Thunderstorms in Seattle

I awoke this morning to the sound of what I thought were airplanes flying over my house periodically.  It's not uncommon--the main northerly approach to Sea-Tac airport goes right over my house.  And they don't call Seattle "Jet City" for nothing.  But the sound wasn't quite right--and it was occuring way too frequently.  Turns out it was something far more exciting--thunderstorms.

KATX 0.5 degree base reflectivity from 15Z, July 25, 2011.

Lacking my usual Gibson Ridge program this morning, I used an internal utility on the UW computer systems to get this radar image.  You can see a lot of yellows with embedded cores of reds, indicating rather intense rainfall in some locations.  In particular there looked to have been a rather intense cell in eastern Pierce County southeast of Tacoma.

But were these thunderstorms or just heavy downpours?  I did a quick plot of the National Lightning Detection Network data of all lightning strikes registered from 7:30 AM PDT to 9:30 AM PDT.  Here's the result:

NLDN Lightning strikes from 1430Z to 1630Z, July 25, 2011.

Sure enough there have been several lightning strikes in the Puget Sound lowlands, particularly over the Kitsap peninsula.  There has been a lot more activity on the eastern slopes of the Cascades, though. 

The visible satellite image from this morning also shows the billowing, tall clouds identifying this as deep convection.

GOES-W 1km visible satellite image from 15Z, July 25, 2011.

Because the sun had just come up in this image, the sun angle on the clouds was still low.  For this reason, taller clouds cast shadows to their wests.  You can really see the shadow to the west of the cloud in the area of that big storm over eastern Pierce County.  Interestingly that storm began developing right in the vicinity of Mount Rainier.  Perhaps the mountain helped promote the lift necessary to get the storm going...

So why thunderstorms today?  First, we got really warm yesterday.  Here's a meteogram image showing plots of various weather variables on top of the atmospheric sciences building in Seattle over the past day. Temperature is the third panel from the top.  You can see that we almost got up to 85 degrees Fahrenheit yesterday.

Meteogram from 17Z July 24,2011 to 17Z July 25, 2011 from the top of the UW Atmospheric Sciences building, Seattle.

All that heating warmed up the lowest layer of the atmosphere.  At night, the surface and the near-surface layer cooled off again (you can see in the above image that last night the temperature got down to the upper 50s).  However, the air above that retained some of its residual warmth from the day before.

Below is a somewhat complex, but still very informative plot.  This shows several soundings taken using a vertical profiling radar at Sand Point in Seattle.  Every hour, the profiler uses a vertical-pointing radar beam to derive the temperature structure of the lowest 1600 meters or so of the atmosphere--up to about 850mb.  On the plot, the height above ground is given on the y-axis and the "virtual" temperature (which is similar to temperature, only a degree or two different because it factors in the humidity) in Celsius is on the x-axis.  However, coordinates of the graph are tilted--temperature lines are slanted up and to the left and are shown by the light gray lines.  So, it's kind of like a skew-T chart.

The actual temperature profiles are the colored curves.  One is taken each hour, and the legend showing what hour each color represents is on the upper right. 

Virtual temperature soundings from the Sand Point profiler in Seattle for July 2, 2011.

The first sounding is the yellow one, which was at about 10Z or about 3 AM local time.  As time progressed, you can see that the temperature really cooled down, at least below 1000 meters.  The surface temperature drives this--notice that the temperatures at the surface for the first two profiles (the yellow and the light blue ones) are much colder than the air above.  The surface radiates away more energy than the atmosphere above it, so it cools faster.

However, by 12Z (around 5 AM) the sun started to come up (this is now the magenta profile).  You can see that the surface temperature is no longer much cooler than the air above it.  Immediately the surface starts absorbing the solar radiation and its cooling slows down.  It eventually starts warming up.  However, the air above the surface (but below 1000 meters) continues to cool.  Just as the atmosphere cannot cool as efficiently as the surface, the atmosphere also cannot warm up as efficiently as the surface.  It has to wait for warm air in the near-surface layer to mix upwards to really start warming up.  As such, even though the sun has come up, the air between around 500-1000 meters continues to cool even though the surface cooling slows down and actually begins to warm.

However, the cool air at the surface overnight had not deepened beyond 700-1000 meters by the time the sun came up.  You can see that the temperature profiles all abruptly get warmer above the 700-1000 meter depth.  The night was just not long enough for the cold air at the surface to deepen up beyond that height.  Therefore, above this level, the residual warmth from the day before remains.  Because we got so warm yesterday, these temperatures above 700-1000m really are very warm--I mean, it's still 23-25 degrees Celsius up there (around 75 degrees Fahrenheit) while at the surface it had cooled to the upper 50s. 

Why do I go through all of this description of the temperature profiles?  I want to point out that even though at the surface we really cooled off last night, residual heat from our unusually warm day yesterday still lingered just above the surface.  It's that still-warm layer that provided the instability for this morning's storms.

Helping this instability is a trough moving in from offshore.  Here's the 700mb temperature forecast for 15Z this morning:

UW 4km WRF 3-hour forecast of 700mb temperature at 15Z, July 25, 2011.

This forecast is for temperatures above the top of the temperature profile I was just showing.  Notice the cooler temperatures just off the west coast of Washington and the westerly, onshore winds.  These cooler temperature are associated with a trough that is now moving onshore.  So, combine these two ingredients:

1. A warm layer around 1000m above sea level that was left over from yesterday's very warm day, even though the surface cooled down overnight.
2. On top of that warm layer, colder air was moving in associated with a trough moving onshore.

And we have cold air moving in over warming air--the right combination to destabalize the atmosphere.  These are not surface-based storms--they are not drawing from warm air at the surface.  Instead they are elevated storms--they're drawing from warm air in a layer above the surface.

That was a look at our fun collection of thunderstorms here in Seattle.  It's a rare event out here, but it makes sense once you look at the setup.  I'm going to bring back those virtual temperature profiles at some point in the future to talk about the development of the capping inversion.  You can really see in those profiles how a capping inversion is developing this morning over the area.  But more on that in another blog.  For now I'll just enjoy the show.