Tug Hill Adventures

I have finally returned from beyond the Internet and my "adventures" on the Tug Hill Plateau and along Lake Ontario.  I had a great trip, with the highlight having the opportunity to meet and speak with so many great people.  We're going to have a lot of fun during the field program next winter.

If you haven't visited the Great Lakes, do it.  Where else can you find a gorgeous beach like the one below (Southwick State Park) with freshwater?  Yeah, the water this time of year is cold, but absolutely beautiful.

Here's a shot from Sandy Island Beach, with an especially humorous mix of summer and lingering winter sign postings.  Those of you venturing out on the pack ice should be well aware that swimming is not allowed and that there is no lifeguard on duty!

OK, back to business.  As discussed in the previous post, the Tug Hill Plateau is one of the snowiest areas in the eastern United States.  It lies directly east of Lake Ontario.  Topographically, it isn't much, with a total rise of about 1750 feet spread out over about 50 km.  This rise is barely perceptible as you look east from near Lake Ontario.  Look carefully and you can barely see the Tug Hill Plateau in the photo below.  The cumulus in the distance sit over the western slope of the Tug.

Snow?  Ya Betcha.  After I left the glorious green of the lowlands along Lake Ontario I ascended the plateau, finding a few snow patches as low as ~1000 feet.  The photos below were taken on consecutive days in North Osceola (~1600 ft MSL) and along the Salmon River Road (~1750 ft MSL), respectively.  Not bad for late April.

Skiing?  Some of the best skinny skiing around.  Embedded in the Tug Hill snowfall maximum is Hugh Quinn's Osceola Tug Hill Cross Country Center

and groomed trails at Winona State Forest, which you can access from Jamie Wilson's Nordic Emporium.  I stayed at the Salmon Hills Lodge, which is a great spot for snowmobilers to stay in the winter.  I suspect my students will be eating breakfast there a lot next winter as they have a good one for pennies on the dollar.

The visit has me really looking forward to our field program next winter.  Hopefully Mother Nature will cooperate and provide us with the goods.

Tug Hill Dreams

"Most climbers aren't in fact deranged, they're just infected with a particularly virulent strain of the Human Condition"

–John Krakauer, Eiger Dreams: Ventures Among Men and Mountains

"Ditto for meteorologists"

–Wasatch Weather Weenies

I embark tomorrow for a mythical place in the eyes of meteorologists, the Tug Hill Plateau of upstate New York.  Unlike the Eiger, subject of John Krakauer's book, there is really nothing topographically imposing about the Tug Hill Plateau.  It rises very gradually to a maximum elevation of about 2000 feet, roughly 1750 feet above the level of Lake Ontario.  It is also covered by dense eastern forests, offering few views.  There are no steep slopes, except for a an escarpment on the east side that rises a few hundred vertical feet and is home to the Tug Hill Plateau's only Alpine ski area, Snow Ridge.  

What makes the Tug Hill Plateau so special to meteorologists is snow.  Despite it's modest elevation, the Tug Hill Plateau is one of the snowiest places in the eastern United States.  The Hamlet of Hooker (yes, that's the name), elevation ~1500 feet, averages 238 inches of snow a year, with greater totals possible in other areas.  Much of this snow is produced by lake effect, which is generated over Lake Ontario with an assist in some events by the modification of cold airmasses by upstream Great Lakes.  If you think the Great Salt Lake produces big lake-effect storms, think again.  A ten-day lake-effect storm in February 2007 produced 141 inches of snow in Redfield on the western slope of the Tug Hill Plateau.  From January 11–12, 1997, an observer in Montague recorded a 24-hour snowfall of 77 inches.  This total was based on six individual snowfall observations, so it is not recognized as the US snowfall record, but storm totals for the event illustrate the dramatic influence of the Tug Hill on lake-effect storms. 

Source: NOAA (1997)

The objective for my trip is to find observing sites for the Ontario Winter Lake-effect Systems (OWLeS) Project, a National Science Foundation (NSF) sponsored field campaign that will take place  next winter to improve our understanding of lake-effect storms.  There are many institutions involved and multiple objectives for this project, but the Mountain Meteorology Group here at the University of Utah is concentrating on the enhancement of long-axis lake-effect snowbands that extend from Lake Ontario over the Tug Hill Plateau.  A conceptual schematic of planned field operations is below and include a number of resources including the University of Wyoming King Air research aircraft and Center for Severe Weather Research Doppler on Wheels radars.  

Although we'll miss part of next year's powder season in Utah, we are pretty excited about this project as it will allow us to greatly advance our understanding of how complex terrain affects lake-effect storms.  

Forthcoming Southwest Climate Assessment

After a very long road, Assessment of Climate Change in the Southwest United States, developed as part of the 2013 National Climate Assessment, will finally be released on May 2nd.  The book contains 20 chapters authored by more than 50 scientists from wide-ranging disciplines.  Along with several other atmospheric and cryospheric scientists, including some from the University of Utah and Utah State University, I helped prepare Chapter 4, which serves as a foundational chapter and describes the current weather and climate of the Southwest.

Copies of the book may be ordered from Island Press at http://www.islandpress.org/nca.  Full color PDF and an archive of all the figures used in the book will be available at http://www.swcarr.arizona.edu on May 2nd.  You can get a sneak peak of the Summary for Decision Makers at http://www.swcsc.arizona.edu/sites/default/files/SWCA_Decision_Makers.pdf.

Earth Day Wishes

Amongst the remarkable scientific breakthroughs of the 21st century is the detection of planets beyond our solar system.  Pluto may have been relegated to dwarf-planet status, but the list of exoplanets (i.e., planets outside our solar system) discovered by clever astronomers using the capabilities of the Kepler spacecraft is growing rapidly.  

Earth-like planets that could support life as we know it are harder to find, but NASA recently announced the discovery of two so-called habitable zone planets in the Kepler-62 system, Kepler-62-e and Kepler-62-f, as well as a third orbiting Kepler-69.  

Source: NASA AMES/JPL-Cal Tech

Perhaps could start getting ready now for a Battlestar Gallactica like exodus to one of these planets (preferably without the cylons).  They are only a short 1,200 light year journey.  It's tough to say at that sort of distance whether or not one of these planets is teeming with life and a breathable atmosphere.  It took a while for the Earth to get to that point.  But maybe it would be worth a shot?  

Alternatively, we could make our home here, on a planet that seems generally well suited for human life.  In general, I find this planet quite hospitable, although there are a few rough neighborhoods (South Pole and the Empty Quarter you know who you are).  

Nevertheless, challenges lie ahead to make this a home for 9 billion given that zoning regulations will not allow for an addition.  In many ways, Earth Day is as much about humans as it is about the environment.  My wish on this Earth Day is that my generation, and those that follow, will find a way to lift the human condition while ensuring that Earth will remain a suitable home for those who follow. Young Thomas Edisons rise up and show us the way.  

Orographic Effects

This morning's radar loops shows the dramatic influence that the mountains of northern Utah can have on winter storms.  Note the persistent radar echoes that are produced over and upstream of the Stansbury and Oquirrh Mountains (identified by red boxes in the radar loop below) as the northwesterly flow is forced upwards by those mountain ranges.

Things are a little more complicated over the Salt Lake Valley and central Wasatch Mountains (ugly oval above).  Here, precipitation develops over the southeast corner of the Great Salt Lake, well upstream of the initial mountain slope.  Lake-effect?  Perhaps.  The contrast in lake and air temperature right now is marginal for lake effect, but it is a juicy airmass, so perhaps it's enough to do the job.  Another possibility is that the funneling of air into the Salt Lake Valley due to the terrain concavity formed by the Stansbury, Oquirrh, and Wasatch Mountains is doing the job.

In any event, it's a nice rainy day out there with snow falling in the mountains.  Enjoy it while it lasts.  You'll be wishing for a day like this in July.

Some Breathing Room

The cool, intermittently wet pattern over the past couple of weeks has likely provided just a bit of breathing room for water managers in northern Utah, with one more storm to come tonight and tomorrow.

We discussed previously the meager snowpack that existed at the end of March.  Not much has changed at lower elevations where much of the precipitation that has fallen in April has either been rain or wet snow that melted quickly between storms.  For example, the Ben Lomond Trail SNOTEL (~6000 feet) bottomed out earlier this month with no real recovery (green line).  This is about a month earlier than their median date of the loss of snow cover (purple line).

Source: CBRFC

Things look better, however, at some (but not all) upper-elevation sites.  In the Cottonwoods, both Mill-D North (8960 ft) and Snowbird (9640 ft) have gained snowpack this month.  At Mill-D North, they are often losing snowpack this time of year (see purple and blue lines), so thats a net positive, putting them well ahead of where they were last year, but still just a bit below the snowpack peak and the average for this time of year (red line).  At higher-elevation Snowbird, however, accumulation rates have only been sufficient for them to keep up with the pace of snowpack growth that is typical until late April.  Nevertheless, that's better than losing ground.  

Source: CBRFC

Skiers should take note that the snowpack at Snowbird is now just a shade ahead of the peak from last year.  Hooray!  The snow depth at Alta-Collins reached 117 inches this week.  So close to 10 feet!  Better late than never.  

Further east, the snowpack in the high-altitude Uinta Mountains actually looks pretty good.  Many sites sit near or above average.  

Source: CBRFC

Major gains have occurred in the Uintas this month.  Check out the increase in snowpack at Trial Lake, which is now sitting just above the median and average peak snowpack and well above last year.

Source: CBRFC

On the other hand, there are some areas in northern Utah that Mother Nature completely abandoned.  Neither Ben Lomond (8000 ft) nor Timpanogos Divide (8100 ft) ever got anywhere near a decent snowpack and they haven't recovered much in April either (they are the red boxes in the plot above).

As we have said many times this year, it ain't over until it's over, but based on the longer range (6-14 day) forecast, this weekend might prove to be this season's upper-elevation snowpack peak.  In most areas, it won't be a great runoff year and the total volume will likely be below average, but it won't be as bad as last year either.  

Sequestration and the National Weather Service

Source: Reuters/Brian Snyder

There was an article in the Washington Post earlier this week discussing one of the impacts of the budget sequester on the National Weather Service: The furloughing of workers for as little as 4 and as many as 10 days.  The latter would be invoked if NOAA is not allowed to reprogram their budget to cover labor costs.

One of the challenges facing the NWS when they furlough is the fact that weather never sleeps.  NWS operations are 24/7.  There is no option for a four-day work week.  Humans are especially critical for issuing and communicating severe weather watches and warnings, including those for tornadoes, tropical storms, and hurricanes.  That being said, NWS forecasters are extremely dedicated and I expect that they will do all they can to ensure this aspect of NWS operations does not suffer.  They will plug many holes in the dike this year, but it is difficult to imagine that this is a "passing storm."

The challenge facing the NWS is how to efficiently advance weather services for the nation in an era of constrained (possibly greatly constrained) budgets.  New NWS Director Louis Uccellini has taken on one tough assignment, but can elect not to "waste a good crisis."  At issue is whether or not he and other managers at NOAA and the NWS can make good strategic decisions to benefit the nation and the weather enterprise during and beyond these difficult times.  I wish them luck.

Disclosure: The author receives research support from the NOAA/National Weather Service Collaborative Science, Technology, and Applied Research (CSTAR) Program.

A Cold Start to 2013

Utah hasn't had a calendar year during which the statewide average temperature was below the 20th century average since 1993.  We came close in 2011, but still came out above it.  2012, as you probably remember, was the hottest year on record for the state.

Source: NCDC

I've often wondered if we would ever see another year below the 20th century average, but maybe we have a shot this year.  The first three months of 2013 produced a statewide average temperature that was 2.6ºF below the 20th century mean, making it the coldest January to March period statewide since 1985.

Source: NCDC

That's a pretty good start, and April has thus far been an up and down month that probably averages out to something that is near the long-term average.  We'll have to see how the rest of the year shakes out.

A Messy Pattern

We have a complicated weather situation over northern Utah today.  As can be seen in the analysis for 1500 UTC (0900 MDT), a 700-mb (crest level) low was centered over northeast San Pete County, with an inverted trough extending northward into southeast Idaho.  As a result, strong contrasts in flow direction existed across the Uinta Mountains and the northern Wasatch Mountains.  Precipitation was concentrated primarily near and west of the inverted trough, although the radar may be unable to see precipitation falling over the Uintas and southwest Wyoming due to blockage and overshooting.  

Note how easterly flow extends from southwest Wyoming to the northern Wasatch Front, as was forecast by the computer models yesterday.  This led to a weak downslope wind event this morning from Bountiful through North Ogden.  Note the predominantly easterly flow along the western slopes of the Wasatch at 1500 UTC (0900 MDT).  

Source: MesoWest

As we suspected yesterday, this has been a fairly benign event.  The peak gust reported thus far by any MesoWest station in that area is 37 miles per hour at a site in Farmington.  It's nice that the model guidance appears to have become more reliable for helping us discern between benign and high-impact downslope wind events.  

Source: MesoWest

If we could only do better with precipitation!

Deja Vu All Over Again (Almost)

Once again, winter's icy grip has returned to northern Utah.  Those with the time and energy to get out ski touring today might find some pretty good conditions in the upper elevations, and I'm sure anyone at Snowbird or Brighton have the resorts to themselves.  Lucky bums!

The large-scale characteristics of the trough that is moving through today and tomorrow is remarkably similar to the one that moved across Utah last week and brought downslope winds to the northern Wasatch Front (previous posts here and here).   In particular, as the trough moves across the state, the forecast models call for a surge of cold, easterly flow to push into northern Utah from southwest Wyoming on Tuesday.

The devil is in the details, however, and as things look right now, it appears that this event will not be as strong as last weeks as the cold, easterly flow is simply not as deep and strong.  This is a scenario in which we might see stronger winds at the canyon mouths, but not necessarily along the entire northern Wasatch Front.  Keep an eye on the forecasts, however, just in case there is a shift in the model guidance.

Once again, there are a wide range of snowfall forecasts being generated by the numerical models for the central Wasatch.  As we discussed last week, location is everything and where the precipitation associated with the trough sets up will strongly influence the amount of snowfall.  If I had tomorrow off, I'd probably be thinking about skiing.  It is cold enough now that today's snow should survive, at least on the upper-elevation north facing aspects, and we'll at least get some more through tomorrow afternoon.  Given the easterly flow, this could be an event where it is best to be in the far upper Cottonwoods and along the Park City ridge line.  How ironic that Park City is finally getting some snow now that April is here.

The NAM is once again more bullish than the GFS.  If snowfall rate tomorrow turn out to be high, be careful out there if you are touring.

Craig Patterson Memorial:

As reported on the Utah Avalanche Center web site, a memorial service to celebrate the life of Craig Patterson will take place Thursday, April 18th at 10am at the Albion Grill, Alta.  In lieu of gifts or flowers, please contribute to the Craig Patterson Memorial Fund at any Key Bank. Or checks can be sent to the Craig Patterson Memorial Fund, 520 Crestview Dr., Park City, UT 84098.

Perspectives on Op/Ed by Rep. Chris Stewart

In an opinion article in the Sunday Salt Lake Tribune (Stewart Cautious on Climate Change), Rep Chris Stewart, newly appointed chairman of the House Sub-Committee on the Environment, argues that “the science regarding climate change is anything but settled” and that “as a leader on these important issues, I will look to ensure that we have conducted a thorough scientific review, then use that information to advocate for reasonable policy decisions.”  As an atmospheric scientist and voter in Rep. Stewart’s district, I agree with both of these statements, but ultimately conclude that the time for action is now (or arguably a few decades ago).  

Rep. Stewart is correct that the science regarding climate isn’t settled.  As is the case in any science, there are always limits to knowledge, and scientists are constantly testing hypotheses, collecting new data, and working hard to obtain a greater understanding.   Knowledge is never absolute, and uncertainty is considered in everything that we do.  This is reflected in key synthesis reports that summarize current understanding of climate change, which I have summarized below.  When not present in the original document, I have italicized phrases that indicate scientific confidence or probabilities.

• “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.” (IPCC 2007)

• “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” (IPCC 2007)

• “The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to natural causes alone.” (IPCC 2007).

• “It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, at the global scale, that is for most land areas with sufficient data.” (IPCC 2012)

• “There have been statistically significant trends in the number of heavy precipitation events in some regions.  It is likely that more of these regions have experienced increases than decreases, although there are strong regional and subregional variations in these trends.” (IPCC 2012)

• “There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities. It is likely that there has been a poleward shift in the main Northern and Southern Hemisphere extratropical storm tracks. There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail because of data inhomogeneities and inadequacies in monitoring systems.” (IPCC 2012)

• Since 1950, “there is medium confidence that some regions of the world have experienced more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, for example, in central North America and northwestern Australia.” (IPCC 2012)

• “Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.” (IPCC 2007)

My interpretation of these statements and the hundreds of scientific papers that I have read regarding climate change is that dramatic changes are occurring to the climate system, that a preponderance of evidence shows that there is a strong human-driven component to this change, and that we are very likely to see an even larger increase in temperature during the 21st century if we continue to conduct "business as usual."  Rep. Stewart agrees that the climate is changing, but argues that this has always been the case and that there is no ideal temperature that the earth is trying to achieve.   This perspective, however, ignores the fact that our society is built around the climate of the 20th century, and therefore, at minimum we should be considering climate change in long-term planning and adaptation efforts.  

The statements above also reflect the fact that linkages between climate change and so-called “extreme weather” vary depending on the phenomenon.  Based on the evidence available to date, there is a well-established link between climate change and the increased frequency of warm days and nights, as well as heat waves, as well as decreased frequency of cold days and nights.  There is mounting evidence of a linkage to the frequency and intensity of large precipitation events, but conclusions in this area are limited by the infrequent nature of these events combined with the limited quantity and quality of precipitation data, especially in areas outside the developed world.  Linkages with trends in tornadoes and tropical cyclones are less clear and, at least for tornadoes, it is unclear if we should even expect a trend.  

Thus, I agree with Rep. Stewart that there have been outlandish connections made between climate change and individual weather events by some groups.  Extreme weather events occur when the atmosphere is in outlier mode and nearly always have a “natural” component.  Separating natural from anthropogenic effects during individual events is incredibly difficult and likely to remain a subject of debate far into the future.  I simply don’t see this as a good litmus test for whether or not humans are causing climate change, or as a good indicator of how severe climate change might be in the future.  

Finally, Rep. Stewart raises concerns about the fidelity of current climate models.  Statistician George Box once said that “all models are wrong, but some are useful,” and this is my motto in both weather and climate prediction.  My concern about future climate change is based not only on my assessment of climate model fidelity (which is not as dour as Rep. Stewart’s) and their projections for the 21st century, but also our theoretical understanding of the climate system, past climate change, and trends in recent decades not only in atmospheric temperatures, but also snow, ice, ocean temperatures, and other indicators.  This assessment leads me to a conclusion similar to that made by M in the Bond film Skyfall:

“Well, I suppose I see a different world than you do, and the truth is that what I see frightens me.”

It frightens me not because I'm certain that we will see disruptive climate change during the 21st century, but because it is within what I consider to be reasonable assessment of what is possible.  Of course, that assessment does include the possibility of a more gradual climate change that might be less disruptive.  At issue is whether or not we should bet on that less disruptive outcome (which I see as a low probability possibility under business as usual) and continue with the status quo, or if we could instead enact policy measures to stimulate innovation and the migration to a low-carbon, high-efficiency energy future without negative economic consequences.  I'd like to think the latter is possible, but it lies outside my area of expertise.  

Be Grateful

Craig Patterson's death in an avalanche on Kessler Peak is a difficult reminder to all of us who recreate in the mountains how merciless the mountains can be.  Now is a good opportunity to give thanks to all those individuals who make it possible for us to enjoy a day of skiing, snowboarding, snowshoeing, or snowmobiling in the mountains.

These folks really do put their lives on the line every day.  According to the American Avalanche Association, 22 ski patrollers, 13 highway technicians, 6 snow rangers/avalanche forecasters, 2 avalauncher gunners, 5 guides, 1 search and rescue team member, and 2 other individuals have been killed while engaged in avalanche and snow-safety related work in the United States.  Most died in avalanches, although 8 were killed in explosive or gunnery related accidents.  Another will now be added to the list.  Let's hope it is the last.

So, keep Craig and his family in your thoughts and prayers, and be grateful for everyone who makes our mountain adventures possible.

Craig Patterson

Craig Patterson above Shingle Mill Canyon, Provo Area Mountains

Please keep UDOT avalanche forecaster Craig Patterson and his family in your thoughts and prayers.  Craig died yesterday in an apparent avalanche accident while conducting work-related duties on Kessler Peak.  Craig was beloved member of a community of dedicated individuals that enable all of us to recreate in the Wasatch Mountains.  My thoughts and prayers are with Craig, his family, and all of his friends in Utah's snow-safety community.

Flaky Science

Source: Tim Garrett, University of Utah

University of Utah Atmospheric Sciences Professor Tim Garrett and Cale Fallgatter have developed a remarkable instrument that they call the Multi-Angle Snowflake Camera (MASC) for taking pictures of snowflakes in freefall.  With support from the National Science Foundation and assistance from Alta Ski Area and Daniel "Howie" Howlett, they have been operating the camera for two winters now in the Wasatch Mountains.  Their data shows one of the dirty little secrets of the snow industry – that the majority of snowflakes made by mother nature, even in Utah, are defective.  The are frequently beat up, broken, and covered with frozen cloud droplets sometimes called rime.  The examples above are some of the nicer images they have captured.  Below are some examples of heavily rimed snow crystals.

Source: Tim Garrett, University of Utah

A great story summarizing their efforts and their significance for improving weather forecasts is now available here.  Have a look.

Storm Update

Thusfar, the downslope windstorm forecast issued by the National Weather Service has verified quite well.  They called for east winds of 35–45 mph, with gusts to 75 mph, in the high-wind watch they issued at 2:55 AM yesterday morning.  Peak gusts over 60 mph reported thusfar to MesoWest include:

UTPR2: 73 mph (Portable weather station installed by UDOT)
Centerville (CEN): 70 mph
Centerville (CENWWS): 65 mph
Centerville (CENEWS): 64 mph
Farmington (AP611): 61 mph

As we discussed yesterday, high-amplitude mountain waves of the type producing the current windstorm feature a descending layer of high momentum air, but also a hydraulic jump across which there is often a very rapid decrease in wind speed and, in some cases, a rotor in which there is a circular motion of air about a horizontal axis that leads to a complete reversal of the flow at the ground.

Source: Whiteman 2000

There is strong evidence of the hydraulic jump and rotor this morning.  Note in the surface map below the strong easterly and northeasterly winds at sites near the base of the Wasatch Mountains from the Bountiful Bench northward through Farmington and Fruit Heights, which contrasts with the much weaker westerly flow observed along the Legacy Highway and just west of I-15 near Farmington.

Source: MesoWest

Meanwhile, in the central Wasatch Mountains, we discussed on Sunday morning how the model forecasts for this period were producing dramatically different amounts of precipitation at Alta with the GFS calling for less than a half inch of snow, the 12-km NAM 11.5" of snow, and the 4-km NAM 36" of snow for the period from 12 AM Monday through noon today.  This forecast spread was in part related to the position of the precipitation band, which did end up far enough north to give a good dump to the central Wasatch.  It looks like 11 inches thusfar at Alta-Collins (the resort reports 12 inches), with about an inch of water.  The big winners, however, are the Park City resorts with about 18" of snow thanks to the predominant easterly flow making them the windward side of the Wasatch during this period.

Potential Downslope Windstorm

Many residents of the northern Wasatch Front recall the major downslope windstorm that occurred on December 1st, 2011, when wind gusts reached more than 100 mph near Centerville.  Discussion of the event from previous Wasatch Weather Weenies posts is available here, here, here, and here.  The latter includes some videos, including a humorous one of some of our students attempting to launch a weather balloon.

Downslope windstorms are produced by high-amplitude mountain waves.  In a high amplitude mountain wave, high momentum air from aloft descends rapidly and accelerates on the lee (downwind) side of a mountain range.  The strongest winds typically occur near the base of the mountain, and one can often find a hydraulic jump in which the strong flow ascends rapidly just a bit further downstream.  A rotor, or an area of reversed flow at the surface, is typically found downstream of the hydraulic jump.

Source: Whiteman (2000)

Most of us, especially whitewater kayakers, have seen similar phenomenon in rivers and streams where the flow moves over a rock, accelerates on the downstream side, and then rises abruptly in the hydraulic jump.  In the atmosphere, as in the stream below, considerable turbulence can be found where the wave is breaking.

The forecast models suggest that we could see a downslope windstorm tonight along the northern Wasatch Front.  As shown in the NAM forecast below, a closed 500-mb low will move across southern Utah today and tonight, with cold air moving southward into Wyoming.  Eventually, a shallow layer of cold-easterly flow penetrates from Evanston westward across the Wasatch Range, where easterly flow penetrates into the northern Wasatch Front.

NAM model forecast of 500-mb heights (black contours) and 800-mb winds (vectors) and temperature (warm-to-cool color contours every 2ºC) from 1200 UTC 8 April –1200 UTC 9 April 2013. 

This leads to a period where strong easterly flow is able to plunge down the west face of the Wasatch Mountains.  The cross section cuts across northern Utah and southwest Wyoming at 1200 UTC (0600 AM MDT) tomorrow.  Note the layer strong flow in excess of 25 m/s (50 knots) indicated by orange-red color fill that extends from southwest Wyoming into northern Utah, and then down the western face of the crudely resolved model Wasatch Mountains.  

This and other model forecasts indicate significant potential for a strong downslope windstorm tonight.  How strong will depend on the details, including the strength and height of the inversion at the top of the cold air, details of how the wind speed and direction vary with height, etc., but it is clear that this could be a significant event.  The National Weather Service as issued a High Wind Warning for the northern Wasatch Front.  Take appropriate precautions and secure loose items that could turn into projectiles in strong winds.

It Ain't Over Yet, but Mother Nature Is Playing Dice

Over?  Did you say over?  Nothing is over until we decide it is!  Was it over when the Germans Bombed Pearl Harbor?  Hell no!

- John "Bluto" Blutarski (Animal House, 1978)

Yup, eventual Senator Blutarski was known for his intellectual insights, but in this case he was wrong.  Nothing is over until Mother Nature says it is over, and she still hasn't closed the door on this winter.

The forecast models are bringing a deep trough across the Intermountain West, ushering in colder weather for the first part of the coming work week.  The trough is forecast to be moving across northern Utah tomorrow afternoon.

Once again, the change in temperature is an easier forecast than snowfall amounts.  In fact, the difference in forecast precipitation for Alta between the various models is mind boggling.  Here's a summary based on the forecasts initialized at 0600 UTC (midnight) 7 April.  Accumulations are for the period from midnight tonight through noon Tuesday:

GFS*:                     .04" SWE; 0.4" Snow

12-km NAM:        0.93" SWE; 11.5" Snow

4-km NAM:          2.66" SWE; 35.7" Snow

Yeah, you got that right, the model spread ranges from less than an inch of snow to almost 3 feet [*The GFS forecast is technically for the nearest grid point to Salt Lake City, but there's not much difference between that and what the model is producing at Alta given the coarse 27-km grid spacing].  

There are two fundamental differences between these model forecasts, which I'll illustrate using the forecasts for 1800 UTC (1200 MDT) Monday 8 April.  Like real estate, the first difference is related to location, location, location.  All the models bring the trough across Utah and generate a surface cyclone over southern Utah with a precipitation band forming in an area of large-scale confluence to the north of the low center.  The GFS, however, has this band just a bit farther to the south than the 12- and 4-km NAM, so Alta just misses out on the action.

GFS (top), 12-km NAM (middle), and 4-km NAM (bottom) forecasts valid
1800 UTC (1200 MDT) 8 April 2013.  GFS precipitation for 6-h period,
NAM precipitation for 3-h period.  Note subtle difference in position of
precipitation band over Wasatch Front.

The second difference is related to resolution and terrain representation.  The high resolution 4-km NAM generates a much stronger precipitation band, and also produces localized precipitation maxima over high terrain throughout the forecast period, which results in more precipitation than generated by the 12-km NAM.  

All of that is fine and dandy, but for that forecast to verify, the 4-km NAM pretty much has to nail the position of the precipitation band, and it is clear from the forecasts above that a small error in that position could yield a dramatically different result.  Betting on the 4-km NAM to verify is perhaps like betting on snake eyes.  It's not impossible, but it is a low probability outcome.  Something closer to the 12-km NAM, which generates about a foot, is perhaps the highest probability event, much like rolling a seven, but it's no guarantee either. 

Just to further highlight the uncertainty in this forecast, below is the probability of more than 0.5" of snow water equivalent during the 24-hour period ending at noon Tuesday based on the Short Range Ensemble Forecast System (SREF).  Of the 21 members that comprise this forecast system, only about 30% generate 0.5" or more of SWE in the area around Alta.  The rest generate less.

So, the bottom line is that this forecast is pretty much a crap shoot, with a wide range of potential outcomes in the central Wasatch.

A Blustery Frontal Passage with Rain to Come

Post-frontal clouds over the Salt Lake Valley @ 7:30 AM

A blustery cold front pushed through the Salt Lake Valley early this morning.  "Cold" in this case is a relative term as the post-frontal environment is still fairly mild.

The leading edge of the front was relatively dry.  As can be seen in the 7:00 AM radar image below, northwesterly flow at the surface is running well ahead of the main precipitation band accompanying the front.

There are some fairly high returns encircling the KMTX radar site on Promontory Point.  Some of that reflects what is known as the radar bright band, an area of locally high radar reflectivity where the radar beam cuts through the melting layer where snow is transitioning to rain.  Even still, there are clearly some pockets of heavier showers and the possibility of a bit of thunder as the frontal band pushes through the Salt Lake Valley.  That would be a nice way to finish the work week.

For those who haven't given up on the ski season, I suspect we'll get a few inches of high density cream   out of this in the upper elevations of the Wasatch.

First Snotel to Bottom Out?

The Ben Lomond Trail SNOTEL is the lowest in the Wasatch Mountains at an elevation of only 5829 feet.  It is, however, located in the remarkably snowy Ogden Valley, one of the snowiest lower-elevation areas in northern Utah (see marker A below).

It is also a beautiful area, and well worth a trip for either backcountry skiing or cross country skiing at North Fork Park.

Looking east from Ben Lomond Peak across the Ogden Valley toward Powder Mountain and James Peak.  The
Ben Lomond Trail SNOTEL is on the lower snow-covered slopes in the foreground on this day.

In a good snow year, you can find snow in the upper Ogden Valley well into April.  Here's how things looked around the Ben Lomond Train SNOTEL on April 26, 2005.  Keep in mind that this site is below 6000 feet in elevation.

This year, however, is another story.  The snowpack at Ben Lomond Trail never reached even half of its average peak, it ripened (meaning it warmed to a constant temperature through depth of 0ºC) in early March, and it is melting fast this week.  The snowpack SWE through this morning was about 2 inches, with a loss of about 0.7 inches per day the past three days.

Source: NWS/CBRFC

The days of measurable snowpack at Ben Lomond Trail appear to be numbered and it looks to be the first SNOTEL that will bottom out in the Wasatch this spring.  Perhaps it will recover for a couple of days if we get a cold storm, but for all intensive purposes, the normally prolific snowpack of the Ogden Valley will suffer an early demise this spring.

James Hansen Retirement

As reported by several media outlets, including the New York Times, noted climate scientist James Hansen will retire from NASA this week.

Hansen is certainly one of the most important climate scientists of the last few decades.  Google Scholar credits him with nearly 35,000 citations and an h-index of 78.

The h-index attempts to provide a measure of the productivity and impact of a scholar's published work.  Although it is an imperfect measure, in the atmospheric sciences, values above 20 generally indicate a productive senior scientist and above 30 an exceptional senior scientist.  78 is off the scale.  Hansen's many important papers can be accessed here.

Publicly, Hansen is perhaps most famous for the testimony he gave to Congress about global warming during the summer of 1998, which after a short introduction began with the following statement.

"I would like to draw three main conclusions.  Number one, the earth is warmer in 1988 than at any time in the history of instrumental measurements.  Number two, the global warming is now large enough that we can ascribe with a high degree of confidence a cause and effect relationship to the greenhouse effect.  And number three, our computer climate simulations indicate that the greenhouse effect is already large enough to effect the probability of extreme events such as summer heat waves."

The full text of the testimony can be accessed here and press coverage following the event in this 1988 New York Times article.

An interesting discussion of Hansen's 1988 testimony, subsequent battles with the US government over censorship, and his increased political advocacy is provided at Roger Pielke Jr.'s political blog.

Positively Cascadian

Low clouds over the Salt Lake Valley

and over the Avenues

Breath deep and enjoy the moisture this morning.  Last night's deluge has brought some cool, moist air to the Wasatch Front, giving the Salt Lake Valley a positively Cascadian feel.

The upper elevations got some of the white stuff.  Alta-Collins received 8 inches overnight with a mean water content of about 10%.  Not the Greatest Snow on Earth, but it probably will ski just fine.  That snow, along with what fell yesterday, got the Alta-Collins stake back over 100 inches again.

Golf weather returns tomorrow.