The End Is Nigh

I've ran out of fingers and toes to count, so thankfully the alta.com snow report states that the storm total as of 4 AM this morning reached 67".  I'm not sure when they started that tally, but suffice to say the past few days have been pretty huge.  A reminder though that this is not unprecedented.  During Thanksgiving weekend 2001, Alta got 100 inches in 100 hours.

The end of this storm cycle is, however, nigh.  Currently we're still seeing snow showers in the northwesterly flow over the Oquirrhs, Salt Lake Valley, and central Wasatch.

Source: NCAR/RAL

Overnight, this led to about 9 inches of snow since 5 PM yesterday.

Source: MesoWest

It's low-density stuff and on the heavier snow that fell yesterday, should be right-side up and ski well.

The models suggest that we will see a break after this morning.  The NAM time height for Salt Lake City shows significant drying out of the lower atmosphere after about 1800 UTC (1100 MST).

Sunday should be dry, although there may be some afternoon high clouds.  The end is nigh.

Giving Thanks

Skiers, snowboarders, and snow lovers have much to be thankful for this Thanksgiving.  Twenty-four hour snow totals reported by the ski resorts this Thanksgiving morning include 22" at Powder Mountain, 23" at Snowbasin, 19" at Brighton, 17" at Alta, 15" at Snowbird and Solitude, 7" at Deer Valley, and 4" at Park City.  I'm a little surprised the totals on the Wasatch Back aren't higher as the flow has been persistently southerly to southeasterly at upper levels and they did well earlier in the storm.  Perhaps the position of the precipitation band was a bit too far west.  The snow depth sensor at the Thaynes Canyon SNOTEL is indeed consistent with the totals reported by the resorts, showing an increase from Tuesday to about 21 inches this morning.

Source: MesoWest

As I type this, the Utah Avalanche Center has not yet updated their forecast for the Salt Lake Area Mountains, but they did issue a backcountry avalanche warning this morning at 5:34 AM for the central Wasatch, northern Wasatch, and western Uintas. 

Source: NWS

Although in effect through 6 AM tomorrow (Friday) morning, I would not be surprised to see it extended through Friday or longer.  Stoke is high, but recognize that a combination of old, weak snow, heavy new snow, and wind are creating remarkably dangerous early season avalanche conditions.

There have also been some impressive lowland and mountain valley totals, although most of these reports are old and haven't been updated since yesterday and include 16" at Liberty through 8 PM Wed, 13" at Eden through 6 PM Wed, 13" at Stansbury Park through 10 PM Wed, 12" in Plain City and Pleasant View through 4 PM Wed, and 11" in Magna through 5 PM Wed.

Radar at 0722 MST (1422 UTC) this morning shows that not much has changed from yesterday on the large scale.  A north-south oriented band extends through northern Utah and is bringing snowfall to much of the Wasatch front from Salt Lake/Tooele northward.

Source: NCAR/RAL

 How about a peak at a couple UDOT cams from Bangerter Highway and Legacy Parkway.

Source: UDOT

Source: UDOT

Model forecasts, such as the one from the NAM below, show this band moving slowly eastward today.  

I'll be spending my day giving thanks, watching football, and monitoring snow reports.  Mountain storm totals will continue to increase today as the band moves across the central Wasatch.  For building an early season base, this storm is indeed looking like the game changer we've been hoping for, although it is creating backcountry avalanche problems and will pose challenges for road maintenance and resort operations.  

Post-Frontal Potpourri and Pre-Storm Perspectives

Storm 1 is in the bag.  Total snowfall at Alta-Collins was 9-11 inches depending on how you read the automated depth sensors.  The resort is reporting 11", which is consistent with measurements collected on the interval board, which reached 5" at 1700 MST, when the board was wiped, and then peaked again at 6" at 2200 MST. 

Source: MesoWest

Most of the snow that fell during the day yesterday was associated with the front.  From about 1700-2200 MST, post-frontal pixie dust fell with a water content of only 3.5%, adding up to 6" of snow with only .21" of water.  Good for chin ticklers and face shots, although our priority right now should be base.  During this period of bone-dry snow radar imagery showed the strongest echoes confined to over the central Wasatch west of the Wasatch crest. 

Source: NCAR/RAL

The overnight post-frontal period was largely quiet, although a little lake and possibly playa effect got going toward morning (more on this in a minute).  If you follow the deterministic models like the GFS, NAM, and Euro, a few days ago they were advertising a moist post-frontal flow and lake effect.  An example is below. 

In reality, flow was more northerly and drier (see lower left-hand panels).

A few years ago we examined the average error produced by 24-hour NAM forecasts for potential lake-effect events.  Although 700-mb (10,000 ft) temperature errors were small (averaging 1.1˚C), the average error in low-level relative humidity was 12% and during some potential events was as large as 25%.  This, combined with the fact that wind direction cannot be precisely forecast, contributes to forecast uncertainty during the post-frontal crapshoot. 

Overnight we did get some lake effect going, but it was weak and due to the northerly flow was confined to over and near the Oquirrh Mountains.

Source: NCAR/RAL

The radar also illustrates some weak cells over the west desert.  This may be "playa effect".  The playa in that area is quite wet and is often warmer than the surrounding desert shrubland at night.  If the GFS analysis above is correct, the ambient relative humidity in that area may have been higher than over the lake, perhaps enabling the convection to develop over the warm playa, whereas it was weaker over and downstream of the lake. 

Today is a transition day before the next storm.  An explosively deepening cyclone is now forming off the Pacific Northwest coast.  Take the time to look at satellite animation as the beast below wraps up during the day today.  As I type this, the cyclone is in the early stages of rapid cyclogenesis with a pronouced dry slot beginning to form and create the classic "comma" shaped cloud mass at 0721 MST/1421 UTC).  I anticipate that we will see this wrap up into a spiral cloud pattern in the next few hours. 

Source: CIRA

This storm is no joke.  The National Weather Service has issued hurricane force wind warnings for the offshore waters and has pretty much issued just about every non-convective warning that exists for near coast areas of northwest California and Oregon.  Subsequent impacts associated with the evolving weather pattern over the next few days has led to a plethora of advisories, watches, and warnings for much of the western contiguous United States. 

Source: NWS, 1441 UTC 26 Nov 2019

The upper-level trough and surface cyclone stall over the western United States Wednesday and Thursday before progressing inland Friday and Saturday.  On Wednesday, the moisture plume ahead of the cyclone will begin to penetrate inland (green arrow below).  This plume of moisture reflects its source in a vapor rich atmospheric river off the southern California coast and a pathway around the southern high Sierra, which leads to less water vapor depletion due to orographic precipitation enhancement. 

The models are advertising heavy precipitation near and along this moisture plume, as illustrated below (especially the lower right panel) by the GFS forecast valid 1400 MST/2100 UTC Wednesday.

Fundamentally, the location of this plume and associated precipitation will be critical for precipitation totals in the Wasatch (and elsewhere in Utah) on Wednesday and even Wednesday night.  The GFS, for example, actually shunts the northern portion of the band back toward the west by Wednesday evening, which might lead to a break in the action for the central Wasatch before the front goes through. 

Then, further out, the GFS calls for a new cyclone to form over southeastern Utah, which would yield easterly flow over the Wasatch. 

The NAM, however, has a somewhat different solution without such a strong cyclone.

I am not endorsing any of these forecast specifically, but I am showing them merely to highlight that the pattern is very complex during this period and the timing and intensity of precipitation locally is going to depend strongly on the flow evolution. 

Which brings us to the downscaled SREF.  Mean precipitation and snowfall produced by the 26 SREF members at Alta through 0000 UTC 30 Nov (1700 MST Friday) are about 2.4" and 40" respectively.  Wooha!  However, the spread is enormous, with one member producing less than 0.5" of water and 8" of snow and another generating almost 4" of water and 65" of snow. 

I think this is about as much spread as I've seen from the SREF for Alta.  That being said, over 90% of the members are generating over an inch of water and 20" of snow.  Thus, it is very likely that we're going to see significant accumulations from Wednesday to the end of Friday, with the main questions being timing and whether or not placement of the various pieces of the system is right to be in the middle to upper end of the SREF forecast.  No matter what, I think winter is on once again and my friends in the snow-safety community should be on alert. 

Jumpy Model Forecasts for Today and Tonight

A few of the students here have commented on how "jumpy" the model forecasts have been for the post-frontal period later today, tonight, and tomorrow morning. 

To illustrate this, the loop below shows a succession of downscaled NAM 6-hour snowfall forecasts for northern Utah beginning with the 84 hour forecast and ending with the 24 hour forecast, all valid at 1200 UTC 26 November (0500 MST Tuesday). You can see quite well that the location and intensity of the snowfall during the period is quite variable. 

In the last three forecasts, there is a bit of "convergence" toward a solution that has a band of snowfall extending from southeast Wyoming across Salt Lake County.  However, the amount of snowfall varies from run to run, with some evidence of a drying trend. 

All of this is evident of the chaotic nature of precipitation and snowfall in the post-frontal environment.  There is a lot of sensitivity to the initial conditions given the model.  An analog is to think of this situation like a random draw from a deck of cards.  You could get anything from the ace of spades to the two of hearts.  Toward the end of the run, perhaps the deck is stacked toward face cards, so the jumpiness decreases a bit, but there's still uncertainty in what you will pull.  This is one of the reasons why I favor the use of ensembles, which give you multiple "pulls" from the deck of cards to provide some idea of the range of possibilities (although current ensembles do not reliably predict lake effect). 

I have no magical insights in situations like this.  I can't tell you precisely how much snow Alta will get tonight, or where and when lake effect might form.  There is a wide range of possibilities.  This is reflected in the jumpiness above.  As noted in the post from the weekend, something in the 5-10 inch range through tomorrow is most likely for Alta, with the potential for more should Mother Nature decide to go big and put Alta in the crosshairs.  That's about the best we can do with the tools available today. 

Bomb Cyclone to Impact West Coast

OMG, my cup runneth over.  So much weather and so little time.  I know you are interested in mountain snow, but it is necessary this morning to look to the west at the forecast of a truly remarkable cyclogenesis event.  In some respects, this is the storm that "stirs the drink" for the weather that follows.

The GFS forecast loop below shows the development of the cyclone off the Pacific Northwest coast.  It forms along the pre-existing warm front associated with a parent cyclone south of the Aleutian Islands as an upper-level trough (indicated by color fill) approaches from the west.  

GFS forecast loop of sea level pressure (black contours), 925 mb temperature (red contours), and dynamic tropopause pressure (color fill)

Deepening rates are quite remarkable and consistent with "bomb" cyclone classification (i.e., > 24 mb in 14 h or 18 mb in 12 h).  Central pressures fall 25 mb in 12 hours, 35 mb in 18 hours, and 40 mb in 24 hours for the period ending at 0000 UTC 27 November, which is just before landfall. 

The GFS analysis wins no beauty contests, but it shows two key features associated with intense marine cyclones.  The first is a warm-core seclusion — a pocket of warm air surrounded by cooler air that is roughly colocated with the low center.  The second is a bent-back front — an extension of the warm or occluded frontal zone into the cooler airsteam behind the low.  

Paraphrasing a wise Norwegian meteorologist, "beware of the poisonous tail of the bent-back front."  The cyclone reaches maximum intensity right at landfall when it features a very intense inner-core pressure gradient.  This is a scenario for producing very strong winds.  Indeed the Eureka National Weather Service forecast office has hoisted just about every non-convective warning you can possibly imagine for their county warning area.  The Oregon coast will also see impacts from this storm, especially the southwest coast should the low take a northerly track.  

So, pardon my digression from the developing snowstorm over northern Utah.  No promises, but perhaps there will be another post later.  

Wild Holiday Week Ahead

All is quiet on the western front this morning as Saturday dawns with clear skies.  It should be a nice weekend for whatever you have planned as Sunday also looks nice, although there could be high clouds in the afternoon.

The coming holiday week looks wild as two complex systems affect the weather and are sure to keep meteorologists, skiers, and travelers on their toes.

The first is a surface cold front that is expected to move across northern Utah Monday. 

Snow levels will be dropping to the valley floor by Monday afternoon and there is a prolonged period of moist, unstable northwesterly flow in its wake with the potential for orographic (i.e., mountain induced) and lake-effect snow showers. 

Through Tuesday morning, the 6Z NAM produced 0.29" of water and 7" of snow at Alta, whereas the GFS was quite excited with 0.81" of water and 17" of snow.  Such is the way it is in the "post-frontal" crap shoot given the fickle nature of orographic and lake-effect convection.  A look at the SREF shows a mean of about 0.6" of water and 8" of snow, but a spread in the latter from 2.5 to 15 inches, with one member really going off. 

My take is that something in the 5-10" range is the most likely scenario for Alta from early Monday morning through Tuesday noon, but that it is worth monitoring forecasts and radar as much will depend on the vigor and placement of the convective snow showers and, if lake effect gets going, where it targets.  It's simply not possible to anticipate such details at these lead times.

On Wednesday and Thursday a deep upper-level trough that will amplify and "dig" (i.e., move southward) along the west coast of North America.  By 1200 UTC Thursday, the GFS forecasts the trough to be over or off the coast of California with Utah in moist, southerly flow ahead of it. 

The corresponding integrated vapor transport diagnostic from the Center for Western Weather and Water Extremes shows very large values of integrated vapor transport moving up the lower Colorado River Basin and into Arizona and southern Utah, with atmospheric river conditions nearly making it to northern Utah. 

Source: Center for Western Weather and Water Extremes

It's looking extremely likely that this will be a major high impact event with heavy precipitation developing first in California and Nevada on Wednesday and then spreading further inland into Arizona and Utah late Wednesday and Thursday.   It's impossible for me to summarize the local details for such a vast region here.  Travelers should monitor official forecasts from the National Weather Service and plan accordingly. 

For the Wasatch, much will depend on the strength, structure, and track of the trough, which cannot be specified precisely at such lead times.  Subtracting totals from the Monday-Tuesday storm period, the NAEFS produces anywhere from about 0.7-3" of water and 10 to 55" of snow from 1200 UTC 27 November (5 AM MDT Wednesday) to 0000 UTC 30 Nov (5 PM MST Friday), although the high totals are being produced by Canadian ensemble members that are known for overforecasting.  The GEFS members have a range of around 10-30 inches which might be more reasonable. 

When big totals are forecast, skiers, like artists, tend to fall in love with their models.  Recognize this is still a long lead time forecast and that while some big totals are being pumped out by the ensembles, so are some more modest totals, although those are even more action that we've seen in some time.  The bottom line is that it is going to be an active holiday week and that we should see improvement, possibly dramatic improvement, in the ski conditions through next weekend.

When the Canadian Goes Off

The North American Ensemble Forecast System (NAEFS) includes forecasts from the Canadian Global Ensemble Forecast System (hereafter the "Canadian Ensemble") and the U.S. Global Ensemble Forecast System (hereafter the GEFS).  At weather.utah.edu, we take these low-resolution forecasts and downscale them based on climatological precipitation distributions to produce higher resolution forecast guidance for the western continental United States.

Users of this forecast guidance and readers of this blog are likely aware that one or more members of the downscaled Canadian ensemble occasionally produce extreme and sometimes non-physical precipitation accumulations (water equivalent and snowfall).  One of the more extreme examples occurred last night when one of the downscaled Canadian ensemble forecasts generated 8 inches of water and nearly 140 inches of snow for Alta, including an unrealistically large 5 inches of water and 80 inches of snow in about 24 hours (Canadian ensemble members below labeled as CMCE members in the left hand graphs).   

Why is this happening?

There are 21 members of the Canadian Ensemble in the NAEFS, including a "control" run and 20 members with either perturbed initial conditions or differing physical parameterizations.   

Below are thumbnails of the 12-hour accumulated precipitation from each member of the forecast highlighted above for the period ending 1200 UTC 27 November, which I obtained from https://weather.gc.ca/ensemble/.  This 12-hour period includes the very heavy accumulation forecast for Alta.  The outlier is ensemble member 11, labeled GEM11 below.  This member produces a localized precipitation maximum over northern Utah, as well as two other spots to the west over northern California and Nevada.  The web site I obtained these plots from does not include a color scale, but it appears the blue shading corresponds to values > 25 mm (about an inch). 

A look at member 11's forecast elsewhere shows some localized precipitation maxima elsewhere, including over the Atlantic near the edge of the plotted region.  These intense, localized precipitation maxima are sometimes referred to as grid-point storms.  It could be that member 11 (and perhaps some other members) has a configuration that is prone to producing such behavior. 

In turn, when we downscale these heavy precipitation amounts, we exacerbate the situation further.  Our downscaling factors for Alta result in an increase in the precipitation provided by the Canadian of about a factor of 2.  Thus, we take an extreme precipitation amount and enhance it further. 

So, it is my hypothesis that these big precip maxima are being produced by these heavy localized accumulations.  We have seen this sort of behavior in some other modeling systems (e.g., West et al. 2007), but further work is needed to understand what is happening in the Canadian ensemble. 

The GEFS does not seem prone to this type of behavior.  Further, we have done extensive verification of the GEFS before and after downscaling.  Those who use these products, have a strong stomach, and would like to know more about the capabilities and limitations of the GEFS-portion of the downscaled NAEFS ensemble can learn more from Lewis et al. (2017). 

Changes Are Coming

Precipitation analyses for the 30-day period ending yesterday at 1200 UTC (0500 MDT) show that northern Utah has clearly gotten the shaft.  Totals are highest across the far northwest, dropping down through Wyoming and Colorado.  To our south, this week's storm has even given them something. 

Source: NWS

Things may, however, be changing for Thanksgiving week as there is a shift in the large-scale pattern expected to begin later this weekend into early next week.  GEFS forecasts valid 0000 UTC 26 November (1700 MDT Monday) predominantly favor a trough  over the west.  

As do most (but not all) forecasts valid 1200 UTC 27 November (0500 MDT Wednesday). 

Source: Penn State E-Wall

Thus, at a minimum, cooler, possibly much cooler temperatures are likely to prevail next week.  It is also likely that we will see some mountain and possibly some valley snow. 

When one here's the phrase "pattern change" there is a tendency to get excited when we've been persistently warm and dry this time of year.  I like to keep calm and carry on.  After deducting totals for today, the NAEFS ensemble forecasts through 0000 UTC 28 November (1700 MDT Wednesday) generate a mean of about 30 inches of snow at Alta Monday, Tuesday, and Wednesday, but this number is jacked up by several Canadian Members that produce huge, unrealistic totals.  The GEFS mean (red line) suggest around 18 inches, but the GEFS range is also between about 8 and 30 inches for the period.

Thus, change is coming, but exactly how much falls before the Thanksgiving holiday depends on  the structure and track of the weather systems as they enter and move across the western U.S.  There is also the potential for snow during the Thanksgiving holiday, although I'm not inclined to speculate at such long lead times about amounts as there are a wide range of possibilities. 

Wait and See Forecast

Tomorrow is a very difficult forecast day due to the fact that we have multiple systems potentially affecting the weather of northern Utah. 

The NAM forecast valid 2100 UTC (2 PM MST) Tuesday shows a complex upper-level pattern with a 500-mb closed low over southern California with an inverted trough extending through Nevada and connecting with a trough in the westerlies over southern California (500-mb contours and wind barbs in black below).  A short-wave trough is moving through northern utah with southwesterly flow south of the Great Salt Lake and southerly flow over southern Idaho.  Meanwhile, at low levels, high pressure is pushing through Wyoming (sea level pressure color contours below). 

Because of that high pressure,  low level winds forecast by the NAM over southwest Wyoming are easterly, and northeasterly, with flow splitting apparent where the cold air impinges on the Uinta Mountains.  The resulting easterly branch pushes across the Wasatch Range and into the Great Salt Lake Basin. 

All of this means we'll see mountain snow showers and valley rain showers pushing northward across the region.  At the same time, we could see downslope flow developing from the east.  The where, when, how strong, or how much is difficult to answer.

For the winds, much will depend on the gory details.  The NAM forecast above suggests we'll see moderate but not strong easterlies.  Other models give different solutions.  It's the sort of pattern where I'd bring in the patio furniture and keep an eye on the forecast in case the low probability outcome of stronger winds were to verify.  Certainly anytime there's a closed low drifting around the Las Vegas area one needs to be alert.  

For precipitation, let's look at Alta-Collins.  Through 5 PM MST tomorrow (0000 UTC 21 Nov), the SREF has a range from nearly 0 to about 7 inches of snow with a mean of 4 inches.  For the event, the range is about 5 to 20 inches.  Note the staircase like structure of the plumes in the lower left, indicative of the storm coming through in bits and pieces, with the timing of those bits and pieces varying.  

My take is precipitation in fits and starts form tomorrow through Thursday.  Total at Alta Collins of 3-6 inches for a best guess.  More if we get a really productive precipitation period going.  These numbers are a bit lower than indicated by the SREF above.  I think that reflects the fact that this is not a pattern that typically produces huge amounts of snow unless something well developed moves slowly over the Wasatch Range.  

That being said, I'm not sure exactly how this will play out, so I'm calling it a wait and see forecast.  

State of the Snow

We're now in mid November, so the time is ripe for taking a look at the snowpack across the western U.S.

In terms of the percentage of median snow-water equivalent as of November 17, a pronounced dipole pattern exists across the western U.S. with most stations near or above median near and east of the Continental Divide and most stations further west below median.  The pattern is a strong one with many stations east of the divide well above median and many west of the divide well below median. 

Source: NRCS

Percent of median can be a somewhat misleading indicator of snowpack robustness since much depends on the station's snowpack climatology.  Many of the stations east of the Continental Divide are relatively dry, and thus 200% of median this time of year doesn't necessarily mean a lot of snow.  If we look at the actual snowpack water equivalent, there are no sites that have crested the coveted 10" mark.  The highest values I could find were in the Lewis Range of Montana [Badger Pass (8.9") and Noisy Basin (8.1")], Gallatin Range of Montana [Shower Falls (8.5")], Teton Range of Wyoming [Grand Targee (7.6")], and Bighorn Range of Wyoming [Bald Mountain (7.6")].

Source: NRCS

Colorado has a few stations at or well above median, but median at many of these stations is low.  The top site there is the Tower Snotel just north of Steamboat (6.5").  Sites elsewhere look to be below 4".  For comparison, the Snowbird SNOTEL sits at 3.4". 

Based on forecasts from the NAEFS, the southern portion of the western CONUS looks to have the best chances for snow over the next week, much of this snow falling with the trough moving across the west through the middle of the work week.  There's a little action to the north as well. 

The Wasatch aren't entirely dry, but don't get much.  Even a few inches would help at this stage.  There are, of course a couple of outlier Canadian ensemble members, including one that puts out about 80 inches of snow in 12 hours!

As I like to say, you've got to love the Canadian, even if it is physically implausible.  We'll have to look into that one and see what the hell is going on. 

On the plus side, the Utah football team is kicking butt and taking names.