Friday, December 31, 2010

It's Bloody Cold!

As discussed in a November post, there are probably only a few days each winter during which the 700 mb  (roughly 10,000 feet) reaches -20C or colder.

Today was one of those days.  The 700 mb temperature in this morning's sounding from the Salt Lake Airport was -21.1C, or -6.0F.

With a low sun angle, it felt really cold out today, and it was.  I somehow survived a 6-hour ski tour, but only by moving almost continuously and stopping only very briefly to eat or drink.   As an example of how cold it was, the Solitude-Apex observing site at 9061 ft recorded an afternoon maximum temperature of -1.5F.

Snow conditions were, however, unbelievable.  Bottomless cold smoke.  To the sufferers go the spoils.

Thursday, December 30, 2010

Overnight Lake Effect

The post-frontal phase of our storm last night produced mostly lake-effect precipitation, which at times organized into wind-parallel bands.

Note how the coverage, structure, and orientation of the lake-effect is quite variable.  There is no computer modeling system or forecaster that can reliably predict such variability.  This is why it is so difficult to provided detailed lake-effect forecasts.  About all you can say is that lake-effect is possible downstream of the lake, but we don't know the details of where, when, and how much.

Wednesday, December 29, 2010

Remarkable LCC Hourly Snowfall

To put it scientifically, it absolutely puked snow in upper Little Cottonwood Canyon during frontal passage.  Check out these observations from the Alta-Collins (CLN) MesoWest site.

Note the observation between 1600 and 1900 MST.  The snow interval board was wiped clean at some point after 1600 MST, with 5 inches falling by 1900 MST.  Thus, we know a5 inches fell in at most 3 hours.  Further, assuming the precip guage is working correctly, only .06 inches of SWE fell from 1800-1900 MST, so much of that snow probably fell during the previous 2 hours.  

We can, however, better pinpoint the snowfall using the Snowbird snowstake.  Check it out.  The snow depth goes up 4 inches between 4:12 PM (1612 MST, 2312 UTC) and 5:12 PM (1712 MST, 0012 UTC).  Four inches an hour.  Impressive!!!

This incredible hourly snowfall occurred as the cold front moved across the Cottonwoods.  Check out the radar loop.  You can see the band of enhanced radar reflectivity associated with the cold front move into the Salt Lake Valley.  Although it becomes more difficult to follow as it approaches the Wasatch, you can clearly see a band of higher reflectivity continuing toward the southeast, especially near the Wasatch Crest.  Thus, there must have been a heck of a pulse of heavy snow not only in the Cottonwoods, but also moving down the Wasatch to at least Mount Timpanogos.  The band appears to move across the Cottonwoods between about 2300 and 0000 UTC, roughly the period when it was puking snow. 

All of this happened just after the close of skiing for the day on a holiday weekend.  Must have been a nightmare getting out of the canyon today.  

Here Comes the Front

Beautiful cold frontal precipitation band evident on radar as the front approaches Salt Lake.  Check out how the front becomes distorted as it interacts with the Stansbury Mountains and other ranges to the west.  In particular, the frontal precipitation band slows near the Stansbury Mountains, leading to the development of a prominent S-shape.

We've seen similar frontal distortions before, including during the 2002 Tax Day Storm (see West and Steenburgh 2010).

Major changes in the spatial structure and intensity of orographic precipitation should accompany the cold front as the flow shifts to northwesterly.  Look out Cottonwoods!

Ruby Mountain Spillover

The Ruby Mountains are a spectacular mountain range located near Elko, Nevada.  They are very similar in appearance to the Wasatch Mountains.  In fact, upper Lamoille Canyon is about as close as you can come to a carbon copy of Little Cottonwood Canyon.

Looking down Lamoille Canyon
from the Ruby Mountain Crest
With peaks over 11,000 feet high, the Ruby Mountains are a formidable barrier to weather systems over northern Nevada.  The Elko radar today showed a remarkable transition in precipitation spillover across the Ruby Mountain crest (red line below) with the passage of the cold front.  Check out how in the prefrontal environment there was a decent extension of higher radar reflectivities into the lee of the Ruby Mountains, but in the post-frontal environment, there is a sharp gradient very near the crest.

Watching this loop makes me think that a ski trip to Lamoille Canyon is needed...soon.

Tooele Rainshadow and Wasatch Mountain Precip

One of the most persistent features in the radar loop this morning is the rain shadow over the Tooele Valley.

Reflectivities are consistently high upwind (south) of South Mountain over the Rush Valley, but there's a sharp gradient in reflectivity over or just to the lee (north) over the Tooele Valley.  Even small, narrow ranges like South Mountain can have a dramatic influence on precipitation if they excite high amplitude mountain waves with strong subsidence in their lee, which appears to be the case today.  I wonder if it almost looks Foehn-like if one looks south toward South Mountain from the Tooele Valley.

Precip totals since midnight near the crest of the Wasatch Mountains are very consistent with expectations during large-scale southwesterly flow.  Big values on Ben Lomond and in the northern Wasatch, and then further south around Timpanogos, Cascade Mountain, etc.  From north-to-south (SWE in inches):

  • Ben Lomond Peak: 3.6
  • Snowbasin Middle Bowl: 1.41
  • Alta-Collins: 0.71
  • Aspen Grove: 1.57
  • Cascade Mountain: 1.5
Bummer that we don't have access to a reliable precip site on the Park City side of the range.  

Border to Border Precipitation and the Wasatch Blocking Front

Radar echoes this morning extend from border to border.  It's a busy weather day in the western United States!  Looks like southern California is once again bearing the brunt of the storm, at least so far.

In Utah, we are presently in large-scale southwesterly flow with warm advection ahead of an approaching cold front.  The morning sounding summarizes the situation fairly well.  The atmosphere is absolutely stable above about 775 mb.  Veering winds with height above this level reflect to some degree the ageostrophic flow in the Salt Lake Valley, but also the large-scale warm advection.

Steady precipitation is falling over the entire Wasatch Range this morning.  The persistent development of radar echoes upstream of the Wasatch strongly suggests that orographic processes are essential for precipitation at this time.

One might wonder why the radar echoes start so far upstream of the initial slope of the Wasatch Mountains.  In stable situations, the flow can be strongly blocked, and a blocking front and related ascent can form upstream of the Wasatch Mountains, typically near Antelope Island.  This appears to be the case today.  MesoWest data show along-barrier south-southeasterly flow extending along the Wasatch Front from Salt Lake to Roy.  In contrast, the flow at Hat Island is SSW.

Cox et al. (2005) examined a similar case of precipitation enhancement near Ogden using dual-Doppler analysis and developed a simple conceptual model for events like this.  The blocked along-barrier flow is found beneath the dashed line in the cross section.    

Situations like this are usually big precip producers on Ben Lomond Peak north of Ogden.  From 2 to 7 am this morning, the snow depth at the Ben Lomond SNOTEL site increased 11 inches, with nearly 2 inches of water.  It appears they are getting hammered with high density snow.

That's what is happening this morning.  The storm dynamics will change today, however, with the approach and passage of the cold front.

Tuesday, December 28, 2010

Making Sense of Mountain Weather Obs

Making sense of mountain weather observations is not always as easy as it looks.  Today provides an example where the temperature observations in the Cottonwood Canyons are difficult to explain and, in some cases, may be influenced by instrumentation problems.

Before looking at the data, let's have a quick review of the overnight weather and morning sounding.  Skies were mainly clear overnight, leading to great conditions for radiational cooling.  Winds were also light.  Under such conditions, cold pools/inversions typically form in the valleys and canyons of the Wasatch Mountains.

Today we also have a large-scale inversion near mountain top level.  This inversion shows up very well in the sounding from the Salt Lake City Airport, which has a surface based inversion as one typically finds on a clear morning, and an elevated inversion located near 700 mb (about 10000 feet).

This inversion further enhances the stability of the mountain atmosphere.  It also allows for inversion conditions to persist until later in the day when the valley cold pools are typically removed by surface heating.  For example, at 11 am this morning, it was 21 at the top of the Collins chair at Alta, but still only 16 at the base.

This makes sense so far, but if you look at the MesoWest plot above, you'll also see some sites that have really warm temperatures:
  • Solitude Summit - 34F
  • Solitude-Apex-Top - 32F
  • Snowbird SNOTEL - 30F
  • Mill-D SNOTEL (near Reynolds Peak) - 31 F
Balmy!  But are these temperatures reflective of the air temperature at these sites?  That's a tough question, and here's why.  Many of the temperature gauges used by SNOTEL stations and ski areas are not aspirated, meaning that they don't have a fan to constantly cycle air through the gauge.  As a result, they tend to "spike" or get anomalously warm when the sun is out and the wind light.   

Although I'm sure it feels quite warm if you are sitting in a sunny calm location in the mountains today, I suspect that the high temperatures above are largely due to poor aspiration of the temperature gauge.  Why?  Note that at the Alta Collins and Mount Baldy sites the temperature is only 21 and 17 F, respectively, which is much cooler than the stations noted above.  At both of these locations, a steady wind of at least 10 knots is blowing, which is sufficient to aspirate the gauge so that its temperature is more representative of the actual air temperature.  

These sort of quandaries are not unusual when interpreting weather observations, which are not ground truth.  Observations have errors and problems.  As such, it's good to remember that "all observations are bad, but some are useful."   

Monday, December 27, 2010

Major Changes Are Coming

Major changes are coming this week, with the Snowmiser making a return to Utah.

We'll have a fairly mild day tomorrow (Tuesday), but then a major upper-level trough and surface cold front approach Utah on Wednesday, which looks to be a day of transition.  

Following passage of the cold front, temperatures in the Wasatch will drop precipitously and we'll see a return to the Greatest Snow on Earth (in contrast to the Cascadian storm cycle we had last week).  In fact, by Thursday, 700-mb (i.e., crest-level) temperatures fall to around -18C over northern Utah. 

What this means in American units (degrees Fahrenheit) is sub-zero temperatures in the high alpine and high temperatures in the single digits near 8000 feet.  Time to break out the down or puffy jacket.

Sunday, December 26, 2010

Downtown Fog

There's a thin layer of fog blanketing downtown and portions of the northern Salt Lake Valley this evening, making a spectacular "San Francisco like" scene looking south from the Avenues.

Looking to south-southeast toward Wasatch Mountains
Looking to southwest toward downtown

Death of a Cold Pool

It's been an interesting few days down here in the valley.   Most cold pools (a.k.a. inversions) start clear.  Then, if conditions are right, fog develops after a few days, followed by a gradual evolution into a cloud-topped mixed layer with stratus.   Pataki et al. (2005) describe one such event.

The current event is unusual in that stratus blanketed the valley at onset, but dissipated over time.  Setting all of this up was the pre-existence of a cloud-topped mixed layer with stratus, which was a leftover of the incredibly wet storm cycle from the preceding week.  Usually, we don't have that layer of clouds during the initial stage of a pool event.  Then as typically happens during cold pool events, a subsidence inversion developed and gradually descended.  This led to a thinning of the cloud-topped mixed layer and ultimately the demise of the stratus.  The descent of the subsidence inversion is very well depicted in the morning soundings over the past three days.

What we are left with this morning is a shallow valley inversion surmounted by a mid-valley stable layer. Aerosols in the shallow valley inversion are quite apparent looking south from the Avenues.

Now, if the current cold pool were to persist for a week or two, perhaps we would see the usual evolution from here with fog forming after a few days and then a cloud-topped mixed layer.  Fortunately, Mother Nature has other plans in store as the coming week looks to be an active one with a bonafide return to winter-like temperatures.

Friday, December 24, 2010

Salt Lake stratus, valley circulations, and mountain surface hoar

If you were in the mountains today, you may have observed a nice example of the diurnal "breathing" of the canyons of the Wasatch Mountains.  Little Cottonwood Canyon, for example, was cloud free at about 9:30 am.

Looking west down Little Cottonwood Canyon
9:30 AM 24 Dec 2010
This afternoon, however, the stratus had pushed back up the canyon to an elevation of about 7000 feet. 

Looking northwest toward Little Cottonwood Canyon
1:30 PM 24 Dec 2010
The cloud evolution is consistent with the diurnal circulations that occur in Little Cottonwood Canyon.  Down valley drainage flows at night erode and advect the stratus out of the canyon at night, whereas up-valley flows during the day transport it back into the canyon.  Those of you on the east bench around Olympus Cove were treated this morning to a brief period of clear skies as the morning outflow from Parleys, Emigration, and Big Cottonwood eroded away that stratus overnight.  When those flows stopped, the stratus returned.   

Also of note is the surface hoar we found at elevations below 9000 feet in White Pine Canyon.

Curiously, there was little surface hoar above 9000 feet.  At issue is if this is a reflection of the large-scale temperature inversion that existed overnight.  The sounding from the Salt Lake airport this morning shows a pronounced inversion and decrease in specific humidity with height right at 700 mb (about 10,000 feet).  

Even below this level, there is a fairly strong vertical gradient in specific humidity, so perhaps the potential for vapor deposition from the atmosphere to the snowpack decreased above about 9000 feet and this led to less surface hoar development?  

Cloud-topped mixed layer

Looks like those of us in the valley will be enjoying a cloud-topped mixed layer for Christmas Eve.  Stratus persists over the valley this morning and the views from the top of Snowbird this morning shows that its a gorgeous day in the mountains with undercast over the Salt Lake and Heber Valleys.

In a cloud-topped mixed layer, radiative cooling at cloud top drives turbulence and mixing in and below the cloud layer.  This is a different type of cold pool than we've had thusfar this year and should make an interesting case for the PCAPS program.  It is also the closest we're going to be to a "white Christmas" here on the valley floor.  

Thursday, December 23, 2010

Stratus sandwich and odds for a white Christmas

I woke up this morning in a sea of stratus that is still hanging around this evening.  In the mountains today,  Alta was in a stratus sandwich between stratocumulus at mountain-top level and undercast that topped out at about 7000 feet over the Salt Lake Valley.

With nearly all the valley snow gone (at least around the University of Utah), and no hope of snow in sight through Christmas, dreary stratus might be the only white we have this Christmas.  Perhaps it will burn off, but stratus is notoriously stingy this time of year.

Wednesday, December 22, 2010

SoCal-Las Vegas-St. George rains

It never rains in southern California, it pours.  At least that's how the 70's pop song by Albert Hammond goes.

Perhaps the same could be said of southern Nevada and southwest Utah.  This morning's radar loop shows widespread precipitation from SoCal-Las Vegas-St. George.  Note that the Cedar City radar is out, so we have a huge hole in radar coverage from St. George to at least Nephi.

Some impressive storm totals thusfar.  First, at 5700 feet in Zion National Park where storm totals are now approaching 7 inches, with more to come today.

Little wonder that the folks in Utah's Dixie are so concerned.  At the Las Vegas International Airport (KLAS), they had only a little precipitation prior to Monday afternoon.  Since then, over an inch has fallen, and it is really dumping this morning.  Keep in mind that their average annual precipitation is less than 5 inches, so this is a real gully washer for them.

Now let's go to SoCal where it never rains, it pours.  Palm Springs regional Airport (KPSP) is now over 3 inches in the past four days, with the heaviest rain coming in the last 12 hours.  This is over half of their annual average precipitation of about 5.5 inches.

Of course, the SoCal mountains are doing even better.  How about the Lytle Creek RAWS station in the San Gabriel Mountains.  Roughly 25 inches in the past five days.

Incredible stuff, fueled in large part by a persistent, slow-moving atmospheric river from the subtropics (a.k.a., the Pineapple Express).  The latest total precipitable water from the SSEC at the University of Wisconsin-Madison shows this quite nicely.

The good news is this event should come to an end tomorrow.  

Tuesday, December 21, 2010

Snow totals and radar limitations

Down south they are getting pounded.  Provo Canyon reporting 62" at 6100 feet for the 90-hour period ending at 6 am this morning.  The southern Wasatch Front lowlands are also getting into the action:  Levan 28", Nephi 26", Santaquin 22" as of noon, most this falling in the past 24 hours.

These storms drive me nuts and probably every other meteorologist in Utah too.  Our radar system provides  no coverage of winter storms over central Utah!  It's too far from the Salt Lake, Cedar City, and Grand Junction radars.  Ugh!

This makes it really tough to monitor what's happening in Millard, Juab, Sanpete, Severe, Emery, and a few other counties.  In particular, much of I-15 and I-70 are "terra incognita." You can only do so much with weather cameras, point observations, and spotter reports.  Radar provides critical time-space information for storm monitoring and nowcasting.

Snowpack looking great

Thanks to the latest ongoing storm cycle, all reporting SNOTEL stations in the Wasatch, Oquirrh, and Stansbury Mountains are reporting above average snowpack snow-water equivalent (SWE).

The big winner in terms of total SWE is Snowbird, with a whopping 24.5 inches, or 222.7% of average.  Throughout northern Utah, the snowpack above about 7500 feet is incredibly healthy, even in areas that are not especially snowy.  For example, the Mining Fork SNOTEL in the Stansbury Mountains has a snowpack SWE of 12 inches, or 267% of average.   This represents 58% of its average maximum SWE and the average snow water equivalent in mid February.

The Mill-D North SNOTEL continues to lag behind other sites in Utah and is "only" at 117% of average.  Thus, relative to average, the terrain between Big Cottonwood and Mill Creek Canyons may be one of the "thinner" snowpack areas.  Still, 10 inches of SWE is quite nice for December.

Monday, December 20, 2010

Steenburgh winter is here!

There are many ways to define winter.  Most meteorologists use 1 December as the start of winter for most climate studies.  Astronomical winter begins at the winter solstice, which this year is December 21.

Steenburgh winter starts the first day that the Alta-Collins observing site hits a snow depth of 100 inches.  This year, Steenburgh winter starts today!

This sounds ridiculous, but there is some method in the madness.  The frequency of hitting rocks at either the ski areas or in the Wasatch backcountry seems to drop quickly at around a 100 inch snow depth.  It's a nice round number and I've found over the years that some backcountry ski terrain is much more passible when the snowpack exceeds 100 inches at CLN (e.g., Maybird Apron).

For skiers, this is great news.  We have a deep snowpack and a low-angle sun to play with for several weeks.

Incredible Mammoth Mountain snows

Looking to the west, there are some incredible storm totals at Mammoth Mountain, which is reporting a storm total since Friday of 8.5 to 12.5 feet.  Yes, they measure the snow at Mammoth in feet.  Here's the snow-depth record over the past five days from their Sesame Snow Study Plot at about 9000 feet.

From 44 to 115 inches in 3 days.  Not too shabby!   According to their snow report, with 134" so far this month, they are 5 inches short of their snowiest December since records were first kept in 1969.  And, the computer models suggest the pounding will continue through Wednesday.

Alta-Collins dumpage mystery

Precipitation rates at the Alta-Collins observing site (CLN) increased dramatically last night, exceeding 0.3 inches per hour of snow-water equivalent (SWE) between 0200 and 0600 UTC (7-11 PM MST).  The peak hourly SWE rate was 0.5 inches from 0400-0500 UTC (9-10 PM MST) This brings their storm-total-SWE to over 5 inches.

Given the frontal passage last night, one might assume that this increase in precipitation rate accompanied the frontal passage, but this is not the case.  In fact, the winds on Mount Baldy were southwesterly during the heavy precipitation period and frontal passage at the Salt Lake International Airport occurred just after 0700 UTC (Midnight MST).  

Instead, during the period of heaviest precipitation, the radar shows a persistent orographic convection band initiating in the southwesterly flow near Lone Peak and then training off downstream to Alta.  It is this band that produces the exceptionally high precipitation rates at Alta, and it occurs in the prefrontal environment.   

At issue are the processes responsible for this band and if it is an example of the orographic convection bands that have been documented over the Coastal Range of the Pacific Northwest (Kirshbaum and Durran 2005a, Kirshbaum and Durran 2005b). 

Sunday, December 19, 2010

Timpanogos pounding continues

Incredible precip totals continue to pile up at the Sundance Mid Mountain MesoWest site (abbreviation SNM).  Over the past 48 hours a total of almost 8 inches of snow water equivalent has fallen.  Precipitation rates this morning and early afternoon frequently exceeded .3 inches of SWE per hour.

Temperatures today hovered near freezing and I suspect the snow was extremely heavy and wet based on what I observed at similar elevations in Little Cottonwood Canyon.

In fact, the 24-hour snowfall at SNM, based on the interval snow depth, was only about 12 inches, with 4.14 inches of SWE.  That's a water content of about 35%.  This is what happens when the subtropics come to Utah.

This morning the Utah Avalanche Center issued an Avalanche Warning for the Provo area mountains and reported that Sundance ski area was closed for the day.   This reminds me more of a Cascade storm than a Wasatch storm and I suspect we'll see evidence of some big, powerful avalanches around Timpanogos when the clouds clear.