Thursday, April 6, 2017

Major Windstorm Heading for Pacific Northwest Coastal Waters

UPDATE at 10 PM Thursday

Confidence is now very high that an unusually strong coastal storm will move northward over the Pacific Northwest coastal waters on Friday morning before it makes landfall on northern Vancouver Island.

The Oregon coast will get hit particularly hard, with gusts reaching 50-70 mph. The latest European Center model forecast for 11 AM (below), shows a large low center due east of Astoria with a central pressure of 973 hPa (or mb).  This is extraordinary low pressure for April and a rare event.  Gusts are shown in color and reach 60-70 knots along the Oregon/southern WA coast.  This means power outages along the coast.

How unusual is this event?   Well, meteorologists have statistical tools to help us with this question.  One way is to use standardized anomalies, in which we look at the collections of all pressures for a long period (at least several decades) at a point and determine the pressure anomaly (how different the forecast pressure is from normal).  Then we divide the anomaly by the standard deviation of pressure at that point.  The standard deviation (abbreviated by the Greek letter sigma) is a measure of the variability of the pressure over the period of record. This process is repeated at many points to create a map of such standardized anomalies.

The larger the standardized anomaly, the more unusual the anomaly from normal.

Here is the standardized anomaly for the pressure forecast for 5 AM Friday.  For the storm center, the standardized anomaly peaks at about 5.9 (or 5.9 sigma)....this is very large, signifying that such low pressure at the point would be expected only once in seveal million hours (assuming hourly data was used and the distribution was near normal).  Bottom line:  this is a unique, first time event.

Now that I have convinced you that this is an unusual event, let's look at some forecasts.  The sustained (several minute average) wind for 8 AM Friday, show winds reaching 50 kt on the central Oregon coasts (again gusts are higher).


This storm will not only produce strong winds, but substantial wave action.  Here are the wave height forecasts from the NOAA/NWS WaveWatch III model... the yellow indicates 8-9 meter (26-30 ft) waves.  The storm/wave watching on the coast will be impressive (but stay safe if you are there)

As the storm moves up the coast, the winds will spread to the Washington coast.  At 2 PM (below), sustained winds will reach 40 kts on the southern WA coast, and Seattle will be blustery with sustained 25-30 knot winds.  Gusts could higher(40-50 mph, particularly near the water).


Fortunately, with a cold winter, the leafing out of the trees has been delayed, which will lessen the damage.  And end of winter storms generally do less damage (because a lot of wind pruning has already occurred).

Finally, how confident am I in this forecast?  Let's turn to an ensemble (many forecasts) system...the NWS SREF.  The SREF ensemble prediction for wind at Astoria, OR suggests that most of the ensemble members have a similar solution of fairly strong wind.


Other ensemble systems suggest the same thing, with nearly all forecasts having a major storm off our coast.  Much more confidence than the ill-fated and much smaller October 15, 2016 storm.

I suspect there still will be some limited power outages around Puget Sound, particularly since the ground is so saturated.

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16 comments:

Tom Wilson said...

Will Puget Sound feel any of this?

Beck Tench said...

Hi, Cliff - any idea how long the winds will last?

Paul England said...

Re: This is extraordinary low pressure for April and a rare event.

Is there a simple way of understanding why winters are stormy and summers are calm in the NW? At first sight it would seem that a lot more energy is being pumped into the system in summertime.

Buddy said...

Cliff I think this has the potential to be very serious. I didn't think the windstorm in October was a "bust." The track and location of the low was forecasted very well a couple days out. But like an eye of a hurricane, the structure of the low can evolve in mysterious ways. This morning it's becoming a little more negatively tilted, potentially driving that poisenous tail into the coast. Maybe making those classic SE sucker winds thru the Sound stronger too.

Eric Blair said...

I'd be interested in hearing how unusual the entire period from October through today the weather patterns have been - knowing Portland natives who've been here for over six decades, they've all remarked that they've never experienced anything like this past Winter.

Scott K. said...

The mention of less damage due to lack of leaves on trees seems to contradict all previous forecast warnings throughout the winter of soft ground due to amount of rain we've had, which increases chances of trees falling.

So...which is it? Did we dry out enough to relieve ground saturation or has recent (yesterday/today) rains reintroduced that saturation giving trees a higher chance of coming down during a wind storm?

Mike said...

How high will this storm be on the Jim Forman scale?

windlover said...

Regarding Buddy's comment about Southeast Sucker winds....Eatonville is one of those locations hit hard by SE winds. Anytime there's a high wind warning for the North Coast and Northwest interior we get hit hard here too. Yet no high wind watch for our area for tomorrow's winds. Will be interesting to see how this plays out. Hopefully it doesn't have any surprises that all the models might be missing.

Corie said...

5 parkas! If he was still doing tv news, he would strap himself to a lighthouse and report live.

dooner said...

Any reason why a wind advisory hasn't been posted yet?

Erin Reetz said...

Is there a likelihood of coastal flooding along shores of Puget Sound? How does this storm compare to Dec of 2012? Thank you!!

Ansel said...

Can we catch a break? Honestly it's about time for a week of sun...

Jan said...

OMG! The Jim Forman scale! ROFL!! WAAAAAY funny

Ansel said...

Paul,

As a former resident of the East Coast I understand, and miss those storms they get. (Though they never go more than a week without seeing the sun.)

But here, we get a persistent summer "Hawaiian subtropical semi-permanent high" that sets up near Vancouver Island, and the subsiding air gives us desert-like dry weather. In winter it moves way south and the jet stream is aimed right as us- or sometimes California. Right now it is aimed at us.

West Coast weather is a strange animal.

sunsnow12 said...

"...knowing Portland natives who've been here for over six decades, they've all remarked that they've never experienced anything like this past Winter."

Here is some history: https://pauldorpat.com/seattle-now-then-archive/a-history-of-seattle-snows-exposed/

Based on that 11-part series, Seattle has. Unusual, yes, but they happen. Of course go back a little further to 1950 - that was a whopper. Or back a little further to 1916, when half of Lake Union froze. Now that was unusual...

Cedarspring said...

Do you mean this large standardized anomaly is extremely rare for this time of year or more broadly for the entire weather year?

Thanks for any clarification