Lake Michigan Mesovortex

1-minute GOES-16

1-minute GOES-16 “Red” Visible (0.64 µm) images, with hourly surface reports plotted in yellow [click to play MP4 animation]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed a well-defined mesoscale vortex (or “mesovortex”) moving southward across southern Lake Michigan on 31 December 2017. The default western GOES-16 Mesoscale Sector provided images at 1-minute intervals. This feature was responsible for brief periods of heavy snow at locations such as South Haven, Michigan KLWA (beginning at 1455 UTC), Benton Harbor, Michigan KBEH (beginning at 1625 UTC) and La Porte, Indiana KPPO (from 2055 to 2115 UTC).

Comparisons of POES AVHRR/Terra MODIS/Suomi NPP Infrared (10.8 µm/11.0 µm/11.45 µm) and Visible (0.86 µm/0.65 µm/0.64 µm) images along with an overlay of the corresponding Real-Time Mesoscale Analysis (RTMA) surface winds (below) provided views of the mesovortex at 1522 UTC, 1714 UTC and 1852 UTC, respectively.

POES AVHRR Infrared (10.8 µm) and Visible (0.86 µm) images at 1522 UTC, with 15 UTC RTMA surface winds [click to enlarge]

POES AVHRR Infrared (10.8 µm) and Visible (0.86 µm) images at 1522 UTC, with 15 UTC RTMA surface winds [click to enlarge]

Terra MODIS Infrared (11.0 µm) and Visible (0.65 µm) images at 1714 UTC, with 17 UTC RTMA surface winds [click to enlarge]

Terra MODIS Infrared (11.0 µm) and Visible (0.65 µm) images at 1714 UTC, with 17 UTC RTMA surface winds [click to enlarge]

Suomi NPP Infrared (11.45 µm) and Visible (0.64 µm) images at 1852 UTC, with 19 UTC RTMA surface winds [click to enlarge]

Suomi NPP Infrared (11.45 µm) and Visible (0.64 µm) images at 1852 UTC, with 19 UTC RTMA surface winds [click to enlarge]

During the preceding nighttime hours, a comparison of Suomi NPP VIIRS Infrared (11.45 µm) and Day/Night Band (0.7 µm) images at 0729 UTC along with an overlay of 07 UTC RTMA surface winds (below) showed in spite of patchy thin cirrus clouds over the region, ample illumination from the Moon (which was in the Waxing Gibbous phase, at 96% of Full) enabled a signature of the early stage of mesovortex formation to be seen on the Day/Night Band (DNB) image. Ice crystals within the thin cirrus clouds were responsible for the significant scattering city light signatures on the DNB image.

Suomi NPP VIIRS Infrared (11.45 µm) and Day/Night Band (0.7 µm) images at 0729 UTC, with 07 UTC RTMA surface winds [click to enlarge]

Suomi NPP VIIRS Infrared (11.45 µm) and Day/Night Band (0.7 µm) images at 0729 UTC, with 07 UTC RTMA surface winds [click to enlarge]

As an aside, it is interesting to note that ice could be seen in the nearshore waters of Lake Michigan — both in the western part of the lake, off the coast of Wisconsin and Illinois, and in the eastern part of the lake off the coast of Lower Michigan. The lake ice appeared as darker shades of cyan in the 250-meter resolution Terra MODIS false-color (Band 7-2-1 combination) Red-Green-Blue (RGB) image from the MODIS Today site (below).

Terra MODIS true-color and false-color images over southern Lake Michigan [click to enlarge]

Terra MODIS true-color and false-color images over southern Lake Michigan [click to enlarge]

A Ridgy New Year

As some of you can tell, I took a bit of a break from the blog for a week, but I am back.

The first week of the New Year promises dry conditions, no snow, no storms and moderating temperatures.

The reason?  The return of high pressure (also known as ridging) over the the West Coast.

The temperatures at Seattle during the last 3 months has had a number of swings, but overall it was a very normal fall/early winter. Note that the number of negative excursions roughly equal the positive ones--a sign of normal conditions for the period.
The cumulative precipitation at Seattle was wetter than normal (19 inches fell when normal was 15 inches).  Above normal, but not that anomalous.
The spigot will be turned off during the next week, with high pressure producing sinking air that dries out the lower atmosphere.  The upper level (500 hPa) chart for Sunday at 4 AM shows the ridge in place.


Monday morning, the ridge is still there over the Northwesr, but with an moderate trough moving into SE Alaska.


Tuesday morning....the West Coast ridge strengthens!

But later in the week (4 PM Wednesday shown), while the ridge holds over the Northwest, a trough (with precipitation) reaches California....they can use the moisture.


Happy New Year....

Not Your "Average" La Niña Pattern

Because of the presence of La Niña conditions in the tropical Pacific, La Niña is an easy target to blame for the wacky weather of the past month or two, but is it the real culprit?  Let's have a look.   

La Niña is a component of the natural see-saw of oceanic and atmospheric conditions in the tropical Pacific characterized by anomalously cold ocean temperatures in the central and eastern tropical Pacific.  These anomalously cold temperatures are evident in sea surface temperature anomalies from earlier this month.  Note in particular, the tongue of anomalously cold water extending along the equator from South America to the dateline. 

Source: Climate Prediction Center
If one looks at composites (or averages) of La Niña winters, one finds patterns similar to those illustrated in the bottom panel below with high pressure over the central Pacific near about 150ºW, cold conditions over northwest North America, wet conditions in the Pacific Northwest, and dry conditions across the southern tier of the United States.  


Another way to look at this is in terms of hight anomalies at upper levels.  Areas in warm colors below correspond to anomalous ridging, and areas in cool colors anomalous troughing.  At upper-levels, the primary circulation features are an anomalous trough near Hawaii, ridging in the North Pacific, and troughing over northwest North America. 

Source: NOAA/ESRL
The pattern over the past month hasn't really looked like that.  Instead, we've had anomalous ridging along the west coast of North America. 

Source: NOAA/ESRL
In addition, while it has been dry across the southwest US and most of the southern US, it has also been dry in the Pacific Northwest.  The only areas of anomalously high precipitation is in the northwest U.S. Rockies and adjoining plains and in lake-effect areas.  


Thus, this is a pattern that doesn't fit the average La Niña pattern all that well.  That isn't to say La Niña isn't playing some role.  It could be playing an important role, with our use of relationships based on averaging past events the real problem.  On the other hand, it is also possible that we need to look at what is happening from a broader, global perspective.  Some discussion of this topic is provided by the California Weather Blog.  This is an area of active research, and one that will probably get even more attention after this winter, which has generated some remarkable weather extremes across the United States.  

Wacky Waves, Winter Warmth, and Pollution Perspectives

Kory Davis sent me the great cloud photo below taken looking east yesterday at about 8:30 AM from Snowbasin.

Photo courtesy Kory Davis
This is a wonderful example of breaking waves produced by Kelvin-Helmholtz instability, named after physicists Lord Kelvin and Hermann von Helmholtz.  Such instabilities are produced by vertical wind shear, leading to breaking waves that are similar in appearance to waves breaking on a beach.  Such instabilities occur frequently in the atmosphere, and produce turbulence, but aren't always easily seen. 

Moving on to today's weather, there's much to discuss.  First, how about our afternoon temperatures.  At 2242 UTC (3:42 PM MST), many sites along the east bench are running in the 50s, including a 56 at the University of Utah.  Alta is 43, Mountain Dell 51, and Kimball Junction 50.  THIS IS NO WAY TO RUN A WINTER.  


If you look carefully, evidence of the valley inversion is apparent, with temepratures in the 40s in Kearns, Taylorsville, and along the Jordan River north of South Jordan.  Also apparent is the strong influence of the lake breeze, with the Salt Lake Airport sitting at only 44.

How about we give the PurpleAir network a little love today and use it to take a look at the distribution of pollution around the region.  Sometimes caution is needed in interpreting the data collected by these low-cost sensors, but they look to be quite useful today.  Note that the highest PM2.5 concentrations are in the downtown area.  Lower values are found on the east bench, the west bench, and in the southern Salt Lake Valley.  One can find predominantly clean air up I-80 to the east.

Source: https://www.purpleair.com

The ups-and-downs in air quality over the past few days have been astounding.  The DAQ sensor at Hawthorne Elementary has had periods each day with PM2.5 at levels ranging from unhealthy for sensitive groups to good.

Source: DAQ
Yesterday in the Salt Lake Tribune, head of EPA region 8 noted that he believes that Utah's air quality can be improved, in part because of Colorado's success in Denver.  He notes, correctly, that part of the trick is figuring out the chemistry.  That's all fine and dandy, but here's my three New Year wishes that I would like to see happen now:

1. Upgrade and improve real-time air quality monitoring.  It is an embarrassment that there is only one "official" real-time monitor in Salt Lake County and that the data from this monitor is often more than an hour old.   There are large spatial and temporal variations in air quality that exist in our county (and other non-attainment counties in northern Utah) and citizens deserve better information about what they are breathing right now.   Alternatively, take greater advantage of lower-cost sensors, accounting for observational issues that arise from their design limitations.

2. More proactively work to improve air quality.  As a scientist, I always cringe at rallying cries for more research.  Yes, we do need more research, but the assumption that because we don't know everything, we know nothing is a bad one.  We know enough to move forward on new initiatives today. 

3. Show greater commitment and be results oriented.  I'm sorry, but I've lived here for over 20 years and I have heard the same song and dance for a long time.  If this state truly cared about the air pollution, we would have bent the curve years ago.  Near as I can tell, the primary difference between states that have made substantial air quality improvements (e.g., California) and those that haven't is political will and the desire to do what is difficult. 

Addendum:

After publishing this post, I realized I may have been overly harsh on points 2 and 3.  There have been long-term improvements in many air quality indicators, however, the worst PM2.5 events (98th percentile) have shown little trend over the past 10 years (see https://deq.utah.gov/Pollutants/P/pm/pm25/trends.htm).  Improvements basically flatlined after 2003.  The shift to tier 3 gas will hopefully help, but more can be done.  It's really a matter of whether or not you are satisfied with the status quo.  I am not.  

Mixed-phase stratiform clouds in an arctic air mass

AWIPS screen capture of GOES-16 Cloud Top Phase (top left), Near-Infrared

AWIPS screen capture of GOES-16 Cloud Top Phase product (top left), Near-Infrared “Snow/ice” (1.61 µm, top right), Cloud Phase brightness temperature difference (8.5 – 11.2 µm, bottom left) and “Clean” Infrared Window (10.3 µm, bottom right) images [click to enlarge]

An AWIPS screen capture showing GOES-16 (GOES-East) Cloud Top Phase, Near-Infrared “Snow/ice” (1.61 µm), Cloud Phase brightness temperature difference (8.5 µm11.2 µm) and “Clean” Infrared Window (10.3 µm) images on 28 December 2017 (above) was provided by Dan Baumgardt and Dave Schmidt (NWS La Crosse) — they were inquiring as to the why the 1.61 µm Snow/Ice imagery appeared bright across southern Minnesota (suggesting cloud tops composed primarily of supercooled water droplets), where light snow was being reported at a number of locations. Note that the Cloud Top Phase product also indicated that much of the stratus cloud deck over that same region was either Supercooled (light green) or Mixed (dark green).

An animation of GOES-16 Snow/Ice (1.61 µm) imagery (below) showed that the high reflectance (brighter white) signature of the lower-altitude stratiform cloud deck persisted across southern Minnesota into western Wisconsin and northern Iowa during the daylight hours, along with widespread surface reports of light snow. In contrast, higher-altitude clouds composed predominantly or entirely of ice crystals exhibited a darker gray appearance (since ice crystals, as well as surface snow cover and frozen lakes/rivers, are strong absorbers of radiation at the 1.61 µm wavelength).

GOES-16 Near-Infrared "Snow/Ice" (1.61 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

GOES-16 Near-Infrared “Snow/Ice” (1.61 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

In the corresponding GOES-16 “Clean” Infrared Window (10.3 µm) animation (below), much of the aforementioned lower-altitude stratiform cloud layer exhibited cloud-top infrared brightness temperatures in the -10 to -20 ºC range across far southern Minnesota into northern Iowa, with colder -20 to -30 ºC values seen in the more northern and eastern portion of the stratus cloud.

GOES-16 "Clean" Infrared Window (10.3 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

GOES-16 “Clean” Infrared Window (10.3 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

Plots of rawinsonde data (at 12 UTC on 28 December) from Aberdeen, South Dakota and Chanhassen, Minnesota (below) showed that the temperature profiles within the low-altitude cloud layers were close to isothermal, with air temperatures generally in the -16 to -22 ºC range.

Rawinsonde data from Aberdeen, South Dakota [click to enlarge]

Rawinsonde data from Aberdeen, South Dakota [click to enlarge]

Rawinsonde data from Chanhassen, Minnesota [click to enlarge]

Rawinsonde data from Chanhassen, Minnesota [click to enlarge]

So how could snow be falling from stratus clouds whose tops appeared be be composed of supercooled water droplets? A journal article titled “Vertical Motions in Arctic Mixed-Phase Stratiform Clouds” demonstrated that in-cloud glaciation can and does occur below the supercooled liquid cloud top in an arctic air mass. This example certainly shows that in an arctic air mass, mixed/supercooled cloud above snow or ice cloud is possible, particularly in temperatures between -20 ºC and -30 ºC — and cloud phase classification for operational decisions must sometimes look beyond the examination of single-band satellite imagery (or even derived products such as Cloud Phase).

Thanks to Mike Pavolonis (NOAA/NESDIS/CIMSS) and Jordan Gerth (CIMSS) for their insightful explanations regarding cloud phase — and thanks to the NWS La Crosse staff for bringing this interesting case to our attention!

Skiing, Inversions, Pollution, and All That

It's been an interesting break.  After hemming and hawing about whether or not to get out of Dodge, we decided that there was enough snow and decent enough air quality that we'd gut out Christmas week at home.  By and large, I think it was a wise decision.

The cross country skiing has been pretty decent and has provided a good cleansing of my aorta.  Back country skiing?  We got in a tour the day after Christmas.  I can't say the skiing was good, but it was an education for my son who had never been out on such a hair-trigger, high-hazard day before.  I made him suffer too, with a 2000 vertical foot climb through heavily brushed slopes to start the day.  It's important that youngsters recognize it's not all fun and games out there. 


We went up for a couple of high-speed laps at Alta this morning.  It was only my 2nd day of lift served this year.  Conditions were surprisingly good for carving.  I'm always amazed that when conditions here are about as bad as they can get, it's still far better than most of the good days I had when I lived in upstate NY.  That being said, it's incredible to think that Mineral Basin is still closed and probably will be through the 1st of the year, unless Snowbird has some real tricks up its sleeve.


Despite the poor off piste coverage, the coverage on the groomers was adequate and by 11, the corrals were full and there was stiff competition for Jerry of the Day honors.  After a few laps of human-breakaway-slalom, we decided to return home and get something done.

Which brings us to the weather.

Here's your pearl of wisdom for the day.  Don't count on a pattern change until you see the whites of its eyes.  Stop looking at the 10-day forecast and buy yourself a fat bike.  Alternatively, go to Jackson.

Now maybe you can provide me with some pearls of wisdom.  Is it just me or is this the weirdest inversion ever?  Remember all that fog, stratus, and pollution on Christmas Day and Boxing Day?  Well, the fog has dissipated, we just hit 44˚F at the airport, and the PM2.5 concentrations have been up and down like a yo-yo the past few days.

Source: DAQ
I'm sure there's still plenty of gunk behind the lake breeze where the cool air over the lake won't release the goods to the free atmosphere, but this situation is far better than I could have hoped a couple of days ago.  More time at the office is needed to decipher this riddle.  Comments on what's going on appreciated.

The Paradox of Sea Level Rise

Sea level is rising.

There is no doubt about it.

But why?

How much of the recent rise is due to greenhouse gas warming?  How much of it is natural? 

The situation is a bit more nuanced than described by some of the media and others (like Zillow in a recent prediction of large numbers of coastal homes flooding in the NW).


Well, let's get our feet wet in this topic by examining sea level records provided by NOAA for several cities.  In each of them, NOAA has also put a "best fit" line for reference.

First Seattle, which has a sea level record going back over a century (1900).  Over the entire period, there has been an average of 2.03 mm increase in sea level per year (.67 feet per century).  The interesting thing is that the upward trend has been going on for a long time, well before the impacts of human emissions of greenhouse gases were significant.  (The radiative impacts of increasingly CO2 became large in the 1970s and later).  And rate of rise has been quite steady, with no hint of a recent acceleration.  In fact, there has been minimal rise during the past 20 years.



San Diego?   A very similar evolution, at a slightly greater rate of rise (2.15 versus 2.03 mm per year)


How about Key West?   A little more:  about 2.40 mm a year (.78 feet per century).  Again, no hint of acceleration of sea level rise during the past decades as human-emitted greenhouse gases have increased rapidly. 


From NOAA's National Climatic Data Center's website I secured this graphic of global sea level rise over the past 140 years.  Pretty steady rise since roughly 1920 and even rising before that.

Sea level can be measured with measurements at the sea surface or from satellites.  The solid line above is a satellite retrieval.  Here is a plot of satellite measurements....but only goes back to the 90s.
Pretty steady rise.



It is important to note that coastal sea level rise is not uniform around the world, with one reason being that the land is not staying at the same elevation!   In some places, the ground is sinking, due to pulling removal of subsurface water or oil, or some other natural process.  For example, the land is rising today in locations that were covered by ice-age glaciers that pushed the land surface down.  After they melted roughly 14,000 years ago, the land rebounded.  That is happening here in the Northwest, particularly for the Olympic Peninsula.

To illustrate all this, here is a a sea level trend map from NOAA.  Some places like the Olympic Peninsula has sea level going down.  Same in Alaska.  But there are large rises where the land is subsiding, such as New Orleans.



So this sea level rise business is pretty nuanced.

Sea level rise is not accelerating appreciably, even thought greenhouse gas concentrations are rapidly rise.  And the rise of sea level began more than a century ago, well before humans could have been a significant cause.    In fact, there was something called the Little Ice Age that occurred during the 1500s to middle 1800s, with subsequent warming that was mainly natural.  The current sea level rise period appears to have its origin in the demise of the Little Ice Age and the warming that followed.

So claims that all or most of the rise in sea level is due to human-emitted greenhouse gases appear problematic because it started before humans could be the main cause.  The casual link is further weakened by the lack of acceleration of sea level rise during the past few decades.

On the other hand, our climate models suggest an accelerated rise of sea level rise due to greenhouse gas warming during this century.  Will our models be correct or are they too sensitive to greenhouse gas impacts?  Time will tell.


So, what sea level rise should we expect in Seattle during the remainder of the century?

Extrapolating the current, steady upward trend implies about a .6 ft rise.  If we include the impacts of greenhouse gas warming, there would be more.  A National Academy of Sciences report did such an analysis suggesting a 4-56 inch increase by 2100, with a mean change of 30 inches (2.5 ft).  But whether such model-driven estimates are reliable is uncertain:  I suspect it will be on the high side considering the slow rise of the past few decades.









Dismal Pattern Continues

The large-scale pattern has not changed.  I repeat, the large-scale pattern has not changed.

Yes, we got a couple of miracle storms prior to Christmas, but on the hemispheric scale, we are still dealing with a highly perturbed, wavy, high-amplitude pattern. 

Note, for example, the dynamic tropopause (jet-stream level) analysis from 0000 UTC 27 Dec (1700 MST Tuesday/Yesterday).  Deep trough over the eastern US.  Deep trough off the coast of Asia.  High amplitude ridge over the eastern Atlantic. 


You want snow, go to the lee of the Great Lakes or western Japan.  Both have been getting pummeled. 

And, there's no end in sight for the dry weather.  Forecasts below are from the 0000 UTC 27 December initialized ECMWF and GFS models through the end of the holiday period (0000 UTC 3 January/1700 MST 2 January).  Storm track to our north.  Dry southwest. 

Source: weather.us
Source: weather.us
There are a few members of the NAEFS that drag the moisture down south enough to give us a little more action around later in the 7-day forecast period.  Thus there is a little hope, but I'm keeping my expectations low.  Plumes below are for Alta-Collins and show a small number of ensemble members giving us some action on or after the 30th of December.  Most generate no more than 4 inches of snow.  Sad!

Is this La Niña?  I'm not ready to endorse that viewpoint.  Yes, there are aspects of this pattern that are consistent with La Niña, but there are other aspects that are not.  Mechanisms affecting the large-scale circulation are multifaceted and complex.  Maybe we'll deal with this in a future post. 

Record-setting lake effect snow event at Erie, Pennsylvania

1-minute GOES-16 "Clean" Infrared Window (10.3 µm) images, with hourly surface reports plotted in cyan/yellow [click to play MP4 animation]

1-minute GOES-16 “Clean” Infrared Window (10.3 µm) images, with hourly surface reports plotted in cyan/yellow [click to play MP4 animation]

GOES-16 “Clean” Infrared Window (10.3 µm) images centered over Lake Erie (above) showed the evolution of lake effect snow bands on 25 December26 December 2017, which produced very heavy snowfall at locations such as Erie, Pennsylvania (station identifier KERI); a Mesoscale Sector provided images at 1-minute intervals. Some noteworthy snowfall records were set at Erie PA:

(27 December Update: additional lake effect snow at Erie on 27 December brought the final storm total accumulation to 65.1 inches: NWS Cleveland summary. NOHRSC plots showed a maximum snow depth of 49 inches just southwest of downtown Erie; the maximum snow depth at Erie International Airport was 28 inches on 26 December, which was still less than their all-time record snow depth of 39 inches on 21 December 1989)

A sequence of Infrared Window images captured by Terra/Aqua MODIS (11.0 µm) and Suomi NPP VIIRS (11.45 µm) is shown below. The coldest cloud-top infrared brightness temperatures associated with the dominant lake effect snow bands were in the -30 to -35 ºC range (dark blue to pale green color enhancement), similar to what was seen in the GOES-16 Infrared Window imagery.

Infrared Window images from Terra/Aqua MODIS (11.0 µm) and Suomi NPP VIIRS (11.45 µm), with surface reports plotted in yellow [click to enlarge]

Infrared Window images from Terra/Aqua MODIS (11.0 µm) and Suomi NPP VIIRS (11.45 µm), with surface reports plotted in yellow [click to enlarge]

Farther to the northeast, these Lake Erie lake effect bands also produced significant snowfall in far southwestern New York, with 32 inches reported at Perrysburg (located 20 miles west of Dunkirk, station identifier KDKK). In addition, lake effect snow bands over Lake Ontario were responsible for even higher snowfall amounts:


1-minute GOES-16 “Red” Visible (0.64 µm) images (below) showed the lake effect snow bands over Lake Ontario on 26 December.

1-minute GOES-16 "Red" Visible (0.64 µm) images, with hourly surface reports plotted in yellow [click to play MP4 animation]

1-minute GOES-16 “Red” Visible (0.64 µm) images, with hourly surface reports plotted in yellow [click to play MP4 animation]

Christmas Brings Gravity Waves and Unexpectedly Poor Air Quality

Christmas turned out to be a fascinating weather day.  The Christmas Eve warm front and precipitation left a shallow cold pool over the Salt Lake Valley with widespread fog and dense haze in the morning.  I noticed some strong gravity waves from my vantage point in the upper avenues, one of which is pictured below.  Note the bulge in the depth of the fog, similar to the crest of a wave on the water. 


Although the fog burned off in many areas, a shallow lens of haze remained over much of the valley during the afternoon. 


My thinking during the day was that this was probably predominantly natural haze as PM2.5 concentrations were quite low on Christmas Eve morning.  However, the PM2.5 observations from Hawthorne Elementary show we reached levels unhealthy for sensitive groups Christmas evening.  In fact, the increase from Christmas Eve morning is astounding.  Normally one sees an increase in PM2.5 during a developing and persistent inversion of about 10 ug/m3 per day.  What happened from Christmas Eve to Christmas was more than double that.

Source: DAQ
Clearly, the DAQ was caught with their pants down on this one as their web site, updated at 1:22 PM on Christmas Afternoon, had us in "unrestricted action." 


So, why was the PM2.5 so high?  I'm not sure why, but there are a few possibilities.  One is that in this case the inversion didn't develop aloft and descend slowly as is often the case when a ridge builds in.  Instead, it developed as the warm front warmed temperatures aloft and precipitation cooled the near-surface airmass, resulting in a shallow cold pool right from the get go.  Thus, any emissions were confined to a shallow layer.  Indeed, the sounding collected in the late afternoon on Christmas shows a very shallow inversion based right at the surface, with a well mixed layer aloft.  Little wonder the upper benches were so clear, with a different story in the valley.  


I'm not entirely satisfied with that as an explanation, however, since the buildup in pollution seemed remarkably rapid.  I wonder if the holiday "sparked" an increase in yule log burning, and that this, along with the shallow nature of the cold pool, resulted in the rapid increase in PM2.5.  Having more wood fires with such a shallow cold pool would be a recipe for a rapid pollution rise.  Being in unrestricted action, perhaps many more people than usual decided to go for the holiday fire.  

A third possible factor is that the high humidity is causing somewhat inflated values, although the sampler used for the time series above I thought was somewhat immune to this effect (some samplers are not as water droplets inflate values).

Regardless, it's unfortunate that pollution along the valley floor was so high during this event.