiences some of the highest winds during Puget Sound windstorms due to the point accelerating winds coming in off the sound, had a peak gust of 64 mph. “Kelp Reefs,” a Canadian station in the Haro Strait just west of San Juan Island, was the windiest location in the lowlands with an impressive 88 mph gust.