WHIFF: A lack of windstorms in Oregon this season

Weather

Portland usually sees about 7-8 wind advisories in the winter; we've only seen two so far this season

PORTLAND, Ore. (KOIN) – Can we call it winter without a Pacific Northwest windstorm?

We may just be gearing up for a busy spring, but there is no doubt that dating back to Oc. 1, 2019, we have only had a few windstorm possibilities for Oregon and Washington. If you’ve been keeping count, you probably know that only two wind advisories have been issued by the National Weather Service in Portland and not one high wind warning.

How does that match up with previous years? If we just take a sample size of a decade, it is below average. This data is based on the fiscal year period of Oct. 1-Sept. 30. Yes, we still have to work our way through the end of winter and then the spring/summer months, but our windstorm activity drops significantly in the summer. That means the numbers may change by the time we wrap up March (which is a month that likes to bring the wind).

You may be asking what the threshold is for the wind alerts in Portland? Wind advisories are typically issued when the forecast calls for sustained wind speeds at 26 kt / 30 mph and gusts to 39 kt / 45 mph. How about high wind warnings? Sustained wind speeds of 35 kt / 40 mph and gusts at 50 kt / 58 mph. Over the last decade, an average of 7.5 wind advisories and 0.7 high wind warnings per year in the zone for Portland.

The last wind advisory that was issued in Portland was on Jan. 10, 2020, and it was a six-hour period from 1:00 p.m. to 7:02 p.m. in the evening. Below you can see the tweet that was sent out from the NWS Portland account canceling that wind advisory for the interior locations.

NWS canceling JAN 10 Wind Advisory (Second Wind Advisory of the season)

If you were to guess what our strongest wind gust at Portland International Airport has been this season, you may have gone above 44 mph. However, that is all we have recorded since the first of October. It happened twice, once in October and most recently the morning of Sunday, Feb. 23. Wind in that threshold is strong enough to cause damage to power and trees. That is what occurred on Sunday morning just outside of Vancouver.

What I find impressive is the lack of strong wind in December. A max wind gust of just 37 mph! You can see the max wind gust broken down by month in the graphic below. I’ve included the month of September 2019 but it is not included in the NWS information above.

MAX WIND GUST BY MONTH

What generates windstorms in the Pacific Northwest? We are typically tracking extratropical cyclones. These are the main culprits for our windstorms and are different from tropical cyclones. With our location in the mid-latitudes and off the cooler water of the Pacific Ocean, our windstorms do not develop like those of a hurricane. They may possess the ability to create hurricane-force wind (+74 mph) from time to time. An extratropical cyclone can be boiled down to the clash of air masses (warm vs cold) and the change in atmospheric pressure. That imbalance is the cause of our windstorms. As the pressure falls rapidly and the tracking of that low-pressure moves closer to the PNW coast, we feel the impact at the surface.

GENERIC EXTRATROPICAL CYCLONE SETUP

With that in mind, here are two of the surface charts for the wind events that did call for wind advisories in the Portland metro area. The event on Jan. 10 will resemble our traditional setup for a wind event in Portland, whereas the surface chart on Oct. 29 is slightly different. The contrast from the high pressure to the east with the extended backdoor cold front created an unstable atmosphere and strong wind funneling through the Columbia River Gorge.

Now there was one major system that could’ve produced very strong wind for Portland and that was the bomb cyclone on Nov. 27, 2019. The rapidly deepening extratropical cyclone took a path that landed around the border of Oregon and California. The route of that area of low pressure was not conducive for hazardous wind around Portland, but it was for southern Oregon and northern California. That system brought a wind gust of 106 mph at Cape Blanco and bottomed out with a low pressure of 974.6 hPa – 28.78″ in Oregon at Gold Beach.

The bomb cyclone was very impressive from the visible satellite (image below). You can spot the traditional comma shape with the dry air slot and the occlusion stage. Truly a beautiful scene that shows off the ability of mother nature. These bomb cyclones are not very common and that goes for the one that developed in November here in the PNW. According to the National Weather Service in Medford, “storms of this magnitude have not been observed on this track in the last 15 to 20 years or more.” Bombogenesis mostly occurs in transition months, where cooler air is met with slots of warm air that is still infiltrating the atmosphere as we discussed above.

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