Thursday, March 31, 2011

Fire and Ice

I'm swamped rewriting our paper on how the Sierra Nevada influence cold fronts over the Intermountain West, so today I'm simply going to say that the next few days will feature fire and ice.

The fire is just beginning to smolder today, but will reach full inferno tomorrow as the GFS forecasts 700 mb (crest level) temperatures of +3C in the afternoon.

Yup, bring your wax if you are going skiing.  Without much of a regular frost freeze cycle, the "snow suck" should be nasty.  Further, those 700-mb temperatures are high enough that we should make a run at 70F for a high in Salt Lake City.

But don't put the puffy jacket away tomorrow because Mother Nature is quite fickle in the spring.  The models have not quite agreed yet on the details, but a major airmass change will likely occur on Saturday evening and the GFS forecast 700 mb temperatures plummet to -11C by Sunday afternoon.

Yup, looks like a period of fire and ice.

Wednesday, March 30, 2011

Riming in Action

In the previous post, I suggested that we might be in for a period of riming in the mountains, which we often see in moist, northwesterly flow downstream of a low-amplitude upper-level ridge (a.k.a., the dirty ridge).

That riming appears to have come to fruition.  The Utah Avalanche Center reports "overcast skies and light snow, rime, and graupel" in the mountains this morning and there are multiple pilot reports along the Wasatch Front of light or light-to-moderate icing.

Maximum icing severity analysis with pilot reports of icing annotated.
Source: NOAA/NWS Aviation Weather Center
Frequent blog reader David clued me into the the product above, which is is produced by the NOAA/National Weather Service Aviation Weather Center and available here.  For you science geeks, the techniques used to produce the analysis are described by Bernstein et al. (2005).    I suspect lead author Ben Bernstein, who was my next door neighbor as an undergraduate at Penn State, never expected that it might be useful for ski forecasting!

Riming and aircraft icing occur in clouds that contain large amounts of supercooled liquid water.  Ice crystals have a difficult time forming at temperatures near freezing but warmer than -10C.  Since there is some snow forming in the mountains today, obviously we're able to form some ice crystals, but there's also a lot of supercooled liquid water.  The morning sounding shows why.  Note the strong inversion at about 640 mb and that cloud top temperatures are near or just below -10C.

Source: NCAR/RAL
The area downstream of a low-amplitude upper-level ridge is where one frequently finds shallow altostratus layers that contain large amounts of supercooled liquid water.  This year we've had several events of this type and now we have yet another.

Tuesday, March 29, 2011

Return of the Dirty Ridge

The Wasatch are facing a couple of days with warm advection in northwesterly flow downstream of a low amplitude upper-level ridge.  The pattern, sometimes referred to as a dirty ridge sets up tomorrow

and is progged by the NAM model to persist through early Thursday.

In this type of pattern, which we've had on a few occasions this year, we often have a shallow altostratus deck that can produce a little snow, but also rime.  Rime is produced when supercooled liquid water (i.e., cloud droplets or drizzle that is below freezing but remains liquid) freezes on contact with the snow surface, trees, the ski lift, your googles, etc.

Riming in the Wasatch is tame compared to that in coastal ranges, but can still be a pain.  We'll have to see how things pan out over the next 48 hours, but it will be interesting to see what happens on the higher peaks and ridges of the Wasatch if things come together as forecast by the NAM model.

Monday, March 28, 2011

The Alta 200

Last week I suggested that we were about to make a run at the big 200 (i.e., a 200" base depth) at the Alta-Collins site.  An active pattern was setting up with several major trough passages.  I thought we would get close.

Indeed, it was an active week, but none of those troughs brought the major one day total.  Instead, we got a few to several inches of snow nearly every day, with 59" falling at Alta-Collins since March 20th.

Source: NWS/Alta
This made for a great week of Spring Break skiing, but daily settlement made a push to 200 impossible.  Instead, base depths increased from about 155 inches on the 20th to 174 inches at present (see light green fill).  Note the retreats after each storm as the new snow settled.

Source: MesoWest/Alta
So we sit at a fairly solid 170+ inches, but climatologically the peak snowpack at Alta-Collins occurs in mid-to-late April, so the "Alta 200" isn't over yet.  Think Snow.

Sunday, March 27, 2011

Jackson Hole Terrain, Weather, and Climate

JH, 26 March 2011
Being a certified Utah terrain and powder snob, I don't leave the Wasatch very often to ski elsewhere, but crossed the border for a fantastic day of late season skiing at Jackson Hole (JH) on Saturday.  In addition to my son Erik, my partners for the day included two of the best mountain weather meteorologists out there, Jim Woodmency, a Jackson icon who provides forecasts for several of the local radio stations and runs, and Jamie Yount, a Utah Meteorology alum who does avy control on Teton Pass.  Jim and Jamie also offer a winter weather forecasting class through the American Avalanche Institute that I hear is top notch.  Keep an eye out for future offerings.

With a good day of skiing to stimulate the senses, I thought I'd take this opportunity to talk a bit about the weather and climate of JH.  Let's begin by putting the resort into its proper topographic context.

Geographic setting and topography

JH is located on the eastern (lee) side of the Tetons and rises abruptly from the plain of Jackson Hole (see above).  In terms of ski terrain, JH is about as good as it gets and is one of the few mountains that delivers the goods promised by all the marketing.  The most radical skiing is on the upper mountain, but there is great fall-line skiing for essentially the full lift-served vertical.

Although not as radical as the upper mountain, even the lower
slopes of JH offer up a solid pitch.
Much of the terrain at Jackson faces east or even southeast.  This is perhaps an advantage during the heart of the cold Teton winter, but can be a disadvantage during the late season, especially at lower sun-sensitive elevations.  Fortunately, conditions held up well on Saturday.

Long-Term Climate

At JH, Altitude is Everything.  Being a lee-side resort with huge vertical means that there is a massive difference in snowfall between the upper and lower mountain.  Long-term climate statistics from nearby Moose, WY, suggest that the average snowfall at the base is probably a shade under 200 inches.  Long-term snow-safety observations of snowfall at about mid-mountain (8250 feet, source suggest an annual snowfall of about 370 inches, which is pretty good for a North American ski resort, but is lower than the almost 500" that falls at a comparable elevation in Little Cottonwood Canyon.  I don't have long-term climate statistics for the upper mountain, but numbers approaching 500 inches near the summit are likely.  These are solid numbers, although not as strong as Alta-Snowbird.


The large climatological contrast in snowfall between the upper and lower mountain is reflected in the current snow depths measured by automated sensors on the mountain.  There is currently about 40 inches of snow at the base, 115 inches at mid mountain, and 140 inches in Rendezvous Bowl (see grey lines below, note scale change).

Source: Jackson Hole Mountain Resort and MesoWest
It has been a great year for snow at JH, and the resort reported last week that the snow depth in Rendezvous Bowl was a record for that time of year.  This is a non-scientific observation, but an important indicator of a good snow year are partially buried trail signs.

Partially buried signs at the top of the Bridger Gondola
Dendritic Observations

The best snowflake I've observed this year fell not in Utah, but at JH on Saturday.  See for yourself.

Check out that stellar dendrite
When the temperature and relative humidity are right, you can get a stellar dendrite anywhere in the world.  Utah doesn't have the corner on these beauties, and JH gets them in spades too.  

Friday, March 25, 2011

Lessons in Radar Interpretation

This morning's radar loop provides a great opportunity for instruction in radar interpretation.  In particular, there are three major precipitation features evident in the loop:
  1. Persistent, quasistationary orographic precipitation.
  2. A cold-frontal precipitation band.
  3. Radar echos generated in the southwesterly flow aloft.
Have a look for yourself.

The area inside the red box provides a nice example of persistent, quasistationary orographic precipitation.  This is an area of pre-frontal orographic precipitation enhancement over Mount Timpanogos and the Little Cottonwood-American Fork divide.  This is a common feature in southwesterly flow.  By quasistationary we mean "almost stationary" meaning it moves around a little, but generally stays fixed to the topography.  Orographic is just a fancy word for mountain-induced. 

If you watch the loop, you can clearly see the cold-frontal precipitation band, which is moving from west to east during the loop.  This precipitation is generated by cold frontal lifting.  Note that the motion of this feature contrasts with many of the other echos in this image which are moving from southwest to northeast and are being generated in the southwesterly flow aloft.

Thus, at least three different dynamical processes are contributing to precipitation today.

Front into Northern Utah

The front is coming in a touch sooner than anticipated last night and at 1348 UTC (7:48 MST) is already moving across the West Desert and impinging on the Cedar Mountains along the I-80 corridor.

The frontal temperature change was quite apparent at I-80 Milepost 29.

 You gotta love MesoWest, which puts data like this at your fingertips.

Thursday, March 24, 2011

Frontal Friday Approaching

Friday is shaping up to be a very interesting weather day.  A cold front is presently moving into western Nevada and bringing heavy snow to the northern Sierra and southern Cascades.
The 0000 UTC 25 Mar initialized NAM brings the front through the Wasatch Front at around 1800 UTC (noon MDT).  Right now, this looks like a bench snow valley "wintery mix" event.  The 700-mb temperature during and following frontal passage is -8C to -10C, which is right on the cusp for valley snow this time of year, especially during the day.  

With a strongly forced cold front coming through, the forecast challenge for this event is related to precipitation type in the Salt Lake Valley.  A degree or two warmer or cooler can make the difference between valley rain and snow, respectively.

A useful product for these types of situations is the Short Range Ensemble Forecast system (SREF), which is an ensemble of 21 simulations by numerical forecast models.  For noon tomorrow, the average 700 mb temperature from all the SREF members is a shade colder than the NAM above (~ -10C), with a standard deviation amongst the members of ~ 1C.

This supports snow making it down to the valley floor, with perhaps an inch or two accumulating on grassy surfaces near the valley floor.

Temperature forecasts are most important when the temperature is near freezing, and tomorrow is such a day.  A degree or two is all it takes to make or break a forecast.   We'll see how things pan out tomorrow.

Of course, the easy forecast is for the mountains where it will snow yet again tomorrow!  This is one of the better patterns I've seen for Spring Break week in some time.  Hope you are enjoying it.

Ruby Mountain Orographic Effects

The Ruby Mountains are a spectacular Wasatch-like range located southeast of Elko, NV that contain some great skiing (heli and backcountry) and frequently produce dramatic orographic precipitation effects.  As shown in the image below, the Ruby Mountains (center of image) are isolated, narrow, and elongated.  There are at least 10 peaks in the Ruby Mountains and the adjoining East Humbolt Range (the high topography immediately southeast of Wells) that reach over 11,000 feet.

The Ruby Mountains stand out like a sore thumb in the Nevada precipitation climatology.  Incredible gradients in precipitation are found on both the windward and leeward side of the range, with estimated average annual precipitation approaching 45" in the highest terrain (for comparison, Alta averages 54").

Source: OSU Prism Group and Wester Region Climate Center
This morning provides a remarkable example of the influence of the Ruby Mountains on precipitation.  With flow out of the west and northwest, check out the persistent area of windward orographic enhancement hanging over the barrier, and then the abrupt rainshadowing just downstream of the crest.

Impressive!  Good skiing to be had in the Rubys today for sure, but of course it is snowing in the Wasatch today as well.

Wednesday, March 23, 2011

The Mammoth Mountain Microclimate

It has been a fantastic snow season in Utah this year, but also in the Sierra, which are going to get the goods yet again today.

The snow depth at the Sesame Snow Study Plot at Mammoth Mountain presently sits at about 180" and has increased over 40 inches in the past 3-4 days.  This site is located at about 9000 feet, not far from the main lodge at Mammoth.  Snow depths on the upper mountain are certainly higher.

As far as Sierra ski areas go, Mammoth is a very interesting meteorologically for a number of reasons.  First, it has altitude on its side, with a summit elevation of over 11,000 feet.  Second, although it is thought of as an eastern Sierra resort, it actually sits on the Sierra Crest in an area where the height of the High Sierra is relatively low and there is a dearth of high topography upstream, especially to the southwest where the San Joaquin River lies.

Mammoth Mountain is located very near Mammoth Lakes in this image.
What this means is that airmasses traversing the Sierra Nevada undergo less loss of water vapor (to precipitation) as they approach Mammoth Mountain, enabling Mammoth to be an exceptionally snowy location.

In addition, the isotopic composition of the snow that falls at Mammoth is somewhat unique.  A small fraction of water molecules that fall from the sky during a snowstorm (or rain storm) are built with a hydrogen isotope known as deuterium that includes a neutron (regular hydrogen does not include a neutron).  Because of this extra neutron, these deuterium-based water vapor molecules are a bit heavier than your standard water vapor molecule, and condense more easily into water or ice.  Thus, airmasses traversing the Sierra typically become depleted of deuterium because it preferentially rains out on the windward slopes.

Mammoth is unique because there is less upstream precipitation and depletion of deuterium.  As a result, there is an unusually large amount of deuterium in the snow around Mammoth.  This was discovered 40 years ago by Friedman and Smith (1970) in a classic paper published in Science.  There is a maximum in the ratio of deuterium to regular hydrogen (D/H) in Sierra snow observed just south of Mono Lake in the Mammoth Mountain area.  This maximum can be seen in the plot below, which includes contours of the departure (in per mil) of the ratio of observed deuterium to regular hydrogen (D/H) compared to that of standard ocean water.  A -100 indicates that the sample has 10% less deuterium than standard ocean water, and more negative numbers indicate less deuterium.

Ratio of deuterium to hydrogen (D/H) in Sierra snow expressed as a per mil
 departure from that found in standard ocean water (Friedman and Smith 1970).
Deuterium is a stable, naturally occurring isotope (i.e., it is not radioactive), so you Mammoth skiers have nothing to fear.  Further, deuterium has no measurable impact on how "heavy" the snow is, so you can't blame Sierra cement on it.  Stable isotopes like deuterium can, however, be used by clever geochemists, meteorologists, and hydrologists to understand the movement of water through the hydrologic system and even long-term climate change.

Tuesday, March 22, 2011

The Big 200 Is in Sight

With 6 inches yesterday and another 5 inches falling overnight, the snow depth at the Alta-Collins observing site now stands at 165".  Looking at the GFS forecast for the next week, it certainly appears that we are going to make a run at the big 200.  The pattern looks quite active through the weekend, with three more major trough passages coming in the next seven days.

Further, for the date, the current snowpack SWE for Snowbird is higher than observed in any water year since 1990 when the online record begins.

We still have a ways to go to match the 75" maximum SWE observed in 2005, when the snow just kept flying until mid May, but the next week is going to help.

Monday, March 21, 2011

Dust here finally

I don't know what took so long given how persistent the wind has been, but the dust has finally arrived in Utah County

Photo and Caption: BYU
and in the Salt Lake Valley.

Photo: MSI
Click here for a nice loop showing the penetration into the Salt Lake Valley, courtesy George Wilkerson and MSI.

Today's Nowcasting Challenges

Today is one of those days where the lack of decent radar coverage in central Utah is a real impediment to nowcasting, the prediction of what is going to happen in the immediate future.

There are only two radars in Utah, one on Promontory Point (KMTX) and the other on Blowhard Mountain just east of Cedar City (KICX).  This makes for a huge gap in radar coverage and, on a day like today with flow from the south, difficulties tracking precipitation features from central Utah into northern Utah.

Today's composite radar image never shows any echos over central Utah.
As a result, when we look at the KMTX radar, precipitation features seem to come out of nowhere before moving into northern Utah.

The useful range of a radar is influenced by a number of factors, but most important during a situation like this is the curvature of the Earth and the tilt of the radar beam that is sent out relative to the horizon.  The lowest-elevation tilt of the National Weather Service radar, plotted above, is about 0.5 degrees.  That's small, but along with the curvature of the Earth, means that the height of the center of the beam relative to sea level (or in the case of the image below, Great Salt Lake level) increases as one moves away from the radar.

Image: Wood et al. (2003)
For KMTX, the radar is located on Promontory Point several hundred meters above the elevation of the Great Salt Lake.  At 100 km from the radar, the center of the lowest-elevation radar tilt is about 2000 m above lake level, and things only get worse as you move further down range.

What this means is that eventually the beam overshoots the storm, and this is what is happening today. Precipitation features appear on the radar only after they have moved moved close enough to the radar to be intersected by the lowest-elevation radar tilt.

This makes nowcasting for northern Utah difficult, but the impediment to forecasting in central Utah where there is no coverage at all is even worse, including along the I-15 and I-70 corridors.

Sunday, March 20, 2011

Wind, Snow, and Spring Break Ski Prospects

The good news is that it snowed last night and some today.  Alta-Collins picked up 8 inches overnight and another inch today.

The bad news is that it remains windy, although today was not as windy as yesterday.  The Mount Baldy site shows gusts today to about 50 mph, compared with 60+ mph yesterday.

Despite the wind, there is good skiing to be had.  We had a surprisingly good day of touring in the backcountry today where the overnight snow fell on the settled powder from Wednesday night's storm and wasn't damaged by the wind.  In fact, it was good enough that we had no problem finding the motivation for 6,000 vertical feet of climbing today.  Sometimes it pays to ski with meteorologists.

It is spring break this week at the U and, although I'll be working most of the week, I'm sure many blog readers are thinking about the prospects for deep-powder skiing.

It will be windy through tomorrow night, but there is a trough moving through that should bring periods of snow to the mountains tomorrow and tomorrow night.

Then, another trough on Wednesday night and another on Friday.

This is a medium-range forecast, so it is too early to discuss the details and timing of the snowfall that accompanies these systems, but it does look like an active week that should provide some of the white stuff.  Looking out as far as next weekend is a bit beyond my confidence limits, but I do like this quote from the National Weather Service:


Let's hope mother nature brings the goods suggested by our computer models.

Saturday, March 19, 2011

Today's Blow and Dust Observations

A "breezy" day today in the Wasatch.  This wasn't an outlier wind event, but there were pretty decent gusts at times throughout the range including 77 mph at Park City-Jupiter Bowl, 76 at Snowbasin-Strawbery, 75 at Deer Valley-Empire Peak, and 66 at Mount Baldy.  It appears most of the ski areas had gusts near their summits of 60 mph or better.  This made for some interesting ground blizzard skiing.  

Sugarloaf Pass@Alta at 11:30 AM
Conditions were generally crappy (that's a scientific term), but with some poking around, you could find some smooth creamy turns in steep wind-loaded terrain.  Some nice snow streamers were observed over the higher peaks, including the Broads Fork Twin below.  In this photo, there is also a banner cloud forming downstream.

Broads Fork Twin Peaks at ~1:30 PM
Thanks to the winds, crowds were light at Alta, especially on the Sugarloaf Chair.  You don't see this many empty chairs very often at mid morning on a weekend during high season.

We've had two dust events this week and dust is finally evident in the snowpack.  The dust events occurred on Wednesday with the frontal passage and yesterday and today with the southerly flow.  It appeared to me that the more substantive dust loading of the snowpack occurred on Wednesday at Alta as the dust was most evident where the new snow from Wednesday night had been skied off.  Hand pits confirmed that the dust was concentrated near the bottom of the new snow layer.

A dusty track.
The event on Wednesday was quite brief, but accompanied with precipitation.  Perhaps what scientists call "wet deposition" is more efficient at scrubbing dust mass out of the atmosphere than "dry deposition" which would have been the primary process for dust to accumulate on the snowpack yesterday and earlier today.  Any thoughts from the dust/aerosol types out there?

Despite the strong winds, emissions from major sources in southern Utah appear to have been fairly limited yesterday and today.  Thus, the dust concentrations were fairly low.  I suspect the land surface is just not ready to go yet this year.  Perhaps in a couple of weeks, although I'd prefer to keep the spring as dust free as possible.

Friday, March 18, 2011

Dust Already?

I've been a bit surprised already at the wide spread dust that appears to be present across much of the region despite flow that is not especially strong.

You can just make out the Oquirrh Mountains looking
west from Olympus Cove.
Although one might be tempted to call this haze, the relative humidity is far to low to produce haze particles.  The relative humidity is presently 30% at the Salt Lake City Airport and typically needs to be above 70% for haze droplets to form.

In the previous post, I noted that the Sevier Lake bed and Milford Flat fire scar are often dust emission sources for Utah, but they don't seem active today, and unfortunately most of the area to our southwest  is covered by clouds, which makes dust source identification nearly impossible.  Thus, I'm working on a mystery without any clues!

Sometimes even with modern technology it's tough to figure out what's going on.  Today is such a day.

Thursday, March 17, 2011

Dust on Snow?

Over the past few years, there have been a number of insidious dust storms that have swept up southern Utah and inundated the Salt Lake Valley with poor air quality and made a mess of our spring skiing.

Tax Day Dust Storm, April 15, 2002
Snow with dust layering, Alta, UT, Spring 2009
(Photo: T. Painter)
Not only does the dust make for ugly snow with poor glide, it increases the absorption of radiation from the sun, leading to "mushier" snow on spring days, an earlier loss of snow cover, and a shorter ski season for diehards who just don't know to quit when the powder stops flying.

Thusfar this year we have escaped the wrath of major dust storms, but that could change this weekend.  The NAM model is cranking up the southerly and southwesterly flow on Saturday afternoon.

1800 UTC 17 Mar initialized NAM forecast valid 2100 UTC
(3 PM MDT) 19 Mar 2011.
These winds are strong enough for dust emission and transport from areas where the land surface has dried and been disturbed.  In past events, dust sources have included areas around the Sevier Lake bed and the Milford Flat fire scar.

MODIS dust identification imagery with possible dust plumes
indicated in pink.  Milford Flat Fire denotes area scared by
fire that has become a dust emissions source for northern Utah
(Courtesy Naval Research Lab)
At issue is if the land-surface to our south is ready to produce dust.  During the last major southerly wind storm it wasn't, but perhaps now that we're creeping into spring it is?  We'll see this weekend.

Storm Post Mortem

Our front delivered last night, but somewhat less snow than I expected.  The Alta-Collins site peaked out at 9" when the snow petered out at about midnight, with a whopping 1.09" of snow water equivalent.  Assuming all that snow water equivalent fell as snow, that's an density of 12%, close to the average found in the Cascades.

The overnight snowfall, as suggested by the computer models, simply didn't materialize.  The NAM model forecast from yesterday morning, for example, kept the precipitation going through early this morning.

Instead, the storm ended at Alta-Collins at about midnight.  As a result, the snowfall was less than expected, and the coating of lower density fluff for faceshots failed to materialize.

Although the computer forecast models have improved dramatically, predicting what happens following the passage of the frontal snowband remains a bit of a crap shoot, as shown last night.  At issue is the best approach to take to improve computer forecasts of these events.  Do we simply need higher resolution models with better representation of physical processes, or is there simply low predictability in the post-frontal environment that is better addressed with a lower resolution ensemble?  These are good questions for graduate students.