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