Columbia Gorge Flows and Freezing Rain in Portland

There's a very active weather pattern at the moment, at least on the edges of the country:

Fig 1 -- Surface temperature (shaded), mean sea-level pressure (contoured) and winds at 21Z, Jan. 11, 2011.  From the HOOT website.

Not only is there a nor'easter building with the merging of those two low pressure centers in the east, but another very deep low pressure center is moving into the Pacific northwest.  For all of you high-pressure chasers out there, there's also that huge ridge of high presure stretching all the way from the Canadian prairies down into Texas (with lots of cold air underneath it).  Today I want to focus on some a certain peculiarity of this storm moving onto the Pacific northwest coast.  So, let's switch our perspective and take a look at a forecasted 850mb map for what's going on right now:

Fig 2 -- 12 hour forecast of 850mb heights (contoured), temperature (colors) and winds (barbs) for 00Z, Jan. 12, 2011 (4PM PST Tuesday).  From the UW 36-km WRF.

Note a nice low-height center west of Vancouver island.  We see that the cut-off low formed right along a baroclinic zone (an area with a strong temperature gradient) and its winds are doing a nice job of advecting warm air up from the south on the eastern side of the low.  So, at 850mb, there's a fair amount of warm air advection.  Let's zoom in and look at the surface/low(er)-level situation:

Fig 3 -- 12 hour forecast of 925mb temperature (colors), mean sea-level pressure (contoured) and winds (barbs) for 00Z, Jan. 12, 2011 (4PM PST Tuesday).  From the 4-km UW WRF model.

Note that we can see the strong pressure gradient (and equally strong winds out of the south to southwest) approaching off the coast.  The winds get a bit crazier on land due to the terrain effects.  One big thing that stands out on this map is the significant dome of high pressure sitting over eastern Washington and northern Idaho (roughly the Columbia River basin).  If you remember from one of my previous posts, I talked about how this area is surrounded by higher terrain on all sides which tends to keep air bottled up in that "bowl".  Here you can see that all that air bottled up is contributing to very strong pressure gradients.  On the ocean side of the Cascades and higher terrain, pressures have begun falling due to the approaching low off the coast.  But inside that "bowl" pressures have remained relatively high.  This is setting up an extraordinary pressure gradient between the interior basin and the coast.

I need to clarify my statement about there being no good way to get out of that interior terrain "bowl".  There is one way--through the Columbia River gorge along the Washington-Oregon border.  The Columbia River is an amazing river--its elevation is way down almost at sea level through most of its journey through eastern Washington and out to the ocean.  That is to say--it stays near sea level even as it flows through the Cascade Mountains.  That's pretty amazing.  It also provides one of the only ways out of the interior basin.

Because there's such a large pressure gradient built up, there are currently very strong winds pushing out of the Columbia gorge, particularly as it goes through the Cascades.  Here are the latest wind observations near Portland:

Fig 4 -- Surface observations from the Portland, Oregon, area at 22Z, Jan 11, 2011.

  Portland is almost right in the middle of this image.  The Cascade mountains would be oriented north to south along the right edge of this image.  You can see that strong winds out of the east are being reported right where the Columbia River is coming through the Cascades and the winds are easterly throughout this entire region (the northern Willamette Valley, if you want the actual name of this region).  This somewhat contradicts the surface pattern we would expect with a surface low off to the northwest--we'd expect winds out of the south or southwest.  But that strong pressure gradient across the mountains is pushing cold air out of the inland Columbia basin and down the Columbia River toward the ocean.

We can see this effect in forecast soundings for the Portland area.  This morning, there was a strong and deep push of cold air out of the Columbia Basin:

Fig 5 -- 3-hour forecast sounding for 15Z (7AM PST), Jan 11, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.

The above sounding is for Troutdale, Oregon, which is just northeast of Portland on the Columbia River.  Note that the above sounding is NOT on a Skew-T chart--the isotherms (temperature lines) are pointed vertically and not skewed to the right like we usually see.  The separation between the air being pushed out of the Columbia basin from the east and the southwesterly onshore flow from the west and southwest aloft is very clear.  Also notice how the temperatures in the layer near the surface where the winds are out of the east are cooler than the air aloft--this shows the difference between the cold air coming out of the interior basin near the surface and the warmer air associated with the Pacific low aloft.

Let's move along a bit in time:

Fig 6 -- 9-hour forecast sounding for 21Z (1PM PST), Jan 11, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.

This was six hours later, or earlier this afternoon.  Notice how the westerly and southerly winds aloft continue with easterly winds still coming out of the Columbia River gorge.  Of course, with easterly winds near the surface and southerly winds aloft, the winds are veering (turning clockwise) with height which indicates warm air advection going on.  This should be maximized where the wind direction is changing the most rapidly, or at around 950mb on the plot above.  And wouldn't you know--there's a bubble of warmer, above-freezing air that is forming right around that level. Amazing.

But now we should start to worry--all that warm air advection is also bringing in more moisture aloft.  In the first sounding, the air was saturated above 700mb.  Now the air is saturated above 850mb--so more moisture is moving in aloft and the level of saturation is dropping.  That warm air advection has also pushed temperatures above freezing just above the surface.  However, note how those cold easterly winds spilling out of the Columbia River gorge are still keeping the surface temperature very close to freezing.  This is a recipe for potential freezing rain.  Water would fall through the above-freezing temperatures just above the surface and melt into rain, but then hit the ground (which is at freezing or slightly below) and freeze.  The question is what particular factor will win out--the warm air aloft or the cold air persisting near the surface.  At this point,our saturation is pretty high off the ground--and according to observations it was not raining yet at this point.  As more warm air advection continues on top of that cold air coming down the Columbia River, the warm air will continue to mix down and erode into the cold air near the surface.  You can already see how much that cold layer (and the easterly winds) has shrunk between those first two soundings.

Six hours later:

Fig 6 -- 15-hour forecast sounding for 03Z (7PM PST), Jan 11/12, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.

This is later on this evening.  Warm air advection aloft continues and with it, more moisture.  Now our saturation level is down to 900 mb. There's a significant above-freezing layer that has built up above the easterly flow and cold Columbia basin air near the surface, but those easterly winds are still keeping the temperature near freezing at the surface.  Freezing rain is definitely a concern at this point.

Finally, by 4AM the next morning:

Fig 6 -- 24-hour forecast sounding for 12Z (4AM PST), Jan 12, 2011 at Troutdale, Oregon.  From the 4-km UW WRF.

By the next morning, all that warm air advection has FINALLY mixed down to the surface.  All that warm air mixing down has warmed the temperature considerably--at the surface the air is now several degrees above freezing.  We're also saturated all the way down.  Except for that one stubborn barb near the surface, winds are out of the south to southwest all the way down to near ground level.  The precipitation falling at this point would be all rain.

So this is why Portland is on the lookout for potential freezing rain tonight.  It all has to do with its geography, sitting there along the Columbia River gorge. It's pretty fascinating to look at how local topography and mesoscale influences can radically change the forecast for a particular location.