Saturday, February 17, 2018

Wind, Snow, and Cold: Very Active Weather in the Northwest

This morning winds are gusting to 40 mph and more around the region (see max gust map below), with scattered power outages in Seattle and vicinity.


The reason?  A low pressure center is moving eastward to the north of Puget Sound, resulting in a large pressure gradient...and thus strong winds.


A strong front is associated with the low, and with powerful westerly winds aloft (some reaching 70-100 mph!), massive snow amounts are falling in the Cascades, with the big trouble in the passes (chains required and delays in Snoqualmie).

But this active weather is just a "warm up" for what is about to happen. (In fact, just the opposite of a warm up!).

Tonight and tomorrow morning,  modified arctic air is going to push in, with a huge pressure gradient down the Fraser River Valley, a conduit between the cold, high pressure in the BC interior and the warmer, lower pressure conditions in western Washington.    Very strong winds will exit the Fraser Valley producing gale-forced flow from north of Bellingham, across the San Juans, and towards the Olympic Peninsula (see forecast gusts at 8 AM tomorrow).  Wow.


The latest forecast model output suggests snow on the windward side of the Olympics and light snow even over Seattle as the leading edge of the cold northerlies push southward during the morning hours.  For Puget Sound residents, LIGHT is the operative word.

Here is the forecast 24-h snowfall (not accumulation) ending 4 PM Sunday.   Lots in the Cascades and less than an inch over parts of Puget Sound, with perhaps 1-2 inches in places south of Seattle. More snow over the San Juans.


But this is only one model forecast.  We should turn to ensembles (running many forecasts to judge our confidence in the snow prediction).   Here is a "plume diagram" showing 15 accumulated snow forecasts for Sea-Tac airport (time on the x axis and snow total on the y axis). A LOT of variation and thus uncertainty, with the ensemble mean being about half an inch.  The heaviest is 1.4 inches. 

But then cold air moves in and the skies clear...and the models are much more in agreement regarding temperature (see forecast below for Sea-Tac).  Dropping to the low 20s on Monday morning--and even colder away from the Sound.

Time to go outside and disconnect all hoses.    This will be coldest air we have had in over a year.

Friday, February 16, 2018

Cold and Snow

We are about to enter the coldest period in over a year (since December 2016 at least), with massive snow in the mountains and lowland snow in "favored" areas such as Northwest Washington and the northern slopes of the Olympics.  Even Seattle might get a dusting. I expect to break some daily low temperature records before it is over.

And if you don't want to support your local plumber, disconnect your outdoor hoses and protect exposed pipes.   And make sure your pets are warm.   Seattle will have to deal with its large homeless population, many of whom will be exposed to below-freezing temperatures for several days.

Let's start by looking at the 72hr snowfall forecast from 4 PM last night until 4 PM Sunday by the highest resolution model we run at the UW.  Two to three feet in the Cascade at the higher elevations of the Olympics.  With a low freezing/snow level, Snoqualmie Pass will get bountiful snow.  Just good news for skiers.  This snowfall will guarantee the rest of the season.


And here is a close up view of snowfall for the same period.  Note the snow around Port Angeles to Port Townsend!   This is due to upslope flow as air from the Fraser River valley jets out to the SW over Northwest Washington.

To illustrate this flow, here are the winds at 4 AM Sunday, showing winds from the Fraser Gap accelerating over the water and then heading toward the Olympics, with some of the cool air peeling southward towards Puget Sound.


You will notice the Puget Sound does not get a lot of snow.  The reason?  We are going to be heavily rainshadowed (snow shadowed) by the Olympics since the general flow is westerly/northwesterly.  But we do have a chance of a snowburst as the leading edge of the cold air from the Fraser (known as the "arctic front" in the business) moves through.  No more than an inch, though.

And then there is the cold air, which moves in Sunday and stays for several days.  Sunday night/Monday morning will bring temperatures dropping into the 20s in western WA and below zero to the higher terrain and eastern WA.   Here is the UW WRF model forecast for 4 AM Monday.  Twenties near Puget Sound and the water, dropping to the upper teens in the Cascade foothills and single digits and below over the Cascades and northeast Washington.   And it will be even cooler Tuesday morning.


The surface air temperature prediction for Tuesday morning is amazing cold, with temperatures below -8F pushing over much of the interior Northwest.   Even northern coastal California will get below freezing!


Stay warming...and be ready for unusually cold temperatures.

Wednesday, February 14, 2018

La Nina Snows Over the Pacific Northwest Followed by Very Cold Air

The atmosphere over the eastern Pacific is thoroughly locked into a La Nina-type pattern, with the snowpack being refreshed for NW skiers and those concerned about the water supply next summer.

The snow water equivalent (the amount of water in the snowpack if melted) is in decent shape (see below), being above normal for the northern and far eastern portions of Washington State, but slipping to about 3/4 normal for the southwestern Cascades.

There was some improvement last night, when 8 inches to a foot of new snow  over the Cascades due to an upper level trough moving southward along the eastern flank of a big ridge over the northeastern Pacific (see below).


As I have noted in earlier blogs, a ridge in the eastern with cool northerly/northwesterly flow moving over the Northwest is typical of mature La Nina events. That sets up the cool temperatures, while the upper level trough moving southward provides the precipitation.

But to get lowland snow one needs to get an upper trough of just the right amplitude and position, which doesn't happen often.  Too far inland, and the cool air and precipitation are too far east.  Too far offshore, we are cold and dry.

Another, more vigorous, trough will approach on Saturday (see below), resulting in another snowy period in the mountains.


The 48h total snowfall ending 4 PM Saturday is enough to make a skier smile, with over two feet at high elevations in the Cascades and a relatively low snow level on Saturday (Snoqualmie Pass the eastern Cascade slopes will get plenty).


Another trough comes through on Sunday morning (see below), with most of the action slipping southward into Oregon, which really needs the snow.


The 48h snowfall total ending


But what really got my attempt was the very cold air predicted to move into our region early next week.  The coldest air in a very long time.  Here is the UW WRF model surface (2-m) air temperature forecasts for 4 AM Monday and Tuesday.

WOW.   East of the Cascades many locations will get below 0F, some locations will be way below.  Twenties near the water and teens elsewhere in western WA.



You might think about protecting your exposed water pipes.

Sunday, February 11, 2018

The Strange Case of the Quinault Blow Down: The Ultimate Solution

It is time to put the facts together and to explain the mystery.   Using high-resolution modeling, theory, studies in other locations, and available observations, we will attempt to solve the compelling scientific puzzle of the massive fall of old-growth and other trees on the north side of Lake Quinault during the early morning of January 27th.

Sherlock Holmes made use of a 7% solution of a certain drug to prepare for such cases.   I will make use of more appropriate drug to heighten my mental prowess, one used by many scientists in our region:   a tall Starbucks coffee.


Science is great fun, particularly for a difficult case like this.   Using limited observations and knowledge of basic physical principles, we attempt to explain natural phenomenon.     The enjoyment of an intellectual puzzle and a detective story.

And when the pieces come together, and when we gain an understanding of something that no one has understood before, the feeling is one of satisfaction and even elation.   A feeling that once experienced, becomes addictive.    A reason why many of us love being scientists.

So as Sherlock would say: "the game is on".  And I will describe my chain of logic, starting with known facts and then examining each possibility until we determine the most probable cause.

Let's review the facts

1.  A large tree fall occurred on the north side of Lake Quinault around 1:30 AM on January 27th. Many of the trees were old-growth, or at least, very large.

2.  The tree fall area was quite limited in size:  perhaps a half-mile on a side and extending from the lake toward the crest of about 2500 ft. 

3.  Several of the trees snapped off and this can only be explained by very strong winds (certainly at least 60-70 mph).

4.  The trees fell to the south and thus the winds must have been from the north.

5.  None of the limited surface observation locations in the area reported any winds even close to those needed to topple the trees.  For example, a site just across the Lake only reported light winds during the tree fall.

5.  The strong winds could NOT have been the result of microburst associated with a thunderstorm or strong convection.   Weather radar showed no such feature and the lightning detection network had no strikes in the region.

6.  An occluded front was approaching the coast at the time of big winds and tree fall.

The first question you should ask was whether the approaching weather system had strong northerly (from the north) winds associated with it.  Or even northerly winds at all.

We know that the surface winds with system did not have strong northerly winds  from the surface weather stations of the region.  But if there were northerly winds aloft, there would be the possibility of a downslope windstorm, as northerly winds accelerated down the slope north of the Lake.

Fortunately, there are sufficient observations to answer this question.  NOAA Earth Systems Research Lab (ESRL) maintains a device called a radar-wind profiler at Forks, Washington  (up the coast a bit) that is capable of determining the wind and temperatures aloft in real-time.  Here are the wind observations for 0000 UTC 27 January through 0000 UTC 29 January from the surface to 9 km above the surface.  Winds are shown by the typical wind barbs and are color coded .  The incident in question occurred at approximately 0930 UTC 27 January.  A blow up at the critical time is shown below as well.  Note that the front came in from the west and hit Forks before Lake Quinault.

The Bottom Line:  No hint of northerly flow during the period in question.  There were southeasterly winds at low levels, with increasing southerly and southwesterly winds aloft.

But we have another observing asset as well:  the Langley Hill radar near Hoquiam.  This radar is a Doppler radar and provides wind information aloft.  Specifically, it provides the radial wind component--the speed of precipitation (and the air it is in) towards or away from the radar.

Here is the Langley Hill radial velocities from the lowest scanning angle at 0927 UTC.  Green and blue indicate flow towards the radar, yellow/red/orange the opposite.  Not easy to read without experience.   But to my practiced eye, the radar suggests southeasterly winds of up to around 30 knots at low levels, turning to southerly and then southwesterly aloft.    Consistent with the profiler.  No northerlies

So if the air coming in off the Pacific was from the southeast east or south, where did the powerful northerlies come from?  Perhaps the comments pushing a secret government project,  aliens, or a meteorite strike were on to something.

Or perhaps not.  There is no evidence of any space object reaching the earth in this region (I checked).  And there IS a possible meteorological explanation:  a rotor circulation associated with a strong mountain lee wave.

But first some atmospheric rotor 101.  If fairly strong winds are approaching a mountain crest, they can undergo wavelike undulations in the lee of the barrier.  A situation in which air surges down the mountain and then suddenly rises up, followed by potentially more down and up motions.  If the wave has sufficient amplitude, a rotor can form underneath the wave, with flow moving in the opposite direction from the flow approaching the mountains.  You see why this is interesting...here is a way to get northerly flow when the general flow is southerly.


Mountain lee waves can increase in amplitude as the winds approaching the mountain strengthen.  But they can also amplify if there is a stable layer near the mountain crest, or if there is what is called a critical level above the crest level.  A stable layer is one where air temperature does not cool rapidly with height, and a critical level occurs when the wind component perpendicular to the mountain reverses direction.  These features help trap and amplify the low-level wave energy, producing stronger waves and stronger rotors.

But it is even better than this.  A large rotor can in turn break down into highly intense subrotors that can have strong winds associated with them.  Two colleagues of mine, James Doyle of the Navy Research Lab of Monterey and Dale Durran, a fellow faculty member at the UW, did a very nice paper showing the results of an ultra high-resolution simulation of these critters.  Here is a vertical cross section across the lee slopes mountain that shows the rotor and subrotors (indicated by the red colors).
And there have several observational studies of rotors, including the T-REX (the Terrain-Induced Rotor Experiment) project and intense studies near the Hong Kong Airport.

Could these conditions have occurred during the early morning hours of January 27th?  I think the answer could be yes.

As the offshore front approached, the wind approaching the crest to the south of the lake increased (see topographic map, which indicates the key terrain features and the direction of the flow).


During the period in question, cooler air near the surface (in  the southeasterly flow) was surmounted by warmer air above.   This results in increasing stability above crest level.  And with southeasterlies at the surface and southwesterlies developing aloft, this led to the development of a critical level, where the flow reversed.   So all the factors supporting a strong mountain lee wave and potentially a rotor were in place.

But do any observations suggest such a development? 

The development of a strong wave would result in substantial sinking along the lee slopes of the terrain feature.  Sinking causes warming and pressure falls.  We happen to have a weather observation just to the north of the terrain slopes (located on the south side of Lake Quinault, see map).    Wow...there was a sharp pressure fall around 1:30 AM, just as the big blowdown occurred (see below).  Suggestive.


But we have a tool that Sherlock would be envious of:  high resolution numerical simulations.  Considering the small scale of the blow down, I suspect we would need to run our model (called WRF) with uber-fine resolution (grid spacing of around 100 meters).   The best the National Weather Service models do is around 4-km.  Our UW WRF is 1.3 km.    But for this case, UW graduate student Robert Conrick took WRF down to 444 meters and fellow student Nick Weber has produced some nice graphics.   

So let us see whether we can simulate this event...or at least determine whether we are on the right track.  I am going to show you a series of vertical cross sections, oriented SSE-NNW, that pass over the blowdown site.  Each cross section will have potential temperature (solid lines), wind vectors  in the cross section, wind speed (color shading) and vertical motion (blue for descent and red for ascent).

At 0400 UTC (8PM), you can see wave-like undulations in the temperature, modest downslope on the terrain and some weak northerlies over the blowdown area at low levels.

As 1240 AM (0840 UTC), the flow had strengthened greatly aloft and a rotor was obvious in the lower atmosphere over the valley.
 The rotor strengthens over the northern side of the Quinault Valley at 0850 UTC
And at 0915 UTC (1:15 AM 27 January), all hell breaks loose with huge amplification of the wave pattern, with stronger northerlies at low level, just as they did in reality.
The amplification at this resolution (444 m) was much greater than for the coarser grids (e.g., 1.3 km or 4 km), and I suspect amplification would be far greater if we ran the simulation at 100 m or less.

But we have seen enough, I believe.  The strong winds were not from UFOs, an angry Sasquatch, a microburst from convection, or some errant meteor.

An approaching front produced just the right conditions to produce a high amplitude mountain wave on the upstream ridge, which resulted in a strong rotor that produced powerful reverse flow (northerlies).   As in the research work cited above, a very energetic subrotor was probably produced, and that resulted in a localized area of intense winds as it rotated down to the ground.

Perhaps we will try going down to higher resolution, but I have substantial confidence that the puzzle is solved.  If I were Sherlock Holmes, I would take out my violin.   But my reward, other than the satisfaction of completing a large puzzle, will be to catch up on the Olympics...or to watch one of my favorite TV shows---Air Disasters--but don't tell anyone.

_______________________________


Announcement:  A very interesting free lecture open to the public

The history of cloud seeding to enhance precipitation, and prospects for the future.  Professor Bart Geerts, University of Wyoming


February 15th, Kane Hall, University of Washington Campus, 7:30 PM
For information and to register go here:

Friday, February 9, 2018

La Nina-Like Cool Period with Some Limited Lowland Snow

(Sherlock Holmes and the Olympic tree fall will return on Sunday---and the answer may be in hand!)

Relatively cool air is now over the Northwest and should in place over the weekend.  And a few folks over the lowlands might seem some flakes before the weekend is over.

With high pressure offshore, northerly winds have developed in the lower atmosphere, as seen in the time-height plot above Sea-Tac airport (below, red is temperature, heights in pressure--850 is about 5000 ft, time increasing to the left).  Over the last 24 h, temperature at 850 hPa (again about 5000 ft) has dropped from 3C to -4C.


The cool air will continue to spread southward today and, as shown in the plot below, will reach the Oregon border by 7 AM (1500 UTC) tomorrow morning.  Note that there is a large pressure change (gradient) in northern CA associated with the leading edge of the cool air.  This makes sense, since cooler air is more dense than warm air, thus a gradient in temperature produces a pressure gradient.


Expect frosty temperatures on Saturday AM with cold air aloft and clearing skies (which allows good radiational cooling to space).

On Sunday morning an upper level trough will approach our region, bringing clouds and some precipitation.  I would be talking about the potential for lowland snow, except the trough is going too far offshore and south of western Washington....not quite the right set up for Puget Sound.


The 24-h precipitation total ending 4 PM Sunday shows plenty of precipitation along the coast, but little over Puget Sound (due to rain shadowing from the Olympics and Mountains of Vancouver Island under NW flow).


The forecast snow total for the same period is disappointing.  Not much on the coast because it is too warm there and nothing over Puget Sound.  Only over far NW Washington and southern BC, will there be sufficiently low temps and enough moisture to get a dusting.  And not much good for the mountains.


During the past few days and this weekend, we have had an area of high pressure offshore, with lower pressure inland, similar to the typical La Nina pattern.  But only similar... strong La Nina's have the high pressure farther offshore.  Here is the typical 500 hPa (upper level) height anomalies (difference from normal heights) for La Nina years, with red indicating above normal heights (pressures), and blue/purple the opposite.

Compare that pattern with the forecast pattern for 4 AM Sunday (below).  Similar, but the features are displaced eastward.  And it looks like La Nina's days are numbered....the latest model forecasts show a transition to neutral (or La Nada) conditions by summer.


In any case, there should be lots of sun on Saturday.

(Again, the third and final blog on the Olympic Mountain mystery tree fall will be released on Sunday....)

Wednesday, February 7, 2018

The Mystery Tree Fall Near Lake Quinault: Why Did It Happen? Part II

Update: The final solution is here.

One important development: Janet Cole of the National Park Service visited the site yesterday. Bill Bacchus, chief scientist of Olympic National Park, sent me this map of the damage, based on Janet's survey of the scene, and the following narrative:

"Most of the trees appeared to be wind thrown, but as you can see from the photos, many were also broken near the base. The amount of trees down was inconsistent, in some areas, nearly every tree is down, but the majority of the area seemed to have lost about 40-60% of standing trees. ... I am attaching an image which shows Janet's rough estimate of the blow down area including arrows showing the direction of most blowdown. Near the drainage outlet, the trees seemed to have fallen southeast, while the western edge trees were oriented more north south. In the eastern edge, the trees were closer to east/west"


And below is a box of similar dimensions to what she drew....with the sides about 1/2 mile long.  The prevalent wind direction was from the north-northwest with a tendency to splay out towards the lake.


Another interesting piece of evidence, in fact something quite unique in such weather detection work, is the seismic record at Quinault, something sent me me by Dr. John Vidale, who used to be WA State Seismologist.  At 1:26 AM that morning there was a lot of activity...and no tremors were noted at other regional stations.  Dr. Vidale suggests that it may represent the tremor produced by huge, falling trees.  Fascinating.  More evidence for the timing.
Now lets examine local weather stations to see if there is any evidence of strong winds or a major weather transition at surface stations.  Using the nice weatherunderground.com interface, we turn to the closest station on the south side of the lake, only a few miles away (KWAQUINA2, see map below).  The station stopped recording at 2 AM.

No real change in temperature or dew point (top panel, right figure).  Wind speeds were quite low (second panel, with blue being sustained winds, dots are gusts).  Steady rainfall (blue line in third panel).  And the pressure shows a weak downward trend (bottom panel).  But look closely.  VERY closely.  There is a downward spike at 1:25 AM.   Something abrupt and short-lived happened.   Is this important, or is it a red herring provided by some meteorological Moriarity?

 


Another station on the map (NE corner), a highly accurate NOAA climatological site, only has temperature and dew point. Here is the plot from that station (MQTCW1).  Nothing happening at the time..

Frustrating....perhaps the Sasquatch or alien visitation explanations should be taken seriously!  OK, let's move out a bit.

The station in the terrain to the SE showed nothing interesting, with moderate easterly flow during the period.  Wrong direction



And the station to the west-southwest was similarly bland, with southeasterly winds:



Our meteorological Sherlock is perplexed.  The trees fell to the south or southeast, implying a very strong northerly wind.   None of the surface locations shows strong wind and most of them are easterly or southeasterly.  Wrong direction.   There is no strong convection or thunderstorms, so no microbursts.

But there had to be something going on around the time of the tree fall...and there was---a frontal zone was approaching, with warm air and southerly flow surging in aloft, while cooler easterly flow dominated near the surface. 

The National Weather Service official surface analysis for 1 AM shows the front approaching, and it was associated with a low pressure system that would bring strong winds to western Washington later in the morning.


Could this be important?   We get fronts all the time without extreme local blowdowns as occurred on January 27.  Sherlock is unsure.

We will investigate this last lead in the next installment.

_______________________________
Announcement:  A very interesting free lecture open to the public
The history of cloud seeding to enhance precipitation, and prospects for the future.  Professor Bart Geerts, University of Wyoming
February 15th, Kane Hall, University of Washington Campus
For information and to register go here: