Subtropical Visit?

After a fantastic couple of days of skiing, a bit of a pattern change is in store for this week.  In particular, over the next couple of days, cyclogenesis will occur over the eastern Pacific, leading to a surge of subtropical moisture into California and the Sierra Nevada late Tuesday and Wednesday.  As this occurs, we will see the large-scale flow over Utah shift to southwesterly with a strong connection into the subtropics.  Note the surge of high precipitable water air into California late in the loop below.

0600 UTC 28 Feb initialized GFS sea level pressure
(black contours), preciptable water (color fill) and 250-mb
(jet level) wind vectors (red).

The Sierra in in store for a round of storminess for sure, although snow levels may rise to as high as 6000 feet near Lake Tahoe during the storm.

Due to the large-scale southwesterly flow, this will be a good case to examine the transformation (drying and warming) of the airmass across the Sierra Nevada and the evolution of the "leftovers" over the Intermountain West.

Mountain Weather Observations

Utah's top meteorologists took extensive observations today and report that the powder was not too thin, not too deep, but just right.

The views weren't bad either.  

Sometimes in Utah you need to pinch yourself to make sure you are conscious and not dreaming.  Today was one of those days.  

Riming and Inverted Snow

Here's a snow crystal image during the period of higher density snow at Alta on Friday night.  Note the crystal on the left is quite rimed, as is the right hand portion of the crystal on the right.

Thus, it does appear there was increased riming that led to the higher density snow, as proposed by David in his comment to my post below.

Inverted Snow Mystery

The Utah Avalanche Center reports this morning that the snow that fell overnight in the Wasatch was much denser (about a 10% water content) compared to what fell yesterday (about a 5% water content).  As a result, the new snow is inverted, and, as most backcountry powder snobs know, that makes for more difficult skiing and trail breaking.  But we are so spoiled!  Skiers elsewhere are going to have little sympathy for us.

At issue is why did the water content of the snow increase overnight?  Looking at the hourly data from Alta-Collins suggests that the density of the freshly fallen snow increased after about 0700 (12 AM MST) when precipitation rates increased dramatically with >0.1" of SWE falling each hour through 1100 UTC (4 AM MST).  During this period, the snow interval increased 7 inches, with .67" of snow-water equivalent, which is a water content of about 10% compared with the 4% that was falling yesterday.

Yesterdays snow was produced by a very shallow orographic cloud.  Snowflake images from Alta show aggregates of nice dendritic (tree or fern like) snowflakes with plenty of pores that are full of air rather than water.  In other words, low density snow.

Snowflake images courtesy University of Utah, Alta Ski
Area, and the Center for Snow Science at Alta

Something about the storm dynamics must have changed to create higher density snow crystals.  There are a few possibilities.  First, since the temperatures changed little between yesterday and last night, it's possible that the storm deepened so that more crystal growth occurred at higher altitudes where temperatures were colder than that favorable for dendritic growth (about -12 to -18C).  Second, winds and turbulence increased, leading to densification as the dendritic arms were broken off either in flight or on the ground (although the latter would not play a role in a rain gauge).  Third, the snowflakes became more rimed.  Typically, this would require a major change in either the storm dynamics (e.g., strength of the vertical motion) or the microphysics (changes in the cloud liquid water content, the concentration and size of cloud droplets, or the number of ice nuclei in the cloud).  Finally, there is the possibility that there are some problems with the Alta-Collins precip gauge and the snow water contents haven't changed all that much, but I suspect the reports from the UAC are also based on manual observations.  

The radar reflectivities broadened and intensified overnight, so the storm dynamics did change from the localized orographic storm that was present yesterday.

On the other hand, it is impossible to say from this data alone that this resulted in a major change in the snow growth processes.  We also know that the winds were quite strong, so this could also have played a role.

What is needed are good observations of snow crystals.  Perhaps Alta will post up some images of the crystals last night or some of our readers can provide their perspectives from the field.  Help us understand this mystery!

Wasatch Mountain Microscale Snowfall Insanity

I love days like today when I can just look out the window in amazement at how the Wasatch Mountains can create an intense snowstorm that pummels Alta and environs but does very little elsewhere.

For most of the day today, it has been snowing in an area confined to the region immediately downstream of the Wasatch from about Provo Canyon to Big Cottonwood Canyon.  Radar echos do not begin over the initial westward slope of the Wasatch, but somewhat downstream.  This is evident in the radar image below and also longer loops covering the afternoon (loops not shown).

Visually, this is what it looks like from the Avenues.  There is an incredible, shallow, cumuliform cloud banked up over Lone Peak.  This is the so-called Lone Peak cap cloud that appears frequently in southwesterly flow, but isn't always associated with heavy snowfall.  

Note that it is not snowing at all at the mouth of Little Cottonwood Canyon, Big Cottonwood Canyon, or along the base of the Wasatch Mountains.  Based on the radar, it has also snowed little in Mill Creek Canyon.  

Within the shallow orographic cloud, it has been snowing all day.  Precipitation began to become localized and the Lone Peak cap cloud emerged from the broader cloud deck at around 8 am this morning.  Since then it has snowed about 6 inches at the Alta-Collins snow stake, with about .24 inches of water.  That's cold smoke at 4% water content and reports are that the skiing today was fantastic. 

All in all, this is a remarkable storm.  The dynamics of the event, especially the shallow convective dynamics, microphysical processes, and local forcing, are worthy of further investigation.  

Visualizing Forecast Uncertainty

Earlier posts this week have discussed the uncertainty in forecasts of front trough and snowband position for this evening.  Here's a nice loop showing a sequence of forecasts from the NAM beginning with the 84 hour forecast from 1800 UTC 22 Feb and ending with the 12 hour forecast from 1800 UTC 25 Feb (today), all valid for 0600 UTC 26 Feb (11 PM tonight).  Meteorologists sometimes refer to this as "DModel/Dt".  Notice how the position of frontal trough and precip band jumps back and forth, from as far south as central Utah to near the Utah-Idaho border.

Now, if you think that now we are dealing with a shorter lead time (24 hours or less), so presumably the forecast is more certain, think again.  The target area has narrowed, but for 1200 UTC 26 Feb (5 AM Saturday morning) the NAM puts the front right on top of Salt Lake City with heavy precipitation to the north, whereas the GFS puts the front roughly over Provo.

NAM Forecast Valid 1200 UTC (0500 MST) 26 Feb 2011

GFS Forecast Valid 1200 UTC (0500 MST) 26 Feb 2011

So, the frontal snowband remains, but the precise positioning remains uncertain.  It's going to be an interesting night.

Alta Cold Smoke

It's time to spend some time looking at the Wasatch Mountains again and, in particular, the remarkable pounding that Alta has taken over the past 24 hours or so.  Looks like a total accumulation of about 17 inches, with only .66" of water.  That's a water content of only 4%.  Cold Smoke!

Although mountain snows were fairly widespread overnight, they have become more concentrated in the upper Cottonwoods over the past couple of hours, with some spillover across the southern Park City ridge line, as indicated by the 1847 UTC (1147 AM MST) radar image.

There is also an incredible Lone Peak cap cloud.  Somebody from the U take a picture and send it to me!

Two Different Cyclones and Ongoing Forecast Challenges

Today provides an excellent opportunity to examine the differences between a western US "Intermountain" cyclone and an eastern US cyclone.

Radar composite with RUC2 sea level pressure (white contours)
and 850-mb temperature (red contours) at 1400 UTC 25 Feb 2011

In the eastern United States, the cyclone is an event.  Midlatitude cyclones are responsible for much of the cool-season precipitation in the east, including most of the major winter storms.  The precipitation shield accompanying today's cyclone is extensive, although those with a good eye may notice that to the south of the low center, precipitation is falling well ahead of the surface cold front.  This is not unusual in this part of the world and indeed overnight a very impressive pre-frontal squall line with numerous severe weather reports rumbled through the southeast.

In contrast, your Intermountain cyclone is a sub-synoptic-scale event.  The frontal structure is presently quite disorganized, although that will change during the day today.  Further, orography is presently playing a dominant role in the precipitation distribution, with heavy precipitation over northern California.  Over the Intermountain West, precipitation is trying to organize along a frontal trough that is developing over northern Nevada and Utah.  It is this trough and its placement has caused meteorologists, especially yours truly, much consternation the past couple of days.  Forecast models have placed the frontal trough and snowband anywhere from central Utah to the Utah-Idaho border.

The observations coming in this morning are consistent with a frontal band developing over northern Utah today during Intermountain cyclogenesis.  We'll see some rain or snow showers in the Salt Lake Valley, but the strong front and snowband eventually develop near the Utah-Idaho border this evening.

As discussed ad nauseam in previous posts, the predictability of the location of the frontal band has been very low.  Nevertheless, I think we can have some confidence now that this evening the real action will be to the north of the Salt Lake Valley, but the subsequent fate of the front remains uncertain.  I'm not making any wagers for late tonight and tomorrow.

My Clueless Forecast

Steenburgh's Intermountain Cyclone Forecast System
Photo: Wikipedia Commons

I'm just about ready to give up.  As discussed in the previous two posts, there's quite a bit of uncertainty with regards to the details of the weather forecast over the next couple of days.  We know there's an Intermountain cyclogenesis event on tap, but the forecast models can't seem to settle on a track or frontal position, especially for tomorrow afternoon and evening.

I was encouraged to see that the NAM and the GFS were converging on a solution this morning, but that doesn't necessarily mean they were converging on the right solution.  Indeed, the SREF showed that there was a lot of spread when one looked at all the available model solutions, so uncertainty remained.

So, perhaps I shouldn't be surprised when I pull up the 1800 UTC 24 Feb NAM and GFS and find that they have shoved the frontal trough and precipitation band for tomorrow afternoon back to the north compared to the 1200 UTC 24 Feb initialize runs.  Oh the humanity!

NAM forecast valid 5 PM MST 25 Feb

GFS forecast valid 5 PM MST 25 Feb

Like the roulette wheel in Vegas, you simply can't beat a chaotic system.  It is a fact of life that some patterns are more predictable than others and in a pattern like this it is simply inappropriate to issue a simple, deterministic forecast.  Snow is coming, but the specifics of where, when, and how much remain uncertain.  The bottom line is to be prepared, but stay tuned.

Intermountain Cyclogenesis and Forecast Uncertainty

As discussed yesterday, we're on track for an Intermountain cyclogenesis event tomorrow and Saturday.  Yesterday the NAM and the GFS forecast cyclone tracks and frontal positions for the event that were quite different.  Today, they have converged on a solution that moves the low center across southern and central Utah.

Over the next 24-36 hours, a surface front intensifies and slides eastward and southward into Nevada and northern Utah.  As shown by the 1200 UTC 24 Feb initialized NAM, the passage of this front through northern Utah (see the cyclonic wind shift in the image below) produces a good round of snow for the Salt Lake Valley and Wasatch mountains tomorrow afternoon.

Concurrently, cyclogenesis occurs over central Nevada, downstream of the High Sierra along the western portion of the frontal boundary.  

Although both the NAM and the GFS eventually move the cyclone across southern Utah, the real forecast problem for Friday night concerns the placement of the frontal trough tomorrow evening and night.  During this period, a band of snow will likely develop along and to the north of the trough.  This can be seen above and in the forecast analyses valid for 0300 and 0900 UTC 26 Feb (8 PM MST Friday and 2 AM MST Saturday).

Vigorous, slow-moving fronts of this type can produce substantial snowfalls in the valleys of northern Utah, but positioning is everything.  The precip band can be 50-100 km wide and a shift in north or south can influence whether or not Ogden, Salt Lake, Provo, or Nephi get the goods.

The National Centers for Environmental Prediction Short Range Ensemble Forecast system (SREF) provides forecasts from several models with different initial conditions.  The SREF can be used to help assess forecast uncertainty.  Meteorologists often will view the mean and standard deviation of a forecast variable to do this.  The 0900 UTC 24 Feb initialized SREF forecast valid 0300 UTC 26 Feb drapes the sea-level pressure trough over central Utah, perhaps a bit south of the NAM position.

Note that the standard deviation of sea level pressure is high across the Intermountain West, indicating that there is quite a bit of spread amongst the model forecasts.  In other words, the forecasts that make up the SREF vary in the intensity and position of the frontal trough.  For these forecast details, the atmosphere has low predictability for a forecast of 36-48 hour lead time.  Be prepared, but stay tuned.

Upcoming Intermountain Cyclogenesis Event

One of the more remarkable transitions that occurs in Utah climate during the spring is the increase in the frequency of Intermountain cyclogenesis.  As shown by Jeglum et al. (2010) using the ERA-Intermim, NARR, and NCEP/NCAR atmospheric reanalyses, the frequency of occurrence and genesis of Intermountain cyclones increases monotonically from a minimum in December or January to an absolute maximum in May.

Mean monthly Intermountain cyclone
frequency and genesis (Jeglum et al. 2010).

The intensity (or amplitude) of Intermountain cyclones also tends to be greater in the spring.

Two-dimensional histogram of peak 850-mb Intermountain
cyclone amplitude vs. month (Jeglum et al. 2010).

Not coincidentally, Shafer and Steenburgh (2008) found that the frequency of strong Intermountain cold frontal passages is also highest in the spring, although the peak is sharper and in June. 

Monthly frequency of strong cold frontal passages over
the Intermountain West (solid) and western United States
(dashed, Shafer and Steenburgh 2008)

If the models are on track, we have a great Intermountain cyclone event on tap for Friday and Saturday.  The 1800 UTC 23 Feb initialized NAM produces Intermountain cyclogenesis in the direct lee of the southern "High Sierra" for 0000 UTC 26 Feb (5 PM MST 25 Feb, Friday afternoon).  

The low center then tracks into northeast Nevada, with Salt Lake City in the so-called "warm sector" and strong southerly flow ahead of the low center at 0600 UTC 26 Feb (11 PM 25 Feb).  

As the low center moves into southwest Wyoming, the cold front rotates across Salt Lake City early Saturday morning.  

The NAM cyclone is as close to a "classical" frontal cyclone as you can get in the Intermountain West.  Although cyclogenesis occurs in the lee of the Sierra Nevada and orographic forcing is important, the low center forms along a pre-existing frontal boundary.  The amplitude of the frontal wave then increases, resulting in a "open wave" cyclone as the low center moves across northern Utah.  The Bergen School meteorologists responsible for the Norwegian Cyclone Model would be quite proud!

This is all fine and dandy, but a look at the GFS illustrates that there is great uncertainty in this case with regards to cyclone track.  In particular, the GFS forecast for 0600 UTC 26 Feb puts the low center in southern Nevada, with a surface trough and frontal zone draped across central Utah, well to the south of the NAM trough and frontal zone.  

In contrast to the southerly warm-sector flow predicted by the NAM, the GFS forecast puts Salt Lake in cold, post-frontal northerly flow for Friday night bar hopping!    Further, the GFS puts a band of what would be heavy snow across Utah Country, whereas the NAM frontal band is near the Utah-Idaho border.

This case provides a great example of how you can have confidence in a synoptic event, but differences in positioning make weather forecasts for specific locations very difficult.   

Name This Cloud (Again)

It's back.  The infamous shallow cumuliform cloud that sometimes drapes itself over Lone Peak and the Little Cottonwood-American Fork divide during periods of southerly and southwesterly flow.  The cloud, which was the subject of a Wasatch Weather Weenies post in October, made an appearance again this morning.

Looking at Lone Peak from the U at 1600 UTC 22 Feb 2011

The display was quite spectacular this morning with snow spilling over Lone Peak into Little Cottonwood Canyon.

The KSLC sounding from this morning provides some insight into the cloud dynamics.  The flow is southerly below 800 mb, but veers to southwesterly at 700 mb (roughly crest level.  Note the steep lapse rate from 750-600 mb, but also the inversion at 600 mb.

Perhaps forced ascent is leading to convection over the barrier that is capped by the inversion?  The cloud-form is fascinating and deserving of a name.  Suggestions?  

Yellow Snow IPA

Looking for that special something to bring an end to a great ski weekend?  Look no farther than Rogue Ale's Yellow Snow IPA.

Photo: thefullpint.com

There's even a Utah connection.  Rogue Ales says it was introduced for the 2000 (sic) Winter Olympics in Salt Lake City.

I wonder if it is as good as my current favorite weather ale, Inversion IPA from the Deschutes Brewery in Bend, OR.

Photo: Deschutes Brewery

Anatomy of a Faceshot

Ryan Murphy, 13, gets the goods today at Solitude

Sure, it's Presidents Weekend, traffic stunk because the canyons were closed at times, and it was packed at the ski areas.  But, you have to admit, the snow today was sublime.  In fact, it was one of the better faceshot days I've had this year.  My cheeks are still burning!   The Greatest Snow on Earth is back after a month of unusually high water content snow in Utah.

Erik Steenburgh, 12, sampling the Greatest Snow on Earth.
Yes, his Dad is raising him right.

It takes special snow to make great face shots.  Yes, you need deep powder, but it also has to be light and dry so that it easily waffles up in your face when you make a turn or pop off a mogul or jump.  For faceshots, you need dendrites stacked deep.

Dendrites are are snowflakes with lots of tree like branches.  The stellar dendrite serves as the official logo for Alta, and the unofficial ice crystal of Utah.

Examples of stellar dendrites from snowcrystals.com, the
ultimate web site for snow-crystal geeks like me.

Other dendrite types recognized by scientists include spatial dendrites, fernlike stellar dendrites, and radiating dendrites.  Check them out on snowcrystals.com.

The significance of dendrites for skiing is that they have lots of pores, cavities, and gaps that are filled with air instead of ice.  When dendrites are stacked deep, you have a snowfall that has a very low water content and snowflakes and snowflake fragments that are easily lofted for faceshots.

The snow that fell overnight and this morning was full of dendrites or fragments of dendrites.  The fragments come from snowflake collisions, which cause mechanical fracturing.

Dendrite potpourri at Solitude this afternoon

Severe fracturing of dendrites can result in snow densification, as occurs when a wind slab is produced by strong winds, but last night and today the wind was fairly light, so that even the fractured dendrites were just fine for bone dry powder.

These dendrite fragments ski just fine thank you.

Dendrites form only under very special conditions.  In particular, temperatures where the snowflakes are forming must be between -12 and -18C.  You also need the relative humidity to be just right.  The upper-air sounding from KSLC this morning shows that temperatures at and above the elevation of the upper Cottonwoods lied in this magic temperature range.

The bottom line is that we were in the dendritic sweet spot.  Wasn't that just grand!  

Moonlight Skiing Anyone?

Photo: Wikipedia Commons

Last night I enjoyed great food and conversation at Alta's Shallow Shaft Restaurant with a group of meteorologists and snow-safety pros from Alta and the eastern United States.  There's nothing I like more than talking Wasatch weather and climate with locals and visitors who are passionate about snow and want to know more about the secrets of the Greatest Snow on Earth.

After the weather and snow geek talk, our attention turned to the incredible moonlit slopes of upper Little Cottonwood Canyon.  Leaving the restaurant at 10 PM, one could easily have gone for a quick lap sans headlamp.  How I wish I had my gear!  I ended up slithering down the canyon to return home to Salt Lake, but wonder if my dinner companions ultimately opted for a lap or two.

The full moon comes today, so what are the chances of moonlit laps tonight?  The clouds have moved in this morning and much (but not all) of the Intermountain West is currently overcast.

There are, however, some gaps upstream over Nevada and this airmass will be moving in for the evening.  In particular, note how the 600-400 mb relative humidity decreases later this afternoon in the NAM model time-height section for Alta.

On the other hand, cirrus clouds are tricky to forecast and the NAM and other computer modeling systems don't do a great job representing them.  Further, we're not dealing with "severe clear" upstream, but the hope that an area of clear skies moves over us in the large-scale southwesterly flow ahead of the upper-level trough.  Thus, I have to waffle this forecast.  The best option is to enjoy a drink or two at the bar this evening and the seize the moment should the opportunity present itself.

Four Horsemen of Yesterday's Weather Apocalypse

Click here to see a great video showing the "Four Horsemen of the Weather Apocalypse" during yesterday's storm: Wind, Dust, Snow, and Lightning.

Another "La Nina" Storm

If you are reading this, you should be skiing.

After yesterday's event, it's a bit of a let down to look at the satellite imagery, radar, and surface observations this morning, but there is something interesting in the forecast.

As many of you know, we are in the midst of a La Nina winter.  During La Nina winters, there is a tendency for the southwest to experience below average precipitation during the heart of winter (Jan-Mar).

Source: NOAA/Climate Prediction Center

We have a saying in meteorology, however, that climate is what you expect, weather is what you get.  Over the next few days, a deep upper-level trough will impact the desert southwest, bringing another round of lowland rain and mountain snow to that region.

The deep upper-level troughs that have moved across the southwest this winter provide a good example of why it is important to avoid over generalization.  Just because the "dice are loaded" for a dry winter, doesn't mean it is guaranteed, and it doesn't mean that a major precipitation event can't occur.  Perhaps being weather resilient in the desert southwest makes sense regardless of whether there is an El Nino, La Nino, or neutral ENSO winter.  

A Surprise Outflow Boundary with a Remarkable Wind Gust

Some remarkable wind observations were recorded as the front penetrated across northern Utah.  In particular check out this meteogram from the UDOT Lake Point I-80 site (UT9).

There is a wind gust of 80 mph at 0040 UTC.  This is a prefrontal wind gust (SSW flow before, during and after) that is accompanied by a brief 4F temperature fall.  The front then comes through at 0110 UTC.

I was ready to dismiss this as a bad ob, and then I looked at the traces for Lake Point (LAK) and KSLC.  They both show the same behavior, although the peak gusts are not as strong.

How do we explain this unique southerly wind burst?  The 0000 UTC sounding at KSLC shows a classic inverted-V sounding with large values of downward convective available potential energy (DCAPE).

Looking at the radar, one can clearly see a pre-frontal precipitation band with bow-echo-like structure moving over the observing sites during the period of peak gusts.  It appears the strong winds were produced by moist downdrafts and related outflow.

With an incredible front approaching from the west, it's easy to overlook such an innocuous feature, yet it produced the strongest lowland gust observed during this event.  You might also check out the cell that develops where the apparent outflow boundary collides with the approaching front.