Major Windstorm On Saturday Looking More Likely

It now appears more likely than it did yesterday that the Puget Sound area will experience a major windstorm on Saturday.

Before we get into Saturday though, let’s take a quick look at what’s happening right now.

Heavy rain is currently falling throughout the Pacific Northwest. All of this rainfall is associated with a very moist atmospheric river plowing into the area driven by an intense 150+ knot jet stream off our coast. With such strong winds aloft, there has been significant orographic enhancement of precipitation over the Cascades and Olympics. The southwestern Olympics have gotten absolutely hammered by rain over the past day, and not surprisingly, the Skokomish river is currently under a flood warning.

Taking a look at satellite imagery, you can clearly see the abundance of moisture streaming into the Pacific Northwest. You can see two lows here, one mature low south of Haida Gwaii, and another low developing along the back side of the atmospheric river (it looks more like a “notch” than a fully developed cyclone.

These lows will combine to form one large, elongated low tonight. While two strong lows combining into one may seem frightening, this will actually weaken the storm. Extratropical cyclones derive their energy from horizontal pressure gradients, and generally speaking, the stronger the gradient, the stronger the storm. Extratropical storms actually help transport warm air poleward ahead of the cold front and bring down cooler, polar air behind the cold front, decreasing horizontal temperature gradients in the process. So when two extratropical storms merge, horizontal temperature gradients are decreased, and the resulting storm is weaker. Additionally, when you have only one low, it doesn’t have to “share” its energy source with the other and can therefore become stronger. Finally, when the lows merge, they create a large, broad cyclone with unremarkable pressure gradients.

Tonight’s storm still promises to bring storm force winds to the coast, and high wind warnings are up for not only the Washington, Oregon, and Northern Californian coasts but for Western Washington as well for gusts up to 55 mph. Additionally, the most recent WRF model run has this storm stronger than previous runs and delivers a second round of strong winds midday Friday as it crosses Central Vancouver Island.

One thing enhancing the possibility of high winds, particularly along the coast, are the very strong 80 knot winds at the 850 mb level of the atmosphere (approximately 5,000 feet), and their ability to mix down to the surface in strong gusts. Accordingly, the Medford NWS has issued the ultra-rare “hurricane force wind warning” for the Southern Oregon Coast, where the strongest 850 mb winds are.

Now, let’s talk about Saturday’s storm.

First, start by looking at the latest water vapor loop over the Eastern Pacific. There’s a TON of moisture currently streaming into the Pacific Northwest, but note the little “blob” of moisture to the south of Kodiak Island, near 155 degrees W and 45 degrees N. These are the remnants of Super Typhoon Songda, and will raise from the grave as a terrific extratropical cyclone off our coast on Saturday.

It is actually not all that unusual for typhoons to become entrained in the westerlies. When this occurs, devastating storms, such as the Bering Sea cyclone that sprung out of Super Typhoon Nuri, can occur. More locally, the Great Coastal Gale of 2007 was powered by Typhoons Mitag and Hagibis and was among the strongest windstorms on record for the northern Oregon/southern Washington coast. And of course, the Columbus Day Storm of 1962 sprung out of Typhoon Freida over the Pacific. So it is not remarkable in and of itself that Songda is transitioning to an extratropical cyclone.

What IS remarkable, however, is the strength of the jet stream over the Pacific. Models show a 200 knot jet stream over the Pacific. Don’t believe me? Look at this afternoon’s GFS model above, and look for the wind barb with four “flags” inside the purple swath in the center. With each flag  equal to 50 knots, that wind barb indicates 200 knot sustained winds at that location. This is absolutely incredible. I’ve never ever seen a wind barb with four flags on it. I know it is extremely difficult to see, but it’s there, trust me.

This is a perfect track for a major, regionwide windstorm. However, the low is not nearly deep enough to be a “historical” windstorm for any one area. Still, something like this could bring the strongest winds to Seattle since the Hanukkah Eve Storm of 2006 and for Portland since the major windstorm of December 12, 1995.