Tuesday, October 18, 2011

Mountain Mixing Brings Warmth

Today is forecast to be a very warm day in Seattle--probably our last time to get into the upper 60s this year.  So what's causing this climatologically unusual day of warmth?

It starts with the upper-air pattern.  We're under a ridge at the moment, and with that ridge is a pool of very warm temperatures aloft along the coast.  Here's this morning's 700mb analysis:
Warm temperatures and high heights are very prevalent across western Washington.  All this warm air has a curious and somewhat counter-intuitive effect, though.  With warm air comes a fair amount of rising motion and a decrease in the density of the air.  This works to lower the pressure at the surface underneath this pool of warm air aloft.  The result is what we call a thermal trough--an elongated area of lower pressure at the surface in response to warming.  Here's the sea-level pressure and temperature analysis from this morning:

That map is a bit cluttered, but you can see the area of lower pressure long the coast.  Contrasting with that, there is an area of higher pressure in the interior of Washington on the Columbia Plateau.  See the large number of isobars (the black contours) that are running north-south along the axis of the Cascade Mountains?  That implies a very strong pressure gradient between the high Columbia Plateau and the Puget Sound lowlands.  The pressure is much higher to the east of the Cascades than it is to the west.  In fact, this morning the pressure in Yakima (east of the Cascades) has gotten to be almost 6 mb greater than the pressure in Seattle (west of the Cascades).  That's quite a difference.

The result is that air wants to push from the east to the west across the mountains.  Of course, the mountains make it difficult for air to move through.  As air crosses the Cascades, it's at a higher elevation than down here near sea-level in Seattle.  As a result, the air pushing across the Cascades this morning has stayed above the cold air that pooled in Seattle last night.  We can see this on the wind profiles from this morning in Seattle:
The above image is a series of soundings taken from the Seattle wind/temperature profiler at Sand Point.  Time increases to the left, so the most recent sounding (as of 11AM this morning local time) is on the left side of the image.  The heights are given in meters on the y-axis.  You can se that starting around 9Z this morning (the time marked by 18/09 on the bottom) that we had easterly winds above 500 meters or so with northerly winds below that.  The easterly winds above are from air coming out the higher elevations over the mountains.  The northerlies near the surface are due to the colder air near the surface moving toward that area of low pressure just off the coast.  So we have two air masses moving different directions.  You can even see that there  is a pronounced temperature difference between the two.  The red contours show temperature in degrees Celsius.  Where the winds are out of the east, the temperatures were in the 14-15 degrees Celsius range.  Below that, the temperatures were down around 10 degrees Celsius.  Two different air masses with two different temperatures.

But what's interesting to watch is how the height of the transition zone has been decreasing as this day has begun.  If you watch over time, the level where the winds switch from northerly to easterly has been dropping rather steadily.  At 11 AM it was only measured at around 200 meters above ground.  The winds aloft have really picked up, too--there are easterly winds measured at around 40 knots just 500 meters above the ground while at the surface they are barely registering out of the north.

As the sun warms the land below, it will basically destroy the cold pool of air that has been near the ground all night in the lowlands.  As the air warms, we'll get more movement and mixing.  The constant high winds just above the ground are also helping to promote this mixing.  Eventually, that warmer air mass with its easterly wins should mix all the way down to the surface.  Not only will this bring the slightly warmer air to the surface, but that air will warm even further as it decreases in elevation.  Since the pressure is higher near the surface, descending air compresses and, consequently warms.  So, as soon as this air aloft reaches the surface, we expect a big spike in temperatures.

Some higher elevations have already experienced this warmer air.  Here's the 11 AM surface map for the Puget Sound area:
Note the temperatures (the red numbers) down in the Puget Sound area--mostly low to mid 50s.  But look just inland to temperature in the foothills of the Cascades.  There are some mid-60s being reported there--with easterly winds as well.  Locations like North Bend, Index and Puyallup are high enough in elevation that they're already in the warmer air and easterly winds coming off of the mountains.  Hopefully that air will get down to us soon and cause our temperatures in Seattle to soar a bit too.  I'm looking forward to a warm day...

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