May 18, 2021

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Storm Approaches With High Winds South of Seattle and Heavy Snow in the Mountains

3 min read

 If you a skier you should be excited and if you live along the southern Washington or Oregon coasts, get ready for a major blow.

Let’s start with approaching Pacific storm.  It is going to be significant.

This is difficult forecast with substantial uncertainty,  less so now that we are within 48h of the action.  The storm that is going to hit the Northwest on Friday is hardly even formed right now and located 2800 miles away.  I have circled its location on a recent infrared satellite picture below.

So have have to forecast the evolution and intensification of this storm over that vast distance.  And yes, get the right location of landfall.  If we are off by 50 miles the forecast will be very, very different for virtually everyone.

To put it another way. Imagine a long bowling alley (typically 60 ft long and 42 inches wide) and demanding that you had to be within six inches of a pin.  And do so while your ball is changing in size, speed, and direction.  Impossible you say?  This is what meteorologists are asked to do.

The latest runs are in and I will show what they predict.  The University of Washington high-resolution prediction system, driven by the NOAA/NWS GFS model has a very potent low (about 977 hPa central pressure) moving across the southern Washington coast around 4 PM Friday (see below).   Those solid lines are isobars, lines of constant sea level pressure, and where they are packed together expect a large change of pressure and powerful winds.  

If this is true expect strong winds over western Oregon and southwest Washington.

And the maximum gust forecast at that time (see below) suggest gusts to 60 mph south and west of the low as it makes landfall.  The Willamette Valley would get a big piece of it.  Not much in Seattle

The latest European Center forecast is now in and its high resolution simulation has a weaker low (991 hPa) crossing the northern WA coast earlier than the U.S. forecast (see sea level pressure forecast map a 8 AM Friday)

In this solution, the strongest winds are still over Oregon.

The big issue is that there is still a lot of uncertainty for this system and that won’t be resolved until tomorrow I suspect.   To show you that, the  European Center runs an ensemble of FIFTY global forecasts every 6 hours.  Here are the predicted positions of the low center at 10 AM Friday morning.  My gods…..the low centers are all over the place and of very different strengths.

At this point, I suspect the UW forecast is quite reasonable, but we will have to keep track of this situation over the next 24 h.

But while there is differences in the exact location and strength of the low center, the general transition to cool, moist onshore flow is quite solid and predicted by nearly all of the ensemble members.

The UW forecast for total snowfall through 4 PM Saturday is impressive, with over 2 feet at the higher elevations.   This is going to be the base needed to make Thanksgiving skiing possible.

November is known as the stormiest month in the Northwest for a reason.

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