Easy Come, Easy Go

Surely you remember it.  That unbelievable low altitude snowpack we had this winter.  I do.  This is what the Avenues foothills looked like on January 11.

Here's the view up City Creek Canyon from the Avenues foothills today.

City Creek Canyon is one of several that provides water for the Salt Lake Valley.  Things are looking grim for runoff this year.  There is a SNOTEL station at 8200 feet in City Creek on the slopes of Lookout Peak, the snowy mountain on the horizon in the photo above.  With a snowpack SWE of 18.8 inches, it presently sits at only 73% of average for today.

Source: NWS/CBRFC

The Joys of Spring

Next to powder, I don't think there's much better about living in Utah than spring.  My guess is that pretty much everyone felt good to be alive as they soaked up the rays today.

For those of you who have given up on the ski season, you're missing out on a decent corn harvest.   The conditions at Alta were pretty good this afternoon, especially on the steeps.  And, without much of a crowd, there's quite a bit of elbow room on the groomers.  If you can't ski good, ski fast, and that's what we did for much of the day.

On the way home we ran across this snowboarder.  Perhaps he is auditioning for Easy Rider II.

Sochi Weather and Climate

The 2014 Olympic Winter Games will be held in Sochi, Russia next February.  Sochi lies on the northeast shore of the Black Sea, near the Russian border with Georgia.

Nordic, Alpine, and Freestyle ski events, along with snowboarding, bobsledding, and ski jumping events will be held within the so called "Mountain Cluster" in the Caucasus Mountains east of Sochi.  I suspect that the organizers will rue the day that they named their venue areas clusters.  Jon Stewart should have a field day next winter.

Source: http://www.sochi2014.com

The weather and climate of Sochi is already getting attention, including an article earlier this week in the New York Times discussing how organizers are already making and stockpiling snow for next year!

Source: New York Times

Much has been made of the fact that one can find palm trees in Sochi.   How can you have a Winter Olympics in a place where there are palm trees?  However, palm trees are not exclusively a tropical and subtropical species.  Wikipedia suggests that they can be cultivated as far north as 50º in maritime climates.  

Sochi is at 43.6ºN, which is farther north than Salt Lake City, so it really isn't at a subtropical latitude.  It does, however, have a fairly mild climate in February that does compare well with coastal areas of the Pacific Northwest.  Here's a comparison:

Sochi:

Mean Feb High: 50ºF

Mean Feb Low: 38ºF

Mean Precip: 5.31 inches

Portland, OR

Mean Feb High: 51ºF

Mean Feb Low: 36ºF

Mean Precip: 3.66 inches

Seattle, WA

Mean Feb High: 50ºF

Mean Feb Low: 37ºF

Mean Precip: 3.50 inches

Vancouver, BC

Mean Feb High: 46ºF

Mean Feb Low: 35ºF

Mean Precip: 4.8 inches

The best comparison might be Seattle (Note: Sochi is much warmer in the summer than Seattle, so overall there are important climate differences during other seasons.  For a more detailed Sochi climo, see this great weatherspark.com page).  

Alpine skiing competition will be held at the Rosa Khutor Alpine Resort in the mountain cluster.  The Men's Downhill, typically the most weather sensitive event of the Winter Olympics, starts at 6709 ft and ends at 3182 ft.  

Source: http://skimap.org

The area has a maritime snow climate, which is characterized by mild temperatures, high density snow, occasional rain (especially near the base), and plenty of clouds.  The low elevation of the bottom of the Men's Downhill really grabs my attention.  An altitude of 3100 feet in the Washington Cascades is extremely marginal.  The only ski areas that operate at that elevation are in Snoqualmie Pass, which benefits from a frequent easterly flow of cold, continental air from eastern Washington.  Even still, the climate there sees frequent warm spells and rain.  Yeah, it can snow a lot there, but what you want for ski racing is cold, dry weather, good visibility, and hard snow.  

Of course climate is what you expect and weather is what you get.  Perhaps Sochi will fare better weatherwise than Vancouver, which had the double-whammy of a maritime snow climate combined with a warm winter.  I suspect, however, that weather is going to pose some challenges for them, through the postponing of events (the Men's Downhill is scheduled for the first day of the Winter Olympics so that it can be postponed for several days if the weather is bad) and difficult snow conditions for ski racing.  

Reality Sets In

Our brief flirtation with winter is over and while the snow was greatly appreciated by skiers, it put only a small dent (if that) in the snowpack deficit that presently exists in the central Wasatch and elsewhere in the western U.S.

The latest snowpack snow-water equivalent analysis shows only a handful of basins presently sit above average.  These basins are located in the Olympic and Cascade Mountains, as well as interior ranges near the Canadian border.  All basins in the upper Colorado and Great Basin are below average.

In the central Wasatch, Snowbird, Mill D North, and Thaynes Canyon are at 77–84% of average.  Although Mill D and Thaynes Canyon are thankfully above last year, Snowbird is pretty much neck-and-neck with it.

Source: NWS/CBRFC

April 1st is not only April fools day, but typically an important date for anticipating the spring runoff.  It is close to the average time of peak snowpack at many locations in the west, although that peak is a bit later at upper elevation sites like Snowbird.

The forecasts through April 1st suggest that the snowpack in Utah and the adjoining upper Colorado Basin and Great Basin will slip further into the red.  It won't be bone dry as we may see some spring showers, but there's little hope of a big dump through the end of March.  A spring miracle is needed in April if we are to avoid a second consecutive year of meager runoff.

Great Coverage

Here are four great reasons to study at the University of Utah, courtesy of Wasatch Magazine.  Photo at upper right should be credited to U alum Trevor Alcott.  I could never take such an iconic photo.  

Source: Wasatch Magazine

Perspectives on Convection

Over the past few days, we have experienced a potpourri of convection, updrafts that transfer heat and moisture vertically in the atmosphere and lead to the generation of cumulus clouds and snow showers.  Depending on the place and time, this convection was produced by the Great Salt Lake, daytime surface heating, and/or flow over the mountains.

After a day of skiing yesterday, I visited Antelope Island State Park today.  Antelope Island is a spectacular place, especially in the spring.  The grass is just starting to green up, so the next few weeks are prime time for a visit.

Here are some perspectives on todays convection.  Orographic convection bubbled over the Wasatch Mountains at about 11 am MDT due to flow over the mountains and/or weak thermal forcing.  The accompanying radar shows the weak echoes over the mountains.

Radar Source: NCAR/RAL

At 11:35 AM MDT, some beautiful cumulus clouds developed over Frary Peak.  They are carried southeastward (left in the photo) by the prevailing northwest flow.

As the day wore on, the convection became deeper and began to initiate over the lowlands upstream of the Wasatch Mountains.  This was our view looking east across Farmington Bay from the Frary Peak trailhead at 1:07 PM MDT.  The KMTX radar showed a greater coverage of radar returns upstream of the Wasatch and the weak evidence of wind-parallel banding.

Radar Source: NCAR RAL

The photo below was taken a few minutes earlier from a viewpoint a few miles to the north.  It shows an outflow boundary associated with a precipitation-induced downdraft spreading northward (left) near the ground at the base of the Wasatch Mountains.  The very fibrous cloud structures are indicative of cloud ice and snow.  More well defined clouds are comprised primarily of supercooled cloud droplets.  In clouds like these, the ice crystals grow very rapidly, often at the expense of the cloud droplets.

This process, known as the Wegener–Bergeron–Findeisen Process, is instrumental in the development of a large fraction of the Earth's precipitation.  Here's a laboratory demonstration of the process produced by the University of Manchester Centre for Atmospheric Sciences.

Alta 100!

The lake effect and orographic snowshowers got Alta-Collins to a 103 inch settled snow depth.  Whoot whoot!

Lake-Effect Miscellany

Some musings about lake effect, beginning with observations of yesterday's lake band.  First, here's a video of the snow squall that came through late yesterday morning, courtesy of George Wilkerson and Steve Krueger.

Here are a few photos from Peter Veals of the event as well.  

Source: Peter Veals 

Today we have snow showers in the area, but the role of the lake is debatable.  There is some evidence of more frequent and persistent returns downstream of the lake, but it is relatively unorganized and there is scattered precipitation in areas not downstream of the lake as well (this precipitation extends northward into southeast Idaho).  I'll call today weakly lake enhanced.  

The models suggest a pattern favorable for lake-effect tonight, with cold, moist, northwesterly flow moving over the Great Salt Lake as shown, for example, in the NAM forecast for 1200 UTC (0600 MDT) tomorrow morning).

Historically, such patterns very frequently produce lake effect.  However, that assumes the NAM forecast is accurate.  In addition, the chaotic nature of lake-effect precipitation makes determining the precise position, structure, and intensity of these storms a crapshoot.  A small shift in wind direction or relative humidity makes a big difference.  About all we can say is that we have a good chance for lake effect tonight, but we will have to see if Mother Nature delivers.  Your guess is as good as mine.  

Lake and Mountain Effects

We're seeing some nice lake-effect snow squalls moving off the Great Salt Lake this morning.  Also evident in the radar loop below are persistent orographic snow showers (orographic is just a fancy word for mountain) in the high terrain surrounding the Cottonwoods.

I suspect the lake-effect will weaken and become more scattered this afternoon, as is typically the case during these events, especially in the spring.  The graph below shows how the frequency of lake-effect tends to decline during the day, especially in the spring (purple bars).

Number of days with lake-effect at a given time of day.
Source: Alcott et al. (2012)

Mother Nature Delivers!

It's good to see a storm finally come through as we've been disappointed one to many times this year and we really needed a refill.  As of 9 am, our Alta-Atwater site near the base of Alta has recorded 0.72 inches of SWE with a settled new-snow depth of about 7 inches.  The Alta-Collins snowstake about 1000 feet higher recorded 0.84" of SWE and about 10 or 11 inches of new snow depending on how you add up the increments.  I've seen higher amounts from a couple of automated high-elevation sensors in upper Big Cottonwood Canyon.  Not a huge dump, but greatly appreciated nonetheless.

Astronomical Spring, Meteorological Reality

Astronomical spring arrives today, but no matter what my daffodils say, meteorological reality will beg to differ in the coming days.

This morning we have some snow falling in the mountains and rain in the Salt Lake Valley.  The precipitation began with your classic "cloud storm" scenario.  An incredibly dry airmass was in place over the Salt Lake Valley this morning, so much of the early precipitation evaporated before reaching the ground.  In this morning's sounding, taken about 5 am MDT, there was a 34ºF dewpoint depression at the surface (which equates to a relative humidity of 25%) and even drier air just above the surface layer.

Source: NOAA/NWS/SPC

A cold front moves in tonight, followed by a prolonged period of northwesterly flow that will gradually bring in even colder air on Thursday and Friday.  By Saturday morning, our 700-mb temperatures are forecast to be about -16ºC, which is below average even for January.

The overall change in temperature is perhaps the easy part of the forecast.  Precipitation is far more difficult.  As we have discussed a couple of times this week, there are wide variations in the timing and intensity of precipitation forecast by the models.  Yesterday's NAM, especially the high-resolution nest, clearly blew the precip forecast for today (see Does Precision = Accuracy), providing a good illustration that high resolution is not necessarily a forecast panacea.  For Alta, here are some numbers produced by the 0600 UTC GFS, 1200 UTC NAM-12-km, and 1200 UTC NAM-Nest-4-km:

GFS

6 AM Wed – 6 PM Wed: 0.18" SWE, 1.3" Snow

6 PM Wed – 6 AM Thu: 0.31" SWE, 2.8" Snow

6 AM Thu – 6 PM Thu: .10" SWE, 1.4" Snow

Total: .59" SWE, 5.5" Snow

NAM-12-km

6 AM Wed – 6 PM Wed: 0.05" SWE, 0.5" Snow

6 PM Wed – 6 AM Thu: 0.13" SWE, 1.3" Snow

6 AM Thu – 6 PM Thu: .02" SWE, 0.4" Snow

Total: .20" SWE, 2.2" Snow

NAM-Nest-4-km (no snow algorithm applied)

6 AM Wed – 6 PM Wed: 0.23" SWE

6 PM Wed – 6 AM Thu: 0.87" SWE

6 AM Thu – 6 PM Thu: .09" SWE

Total: 1.19" SWE

So the SWE forecast produced by these three models varies from .20" to 1.20".  Good luck!  We don't run our snow algorithm on the 4-km NAM yet, but using something like a 10-to-1 ratio, which is close to what we are getting from the other models and consistent with the warm temperatures, gives us a range in snowfall of about 2–12 inches.  How's that for uncertainty.  

My read of this event is that something in the 4–8" range is the most likely scenario for the total accumulation in upper Little Cottonwood through tomorrow afternoon.  That accumulation will probably come in a few pieces, with this morning being the first.  Much of the snow will be high density, which we really need to stick to the bone rattling frozen granular that covers the upper elevations of the Wasatch.  

All of this being said, this is a situation where forecast uncertainty is high, as indicated by the model spread above.  

Does Precision = Accuracy?

Public forecasts issued by the National Weather Service and most news broadcasts are usually fairly vague with regards to snow accumulations.  They usually provide a range (e.g., 4-8 inches) for a broad period of time (e.g., tomorrow).

There are, of course, many users who need far greater precision than that.  Determining how much salt to apply to highway for the morning rush hour, or whether or not to close a canyon to traffic to perform avalanche control work requires knowing very precisely how much snow is going to fall and when.  The temperature and the strength of the wind might also be important for these applications too.  

With those examples in mind, let's think about the ramifications of the forecasts being produced for tomorrow (Wednesday) afternoon through Thursday morning by the 12-km NAM and its 4-km high resolution nest, which are remarkably different in terms of the timing and amount of precipitation.

For the three-hour period ending at 3 PM tomorrow afternoon, the 12-km NAM sags precipitation into Salt Lake County and the central Wasatch.  In contrast, the 4-km nest keeps the precipitation to the north.  For Little Cottonwood Canyon, this affects how early tomorrow road crews need to start treating and plowing the highway as skiers are preparing to leave the resorts (Note: it is going to be warm tomorrow, with snow levels near 7500 ft, which will help some, but I'm using this as an example, so go with it).     

Once the storm sags into northern Utah, the 4-km NAM generates a stronger storm, that lasts longer, as illustrated by the 3-h accumulated precipitation for the period ending at 9 PM tomorrow night.  

Here's another way to look at it.  Below is a time series of 3-h accumulated precipitation from the 12-km NAM (green) and the 4-km nest (white) from 1200 UTC (6 AM MDT) this morning through 0000 UTC 22 March (6 PM MDT Friday).  There is a tendency for the 12-km NAM to smooth out the precipitation, generating less abrupt peaks and valleys, whereas the 4-km NAM has greater variability, with a more pronounced peaks and valleys, but also more overall precipitation.  

What you are seeing in the 4-km NAM is the direct influence of resolution, which allows it to produce precipitation features with greater structure and detail.  That's great, but precision does not necessarily equal accuracy.  A high resolution model can produce intense, narrow precipitation bands that may look more physically realistic, but if they are in the wrong place, the forecast is arguably worse.  A time series like the one above can be very misleading in such an instance.

These are the growing pains that meteorology is presently experiencing.  Eventually, we will have ensembles of high resolution models, which will allow us to deal with some of the predictability challenges posed by precision.  Until that happens, we have to carefully interpret high resolution forecasts produced by a small number of models.  

Reminiscing

On this day last year, during one of the better storm cycles we had all year, the snow depth at Alta-Collins hit the coveted 100 inch barrier.

And the snow depth kept climbing, eventually topping out at 118"on March 20th.  Ah, we'll go dancing in the dark, walking through the park, and reminiscing.  I bet you never thought last year would look so good.

Currently we sit at 84 inches at Alta-Collins.  Maybe we can make a run at the coveted 100 inch barrier later this week, when the models advertise a bit more action.  I don't see us catching up to that 118" snow depth, but any refill will be appreciated.

And, in case you were wondering, on March 18, 2011 we were sitting on 153 inches, on our way to 200+.

Post Mortem

My subjective impression of the 4-km NAM forecast for this morning's 0900–1500 UTC (0300–0900 MDT) period discussed in the previous post is that it was far too wet in both the mountains and the lowlands.  Here are a few precipitation observations for the period, which are all less than what was produced by the 4-km NAM.

Ogden: T
Salt Lake City: T
Alta-Collins: .09"
Alta-Atwater: .12"
Alta-Guard: .16"
Lookout Peak: ~.20"
Ben Lomond Peak: ~.10"
Powder Mountain: ~.11"

Call this false alarm (i.e., overforecast) #1.  We'll see how it does in future events, but I'm going to guess we are going to see more of this, especially in the mountains, although there will eventually be a moment of glory when it nails a big one.

Risks and Rewards of Resolution

Earlier this week, we started downloading and processing forecasts from the NAM high-resolution nest, which covers the continental US at 4-km grid spacing, which is three times the resolution of the regular NAM.

We will start to see if the greater resolution proves useful for Wasatch forecasting over the next few weeks.  During the 2002 Winter Olympics, we found that a 4-km model did produce more accurate precipitation forecasts over northern Utah than a 12-km model, but also with more false alarms.   The 4-km model got more forecasts right, but when it was wrong, it was a disaster.  Thus, like a well-designed golf hole, resolution offers up risks and rewards.

We have a dust-on-crust storm in the offing for tomorrow, so I thought we would do a quick comparison.  First the 12-km, which lumps all the major mountains of northern Utah, including the Stansburys, Oquirrhs, Wasatch, and Uintas, into a single broad mountain that is highest where the western Uintas sit.  As a result, the greatest precipitation during this event is forecast to fall in that area, rather than over the Wasatch range.  Alta gets .16" of snow water equivalent in the three hours ending at 1200 UTC (0600 MDT) and .06" in the 3 hours ending at 1500 UTC (0900 MDT).

The 4-km nest, however, begins to resolve the Stansburys, Oquirrhs, Wasatch, and Uintas.  They aren't well resolved, but they are more independent.  This leads to separate precipitation maxima over these ranges in the 3 hours ending at 1200 UTC (0600 MDT) and, after things taper off over the Stansburys and Oquirrhs, maxima over the Wasatch and Uintas in the 3-hours ending at 1500 UTC (0900 MDT), including a maximum over the Cottonwoods.

That certainly looks better compared to what we would expect climatologically, which features more precipitation over each of the mountain ranges with less precipitation over the intervening valleys and lowlands.  At issue, however, is whether or not the 4-km NAM can reliably produce those spatial patterns.  For instance, if this storm comes in a bit drier, or ends up ging to the north just a bit more than forecast, the added detail and precipitation will cause a bigger forecast bust than the 12-km NAM.  That's the risk and reward of resolution.  

One case isn't a big enough sample, but let's what happens late tonight and tomorrow morning and during storms in the future.  Personally, I hope it verifies and perhaps even underdoes the precipitation.  We need every flake we can get.  

DModel/DT

DModel/DT is calculus-based slang that meteorologists use to describe a trend in model forecasts.  For example, the GFS forecast from 0600 UTC 12 March showed a less than favorable forecast for snow in the Wasatch late Saturday night and Sunday morning, with a short-weve trough over the Pacific Northwest and Utah under the influence of a short-wave ridge.  Subsequent forecasts, however, have pushed the short-wave trough further east and sagged the storm-track southward, resulting in snow over the Wasatch late Saturday night and Sunday morning.  This model trend is illustrated in the DModel/DT loop below, which is based on 0600 UTC initialized GFS forecasts from the past four days all verifying at 1200 UTC 17 March (Sunday).   Note how each successive forecast produces more precipitation over northern Utah.

You have to be careful when using DModel/DT because the existence of a trend doesn't necessarily mean that the trend will continue in future forecasts.  In longer range forecasts, it's not uncommon to see the forecasts flop back and forth from run to run.  Nevertheless, DModel/DT can be a useful forecast tool and, in this case, might explain why the chances for precipitation on Sunday have been on the increase the past few days.

Records Fall

Following up on the previous post, the National Weather Service reports that we've hit 71ºF at the Salt Lake City airport, breaking the previous record that was set in 1935.  If you are wondering, I think the all-time record for March is 80ºF, which we set March 31, 2012.  That record is safe.

That 70s Show

That 70's Show airs today, but it might not be available from your cable provider.

For example, you might want to avoid going near the Great Salt Lake.  The lake is quite cold right now as illustrated by temperatures from 4 PM yesterday afternoon.  While it was 67ºF at the Salt Lake City airport, it was only 54ºF at the Saltair marina, 53ºF at Hat Island, and 51ºF at Gunnison Island.

Source: MesoWest

That's an impressively cold airmass sitting over the lake and it will be there today too.

Another cool spot was the Cache Valley where the high at the Logan Airport only reached 50ºF.  The issue there is snow cover, as can be seen in yesterday afternoon's MODIS image.

Source: NASA

They even had some fog yesterday morning and this morning, which can just be made out along the valley floor in this Utah State webcam image.

Source: Utah State University

All of this provides a good example of how the state of the land and lake surfaces can affect the local microclimate.  Today I'll stick to the east bench, where That 70s Show will provide an episode worthy of an Emmy.

Happy Anniversary!

Asheville, NC, March 14, 1993.  Source: NOAA

This week marks the 20th anniversary of the 1993 Superstorm, one of the biggest snowstorms ever in the eastern United States and a seminal moment in modern weather forecasting.

As summarized in the now classic paper by Kocin et al. (1995), the Superstorm began as disorganized low over the Gulf of Mexico, but deepened 17 mb in 12 hours to reach 984 mb when it was just south of Louisiana.  As it moved up the eastern seaboard, the central pressure dropped to 971 mb over Georgia and 960 mb over Delaware.  The storm set record low sea level pressures at locations spanning from Georgia to Maine.

Source: Kocin et al. (1995)

Measurable snow stretched from the Florida Panhandle into southeast Canada, covering an area containing 90 million inhabitants in the eastern U.S.  Accumulations of more than 40 inches were observed at locations in Maryland, New York, North Carolina, Pennsylvania, Tennessee, Vermont, and Wast Virginia.  The big winners were Mt Mitchell, North Carolina (50 inches) and Mt. Laconte, Tennessee (60 inches).

Source: Kocin et al. (1995)

Kocin and Uccellini (2004) show that the area and population affected by a snowfall of 10 inches or more was likely the largest of any east coast storm during the historical record.  The storm was the highest rated of any since 1888 on their Northeast Snowfall Impact Scale (NESIS) and one of two (the other is the January Blizzard of 1996) classified as category 5 for extreme (see also this NCDC web site).

Prior to the Superstorm, forecasts for nor'easters were not known for their reliability.  As a budding young skier in the northeast, I recall many surprise storms and great forecast disappointments.  Storms such as the 1979 President's Day Cyclone, which produced the largest snowfall accumulation in Washington D.C. in 50 years, were poorly forecast by the numerical models of the day (e.g., Uccellini et al. 1984).

By 1993, however, the resolution and capability of numerical models had reached a point where they were becoming more skillful at predicting nor'easters and forecasters were able to process and take maximum advantage of these forecasts.  The forecasts for the Superstorm were especially noteworthy.  As summarized by Uccellini et al. (1995), the Superstorm cyclogenesis was predicted up to 5 days in advance, the unusual intensity of the storm 3 days in advance, and the excessive snowfall amounts and coverage two days advance.   It was a major forecast victory.

Headline from the Boston Herald prior to the Superstorm.
Source: Uccellini et al. (1995)

Over the two decades since the Superstorm, winter storm forecasts along the east coast have continued to improve.  However, there is still work to do, as illustrated by the recent "Snowquester" forecast in Washington DC (see Capital Weather Gang and Cliff Mass Blog posts).  Nevertheless, I'm proud of what the profession has accomplished the past two decades and excited about where we can go in the future.