Wednesday, February 21, 2018

The Olympic Mountain Snow Shadow and Prospects for Snow this Week

Some light snow is falling over Seattle now, as a weak upper level disturbance moves though.  Perhaps a half-inch on the ground in North Seattle.

But in general, Seattle has been left out of the bounty, even though we are cold enough to snow.  More snow north, south, east and west of us.  But why are we being short changed?

There IS a reason.

Let's start by showing the distribution of snow depth tonight at 10 PM from the NOAA National Snow Analysis.   On the left, is last night and on the right, 10 days ago.

Quite a bit more over the west lowlands, particular south of Olympia, and east of the Cascade crest.  Very light amounts over Seattle and south King County.



A year ago? More snow than this year over the Oregon Cascades, less over the western lowlands and slightly more over the WA Cascades.

So why has snow been generally light or absent over central Puget Sound?

A major reason:  winds in the lower atmosphere have been too persistently out of the Northwest, leaving Seattle in the rain shadow of the Olympics.  To illustrate, here is the National Weather Service precipitation analysis for the last week....lots of precipitation in the mountains, but major rain shadowing over Puget Sound. And with cold air over us, that means snow shadowing as well.


Another issue is that the atmosphere is relatively dry west of an upper level ridge--and that is the pattern we have had for days (see map).


Only when a strong trough works its way around the ridge and drives southward can we get real snow in Seattle in such a situation.

Getting back to the current snow situation....snow should end during the next few hours as the upper trough moves by.  Thursday should be dry.

But we have another chance of snow on Friday, as two weak disturbances make their way south from SE Alaska to our region on Friday/Friday evening, and another on Sunday (see upper level maps for these times).




There is a good chance for light snow over the lowlands on Friday...and much more in the mountains.   Here is the European Center  high-resolution 24-h snow total forecast between 10 AM Friday and 10 AM Saturday. Over a foot in the mountains and about an inch in the lowlands.


In their ensemble (many model runs), the Seattle accumulated snow forecast starting 4 AM this (Wed) morning shows the light snow today (good prediction of about a a half inch), but more (about an inch) on Friday.  I should note that there is some uncertainty in the exact amounts, but some snow looks likely.

So at this point, no big snowstorm in view (at least through Saturday), but enough to think about getting that hot chocolate and enjoying the flakes.



Monday, February 19, 2018

Record Cold and the Northwest Stays in the Freezer for a While

The air over us now is unusually cold for this time of the year, in fact, record breaking cold in some ways.  For example, let's start with the temperature around 5000 ft (850 hPa pressure) at the radiosonde station at Quillayute, on the Washington Coast (see below).   In this plot, the blue line shows the daily record low 850 hPa temperatures.  Today's temperature (-12.5C, shown by a circle) was not only a record for the date, but the lowest for the surrounding dates as well.  Really unusually cold.

The surface air temperatures last night (see below) were held up by the windy conditions (which mix down warmer air above the surface) and some clouds, but still temps dropped into the mid 20s F in much of western Washington and single digits in the mountains  Some valley locations dropped below 0F.  Teens dominated in eastern WA.


Tonight is going to be much colder, with colder air aloft, clear skies, and less wind. The temperatures predicted for tomorrow morning suggest mid-20s in the west and single digits east of the Cascades, with some valleys in eastern WA dropping below -8F.


The latest forecast model output predicts another four days of the really cold stuff.  Here is the NWS GEFS ensemble forecast for Seattle.   Several more days of lows in the mid-20s ahead.


The large scale atmospheric pattern is really locked into a super La-Nina configuration with high pressure offshore and cool northerly flow over the NW.   To show you this, here are the upper level (500 hPa) weather maps for Wednesday and next Tuesday. Quite similar really.


There are occasional disturbances moving southward in to the flow east of the upper level ridge of high pressure that will bring some occasional precipitation, and in some places, snow.

To illustrate, here is the total snow fall forecast for the next 72 h.  Most of the snow is heading for Oregon and SW Washington.  None over Seattle or the WA Cascades.


The next 72 hour is quite different, with massive snows in the WA and Oregon Cascades. Skiers will be pleased.

Keep warm....

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: