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.

Windy Western Washington

Time for me to return to Pacific Northwest weather for a day.  Unfortunately (or fortunately if you like highly dynamic weather) for us, this impending "arctic outbreak" isn't the only major pattern change going on. Here's a look at our current hemispheric plot like we've been looking at over the last several blog posts.

Fig 1 -- Northern Hemispheric plot of 500 mb heights (shaded) and mean sea-level pressure (contoured) from 12Z, Nov. 16, 2010.  From the HOOT website.

First, I'd mention that the really cold air mass is continuing to slowly move south from the pole in our direction.  But turning our attention to the northern Pacific, we can see some very broad areas of high pressure at the surface which are mostly mirrored in the 500 mb height shadings above.  With high 500 mb heights in this region, we expect the mean temperatures in the lower atmosphere to be relatively warm.  However, as cold air continues to slowly creep southward over the continent through Canada, it's going to be increasing the temperature gradient between the relatively warm ridge over the eastern Pacific and the cold continental polar air.

See the band of green shading where the heights (and consequently temperatures) rapidly change from warm (the yellows and oranges) to cold (the blues and purples)?  This is the region where we would expect to be find the strongest winds aloft, also known as the jet stream.  What have we seen that connects temperature gradients with winds aloft?  If you guessed the thermal wind, you'd be correct.  But I'm not ready to return to that topic yet...

Basically, you can see how the height (or temperature) gradient seems strongest over the Pacific Northwest, and it would only be expected to strengthen as cold air moved south.  This region is an area where shortwave storm systems are likely to form.  It only takes a small pocket of colder air perturbing that interface to start cyclogenesis at the surface and eject a shortwave.  Last night the Pacific Northwest saw one of these shortwaves move through (you can see in figure one above how the surface cyclone associated with this has already moved inland over Idaho and Montana).  While not the biggest precipitation producer, this little storm did bring some powerful winds.

Fig 2 -- 24-hour meteogram from UW rooftop weather station.  As of 1847Z, Nov 16, 2010.

The figure above is what is called a "meteogram" from the weather station on top of the University of Washington atmospheric sciences building.  It shows a time series of several different meteorological variables over the last 24 hours, with the most recent time at the right.  This allows us to see how several different variables have evolved over time and also identify any correlations between them. (Click the image to get a full-size version.)

In the top panel, we can see the wind speeds over the last 24 hours.  Note how winds remained relatively strong, varying between 10-25+ knots from 00Z to 06Z last evening (from about 4 PM to 10 PM).  Gusts were often well over 30 knots.  Strong winds like these were seen throughout the Puget Sound region and were responsible for some moderate damage and power outages last night.

Let's notice one additional thing about this data.  Many of the news sources last night were saying that the strong winds were in association with a cold front moving through as this cyclone came on shore to the north.  Is this so?  Because of the effects of mountains, wind direction is not the best indicator of frontal passages out here (take note of this, meteorology students!).  Instead, we remember two other features of fronts:

1. A cold front typically lies within a local pressure trough such that pressure falls as the front approaches and rises after the front has passed.
2. Fronts are technically defined as regions of strong potential temperature gradients.  But, since we're at the surface and near 1000 mb, ordinary temperature gradients will do.  So we'd look for temperatures to cool behind a cold front  Common sense.

Where do we see those two features occuring?  Right around 530Z (930 PM, PST).

This is the most likely time that the cold front passed through the area--toward the end of the time of maximum winds!  This means that these strong winds were in the warm sector of the cyclone ahead of the cold front.

Another interesting feature of this meteogram is how the temperature profile changed (or didn't).  Note in the bottom panel of these meteograms there is a "solar radiation" chart showing how much radiation was received by the sensor on the roof.  We can guess where the sun went down by seeing where that graph finally went to zero--around 100Z.  However, even though the sun went down, the temperatures did not drop much at all until the front came through four and a half hours later.  Three factors probably contributed to this:

1. Since we've already concluded that these strong winds were in the warm sector of the cyclone, there was probably some good warm air advection going on which canceled out much of the cooling.
2. Cloud cover over the warm sector helped to insulate the lower atmosphere so that longwave radiation from the surface did not escape to space.  This would help keep the near-surface layers from cooling significantly.
3. The strong winds through the lower atmosphere kept the boundary layer well-mixed, preventing a strong surface inversion from forming by mixing warmer air (in terms of potential temperature) aloft down to the surface.

So what about these crazy wind directions and speeds? What caused them?  The biggest factor seems to be the interaction with the terrain, particularly the Olympic Mountains to the west.  Below is a 3-hour forecast graphic from last night's UW WRF model run.

Fig 3 -- 3-hour forecast of 950 mb temperature, MSLP, and 10 m wind barbs from UW WRF model initialized 00Z, Nov. 16, 2010.

There are two areas where winds in the Puget Sound region were particularly strong.  One was over and to the east of the Strait of Juan de Fuca  and the other was over the south Puget Sound and Seattle areas.  Why were the winds stronger in these regions?

1. To the north, there is a relatively narrow gap over the Strait of Juan de Fuca between Vancouver Island and the Olympic Peninsula.  Both of these land masses quickly rise to mountains, and as such any westerly flow approaching them (like we see here) is going to be channeled and accelerated through the gap.  Thus we see really strong winds through the strait and any land mass downstream of that gap.
2. As strong westerly flow approaches the Olympic Mountains, a lot of the low-level air is forced to split and go around the mountains.  On the other side (the eastern side), this creates an area of relatively low pressure (which we see very, very nicely in the figure above).  As such, once air gets around the mountains, it is accelerated toward that area of lower pressure (the air wants to fill this relative "vaccuum").  Therefore, we see strong winds across southern Puget Sound being turned from westerly to more southerly as they are accelerated by this pressure difference.

So there is a brief look at some of the details surrounding this little shortwave passage through western Washington.  With this particular upper-air setup, though, we're expecting even more action as another shortwave moves through on Wednesday...then again on Friday...then again over the weekend...  It promises to be an exciting week.

Gap Winds and QG Theory

Being in Seattle now, I have developed a healthy interest in our own local weather phenomena.  Today I woke up to a new weather alert that there was a wind advisory out for the passes and western slopes of the Cascades.  Sure enough, observations this morning are confirming 20-30 knot winds through all of the mountain passes in the central Cascades and in some foothills towns like Enumclaw.

But what's causing such strong winds through the passes and on the western slopes?  This is a phenomenon known as gap winds.  Think about water or air being forced through a funnel.  A lot of water moving through a narrower space is going to make the water accellerate.  That's what happens with gap winds.  Air wants to cross the Cascades, but can only find a way through in the low, narrow passes.  Since large volumes of air are trying to move through such narrow corridors, the flow is accelerated and we see really strong winds as the air goes through and comes out on the other side.  This is a crude description of what happens, but it gives you a good general idea.  Justin Sharp, a recent student of Cliff Mass (one of my advisors at Washington), did a study on some of these gap winds that can be found here.

But what is making the air "want" to cross the Cascades today, particularly from the east to the west? The answer is differences in air pressure.  Air tends to flow from regions of higher pressure to regions of lower pressure.  Thing about pumping up a flat basketball.  Initially after you attach the pump nothing happens.  But as you press down on the pump, you increase the pressure of the air in the pump.  There is then a pressure imbalance, as there is higher pressure in the pump but the same, original air pressure in the ball.  Therefore, the air starts flowing from the pump into the ball to try and decrease this pressure difference.  It flows from high to low pressure.

So how do we see this today?  Take a look at the sea-level pressures from several stations on both sides of the Cascades for the past 24 hours:

Fig 1--Sea-level pressures in inches of mercury for the past 24 hours at several Washington stations.  Figure courtesy of University of Washington Dept. of Atmos. Sci. website.

In the graph above, the left side represents almost 9AM PDT yeseterday and the right hand side represents this morning at around 8AM.  The red, green and blue lines represent Seattle, Bellingham and Tacoma respectively, all on the western side of the Cascades.  The pink, teal and yellow lines represent Wenatchee, Ellensburg and Yakima, all on the eastern side of the Cascades.  Yesterday morning, all the pressures were about the same.  But look at what has happened in the last 24 hours!  All of the stations west of the Cascades have seen their pressure drop while those on the eastern side have seen a slight pressure rise.  No wonder air is moving from the east to the west with a pressure imbalance like that!  If you're curious about some more details about this phenomenon, here is a study by Richard Reed (1981) that highlights the kinds of pressure differences required to produce winds of a certain strength in the Cascades.

Let's go a step further (and into a far more technical area) and ask one question more--why are we seeing such a pressure difference?  The answer partially lies in the fact that there is a strengthening ridge to the east of the Cascades (also due to a trough approaching from offshore, but that's a story for another time).  Those without a good meteorology background should probably stop here, but you can read on if you would like.  We can diagnose this strengthening by remembering the quasi-geostrophic height tendency equation.  Recall that a change in the geopotential height of a pressure surface (and correspondingly changes in surface pressure) is related to vorticity advection on that surface and the gradient of temperature advection through through surface.  A quick look at a vorticity advection map (not shown) didn't reveal much coherent going on in the area, so particularly, we will recall that:

Warm air advection decreasing with height contributes to a rise in geopotential heights.

Let's look at an objective analysis of warm air advection at 850 mb from this morning:

Fig 2--850mb temperature advection and geopotential height contours from 12Z, Nov 3rd, 2010. From the HOOT website.

We can see this soaring ridge across much of the inand northwest, as well as the strong height gradient offshore.  If we translate this to the surface, we can already see that pressure gradient we saw from the surface observations manifesting itself in the upper air pattern.  But note the very strong values for warm air advection along the British Columbia coast as indicated by the blue arrow.  That's a maximum of over 5E-4 degrees Celsius meters per second (crazy units, I know...).  And this area seems to extend near to or just west of the ridge axis.  Let's look a bit higher and see temperature advection at 500 mb:

Fig 2--500mb temperature advection and geopotential height contours from 12Z, Nov 3rd, 2010. From the HOOT website.

The same location near the British Columbia and Washington coasts is denoted by the blue arrow.  The warm air advection up at 500mb is considerably weaker! Only a maximum of around 3E-4 degrees Celcius meters per second, and over a somewhat smaller area.  So as we go up in height from 850mb to 500mb, our warm air advection is decreasing.  What does our "adage" from above say that this means about heights?

Warm air advection decreasing with height contributes to a rise in geopotential heights.

So we can expect that in the vicinity of that ridge axis area and to its immediate west, heights will rise.  This translates to an even stronger ridge and with that height gradient offshore--winds could really kick up.

A really astute observer might also notice that the cold air advection over ther northern plains is increasing with height.  This is the same thing as warm air advection decreasing with height--so we once again would look for height rises over the next day or so across the northern plains.  Tomorrow we'll see if the ridge did indeed evolve in this way...