Friday, March 23, 2018

Cold, Unstable Air Brings Snow Down to Higher Hills Around the Puget Sound Region

Unusually cold air has moved over the northeast Pacific and the Pacific Northwest, creating unstable conditions and snow descending to the higher hills.

During the past hour (10 AM), snow was reported at Paine Field in Everett (604 ft) and at Sea-Tac Airport (452 ft).  At Peter Benda's house in Bellevue (1300 ft), snow was accumulating in a winter-like scene.

Using the capabilities of the City of Seattle's SNOWWATCH app, we can see that the freezing level this morning has dropped to about 1300 ft (see plot of temperatures above SEA-TAC airport below), which means the snow level would be roughly 300 ft (below that level there would be only rain).

Cold air has moved in aloft, associated with an upper-level low pressure center that is centered west of Vancouver Island (see 500 hPa--around 18,000 ft-- weather map below for 5 AM Friday morning).

Here is a temperature map at roughly 10,000 ft (700 hPa)--you can see the cold air aloft (purple colors) associated with the low.

With cold air aloft and warming temperatures at the surface (it IS late March), there is a very large change of temperature with height and that promotes instability:  having the lower atmosphere break up into upward moving thermals and sinking air around them.   Just like in your hot cereal pot when you turn on the burner.    Upward motion results in cumulus, cumulus congestus, and cumulonimbus clouds, resulting in intermittent heavy showers.  Maybe some soft hail or graupel if you are lucky.

You can see the instability in the visible satellite image this morning.  The small elements are the anvils from convective clouds.

Perhaps a more impressive view is from the infrared satellite image (the shading shows the temperature of the cloud tops, with white being the highest).  Cold, deep convection over and off our coast.

The problem for lowland snow lovers is that low-level air is coming off the warm ocean (which is about 50F), making it too warm for snow, and particularly accumulating snow, to reach sea level.

The threat of light lowland snow over the hills is not over yet.  Tomorrow morning, the upper level low and its surface reflection will approach and move south of us (see surface weather map at 8AM Saturday), with cool air over western Washington.  Very weak onshore flow.  There will be enough upward motion to promote some showers of snow and rain.

The latest UW WRF model run shows some light snow getting close to sea level for the 24h ending 5 PM Saturday.  Enjoy the snowflakes...this could be your last view of them until next winter.

Wednesday, March 21, 2018

Unusually Cold Air and Even the Chance of Light Lowland Snow will Hit the Pacific Northwest

By late March, the threat of lowland snow is normally over, temperatures are warming, and one starts thinking of gardening and hiking.

But during the next few days our relatively normal temperatures of the past week will cool substantially and some portions of lowland Washington and Oregon may get some light snow.

The reason?  A deep upper-level trough moving southward into the eastern Pacific.

The current (5 AM Wed)  upper level map at around 30,000 ft (see below, with wind speeds shown by color and wind parallel to the height lines) already shows substantially troughing (lower pressures or heights) over the eastern Pacific, with a jet stream (stronger winds, yellow colors) well south of the Northwest. Strong southwesterly flow will bring a lot of moisture into California and large amounts of precipitation there.  The jet stream is associated with a large horizontal temperature contrast (cold north, warm south of it), and thus we are on the cold side.

But the situation is about to get more exciting for us as the low to our north greatly amplifies and moves southward (see the same map for 11 AM Friday below).  The jet stream (yellow areas) will still be south of western Washington and Oregon and northern California will remain wet.

Now lets turn to a surface chart (sea level pressure, 3000 ft temperatures--colors, and surface winds). The cold blue temperatures show a lower atmosphere that is cold enough for snow to fall below roughly 1000 ft.  The problem for getting snow at sea level with such a pattern is that the flow is onshore (the ocean is a WARM 50F).

So what do the model snow forecasts show?  Here is the 24 total snowfall (not accumulation) ending 5 AM Friday. Some very light snow (flurries) near sea level and maybe an inch or so on the Kitsap where upslope flow will produce greater intensities (which helps bring snow to the surface).  Plenty of snow in the mountains and particularly the Olympics.

The next 24hrs?  Snow in the mountains extending down to roughly 1000 ft.

Looking at the total over the next 72 hours will bring smile to all skiers, snow-shoers and thus enjoying winter recreation.  Our regional mountains, already with an above-normal snowpack, will get a substantial hit of new snow (1-2 feet at higher elevations).   Northeast Washington gets more snow.  

The bottom line:  we are now guaranteed of going into the summer with a very healthy snowpack.  And with our reservoirs in excellent shape, water supplies for the population and agriculture are secure.

Such a strong end to the season is typical of La Nina years, and this one is not disappointing.

Monday, March 19, 2018

Four Women Will ROW From California to Hawaii To Highlight Climate Change and Sustainability

In June, four women plan on rowing from California to Hawaii to highlight their concerns about global warming and the degradation of the world's oceans.   We are talking about journey 2400 miles on human power alone, and they are planning on doing this in less than 50 days, thus breaking the record for a women-only team to cover this huge distance.

One of the women is an undergraduate in my department, Eliza Dawson, an excellent student who plans of going to graduate school next year to study the earth's climate system.  Eliza is also a member of the UW's fabled women's crew team, and thus is an accomplished athlete as well.  Such an effort is not inexpensive and thus she is raising funds to support this effort (here and here).

I am intrigued by this ambitious project on many levels. 

Eliza and compatriots will view and document the current state of the northeast Pacific at close hand.  As many of you know the world's oceans have become a giant junk yard, particularly for floating plastic, and they will go through one of the worst garbage collection areas--the center of the east Pacific gyre (see images).  They plan on taking pictures and documenting the debris they pass through.

They are also concerned about the impacts of climate change on the oceans and the planet, and Eliza plans on documenting the ocean surface temperatures and other characteristics as they row from the mainland to Hawaii.

I am also fascinated by the anthropological  implications of the passage of humans from the mainland to the islands without the benefit of motorized transport.  Could ancient Hawaiians have visited North America and vice versa?  Eliza and friends will have a harder trip than native Americans, since they are not using sails, which would be quite useful for the route they are taking.
Boat and Supplies
And I am intrigued of the athleticism of this challenge....making this journey using the power of their arms.  Imagine, a month and half of rowing, 2 hours on and 2 hours off.   Imagining this is as close as I am going to get to doing it.

In order to participate in this effort, Eliza will have to raise about $20k during the next few months.  So far she has raised several thousand dollars and is turning to crowdfunding to secure the remainder.  If  you would like to assist her, please go to her fundraising web site.  I will certainly do so.

Finally, take a look at a video they produced about their upcoming trip.

Dealing with anthropogenic climate change and learning to live with our planet in a more sustainable way are two of the great challenges of our species.  It is very encouraging to see a future young scientist willing to make such a tremendous exertion to bring attention to these important issues.

Saturday, March 17, 2018

California's "March Miracle" is Not Over

Some call it the "March Miracle"--the large amounts of precipitation and snow that fallen over California the past month.  The Sierra Nevada got hit by 2-5 feet this week alone. And this is a miracle that is not over, with a strong atmospheric river poised to occur over the next week.

Here are snow water equivalent maps (amount of water in the snowpack) from the National Snow Analysis for February 17 and March 17.   A huge increase in both depth and coverage.

The origin of the wet bounty over California was a shift in the large scale atmospheric circulation, with high pressure moving further offshore and persistent troughing (low pressure) developing over the West Coast.   To illustrate, here is mean upper level (500 hPa, about 18,000 ft) heights for March 10-16th. The left shows the heights (analogous to pressure) and the right shows the deviation (or anomaly) from normal.  A trough (low heights) is found right off the West Coast...that is culprit.

Well, this pattern is not going away...and in fact, it will amplify in a few days and our friends in California need to get prepared.  A trough will form west of California with strong southwesterly flow on its south side, which will entrain large amounts of tropical moisture (see map for 11 PM Tuesday below)

As a result, a strong atmospheric river will develop, which will bring tropical moisture into central and southern California (see plot of total moisture in the vertical for 5 PM Tuesday)

How much precipitation will those folks in California enjoy?   Here is the forecast accumulated precipitation over California for the next 7 days from the European Center Model:  up to 6-8 inches in the Sierra Nevada and coastal mountains.

And plenty around southern CA, such as the mountains that surround LA.

 Snowfall?  Up to 3- 4 feet in the Sierra Nevada (see below)

With all the recent snow, the Sierra Nevada snowpack is about 60% of normal.  It will get much closer to normal after the next few weeks.  And the reservoirs, already around 100% of normal, will get topped off for the upcoming dry summer.

The only negative of all this precipitation is that it will encourage the growth of grasses, which can enhance the potential for fire danger next fall after it has dried out.  

Here in the Northwest, our situation will be far less exciting than for California, with much of the big action going south of us.  The 7-day totals show the heaviest precipitation over western Oregon but only light precipitation over the Puget Sound area and the San Juans.

Thursday, March 15, 2018

Is the Western U.S. Snowpack Declining "Dramatically"?

Last week a paper (Mote et al., 2018)  was published in the journal Climate and Atmospheric Science claiming "dramatic" declines in western U.S snowpack (you can access it here).

The article had all kinds of scary details. The loss in water resources would be "comparable in volume to the West’s largest man-made reservoir, Lake Mead" and the losses would be so great that "new reservoirs cannot be built fast enough to offset the loss of snow storage."

The principal author's (Phil Mote) institution put out a press release that amplified the message, with the lead author noting that:

It is a bigger decline than we had expected,

The media headlined this "dramatic" loss of western snowpack, with hundreds of stories in major outlets around the world, allowing millions of people to learn about the bad news. Here are a few examples. I could show you a hundred more, but you get the point.

But fortunately, the "dramatic" headlines and all the hype are not correct.  

There has been no "dramatic" loss of western snowpack during the past century, but rather a relatively slow, steady decline.  And as I will demonstrate, the scary paper's own research supports a less apocalyptic interpretation.  As does other research in the peer-reviewed literature.

But let's get our terminology down straight.  What does dramatic mean?  Let's look at a typical definition (Oxford)

Since we are not talking about a thespian document, definition (2) is the one we want:  an event or circumstance that is sudden and striking.  Has the trend of snowpack over the western U.S. during the past decades been sudden and striking?

The answer to this question is really important.   Many politicians and activist organizations are claiming that we have experienced a rapid decline in western snowpack driven by global warming.  And an accurate knowledge of snowpack changes is clearly important for making decisions about water resources. And what about the future of western U.S. snowpack?

The Mote et al. paper uses two approaches to evaluate past snowpack changes over the western U.S.  The first is to examine snowpack changes based on direct measurements.  The problem with that approach is that there are only a limited number of stations and the number and distribution of such stations have changed considerably over time.

The second makes use of a snow/hydrology simulation model called VIC (Variable Infiltration Capacity) model, developed by Professor Dennis Lettenmaier of UCLA (and formerly the UW).  This model uses precipitation and temperature inputs (there is a LOT more of these than snowpack measurements) to simulate the changing snowpack.

Below is a figure from the Mote et al paper showing the snowpack (actually the snow-water-equivalent or SWE) over the western U.S. on April 1 each year from 1915 to 2014 using the VIC model approach.  They also fitted a line to the variation over time.  You will note that there is huge amount of variability year to year, with an apparent slow decline in snowpack over the past century.    Specifically, they found a 21% decline over the past century or 2.1% decline per decade.   Hardly seems dramatic.  I should note that April 1 snowpack is a frequently used measure, since in the west snowpack generally peaks around then, and thus April 1 snowpack is a good measure of the water availability for the upcoming warm season.

Now imagine their line was not there.  In fact, you don't have to imagine, I have done it for you!  There doesn't seem to be any decline during the past few decades...if anything, the snowpack seems to be increasing.

In fact, here is the same figure, with only the last 40 years shown.  No decline, dramatic or otherwise is apparent.  Where did that headline come from?

Now a completely independent analysis of long-term snowpack trends over the Northwest U.S. is found in a peer-reviewed paper in the Journal of Climate (A New Look at Snowpack Trends in the Cascade Mountains by Stoelinga et al...found here).   They used a statistical approach to secure the snowpack from temperature, precipitation, and streamflow instead of the physical model (VIC) mentioned above.  But the same general idea.  Their results for 1930 to 2007 are quite similar to those found in the Mote et al (2018) paper, with a 23% decline for the entire period, and increasing snowpack since 1975.  I repeat, increasing.

Furthermore, they went one step further and tried to remove natural variability (like the Pacific Decadal Oscillation) and got the April 1 snowpack trend shown below. Plenty of variability and a very slow decline (16% over the period).  About a 2% decline per decade...similar to  the Mote et al. VIC results.

Nothing large, nothing sudden, nothing dramatic.  2% loss per decade.  No acceleration of snowpack loss.    And as I will explain late, this make perfect sense considering that the Pacific Ocean is just west of us.

But what about snowpack observations over the West?

As noted by Mote et al., there is a major problem using such observations:  the number of measurement sites is small and their numbers and distributions have changes substantially over the past 50 years.  To illustrate, here is a figure from the supplementary material from the Mote et al paper, showing changes in the number of observations for various subregions.  Few observations before 1940 and a major increase in the 60s and 70s.  The numbers have been relatively stable since roughly 1975-1980.

With those issues noted, below is a plot from Mote et al of the observed April 1 snowpack for three western regions: the Cascades, the Rockies, and  California.  The circles are the average snowpack for each region (ignore the red xs and red line...that is for the VIC model which we already covered).  I removed a blue trend line from these figures--I want you to make your own appraisal of the trends.    Specifically, look at the period since 1980, when the observational network as relatively stable.   

It is clear that there is little April 1 snowpack trend in the observations for the last 35 years.  Yes, 2015 has a very poor snowpack...but that was an isolated outlier....for climate studies we must look at the trend...and there simply has not been much of trend.  Just a lot of variability.

As an independent check on the observed trend in April 1 snowpack, research meteorologist Mark Albright, past WA state climatologist, did his own analysis of the April 1 snowpack changes over Oregon, Washington, Idaho, and Montana using the USDA Snotel observing stations.  He considered the period 1984-2017, since the SNOTEL network expanded into the early 1980s.  A seen below, there is virtually no trend over that period (and I might note that 2018 looks like an above-normal year).
Now to beat a dead horse, here is one more observed record of mountain snowpack, one encompassing a very long period (1879-2017):  the snowpack at Donner Summit, high in the Sierra Nevada (this is from the Central Sierra Nevada site associated with the University of California, Berkeley).  The color bars are April 1 snowpack).   More snowpack in the late 1800s, but only a slightly downtrend during the past several decades.

Now you might ask, why has western snowpack been so stable if the earth is warming?  A very good question.

A major part of the answer is that the eastern Pacific has NOT warmed very much and our temperatures are substantially controlled by the eastern Pacific surface temperatures.  To illustrate the lack of warming, here are the surface air temperatures from the NASA/GISS website, showing the trends from 1977-2015.  The eastern Pacific actually cooled during that period.

This pattern is similar to that indicated in climate models driven by increasing CO2.  The Arctic warms up more than anywhere, land warms more quickly than oceans, and eastern oceans generally warm the slowest.

I know that some of you are unhappy with the above analysis, even though the evidence is pretty compelling that the snowpack has not been dramatically reduced the past few decades.  You have heard the constant drumbeat from the media, some activist groups, and a few scientists who should know better.

But before some start calling me names (e.g., climate contrarian or denier) or the Seattle Stranger does another hit piece, or someone complains to my Dean, let me explain that global warming will have major impacts on snowpack during future decades and especially after 2050.  Increasing CO2 will cause increasing warming during the upcoming century that will reduce snowpack substantially.   Some regional climate runs that my group and Professor Eric Salathe completed a few years ago, show major snowpack reductions (see graphic).

But the loss of snowpack during the snowpack has been modest and slow, and certainly not dramatic. And the fact that it has been going on for over a century suggests that part of it is probably natural and not driven by anthropogenically driven increases in greenhouse gases such as CO2.  The planet experienced a cool period (the Little Ice Age) from the 1600s to the late 1800s, that produced more snow over our region.  With the end of the cool period (probably due to natural causes), snowpack has slowly declined.

Scary headlines and claims of dramatic snowpack loss are counterproductive in many ways.   They are clearly not true and thus undermine the credibility of those claiming such losses (activist scientists, politicians, and advocacy organizations).   They can result in poor public policy and infrastructure planning.   They unnecessarily scare people and make them anxious, an increasing problem (two days ago the Seattle Times had a front page article about a UW Bothell class dealing with dealing with anxiety about climate change).

And then there is the moral/ethical dimension.  Scientists and the media must communicate our best understanding of the truth faithfully and not exaggerate/hype to get people to "do the right thing."   As I have learned personally, there is a real cost to telling "inconvenient truths", but society can only make wise decisions if it is provided with unvarnished information based on the best science, and including information about uncertainty.

Another issue regards the press releases of universities and other research institutions.  There is a tendency to go for dramatic headlines and hype to secure the "currency of the realm" for PR people--lots of clicks and attention.   But the contents of the research papers are often distorted in the process.  This was clearly an issue for the Oregon State University press release regarding the Mote et al paper, and it occurs all the time here at the University of Washington and at other instituions.

Finally, the spread of such hyped material says something about the current state of online and print media.  Apparently, few "reporters" bothered to read the paper they were headlining.  Few completed a reality check on the claims.   But they were attracted to the big "dramatic" headline and were happy put out the excessive claims as a way of getting attention and "clicks."  This is  more than an inconvenient truth, but is a challenge for our democracy, since a misinformed public will not make good decisions.

Tuesday, March 13, 2018

Extreme Morning Heat Wave Hits the Puget Sound Region

Major, long-term, temperature records are being broken this morning, with amazingly high temperatures around Puget Sound.  Stunning records.

I knew something was up when I walked my dog this morning around 6:15 AM.... it felt amazingly warm.   Then I checked and was floored.

Temperatures at 6 AM were in the mid-60s from Seattle to Tacoma (see graphic, click to expand).  50s over north Seattle.  Normal lows this time of the year are around 38-39F.

Sea-Tac Airport was 63F at 6 AM, with an overnight minimum of 62F.    The previous minimum temperature temperature record for the date was 50F--but temperatures will fall we will have to see if we beat it.

We are talking about a stunning record low temperature.  The plot of temperature at Seattle illustrates this (normal highs and low temps plotted with purple and cyan color lines).  The low this morning was roughly 10F higher than our normal high temperatures.
According to my colleagues at the National Weather Service, the 5 AM PDT temperature at Renton, Washington (at 66 F) was the 2nd warmest spot in the lower 48 states....just behind Palm Springs, California (69 F). 
So what is going on?   A very strong easterly wind event with powerful downslope winds.   In fact, there is windstorm going on right now in the Cascades foothills, particularly around Enumclaw.   And strong downslope winds cause powerful compressional heating of the air, like in your bicycle pump.  Thus, the extreme temperatures.

Here are the temperatures and winds around Puget Sound.  Look closely (click on image to expand) and you will see that the warmest places have easterly winds.

The pressure gradient across the Cascades is very large now (7.5 hPa across the mountains, higher to the east) and the winds above Sea-Tac Airport (see below) shows a deep layer of easterly flow...and easterly flow that is very strong at low levels. ( the figure shows temperatures in red and wind barbs, time is on the x-axis and height in pressure on the y-axis, 850 is roughly 5000 ft).

The turbulent easterly flow is mixing down to the surface, working against any nighttime cooling by the emission of infrared radiation to space.  The strong easterly winds have gusted to 52 mph in Enumclaw and 30-40 mph as far west as the Sound and as far north as Issaquah.

Why are there strong winds and warmth between Seattle and Tacoma?  Because of a "weakness" in the Cascades to the east....what meteorologists call "Stampede Gap"---- see the terrain map below.

The winds and high temperatures will fade today....but those of you living between Seattle and Tacoma are experiencing an unusual event today.