Blowing dust in Texas and Oklahoma

GOES-16

GOES-16 “Moisture” Infrared brightness temperature difference (10.3-12.3 µm) images, with hourly surface reports plotted in cyan [click to play animation]

Strong winds in the wake of a cold frontal passage created large areas of blowing dust across West Texas on 21 January 2018 — GOES-16 Infrared brightness temperature difference “Moisture” or “split window difference” (10.3 µm12.3 µm) images (above) showed that the leading edge of this airborne dust moved over far southwestern Oklahoma after 20 UTC.

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Cirrus” (1.37 µm) images (below) also displayed blowing dust signatures; surface visibility dropped to 2-3 miles at some locations, with one site reporting only 0.5 mile. The signature was apparent on the Cirrus imagery since this spectral band can be used to identify any airborne particles that are effective scatterers of light (such as cirrus ice crystals, volcanic ash, haze, or dust/sand).

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with hourly reports of surface weather plotted in red and surface visibility (miles) plotted in red [click to play animation]

GOES-16 Near-Infrared

GOES-16 Near-Infrared “Cirrus” (1.37 µm) images, with hourly reports of surface weather plotted in red and surface visibility (miles) plotted in red [click to play animation]

 

 

Very Strong Cold Front Crosses the Coast

Sometimes you see an image that really gets your attention...and the radar image this morning from the Langley Hill radar (near Hoquiam) was one (see below).
This image shows reflectivity, which is related to precipitation intensity--with higher values (pouring rain or hail) shown by red.

A very strong front is seen, with the red lines indicative of a narrow cold frontal rain band, which is found at the actual location of the cold front.

Do you see how the red line is corrugated, with heavier areas and gaps?   This is characteristic of the very strongest fronts, and produced by an instability associated with strong shear across the front.   Just amazing.


Below is a blow up to give you a closer look.  This intense cold front is associated with a temperature change (colder behind), a trough of low pressure, and a very rapid wind shift--from southerly in front to westerly behind. 

The frontal characteristic are not uniform along the front, but are much stronger in the red areas, compared to the break locations (known as gaps).



The National Weather Service has some coastal buoys that can show us what the transition was like at the surface. At Buoy 46041, just off the WA coast, the pressure (green line) fell rapidly until 15 GMT today (7AM), and then rose very rapidly.   Wind gusts (red lines) increased to 47 knots, before plummeting after frontal passage.


At the same buoy, there was a sharp wind shift  from southerly (about 175 degrees) to southwesterly (roughly 240 degrees) with frontal passage (see below).  Temperature dropped several degrees with frontal passage (not shown).


As suggested by the 2 PM visible satellite figure, the front was over the Cascades at that time, with very cold unstable air behind it (strong convective cells are seen over the Pacific as colder air went over warmer water...which makes for big instability).


As a numerical modeler, the first thing I looked at was the simulation that started last night at 4 PM.  

Did it get the strong front?  It did.  

Here is a 16 hour forecasts for the very highest resolution run at the UW (1.33 km grid spacing) for pressure/wind and precipitation/wind, valid at 8 AM Sunday.  Sharp wind/pressure transition and even the hint of the corrugated precipitation pattern in the right hand figure.  Impressive

 
All that instability and convection should produce significant snow over the Cascades during the later afternoon and evening.

About Last Night…

If you want blower pow, last night was your storm.  From 5 PM yesterday to 6 AM this morning, the Alta-Collins observing site picked up a paltry .32" of water equivalent, but 10" of snow.  That's a mean water content of only 3.2%!   That's a snow-to-liquid ratio of 31-to-1.  If you want a big dump, but don't have a lot of water to play with, that's how to do it. 


Really, conditions overnight were only marginally better for orographic precipitation generation than they were yesterday.  The 0000 UTC (5 PM MST Saturday) sounding still showed stable conditions at mid level with strong wind shear from 700 to 600 mb. 


The morning sounding is a bit better with northerly flow through depths and colder temperatures aloft, although a weak stable layer remained just above 700 mb. 

But the secret to the overnight dumpage wasn't large orographic precipitation enhancement.  There was some enhancement, but .32" of total water and a maximum water equivalent rate of 0.05" per hour isn't much.  The secret was the huge snow-to-liquid ratio.  If the snow-to-liquid ratio for this storm was Alta's average of 13-to-1, the snowfall would have been 4 inches.  But at 31-to-1 you get 10 inches and the stuff that Kodak moments are made of.  Yup, this was a snowfall that will bring smiles to skiers, but continue to give water managers, who need to see higher water content dumpages, heartburn. 

A Good Front Followed by Bad Orographics

A cold front moved across northern Utah last night bringing much needed snow from the valley floor to the highest peaks.  Powder panic brought gridlock to most (probably all) routes to the Cottonwoods, providing an all-too-frequent reminder that we are loving our canyons to death. 


Those who braved the traffic were rewarded with knee deep powder.  I would describe the skiing as good, not great.  A spongy layer of high-density snow to start would have helped reduce the bottom feeding, but this year, beggars can't be choosers.  



Snowfall produced by the front went largely as advertised by the models, at least in the upper Cottonwoods.  Alta-Collins had .59" of water and 7 inches of snow through 8 AM this morning.  This compares very well with the NAM forecast we discussed on Tuesday, which put out .64" of water and 8 inches of snow (see Frontal Snowfall Event on Tap for Late Tomorrow and Tomorrow Night).  The observed water totals are also near the middle of what was advertised by the University of Utah downscaled SREF ensembles.  The model wizards can be happy about this period.

I suspect that those hoping for a true storm ski day were a bit disappointed, however, with today's offering.  Light snow fell for much of the day, but since 8 am it added up to only .15" and 2 inches of snow at Alta Collins, which is probably a bit more than what we saw where we were ski touring in Big Cottonwood Canyon.  

Quite frankly, the orographic forcing today simply sucked, which we discussed as a possibility yesterday (see Probabilistic Snowfall Forecasting).  The morning sounding shows the situation quite well.  The atmosphere was quite moist, but also generally stable below 700 mb (10,000 ft), with a strong stable just above, which is associated with the front aloft.  The flow at low levels was northerly, but swung to southwesterly in the strong stable layer.  This is simply not a recipe for orographic enhancement.  

Source: NCAR/RAL
During the afternoon, the flow on Alta-Mt. Baldy slowly shifted to northwesterly to westerly, but remained weak and with high atmospheric stability, wasn't generating much at upper elevations.  


Instead, the radar loop below shows the development of the strongest echoes along the east bench and within the lower canyons through 0028 UTC (5:28 PM MST).  


Driving down Big Cottonwood late this afternoon, it was clearly snowing harder in the very bottom of the canyon and along the east bench than it was in the upper canyon.  Note that you can also see this effect in the Oquirrh Mountains where the radar returns, especially later in the loop are stronger on the lower and mid elevation western slopes.  

The devil is in the details.  

Newest Cloud Types Seen in the Pacific Northwest

In  March 2017, an updated International Cloud Atlas was released and several new clouds were added. 

One new cloud is Asperitas, characterized by a complex wave-like base.   This week (January 17th), James Dearman sent me this wonderful shot of an Asperitas cloud that he took near Vancouver, Washington (see below). 

Pretty amazing cloud, with undulations looking like a sea surface turned upside down.


Here is another example from Virginia:


These clouds result from wave-like motions in the atmosphere that distort a pre-existing cloud deck.   Such waves can be initiated by fronts, thunderstorms, or some other type of atmospheric disturbance.  Generally no precipitation or severe weather with these clouds--but they do look scary.

Folks normally don't think about it, but the atmosphere is full of waves.    Generally, the atmosphere is stable, meaning if you push an air parcel upward, it will want to return to where it started.  But like a swing set, the air parcel tends to overshoot and goes into an oscillation.  A pendulum is another example.


Another new cloud is one we have talked about in this blogCavum, or the hole punch cloud (see below).   These "holes" are produced when an aircraft goes through a cloud made of supercooled water (liquid water below freezing).  The passage of the aircraft through the cloud causes a transition to ice crystals, which subsequently fall out, leaving a hole.




Ice dam in Lake Erie

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with hourly surface wind barbs plotted in yellow and wind gusts (knots) plotted in cyan [click to play animation]

Thanks to Dave Zaff (NWS Buffalo) for the email alerting us to an ice dam that had formed across the eastern portion of Lake Erie on 19 January 2018 — GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed that the northeastward drift of ice floes was effectively being blocked by this ice dam feature.

A toggle between 250-meter resolution Terra MODIS True-color and False-color Red-Green-Blue (RGB) images from the MODIS Today site (below) provided a more detailed view of the Lake Erie ice dam and upwind drift ice at 1615 UTC. Snow and ice appear as shades of cyan in the False-color image, in contrast to supercooled water droplet clouds which are shades of white.

Terra MODIS True-color and False-color RGB images [click to enlarge]

Terra MODIS True-color and False-color RGB images; red arrows denote the location of the ice dam [click to enlarge]

The Terra MODIS Visible (0.65 µm) image with an overlay of RTMA surface winds (below) showed the southwesterly flow across the long axis of the lake.

Terra MODIS Visible (0.65 µm) image with surface METAR reports and RTMA surface winds [click to enlarge]

Terra MODIS Visible (0.65 µm) image with surface METAR reports and RTMA surface winds [click to enlarge]

A toggle between 1607 UTC Terra MODIS and 1757 UTC Suomi NPP VIIRS Visible images (below) showed the motion of the lake drift ice during that time period.

Terra MODIS and Suomi NPP VIIRS Visible images, with METAR surface reports [click to enlarge]

Terra MODIS and Suomi NPP VIIRS Visible images, with METAR surface reports [click to enlarge]

Probabilistic Snowfall Forecasting

For decades, snowfall forecasts have typically involved the issuance of a range of accumulation amounts, typically (but not always) based on a factor of two.  For example, 3-6 inches, 4-8 inches, etc.

I have no idea why.  Perhaps it is a convenience thing.  Maybe people like it that way.  I don't know what that range even means.  Does it represent the middle 50% of possible outcomes, with a 25% chance of more and a 25% chance of less?  Does it represent the middle 80%?  Why always use a factor of two?  Sometimes the range needs to be bigger, especially in longer range forecasts. 

And then there is my favorite, "higher amounts in favored locations."  What the hell does that really mean and how do you verify it? 

There was a time when snowfall forecasting was truly guesswork, but things are changing.  Computer models are now capable or will soon be capable of simulating smaller storm details, what meteorologists call "cloud scale."  Ensembles can be used to better estimate the future outcomes.  There remains much work to do, but there is great potential to dramatically improve snowfall forecasting. 

The National Weather Service is now producing experimental probabilistic snowfall forecasts, and they are available at https://www.weather.gov/slc/winter.  They provide much richer information about storm potential than a simple range.  For example, ,aps are provided showing the likelihood of snowfall above several thresholds, an example of which is the probability of 6" of snow or more for the period from 5 AM today to 5 PM Sunday, shown below. 

Source: NWS
They also provide a table with snow amount potentials and probabilities of snow within certain ranges, shown below, as well as above certain thresholds. 

Source: NWS
Readers of this blog are snow lovers.  Start perusing these forecasts and provide feedback through the links on the page. 

Now, to clarify some of my scattered comments to yesterday's post about the situation on Saturday.  Although we have a front pushing through tonight, it is a very slow mover.  As a result, this is not a frontal passage in which we quickly get into deep, unstable, northwesterly flow on Saturday morning. 

This is evident in the NAM time-height section for Alta below.  The front at Alta is a late arriver (light blue line), in this case moving through at or just after 0600 UTC (11 PM MST tonight).  Then, look at the winds behind the front on Saturday (circled).  They are NNW at low levels, but NNE near 700 mb (10,000 ft) and then SSW at 600 mb. 


This reflects the slow movement of the front through the area. 

If we look at the sounding for 1800 UTC (11 AM MST) Saturday morning, we see the low level northerly flow, but note how the winds shift to NNE and then SSW with height.  The temperature and dewpoint traces show a sharp inversion just above 700-mb, or 10,000 feet.  
This is not a recipe for our classic northwesterly instability snow showers over Alta for two reasons.  First, the flow direction isn't right.  Second, the instability is too shallow.  

However, if you look at the sounding for 0000 UTC (5 PM MST) tomorrow afternoon, the low level flow is NNW through a deeper layer, although a capping inversion remains based just below 600 mb.  This is closer to what is needed for the NW instability showers, but the capping inversion height is right on the edge of what I would like to see.  Tough to say if it's high enough that Alta can benefit, or just a bit too low so that the mid and lower canyons and east bench do better.  
And that's just one model run.  There are variations in the timing of these changes, wind directions with height, etc., if one looks at other models.  

All of this illustrates what a complex mess this is for Saturday and why probabilistic forecasting is necessary.  The good news is there's enough going on that in the end, this will be a decent storm for the mountains and even the mountain valleys after snow levels lower today and this evening.  

Big Waves Hit the Northwest Coast from an Immense Storm Offshore

The Washington Coast is being pummeled by unusually big waves, some as high as 30-40 feet!  For example, buoy 41, right off the central WA coast observed a significant wave height of over 30 ft (see plot and map below)



Buoy 50, right off of Newport, Oregon, got to 35 feet this morning.


And a Westport surf cam shows an angry sea.


Or this scary video from Westport taken by Bryan Benoit:


My colleagues at the National Weather Service currently have coastal flood and high surf warnings up for the Washington and Oregon coasts.

 The National Weather Service runs a wave prediction model, called WaveWatch III, the predicts wave heights (using weather prediction models to drive the wave model).  The WW3 forecast of significant wave height for 1 AM last night predicted a slug of very high waves of 13-14 meter height (43-46 ft) approaching our coast.

No wonder my friends at the National Weather Service had warnings out!

Here is a blow up of the predicted waves at the same time.
Why such big waves?  Because there is a HUGE, intense, and slow moving storm (midlatitude cyclone) over the northeast Pacific.

I mean a stunningly big storm.  Here is an infrared satellite image last night...you can see the immense swirl of clouds circling the system.


The sea level pressure analysis at 4 PM Wednesday shows the extraordinary system, with a central pressure of 964 hPa....very deep for a low that far south.

You notice the extreme pressure gradients around the storm, particular on the south and west sides?  Those are associated with very strong winds.
In fact, the surface wind forecast from the UW WRF model for 7 PM Wednesday, indicates SUSTAINED winds of 50 knots around the south side of the storm.

Wind waves depend on the strength of the winds, the distance the winds are blowing over the water (the fetch), and the length of time the winds work on the water.

With a huge, intense, slow-moving storm like this, all of these elements are maximized to produce big waves offshore, which propagate away from the storm (and towards us) as swell.

Be very careful if you go wave-watching on the coast.  Sneaker waves can come in and inundate previously dry locations. 

And I hope that no cruise ships are in the area....

Frontal Snowfall Event On Tap for Late Tomorrow and Tomorrow Night

After another 10-day or so stretch with limited to no accumulations, our next storm will be served up late tomorrow and tomorrow night. 

Most of the precipitation for the central Wasatch looks to be primarily frontally forced.  The large scale setup is shown below and features an upper level trough that is initially tilted from southwest to northeast (referred to as "positively" tilted by meteorologists) that closes off and becomes more north-south oriented as it moves inland across the western US.  




This has both pluses and minuses for snowfall prospects in the central Wasatch.  The plus is that the front may slow as it drags through northern Utah, extending the period of frontal snowfall, as depicted below in the 1200 UTC NAM forecast.  At 000 UTC 20 January (5 PM MST Friday), the surface front is over Utah County with precipitation over the northern Wasatch.  


Frontal precipitation fills in, however, as the front phases with moisture sneaking around the southern end of the Sierra Nevada over the next 3 hours. 


That precipitation continues for another 3 hours as the front makes slow progress into southern and eastern Utah. 


By 0900 UTC 20 March (2 AM MST Saturday) the main frontal band is just downstream of the central Wasatch, with some post-frontal snow showers persisting. 


The minus for snowfall prospects is with the low closing off, the post-frontal winds shift very quickly to northerly, when we would prefer a period of northwesterly flow for better orographic forcing.  Note in the Salt Lake City time height section below that the post frontal flow is predominantly northerly and deepens gradually from about 0Z Saturday through 6Z Sunday.  



Actual numbers derived from the 12Z NAM show the wet bulb zero dropping during the day Friday (snow level is usually about 1000 ft below this level), with values low enough that most of the precipitation produced during this event should fall as snow in the mountain valleys.  Perhaps Mountain Dell might see a bit of rain to start, but then turn over to snow.  Total water equivalent at Alta is 0.64" through Saturday at 8 AM, with snow densities decreasing during the storm for a right-side up snowfall.  



Looking more broadly at the ensembles shows that the NAM is roughly in the upper half of the SREF plume for Alta.  Through 18Z 20 January (11 AM) the SREF members put out anywhere from 0.3 to 0.8 inches of water, the former being a slightly better than dust on crust event adding up to perhaps 4 inches of snow, the latter representing a lower end deep powder day with perhaps 10-12 inches of snow.  

That spread represents variations in the strength and speed of the front.  Increases in precipitation after 18Z 20 January occur in some model runs that are more bullish on the post-frontal precipitation.  

I continue to keep expectations low and hope for the best.  

What Happens to Fronts over the Western US

It's a question that fascinated me 30 years ago and continues to fascinate me today.

What happens to fronts over the western US?

The short answer is a lot, and it varies from case to case.

Let's begin with a loop of the front making landfall later today and tonight.  This loop presents sea level pressure in black contours, 850 mb (1500-m) temperature in red contours, and either radar or 3-hour forecast accumulated precipitation in color fill.  In this loop, note how the front appears to decelerate as it pushes inland across Oregon and northern California and ultimately takes on an "S" shape with the inflection near the trough that develops downstream of the Sierra Nevada. 


These and other effects are more apparent if we look at individual times.  At 1800 UTC 17 January, the system has the appearance of a classic occluded cyclone with a relatively smooth and continuous cold front extending from just off the Washington coast into the subtropics. 


Eighteen hours later, the front is pushing into Oregon and northern California (I've given up analyzing the fronts north of this location as they have become difficult to track).  The front at this time is beginning its inland transformation and is decelerating over southern Oregon and northern California.  At the same time, there is a temperature contrast just ahead of the front over California and the eastern Pacific (circled with a thin light blue line).  This is an important feature as a new front is beginning to form at the leading edge of this zone of temperature contrast.   


By 1800 UTC 19 January, the front and the temperature contours have taken on an "S" shape with the inflection over northern Nevada.  This S-shape is the result of several processes, including the deceleration of the front over northern California and Nevada, and the development of a new front ahead of the old Pacific cold front over the eastern Pacific.  Note in particular that the precipitation band accompanying the old Pacific front is well behind the leading edge of cold air at this time, consistent with a new front forming ahead of it. 


By 1800 UTC 19 January, the front has a strongly distorted into an S-shape.  One can quibble with the precise position of the cold front in my analysis over southern California and Baja California, but the S-shape is very clear in the temperature contours.  Again, note the separation between the cold front and the model precipitation. 


The processes responsible for this evolution are complex and not well captured by simple models of frontal evolution.  Where to position a front in an analysis is always a subject for debate, but the development of the S-shaped appearance is common as cold fronts make landfall in this part of the world. 

If you are wondering what all this means for snow, tough luck.  I've already spent too much time on this post and need to get some work done!  Maybe tomorrow.  Maybe.