Weekend Snow In Gorge

December 15, 2010

It appears the Columbia River Gorge will see a significant snowfall for the 3rd or 4th time this season; beginning late Friday night or early Saturday morning. 

We’ve been a bit busy (about 100 emails unanswered right now) in the weather center with the tornado excitement, but now I’ve found some time to look forward into the weekend and beyond.  Interesting weather pattern the next 5 days!

First, not much of anything tomorrow and Friday as an upper-level trough digs down over the Gulf of Alaska, turning our upper-level flow from the southwest.  At the same time COOLER, (not cold) air settles in along with high pressure at the surface over most of the western USA to our east.  That allows the usual cold air inversion begin to develop in the Columbia Basin.

Then on Friday evening and Saturday morning a deep surface low pressure system develops, races north along the California/Oregon coastline and swings back to the west (back into the upper-level trough location) just about due west of Astoria.  This induces a tremendous offshore flow over our area.  Cold air piles up at least 5,000′ thick over the east slopes of the Cascades in southern Washington and northern Oregon.  Of course the deep cold air means a big pressure difference between Portland and the Columbia Basin.  Maybe 9-11 millibars through the Gorge.  The east wind will begin raging Friday evening and continue until sometime next week.  Clearly it’ll be cold all the way up through the atmosphere (for snow) from about Bonneville/Cascade Locks eastward…all snow out there.  Maybe 3-6″ snow in the central/eastern Gorge.  Of course we’re still 48+ hours away, so that snow outlook can definitely change.

Is it cold enough for snow in the Portland Metro area?  Probably not…the cold air flowing through the Gorge will be quite shallow at this end (that’s normal), maybe just 1,000′ or so deep.  However, the actual snow level above will be around 3,000′ or a little below.  If the two meet (main snow level is a bit lower, or Gorge cold air is deeper than expected), we wouldn’t be far from snow.  Hopefully that makes sense…maybe I should have drawn a diagram?   Because of the strong easterly flow, I do see one other possible snow area…western Washington County or even western Yamhill counties.  There is a signature of cold air damming up against the east slopes of the Coast Range early Saturday morning for a few hours.  This is the “Forest Grove Effect” I’ve talked about for 8 years or so.  This pattern is the setup for that.  So MAYBE we’ll get snow down to the lowest elevations briefly.  Something to watch…

Of course there may be a band of freezing rain at the west end of the Gorge again east of Troutdale.  That’s where the cold air flow is only 1,500′ thick or so.  But between 1,500′ and 3,000′ it’s above freezing.  So you get freezing rain in the area around Corbett, Bridal Veil, Cape Horn, and Skamania.

Here’s another interesting angle…there is no push to get the cold air out of the Gorge, just a VERY gradual moderation Sunday and maybe Monday, then the pattern appears to repeat Tuesday.  So the snow Saturday in the Gorge may stick around awhile and there MIGHT be more early next week.

Chief Meteorologist Mark Nelsen

Final NWS Tornado Images and Wrapup

December 15, 2010

The National Weather Service has issued a detailed report of the Aumsville Tornado, including this great damage map.  Make sure you click on the image for a much better view.