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.

Severe weather for the south-central US

A lot is happening in the weather across the country over the next few days.  Today I'm going to focus on the severe weather threat Thursday into Thursday night for the south-central part of the country.  Tomorrow I'm going to talk about the snow event that is shaping up for Wednesday night into Thursday here in Seattle.

The shortwave trough that moved through a week or so ago that initially spawned some severe weather hype passed through without much of a fuss in terms of severe weather--big snow event, though.  This shortwave moving through this week, however, has a bit more certainty to it with regards to it being a severe weather producer.  So what's different about this week?

Let's look at the current 500mb map:

Fig 1 -- RUC 18Z analysis of 500mb winds (shaded and heights (contoured) on Tue., Feb. 22, 2011.  From the HOOT website.

 Right now we have pretty zonal (west-east) flow across the eastern half of the country with a bit of a ridge building over the Great Lakes.  However, two troughs are approaching from the west.  One is somewhat off the map but just entering the Pacific northwest (more on that one tomorrow).  The second is the cut-off low height center off the southern California coast.  This cut-off low is forecast to be picked up into the main west-to-east flow and move into the southwestern US tonight and tomorrow.  By Thursday morning, the GFS shows this at 500mb:

Fig 2 -- GFS 42 hour forecast of 500mb winds (shaded) and heights (contoured)  for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

Notice that there seem to be two main jet streams in the forecast map above--one involves jet streaks oriented west to east across the northern tier of the country.  However, another is oriented southwest to northeast stretching from northern Mexico into the midwest.  The presence of two jet streams like this is important--it makes this a potential severe thunderstorm event instead of another snowstorm.  Why is this?

During much of the winter, we typically see only one major jet stream across the US--the polar jet.  Remember that strong winds aloft form as a response to temperature gradients below.  As such, this one polar jet lies over the "polar front"--the boundary separating frigid arctic air from warmer, moister subtropical air.

Fig 3 -- Schematic of the single polar jet.

As warm, moist air is advected northward to the east of a low pressure center, it moves into areas that were previously underneath frigid arctic air.  This not only limits instability (we'd want it to be really warm at the surface and cold aloft to get good instability) but also causes the warm air to cool off considerably as it heads north.  As long as you are north of the polar jet, you're usually having very cold weather.

So what happens when the polar jet moves further north, particularly as we start warming up during the spring and into summer?  It leaves behind a somewhat cooler (but not frigid) air mass over the country.  However we also still get warm, moist, subtropical air trying to intrude north.  This time, however, it's moving into a less-cold air mass.  Furthermore, there's still a boundary between the cool air over the continent and the frigid air up north underneath the polar jet.  So we have two areas of temperature gradients--one between the frigid air up north and the cool air over the central continent and another between that same cool air over the central continent and warm, moist air coming from the Gulf of Mexico.  With two temperature gradients, we see two jets:

Fig 4 -- Typical spring time pattern with the polar jet further north and the "subtropical" jet to the south.

The southern jet is typically called the "subtropical jet", though usually not until it has become more organized later in the year.  Why is this setup better for severe weather?  Instead of the warm, moist semi-tropical air moving into frigid arctic air, it's moving into a moderated air mass that's typically cool and very dry.  Since the middle part of the country has warmed up some (as compared to when it was under the arctic air), it's much easier to warm the surface enough to generate strong instability.  It also means that frontally-generated precipitation can usually fall as rain instead of us worrying about temperatures near the surface being cold enough for snow.

So we have a jet pattern similar to this forecast for this Thursday morning.  Does the surface temperature profile match this model?  It certainly does.

Fig 5 -- GFS 42 hour forecast of surface pressure (contoured), temperature (shaded) and winds (barbs) for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

The cold, arctic air is confined to the northern part of the country--north of the polar jet.  There are cool temperatures in a region extending from the central plains through the upper midwest, and the warm, moist tropical air is located in the southern plains and lower Mississippi valley.  We can see a surface low also forming in west Texas underneath the divergent exit region of a jet streak in the subtropical jet stream (compare it to figure 2 above).  We still see strong thermal contrasts around the developing surface low, indicating a cold front stretching south and a warm front stretching east.  However, the frigid arctic air we would be expecting for a winter storm is left well to the north.

Is there moisture in that expanding warm sector across the southern plains?  The GFS seems to think there will be:

Fig 6 -- GFS 42 hour forecast of surface dewpoint temperature for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

There are 60+ degree Fahrenheit dewpoints forecast for much of east Texas , Louisiana and Mississippi.  As the low moves eastward, the moisture moves too:

Fig 6 -- GFS 60 hour forecast of surface dewpoint temperature for 06Z, Friday, Feb. 25, 2011.  From the HOOT website.

So we have moisture, and the frontal boundaries will definitely provide some lift.  What about instability and wind shear?  Here's the GFS forecast sounding for Thursday evening in Jackson, Mississippi:

Fig 7 -- GFS 60 hour forecast sounding for Jackson, Mississippi (KHKS) at 00Z, Friday, Feb. 25, 2011.  From Earl's Skew-T Page.

This forecast sounding shows strong directional wind shear from southerly winds at the surface to southwesterly winds aloft--a good directional shear for severe storms.  There's also speed shear--from 10 knots at the surface to 40 knots at 10,000 feet.  With this combination, the hodograph has a nice, clockwise curvature that we like to see for severe weather.  The lapse rate isn't extremely steep--in fact it looks close to moist adiabatic for much of the way up.  Correspondingly, only 120 J/kg of surface-based CAPE is being forecast for that sounding.  This is something to watch--we'd want stronger instability for very severe storms.  However, the lift provided by the front should be enough to fire off some storms.  This is also just a sample sounding at one time at one point--instability can be greater elsewhere.  In fact, the GFS SB-CAPE forecast map looks like this:

Fig 8 -- GFS 54 hour forecast Surface-Based CAPE forecast for 00Z, Friday, Feb. 25, 2011. From the HOOT website.

The best CAPE values are forecast to pool along the frontal boundary from southeastern Oklahoma down through Texas--and even then they aren't terribly impressive. There's also a small area out in front of the front in southern Arkansas and northern Louisiana where the instability seems to be enhanced.  This area would probably see the best chance for tornadic storms--with wind shear like what we saw in that forecast sounding, there's definitely the potential for rotating storms.  However, if the front becomes the main lifting mechanism, this will probably reduce to a hail-wind event, at least in the storms along the front.

The OWL-WRF model does fire some storms out ahead of the main front, but the structure seems very unorganized:

Fig 9 -- OWL-WRF 60 hour forecast of simulated composite reflectivity at 00Z, Friday, Feb. 25, 2011.  From the HOOT website.

So this will be something to watch as we get closer.  Quite understandably, though, the SPC has issued a slight risk of severe weather for Thursday in that general area:

Fig 10 -- SPC Day 3 Convective outlook, issued 0824Z, Feb. 22, 2011.  From the SPC website.

And so we're right on schedule for our first severe weather event of the spring season.

Tomorrow I'll turn my attention to the snow event up in Seattle that may even be ongoing by tomorrow evening.  Lots of weather going on...

This Friday's trough and January 7th, 2008

After a few days off to celebrate the holidays, it's once again time for me to look at the weather.

Of course, in my days off, a powerful nor'easter swamped the east coast with rain and heavy snow.  Blizzard conditions were experienced in New York City.  It was quite the event.

But I kind of missed that.  So, instead I'm turning to the potent trough that models are indicating should move through the country late this week.

What trough is this?  Here's the 96 hour GFS 500 mb height forecast for Friday morning--

Fig 1 -- 96 hour GFS forecast of 500 mb geopotential heights and winds for 12Z, Friday, Dec. 31, 2010. From the HOOT website.

Now that's a very deep trough for this time of year--the minimum close contour is at 5280 meters and is all the way down over Kansas.  Based on this cyclonically-curved jet streak pattern, one might imagine a deepening surface low pressure center somewhere near Kansas City, Missouri, with a trailing cold front back through central Kansas and western Oklahoma (underneath the jet streak).  And, based on the GFS surface forecast for that time, you'd be pretty accurate with that imagination--

Fig 2 -- 96 hour GFS forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) for  12Z, Friday, Dec. 31, 2010. From the HOOT website.

 We can also see a warm front in the above image extending northeastward from the surface low pressure center through eastern Iowa and into southern Wisconsin.  Superficially, this reminds me of the surface synoptic-scale setup from the first week of January in 2008:

Fig 3 -- HPC surface analysis from 15Z, January 7, 2008.  From the SPC Severe Thunderstorm Event Archive.

At this time, there was also a surface low near Kansas City with a warm front extending into southern Wisconsin and a cold front trailing into western Oklahoma.  With this particular event, several tornadoes were reported later that day in the lower Mississippi valley in addition to strong EF3 tornadoes that moved across northern Illinois and southern Wisconsin--on January 7th.  Not what you'd expect from the weather at this time of year.

However, there are some noticeable differences between these two events (or rather, from this theoretical GFS forecast and this event two years ago).  First, the surface temperatures are rather different.  In the GFS forecast, northern Illinois is only in the upper 30s in the morning, where as in the map from January 7, 2008, the dewpoints (and consequently low temperatures) were in the 50s--so not only is it much cooler, there also doesn't seem to be as much moisture.  In fact, looking at the GFS dewpoint forecast for the same time:

Fig 4 -- 96 hour GFS forecast for dewpoint temperature (shaded) and winds at 12Z, Dec. 27, 2010.  From the HOOT website.

We see that dewpoints are only in the 40s (which is still high for this time of year, but not as high as it was in 2008) in much of Missouri, Oklahoma and Arkansas with the really deep moisture confined to the far south and gulf coast.

However, in looking at the 500mb chart from 12Z, January 7th, 2008, we see more differences:

Fig 5 -- Objective analysis of 500mb heights, winds, and temperatures from 12Z, January 7th, 2008. From the SPC Severe Thunderstorm Event Archive.

It's immediately obvious that that the 500mb pattern looks rather different.  There is no cutoff contour of low heights like we are seeing in the GFS forecast.  Winds are more out of the southwest over the warm frontal zone in Iowa and southern Wisconsin as opposed to the more southerly winds being forecast in the same area by the GFS for the end of this week.  This may seem like a small difference, but changes in directional wind shear can have a huge impact on the ability of any storms that form to rotate.

We can also look at the upper air temperatures to get a hint at possible instability.  In figure 5 above, on January 7th, 2008, the 500mb temperatures over the surface baroclinic zone (over the fronts) ranged from the -10 to -15 degree range in the lower Mississippi valley to around -20 degrees Celsius over northern Illinois and southern Wisconsin.  Looking at the GFS 500mb temperatures forecast for this Friday shows surprisingly similar numbers:

Fig 6 -- 96 hour GFS forecast of 500mb temperatures (shaded), geopotential height (contoured) and winds for 12Z, Friday, Dec. 31, 2010.  From the HOOT website.

Those temperatures line up rather well.  This would theoretically say good things about the potential instability of the atmosphere, as colder temperatures aloft tend to imply steeper lapse rates.  However, remember that our surface temperatures are around 15 degrees cooler or more with significantly less moisture in this forecast than it was on January 7th, 2008.  That does not make it a good environment for being able to get surface-based convection.  Forecast soundings agree with this, still showing a deep isothermal layer (which is a stable layer) near the surface even after all the heating that will go on during the day on Friday.

Fig 7 -- BUFKIT GFS 108-hour forecast sounding for KRFD for 00Z, Jan. 1, 2010.

 We can see that we're saturated well up to 700 mb, which does imply a good rain event.  Wind directional shear actually looks all right near the surface.  But that isothermal layer near the surface and lapse rates that in general really aren't that impressive would seem to inhibit any good, deep convection from going on.  But we'll see...

Anyhow, just thought I'd do a quick run down comparing this long-term forecast to the January 7th event after noting the striking similarities in the surface patterns.  However, as we can see here, just because the conditions look similar from being viewed one way doesn't mean that they are similar in other ways.  We'll still see unusually warm temperatures throughout much of the middle of the country and probably some thunderstorms somewhere this Friday.  But at this point it doesn't look like quite the severe weather event we had two years ago.

Things could change in the models, though.  And they most definitely will.  96-hour forecasts are still way, way, way out there and there's no doubt that that GFS forecast will change in the days to come.  Already there's disagreement with the ECMWF model.  Here's the ECMWF 500mb chart for the same 96-hour forecast:

Fig 8 -- 96 hour ECMWF 500mb forecast winds (shaded) and geopotential heights (contoured) for 12Z, Dec. 31, 2010.  From the HOOT website.

The ECMWF model is slightly slower than the GFS with this trough--note that the main jet streak is over central and western Kansas instead of eastern Kansas.  It also brings the jet streak further north--now we'd expect to find a surface low somewhere near Sioux Falls, South Dakota, with a trailing cold front over western Kansas and the panhandles of Oklahoma and Texas.  The trough itself is also deeper--note the minimum closed contour is still 5280 meter (like it was in figure 1 above) but this closed contour extends over a much, much larger area.  So there are definite and distinct differences.

We'll just have to wait and see how this one all comes together.

Convection in Seattle

This morning, as I was waiting at my bus stop, I was somewhat startled by a sudden flash and the familiar, but so strangely foreign sound of thunder here in Seattle.  The ensuing downpour was unlike any I have seen in Seattle so far.  The severe thunderstorm warning that followed for Snohomish County to the north was even more unusual.  So I thought I'd grab some images from this event for a quick discussion.

In the synoptic setup, a large, cut-off upper-level low is rotating around in the northeastern Pacific.

Fig 1 -- GOES-W infrared satellite image from 1500Z, Dec 8, 2010.  From the HOOT website.

Note along the northern Oregon coast how you can see little "blips" of clouds firing off in a line extending southwestward from Puget Sound.  Convective clouds like that all forming along a line point to a frontal boundary pushing onshore and providing the lift for a more convective rain.  Here are a few glimpses at this rain as it came through on radar:

Fig 2 -- 0.5 degree base reflectivity from KATX radar at 1556Z, Dec. 8, 2010.

Note the high reflectivity values in the core of this precipitation--red colors showing dBZ values well in the 50dBZ range.  That's some very, very heavy rain--far heavier than you would usually expect in a non-convective system.  Also notice (where I inserted the little purple arrow) how the leading edge of the precipitation area is forming up into a relatively tight band with a strong reflectivity gradient on the leading edge.  This is usually indicative of strong winds being brought down to the lower levels helping to orgnaize the convection.  And that spike shooting out to the southeast?  That's where the radar is pointed directly at the rising sun.  The sun emits energy at all sorts of wavelengths--including some in the 10cm wavelength band that the radar uses.  So, when the sun is low in the sky and the radar scans toward it, we get strong, continuous returns from the direction of the sun.

Now about 20 minutes later:

Fig 3 -- 0.5 degree base reflectivity from KATX radar at 1615Z, Dec. 8, 2010.

Here we can see that that leading line has really gotten together, now evident as a narrow band of high reflectivities stretching down the Cascade foothills.  This almost looks like a sqaull-line structure--a leading edge of deep convection and strong radar returns followed by a region of weak reflectivity and then an area of enhanced trailing stratiform precipitation.  A wind gust of 58 mph in Everett behind that leading line prompted the severe thunderstorm warning for Snohomish County.  Let's take a look at the base velocity image for this time:

Fig 4 -- 0.5 degree base velocity from KATX radar at 1615Z, Dec 8, 2010.

Remember in interpreting radar velocity images that cool colors (the greens) represent velocities toward the radar and warmer colors (the reds) inidicate velocities away from the radar.  In that leading line, particularly in the area within the severe thunderstorm warning, notice the shift in wind directions.  To the east of the line, that tongue of green represens an area of air moving toward the radar.  To the west of the line, the reds indicate air moving away from the radar.  Therefore we see a zone of convergence right underneath our leading line--exactly what we'd expect to find under deep convection.  Convergence near the ground forces air to rise and, if the air is unstable aloft, that rising motion can blossom into thunderstorms.

Also of note in the above image are the strong inbound velocities in the southern blob of the radar returns and the strong outbound velocities in the northern blob. This is the radar sampling a jet at low-levels aloft--remember as the radar beam moves away from the radar, it tends to increase in height above the ground.  So at the southern and northern parts of the domain, the radar beam is relatively high off the ground.  Those high velocities like that point to a strong south-to-north jet over the area.  Do we see this in analyses?

Fig 5 -- UW 12-km WRF model analysis for 12Z run of winds at the 4000 ft. level.  From the UW model page.

The above image repersents a 12Z analysis of winds at the 4000 foot level from the UW-12km WRF model.  There is a nice, south-to-north oriented jet of wind in the 45+ knot range in the central Puget Sound region.  So everyone agrees.  Like I mentioned before, strong winds in the low levels like this can enhance wind shear which helps the convection to stay organized and develop into something severe.

Another observation to look at is a meteogram from the roof of the UW Atmospheric Sciences building:

Fig 6 -- 24-hour meteogram ending at 1919Z from the roof of the UW Atmospheric Science building.  From the UW NW Obs. page.

Our front with its line of convection moved through at around 16Z, and that time immediately stands out on te right side of the meteogram.  We can go through each of the variables individually. Note the wind speeds peaked with sustained winds over 20 knots and a gust measured to near 30 knots right as the leading edge came through.  At the same time, the wind changed direction from being southeasterly to southwesterly.  Very typical wind changes at the leading edge of a convective line.

At the same time, the temperature dropped sharply--from 54 degrees down to around 46 degrees.  Some of this cooling could be due to colder air behind a cold front, but I think a lot of the cooling has to do with convective downdrafts out of this line.  Warm air rising and cold air sinking provide the dynamical mechanism that gives thunderstorms their strength.  Thus, in very strong thunderstorms, we'd expect a lot of sinking cold air in the downdraft region of the storm. 

Now, the plot above plots relative humidity as a moisture variable instead of dewpoint temperature or something more specific.  Relative humidity is a function of the temperature--so as the temperature drops, even with the same amount of moisture in the air we'd expect the relative humidity to rise (we cool closer to saturation).  Therefore it's difficult to tell much from the relative humidity trace--yes, it rises sharply when the line comes through, but that's probably mostly due to the temperature falling rapidly at the same time.  However, with all that rain falling, a case could be made for an increase in the ambient miosture content of the air near the surface...

Finally, we see that the pressure had been falling steadily all night until it jumped higher as the line came through and has been slowly rising since then.  This further supports this line being associated with a cold front, as I've talked about in previous posts how cold fronts tend to lie in local pressure troughs.  Finally, the precipitation accumulation jumped as the rain began to fall--all to be expected.

You can see from the solar radiation plot that much of the morning has been cloudy, but then there was a very sharp transition to rather clear skies once the rain stopped--lots of solar radiation suddenly reaching the surface.  With all the rising motion in the convective line and behind the cold front, compensating subsidence (sinking motion) promoted an area of clear skies and sunny weather for at least part of the morning (though now more clouds seem to be moving in.

So there was a quick look at this morning's convection in Seattle.  Just to confirm that there was thunder, here's Vaisala's map of the National Lightning Detection Network from this morning, showing there was indeed lightning in the Puget Sound area...a magical sight.

Fig 7 -- Lightning strikes from the NLDN for the previous two hours as of 16Z, Dec 8, 2010.  From Vaisala's Public Lightning Data page.