Living on Scraps

I keep hoping for a big storm cycle to add to the current snowpack, but the forecast is pessimistic.  Below is our experimental downscaled forecast system (see Snow is Coming but How Much for more information) and through 0000 UTC 7 December the downscaled average precipitation produced by the 22 ensemble forecasts is less than an inch of water for the central Wasatch and less than 12 inches of snow.  Only 80% of the downscaled ensemble forecasts produce more than 6 inches of snow in the higher elevations of the central Wasatch and none produce more than 24".

Although I emphasize that this is an experimental product and we don't know the biases all that well (I suspect if anything these forecasts are too wet and snowy on average), it is sufficient to illustrate that we'll be living on storm scraps for the next week, with our best chance of snow late Tuesday and Wednesday.  Let's hope one of the snowier ( > 12 inch) forecasts verifies, although I suspect a more modest event is more likely.

Do Something Already!

I can't decide if cirrostratus or fog is my least favorite cloud, but after three days of high overcast and flat light, I'm leaning toward the former.

Here's how things looked on Thanksgiving. 

And here's how things looked today.

Yup, cirrus monotony.  Come on Mother Nature, do something already!  Give us snow or give us sun!

An irony of thick cirrostratus is that it produces flat light and difficult skiing, but it also results in outstanding visibility.  In fact, the best visibility is usually on days with high overcast.  This is because cirrus clouds reflect some of the incident sunlight back to space, so there's less scattering of sunlight by atmospheric gases and aerosols at low levels.  Below is the view from the Avenues foothills yesterday.  One can easily see Mt. Nebo, roughly 100 km distant, on the horizon to the right of the central Wasatch.

Screwed by the High Sierra

The Wasatch are a remarkably snowy interior mountain range, but they would be even snowier if not for the High Sierra.

One way to illustrate this is to look at the fraction of cool-season (November to April) precipitation produced over the western U.S. when atmospheric rivers (ARs) are present.  ARs are filaments of strong water vapor transport characterized by high water vapor content and strong winds.  As can be seen in the figures below, more than half the cool-season precipitation along much of the U.S. west coast is produced when ARs are present.  ARs sometimes extend inland, especially over the Columbia Basin and panhandle of Idaho, the Snake River Plain and southwest central Idaho Mountains, and much of Arizona, where >30% of the cool-season precipitation occurs when ARs are present.  In contrast, AR penetration into central Nevada and Utah is less common and AR-related precipitation is less common.

Source: Rutz et al. (2014)

This is primarily the result of the High Sierra, which form a formidable barrier running through eastern California south of Lake Tahoe.  The High Sierra directs low-level airmasses to the north or south and generates heavy orographic precipitation that removes water vapor from any airmasses traversing the range.  As a result, it is very difficult for ARs to penetrate directly across the High Sierra.

One can see this very clearly in the forecasts for the coming week.  In the GEFS ensemble mean integrated water vapor transport forecast for 0600 UTC 2 December 2004, there is a broad area of strong water vapor transport over and upstream of the U.S. west coast.  This is basically a fat AR.  The AR able to penetrate inland across portions of Oregon and Idaho, but can't survive transit across the High Sierra.  What a pity!

As the system moves southeastward, higher integrated water vapor transport values can sneak inland to the south of the High Sierra, but once again, there's not penetration inland across the highest portion of the range.

And here's the GFS total precipitation forecast for the next week showing a band of heavy precipitation to the north of us and an arm of heavier (but weaker than that to the north due to the shorter duration) precipitation to the south.  In this forecast, we get clipped by the occasional system so we do get something, but it would have been much more were it not for the High Sierra.

To do better, we need a system to draw moisture in from the north or south and concentrate it over northern Utah (a weak front sagging in on Sunday does this to some degree, but right now doesn't look like it will produce a major snowfall in the southern Wasatch).  A passing system on Wednesday might do the trick too, although we'll have to see it it comes together as the event approaches.  Alternatively, you could drive north and out of the shadow of the High Sierra.

Announcement: I'll be giving a talk on my new book Secrets of the Greatest Snow on Earth at 12:15 on Tuesday, December 2nd in the Sutherland Moot Courtroom at the University of Utah College of Law, with a book signing immediately following.  The talk is free and open to the public and King's English Bookstore will have books available for purchase.  Click here for more information.

Beautiful Thanksgiving in the Offing

Ideal weather will prevail over Utah tomorrow for your Thanksgiving festivities with high pressure firmly in control.

I can think of only a few individuals who might be unhappy.

Source: visualparadox.com

Happy Thanksgiving!

Valley Cloud Storm

The radar loop for this morning shows some very weak echoes forming over and upstream of the Wasatch Mountains.  If you look specifically at the eastern Salt Lake Valley, you'd probably think it is raining or snowing.

Very little of that precipitation, however, is reaching the valley floor (although it's beginning to get close as I write this).  It's mostly snow that is falling and sublimating (when ice turns directly into vapor) before it reaches the ground.

The reason that this precipitation shows up on radar is that the beam originates on a mountain peak (Promontory Point) and is oriented with a slight tilt relative to the horizon (0.5º).  The starting elevation, combined with the slight tilt and curvature of the Earth, overwhelms the slight tendency of the atmosphere to bend the beam back towards the Earth's surface.   As a result, the KMTX radar samples the atmosphere well above the valley floor, meaning it sees the snow aloft rather than what's happening at lower levels.

Source: Secrets of the Greatest Snow on Earth

This is an important consideration when interpreting radar imagery over the Salt Lake Valley and many other valley locations over the Intermountain West.  It pays to consult web cams and other sources in addition to radar.

Natural Snowmaking Today

“It doesn’t need a reason to snow in Little Cottonwood Canyon; it needs a reason to stop.” 

- Alta Meteorologist Mike Kok

The natural advantages of upper Little Cottonwood Canyon and the high terrain surrounding the Cottonwoods are on display today.  It doesn't look like much, but notice the weak radar reflectivities that have been lingering in the central Wasatch this afternoon.  Alta and upper Little Cottonwood Canyon identified with a red box.

Those weak radar returns have been producing about an inch an hour of low-density fluff at the Alta-Collins site this afternoon.  

Most of that snow growth is probably occurring at very low levels, some of it below the radar beam.  This is often a problem in shallow orographic storms, as illustrated conceptually in the figure below from my book Secrets of the Greatest Snow on Earth.

Source: Secrets of the Greatest Snow on Earth

If you look carefully at the radar loop above, you can see another area of light but persistent radar returns in the mountains southeast of north Salt Lake and Bountiful.  Here and in the Cottonwoods further south, the northwesterly flow is encountering pronounced concavities in the terrain thanks to the Avenues foothills and the Traverse Range, respectively, which extend westward from the main body of the Wasatch.  Coincidence?  I think not.  

Storm Update

Pretty good storm totals in the upper Cottonwoods so far (as of 8 am) including 20 inches at our site in upper Albion Basin and 19 inches at Alta-Collins.  Water totals are also great for base building with 1.96 inches of water equivalent at the former and 2.21 inches at the latter.  That works out to a water content of about 10-12%, which is high by Utah standards, but it's stacked right and should ski great this morning, although there's lots of buried monsters (stumps rocks, etc.) and high backcountry avalanche danger to contend with.  Let's be careful out there!

This morning we've had a nice round of orographic snowfall add to the totals, with some possible lake enhancement to the southeast of the Great Salt Lake.  This particular period of snowfall appears to be winding down as I write this (about 8:30 am).  If you look carefully at the radar loop, you can see one of the reasons why Park City gets so much less precipitation than the upper Cottonwoods.  In shallow, northwesterly flow storms like this one, very little of the precipitation is able to spill east the Wasatch Crest.  Note how widespread and persistent the radar echoes are over the Cottonwoods compared to east of the Wasatch crest.

Forecast and Secrets Update

Not much change from yesterday in the forecast for today and tonight (compare time-height section below with that in the previous post).  Looks like a pretty good dump for the Wasatch.  I suspect most of you will be happy in the morning.   I'm still perplexed about what to expect thereafter. It's the weekend, so I'll let you surf around and look a the divergent forecasts out there and pick what you want.

A quick update on my book Secrets of the Greatest Snow on Earth.  Amazon is finally reporting that it will be in stock on November 25.  I've heard delivery dates ranging from November 28th to 30th.  Thanks for your patience!  In addition, it is now available in Nook and paperback (shipping with 24 hours) from Barnes and Noble.  I'm a huge fan of e-books, but in this case, I recommend the paperback.  For those of you on or near the University of Utah campus, I will be doing a lecture and book signing (Thanks to King's English who should have the book in stock, but call ahead to be sure) at the Wallace Stegner Center at 12:15 PM on December 2nd.  Click here for more information.

Peering into Forecasts of the Coming Storm

Time-height sections are remarkably powerful tools for weather forecasting (see Forecast Tools: The Time-Height Section for a basic introduction).  The forecast NAM time-height section for Salt Lake City for the next couple of days is an absolute classic, showing beautifully the likely storm environment changes over northern Utah.

With time-height sections, we typically reverse the x-axis so that time increases to the left, as is the case above.  So, scan from right to left to see the passage of various weather systems between now and Monday afternoon.  Instead of temperature, we also use a thermodynamic variable known as equivalent potential temperature, also known as theta-e or θe, which considers temperature and moisture and can be useful for examining atmospheric stability.

As can be seen above, we begin today with a mainly dry (and polluted environment).  The leading edge of the precipitation system is marked by a pronounced warm front that descends overnight and passes at mountaintop level prior to noon (Sat 18Z) tomorrow.  As the warm front approaches, southwesterly low-level flow will increase and potentially mix out our the inversion overnight.  

Moisture moves in with the approach of the warm front at low levels, and precipitation will develop over the mountains tomorrow morning.  Snow levels will max out around the time of warm-frontal passage, reaching 7500 feet (note that the blue line is the freezing level and that the snow level is typically 1000 feet below this).  Later in the day, a surge of low-θe aloft will precede the surface cold front and destabilize the lower atmosphere.  Thus, I would expect the precipitation to become more convective tomorrow afternoon, with even the potential for a bolt or two of lightning and rumble of thunder.  During this period, there might be graupel at times, and snow levels will lower, but remain above the valley floor.

Cold frontal passage occurs late tomorrow or early tomorrow evening, at which point snow levels will crash to the valley floor.  This will be the first full-bore period of winter weather at all northern Utah elevations this year (hooray!).  Get your old bald tires replaced today!

Following frontal passage, we see a prolonged period of unstable northwesterly flow.  This is what I call the post-frontal crap shoot since much of what happens during this period depends on factors that are difficult to forecast, including lake-effect.  

Of course the hard part is figuring out how much snow is going to fall.  The graphs below show what the NAM is producing for Alta.  Precipitation develops during the day tomorrow, and becomes heavy during the frontal passage, and then tapers off overnight.    

Based on this, I'd probably go for a total accumulation of 12-20 inches tomorrow and tomorrow night (i.e., through  5 am Sunday) in upper Little Cottonwood (typically the NAM underestimates total snowfall, hence the increase, although there are unfortunately exceptions). After that, based on a survey of the various models, I think it's likely we will see periods of snow through Monday afternoon and that we'll get more snow than indicated by the NAM above.  Nevertheless, I consider the post-frontal environment to be more of a crap shoot, so I'm not sticking my neck out for specific accumulations in a 48-84 hour forecast, especially given the fickle nature of the lake effect, which could play a role beginning on Sunday morning.  I think you should be optimistic, but keep your fingers crossed.  

Snow Is Coming, But How Much?

I've been distracted by Buffalo's snowmaggedon the past couple of days.  Incredibly, they are getting more again today.  Never has so much powder gone to waste.  If only Mother Nature could have arranged for the Adirondacks, especially the high peaks, to be located more strategically relative to Lakes Erie and Ontario.

Of course, there is a place where Mother Nature has fortuitously aligned the weather, topography, and geography for great skiing and that's the Cottonwood Canyons.  And, the good news is that we are going to be getting some beginning this weekend.  The storm will be warm on Saturday with a passing warm front.  Following the cold frontal passage late Saturday, temperatures will fall and we are looking at what will likely be an active northwesterly flow period through Monday Night and possibly Tuesday.

The image below shows the 1200 UTC GFS forecast accumulated water-equivalent precipitation through 0000 UTC 25 Nov (6 PM Monday MST).  One can see the moisture corridor extending across the Pacific Northwest with precipitation maxima over the Wasatch–Teton ranges and the western Colorado Rockies.

Source: NCEP

If we extract a time series of accumulated precipitation for Salt Lake City from the GFS we see some healthy numbers, with a total of about 1.2 inches by late Monday.

There are, however, at least three critical factors to consider when interpreting such a forecast:

1. The GFS does not adequately resolve the topography of the western U.S.  As a result, we typically have to adjust these precipitation amounts for topographic effects.  This is typically done in ad hoc fashion.  For example, the standard approach for estimating the total precipitation in upper Little Cottonwood is to multiply the GFS forecast for Salt Lake City by 2 or 2.5.  Using the latter would be a phat 3.6 inches of water, which would be marvelous for base building.
2. For snowfall amount, we need to also forecast the snow-to-liquid ratio, or how much snow we'll get out of each inch of water.  There is no model that does this today, although we do have some algorithms that we can apply to model output to get a pretty good estimate for the upper Cottonwoods.
3. Can I trust this forecast?  The atmosphere is quite chaotic.  A slight shift in the position of the jet stream and moisture plume in a situation like this could leave us either high and dry (north shift) or right in the thick of it (south shift).

Through the National Weather Service Collaborative Science, Technology and Applied Research (CSTAR) program, I've been working with Trevor Alcott to develop approaches for dealing with these issues.  More correctly, Trevor has been coming up with great ideas and implementing them, while I sit back, pat him on the back, and otherwise get the hell out of the way.  

The approach uses forecasts from the North American Ensemble Forecast System (NAEFS), which includes the Canadian ensemble system and the U.S. Global Ensemble Forecast System (GEFS).  These low-resolution forecasts are then downscaled using climatological precipitation –altitude relationships to generate high-resolution precipitation forecasts.  Then we apply a snow-to-liquid algorithm to estimate snowfall.  We do this to all members of the ensemble.  This allows us to:

1. Calculate a mean of all the members, which over the long run is typically a better forecast than that produced by a single modeling system such as the GFS
2. Evaluate the range of possibilities for a given period, including probabilities of exceeding specific thresholds.

Now, I'm going to show you these results, which are literally a day old.  I need to emphasize that this is an extremely experimental system, so don't plan your powder days or backcountry travel based on it.  The system has not been tested or calibrated, so right now, this is just great eye candy, but hopefully over time we can assess reliability and make refinements. I'm going to hold off on the snowfall estimates as I'm not sure we're even in the ballpark on those yet.  

The ensemble mean water equivalent forecast below is from last nights NAEFS and covers the period through 0000 UTC 25 November (5 PM Monday).  It shows a strip > 3 inches along most of the high Wasatch with a maximum of just over 5" on Ben Lomond Peak.  Structurally, this looks reasonable, but will it be skillful?  Time will tell.  I am skeptical of the Ben Lomond max.  Since we are using climatological precipitation-altitude relationships, they don't account for flow directions.  Typically Ben Lomond doesn't do well in northwesterly flow, so perhaps it will fall short.  Elsewhere perhaps we have a shot, but the storm will need to be productive.  If this does verify, we'll be in good shape for Thanksgiving skiing.

With an ensemble, however, we can take a look at many many forecasts to get a handle on the range of possibilities, as well as probabilities. Below is what we call a plume diagram and it shows the total accumulated precipitation at Alta-Collins from all the ensemble members (Canadian model light green, GEFS dark green) and the mean (thick green line). 

You can see that the mean gives almost 4 inches of water for Alta-Collins.  What a godsend that would be.  The range, however, is quite large.  The driest members produce around 1.5 inches of precipitation, and the wettest over 7 inches.  That's a lot of spread.  The Canadian model is in general wetter than the GEFS.  This appears to be due to the Canadian simply being more productive than the GFS since their large-scale forecasts are similar.  Canadians do like snow, eh? Of course, there is a member of the GEFS that goes for five inches.  

 Now, if this were a reliable and well calibrated system, one might draw an analogy of that plume to rolling two dice.  The most likely outcome is a 7 when you roll two dice.  In the case of the forecast above, the most likely outcome is probably something near the mean.  The extreme low and high precipitation amounts are more like rolling snake eyes or two sixes.  Less likely, but still possible outcomes.  Surely you see why it can be foolhardy to put specific numbers on winter storms many days in advance.  

Of course, we don't know yet how reliable and calibrated this system is, which is why you should continue to listen to official forecasts and not mad scientists like me.  I thought, however, that it might be interesting to share and perhaps we'll get some of this running on weather.utah.edu in the near future.  Putting all this together, it looks like we're going to see a good storm cycle through Monday, but we'll have to see how it all comes together in the coming days.  

Buffalo to Get Even More

Do you ever wonder what it would be like if it started snowing and never stopped?

- Steve Casimiro, Editor, Powder Magazine, 1987-1998

The quote above is one of my favorites and one that I often think of during major snow events.  It came to mind this morning as I examined the snowfall totals for the Buffalo area so far, with maximum storm-total accumulations of more than 60 inches.  

At the moment, it's actually not snowing in Buffalo.  The camera image below shows the New York State Thruway near West Seneca, near the heart of the most intense snowfall.  This is a major thoroughfare and one of the best maintained highways in America.  They have finally carved a trough through the snow, but there's a lot of work still to do.  

A few miles to the north accumulations are much lower and the road looks fine.  

Although Buffalo can exhale this morning, more snow is expected later today, tonight, and tomorrow.   At first, the snow will be of the widespread "garden variety" type, but later tonight, the lake-effect will return with another raging lake band possible.  Below is the 4-km NAM forecast for 1200 UTC (0700 EST) tomorrow).

Bets on maximum total snowfall?  

Buffalo in the Lake-Effect Crosshairs

Note: This post has been updated to include a remarkable video of the lake-effect band at the end. 

You don't see this every day.  Even in metric, 40-60 (cm) is a pretty good storm.  But this is the forecast in inches.  Source: WIVB.com

Contrary to popular belief, Buffalo is not the snowiest city in the United States, it's Syracuse (96 vs. 126 inches).  However, when it comes to behemoth storms, Buffalo is king.  Why?  It sits on the eastern end of highly elongated Lake Erie and thus can be pounded by intense snowbands that form along the major lake axis.  Such snowbands are sometimes called Long-Lake Axis Parallel or LLAP bands and are responsible for some of the most intense snowstorms in the world.  Last night and today portions of the Buffalo Metro Area have been getting pounded by a persistent LLAP band, as illustrated by the 18-hour-long radar loop below.  The approximate position of the Buffalo metro area is outline by the red box.

There are a number of reasons why these bands are intense.  First, they form when the large-scale flow is moving over the longest axis of a lake, which means the airmass experiences the maximum heating and moistening possible.  Second, the local heating over the lake surface generates land breezes that converge near the center lake axis, triggering and organizing the snowfall in an intense, localized band. The schematic below from my recently released book shows how this process works over the Great Salt Lake, but the concept is the same over Lake Erie.

Source: Secrets of the Greatest Snow on Earth (Steenburgh 2014)

LLAP bands can be very narrow with sharp contrasts in snowfall rates and accumulations.  If you look carefully at the loop above you'll see that once the band sets up, it snows only briefly in the northern portion of the Buffalo metro area.  In contrast, the southern portion is getting absolutely pounded.  A great photo of what I think is the northern edge of the band was taken by the Buffalo News 4 staff.  On the right you are in the clear.  On the left, snowpocalypse.

Source: WIVB.com

The Weather Channel is currently reporting that up to 4 feet of snow has fallen already in the Buffalo area.  In the satellite imagery, you can see very clearly the elongated, corrugated nature of the cumulus clouds in the primary lake-effect band, with clear skies immediately to the north.

Source: College of DuPage

In northern Utah, the Great Salt Lake occasionally produces LLAP bands.  In fact, about 20% of lake-effect periods feature such bands.  However, they lack the intensity and duration of the LLAP beasties that form downstream of Lake Erie.  Of course, as good as the Lake Erie bands are, it is Lake Ontario and the Tug Hill Plateau that see the biggest and baddest LLAP bands in the world.

Addendum @ 1:40 PM 18 Nov: Great video below from YouTube user Alfonzo Cutaia showing remarkable updrafts and northern edge of the band.

 

Update on Secrets of the Greatest Snow on Earth

I've been told by some of you that you've received e-mails from Amazon stating that they are having trouble obtaining copies of Secrets of the Greatest Snow on Earth.  I'm not sure what the problem is with their cyborgs but I suspect that there's been some minor glitch in their receipt or processing that's pushed their ship date past November 14.  Sit tight as I suspect it won't be long now.

I do know that copies are available currently at Weller Book Works in Trolley Square.  You can find them on their signed copies table and in their winter sports section.  You should also be able to order directly from Utah State University Press or by calling their customer service center at 1-800-621-2736.  I just bought 20 copies and they will be here by the end of the week.  Alternatively, check with your favorite book store as the book is beginning to appear at other retailers.  

Thanks for the interest and patience!

Down One Good Storm

I know everyone is anxiously waiting for more snow, but it's worth remembering that even though the odds are better here than elsewhere, it's still pretty much a crap shoot for having a decent natural snowpack by mid November.

Let's take a look, for example, at the snowpack statistics at the Snowbird SNOTEL site, which covers a 26-year period.  The snowpack snow water content at this site currently sits at 2.4 inches, compared to a median of 4.0 inches (and an average that is also right around 4 inches).  That basically means that the difference between what we have right now and average is one good storm (think a wet 15 incher).  Years with big snowpacks in early to mid-November, like 2004-2005 (purple) are really fantastic, but they are the exception, not the rule.  What we have right now is not all that unusual.

When I moved to Utah, I was always surprised to see snow rich resorts like Alta making snow.  It seemed like an embarrassment of riches, but it clearly is a good investment for them when the early season starts slowly.  One of the blessings of the high-altitude Utah climate is that when it's not snowing, conditions are often ideal for snowmaking.  That was certainly the case today with cold, dry weather and minimal wind.  Between the recontouring of Corkscrew and their snow cannons, Alta looks to have a pretty good base of man made laid down.

We will remain down one good storm probably through the work week.  A couple of weak systems may clip us on Thursday or Friday, but right now they don't look like they will produce much.  The models are hinting at some snowier weather next weekend, but that's still a ways beyond my predictability horizon so we'll have to see how things shape up in the coming days.

Alta Snowfall

Thanks to a few hours of low-density snowfall this morning, the overnight and morning totals in upper Little Cottonwood aren't looking too bad.

Automated interval board observations at the Collins site show the 3 inches that fell Thursday night.  The board was wiped between the 1600 MST and 1700 MST observation, then you can see the accumulation of about 10 inches of snow through 0900 MST this morning (ignore the spurious 34 inch observation at 0600 - one of the hazards of automated snow measurements are the occasional spurious observations).  It appears the board was wiped this morning between 0900 and 1000 MST with a 3 inch accumulation after the wiping.  That observation seemed a little odd given that only .01" was observed by the rain gauge.  If it is legit, that would give us an event total of 16 inches and an overnight total of 13 inches.  If not, it's more like 13 and 10.

I believe that Collins Gulch is currently closed to uphill skiing.  If you are considering venturing up the summer road, we have a new snow study station installed in upper Albion Basin that we are calling "Top Cecret."  Since we're not currently wiping the interval board, it is essentially acting as a total depth sensor.  Note the increase since Thursday from about 8.5 inches to a peak of about 21 at 0900 (ignore the spikes > 45 those are also spurious) for an event total of about 12.5 inches and an overnight accumulation of about 11 inches.

Water equivalent at the two sites was 0.74" at Alta-Collins and 0.7" at Top Cecret.  I wish I could move the decimal point one digit to the right as we really need a good wet, heavy base-building snowfall.  With an unsettled snow depth of 20 inches at Top Cecret, it's best to call this pre-early-season conditions.

24

Imagine if Jack Bauer wasn't a secret agent, but instead was a meteorologist.  Fox would have an endless treasure trove of plot ideas to tap into.  Forecasts for severe and unusual weather.  Tornado out breaks.  Snowstorms.   Let's start season 1 this morning since the next 24 hours are pretty interesting.

Instead of terrorists, we have a band of precipitation precipitation near the Utah–Idaho border.  There are a few scattered showers over the Wasatch Range to the south.  This pattern will likely persist for most of the day today.  The north gets most of the action, while we get some intermittent mountain snow showers to the south.  It's not too exciting, but the climax is of course tonight.

The NAM forecast for 0300 UTC (2000 MST) shows things picking up in the evening.  The main precipitation band associated with the approaching front remains to the north, but the orographic snow showers pick should be picking up in frequency and intensity in overnight, first in the northern Wasatch and Bear River Range, and then eventually in the central Wasatch.

Bauer's best forecasting work will need to be done late tonight and early tomorrow morning as the cold front approaches and moves through the area.  Although not a long lived event, it will bring a period of heavy snow to the mountains, some snow on the east bench, and maybe even an angry inch on the valley floor.

Accumulations will likely be greatest in the northern mountains.  The upper Cottonwoods got 3" yesterday and I'll go with another 6–12" through noon tomorrow (things may wind down a couple hours before then, but that's an easy time to peg).  After that, the front is through and season 1 concludes.

Another Difficult Forecast

With regards to the forecast through the weekend, we're in a tough spot.  The pattern in the higher latitudes is completely wacked out with a high-amplitude ridge centered currently over the Yukon Terrritory and a deep trough over the eastern United States.  Meanwhile, southerly flow is cutting underneath the high-amplitude ridge and penetrating over the southwest U.S.

Downstream of the col in the height pattern, Utah is in an area of confluence between westerly flow from over the Pacific and flow originating over western Canada, as illustrated above.   In terms of the timing and intensity of precipitation features moving through northern Utah, there's a lot of sensitivity in a pattern like this and I've been watching the models struggle coming to a consensus over the past couple of days.  Look at the difference, for example, in the GFS (top) and NAM (bottom) forecasts for 11 PM MST Saturday night.  The GFS has the upper-level trough axis over northern Utah with a cold front and associated precipitation over central Utah.  In contrast, the NAM has the trough axis over southern Idaho (extending all the way to Portland, OR), a less well defined front, and a plume of moisture remaining over northern Utah.

Since the gold-standard ECMWF is similar to the GFS, I'll lean toward the GFS on this one.  The GFS time height-section shows a decent plume of moisture and an upper-level front moving through today, a bit of a break tonight and early tomorrow, and then another pulse of moisture later tomorrow through Saturday morning.  After that, cold, dry air moves in.

I'm still not sold on the idea that the models have a great handle on all these modulations in precipitation, so periods of mountain snow through Saturday morning is perhaps the most reasonable forecast.  I'm leaning toward a total accumulation by noon Saturday of 8–14 inches in the upper Cottonwoods, but confess that the error bars on this one are fairly wide.

Coverage of Secrets

My first interview about Secrets of the Greatest Snow on Earth occurred this morning on Utah Public Radio.  Click here for a listen.

Detailed Look at the Cold Surge

The new High Resolution Rapid Refresh (HRRR) modeling system is now providing remarkably detailed analyses and short-range (0-15 hour) forecasts operationally at 3-km grid spacing.  This is a major advance for weather analysis and forecasting in the United States and I'm hoping it should prove useful for ski forecasting this winter.

Below is the HRRR analysis for 1300 UTC (0600 MST) this morning showing the remarkable structure of the current cold surge interaction with the high topography near and along the Continental Divide.  Analyzed surface temperatures are below -12ºF across much of central Wyoming and southwest Montana.  The strong pressure gradient associated with this cold airmass is driving easterly and northeasterly flow into the Columbia Basin, Snake River Plain, and northern Utah.  In the case of the latter, one can see how the Uinta Mountains splits the cold flow, with easterlies pushing across Evanston to the Wasatch Mountains and northeasterlies pushing across Dinosaur National Monument into the Uinta Basin.

HRRR 1300 UTC 12 November 2014 surface temperature (ºF, color fill), wind vectors, and sea-level pressure (contours)

A close up of winds over northern Utah shows remarkable structure and detail including an area of strong northerly over the northwest Great Salt Lake, a remarkable anticyclonic (clockwise) turn of flow around Promontory Point, outflow from Weber Canyon extending over the Great Salt Lake just north of Antelope Island, downslope winds near Farmington and Centerville, downslope winds near the University of Utah, and strong northerlies on the Wasatch Back extending into the Heber Valley.  I've identified each of these with red boxes below.

RRR 1300 UTC 12 November 2014 surface wind speed (miles per hour, color fill) and vectors

The analysis above is nice, but it does have some shortcomings.  It has overdone to some degree the easterly component of the flow over the Great Salt Lake.  Note how the observed winds at 1300 UTC over the northern Great Salt Lake were strong but northwesterly rather than northeasterly.  In addition, the flow on the northern end of Antelope Island is northerly rather than easterly.

Source: MesoWest

Near the Wasatch, the HRRR is unable to capture the fine-scale details of the flow, even at 3-km grid spacing, but one can find areas of easterly flow along most of the Wasatch Front include the areas noted above, as well as northerlies penetrating into the Heber Valley (latter not shown).

Source: MesoWest

Even with these shortcomings, the HRRR is a major improvement over previous lower resolution analysis and forecast systems.