Looking Back

I enjoyed my ride to work this morning as there was a wonderful view of the snow-covered Wasatch capped by a lenticular cloud with the scene enhanced by fall colors in the valley.  

We are truly blessed this October as there's ski touring in the mountains and, once the trails dry a bit more, mountain biking in the valley.  Our cup runneth over. 

Let's take a quick look back at the storm.  Total snowfall based on automated observations from Alta-Collins was 14 inches with 1.98" of water.  Approximately 0.2" of that water fell as rain before snow levels dropped with the arrival of the cold front.  One aspect of the storm that I did not anticipate correctly was yesterday's snow density.  With decreasing temperatures, I thought we would see a transition to lower density snow, but the post-frontal environment was too unstable and convective.  There's an example below with four strong cells moving into or over the central Wasatch range from the northwest.  

Such cells contain strong updrafts, potentially of a few meters per second, which enables substantial riming of the snow and the formation of graupel, a styrofoam-like snow pellet.  It appears graupel was plentiful during the storm as suggested by all the pellets evident in yesterday afternoon's Alta snow-cam photo.  

Less instability and the riming would more limited and the snow lower density.  These are the tipping points that we deal with as meteorologists.  On the other hand, in such an environment, the snowfall amount might not have changed much, but the water amount would have been lower.  Right now, we want as much water in the snowfall as possible for cover and base, so all in all, yesterday was a good thing.

We're entering a break for a couple of days before the next system approaches Friday night and Saturday.  Right now, that looks like it might produce some flurries and not much more, but maybe we can get lucky again.  Continue doing whatever Pagan rituals you've been doing.  

Storm Update

It was quite a night last night with many sites in the Salt Lake Valley recording over an inch of rain.  Amounts were greatest in the Olympus cove area where just over 1.5 inches fell.  Even the airport recorded 1.21 inches.  And it's still raining.

Meanwhile in the mountains we're up to about 8 inches of dense snow based on the Alta-Collins and the Snowbird snow stakes, the latter below.  

Source: Snowbird.  Downloaded 7:38 AM 26 Oct 2021.

Observations from Alta-Collins show that precipitation began between 1600 and 1700 MDT yesterday with temperatures near 40˚F.  About 0.18" or so fell at temperatures above 34˚F.  Temperatures reached 34˚F at 2000 MDT afterwhich snow fell at this elevation (it wasn't long after that it was falling to the Town of Alta.  I guestimate that the snow 8" of snow contains about an inch of water, for an 8:1 snow-to-liquid ratio or a 12.5% water content.  Total precipitation 1.22".  

The GFS-derived forecast from yesterday is doing pretty well through 7 AM this morning.  The forecast presented in yesterday's post called for a total of 1.29" of total precipitation and 11" of snow through 7 AM.  The former is just .07" higher than observed and the latter 3" too high.  The main shortcoming of the GFS was that it was a bit drier before temperatures fell and a bit wetter after.  Hence the 3" snowfall overforecast.  Still, that's a pretty good forecast.   

We are now transitioning to the unstable, post-frontal regime.  The latest radar image shows precipitation over the eastern Tooele Valley, Salt Lake Valley, Central Wasatch, and Bountiful-Area Wasatch.  There's also a weak lake-band over the Great Salt Lake.

It is likely that snow will continue in the Cottonwoods for the next hour or two and snowfall should transition to lower density.  After that, periods of snow.  I'll go for another 3-6" at Alta Collins from 7-5 PM, although I confess it's a bit more of a crap shoot for this period than the frontal period and we could do better if the instability or the lake goes off (the latter is less likely in the afternoon).  That's some waffling to keep a friend of mine happy.  

Suspect the skiing will be good later today.  Stay safe out there. 

Deja Vu All over Again

The forecast has largely been locked in now for a couple of days so there are few changes to report this morning.  Thus, this post is deja vu all over again.  

Today brings the warm before the storm, as well as wind.  At 7 AM this morning it was a balmy 60˚F at the University of Utah and 35˚F on Mt. Baldy (11,000 ft).  I bet you forgot it's only late October.  Overnight wind gusts through 8 am include 87 mph on Ogden Peak, 77 on Mt. Baldy, 73 at the top of Jupiter Bowl (PCMR), and 73 near Point of the Mountain, the latter a lowland site.  

Expect more of the same today as we are in the warm and windy southerly flow ahead of cold front.  Below is the HRRR forecast valid 0000 UTC 26 November (1800 MDT Monday) showing what has been expected for some time with strong southerly pre-frontal fro, a very well defined cold front, and heavy precipitation near and behind the front.  In the Salt Lake City area, pre-frontal precipitation is limited, although there could be a shower here or there mainly in the upper elevations.  

Our latest GFS-derived forecast guidance for Little Cottonwood Canyon is remarkably similar to what we've seen the past two days with temperatures and snow levels dropping with the arrival of the cold front, heavy precipitation immediately behind the front, and then a period of post-frontal instability showers through Tuesday night.  Total storm water of about 2" and snowfall of about 20" at Alta-Collins.  

Thus, windy but mainly dry today with perhaps a shower or two (snow levels near 10,000 ft), but then as the front approaches this evening, perhaps a brief period of rain to 10,000 ft, but then very quickly snow levels drop to 7000 feet with heavy snow at upper elevations.  After midnight, snow levels will gradually lower to near 5500 feet by morning.  

Below is some tabular output.  I've highlighted the critical period from about 6 PM MDT this evening through 5 AM MDT tomorrow morning.  At Alta, some light precipitation falls from 6 PM to 9 PM (totalling 0.04" water equivalent) and the wet-bulb zero level drops during this period from 10,500 to 8300 ft.  Snow levels are usually a bit below this level, although the drop in snow level might be a bit delayed compared to this for reasons noted below.  After that, the front arrives with the heavy precipitation and high-density snow overnight.  After that is an extended period of post-frontal snow showers.   

A quick comment that the drop in snow levels might be a bit slower than indicated by that table for this reason.   We can't estimate snow levels at lower elevations using the modeled atmospheric profile at Alta because the bottom of the profile is too high and starts at about 8200 feet.  As a result, we need to use an upstream profile from over Salt Lake City.  In a situation like this, with a rapid drop in snow level, there can be a difference in timing between Salt Lake and Little Cottonwood Canyon. 

The latest GFS also is forecasting fairly moist, unstable, northwesterly to develop behind the cold front for Tuesday and Tuesday night.  

It should be noted that the GFS is one of the wetter models right now.  The ECMWF model has about 1.28" of storm total water for Alta and a maximum of 1.63" just upstream.  Our downscaled SREF has a mean of 1.5" of water and about 17 inches of snow with a somewhat skewed distribution in which most members are between about 1.1 and 1.75" of water.

The NWS forecast below is for 12-18" at Alta.  I'm inclined to go a bit higher than that with a storm total of 10-20", although that might reflect my focus on Alta-Collins (9600 feet) whereas they may be thinking about the town of Alta, which is about 1000 feet lower and that would yield somewhat lower numbers in a situation like this.  

I suspect the skiing on Tuesday afternoon and Wednesday will be good as this will be a well-stacked storm with low density on top of high density, but avalanche danger is rising, the green light is not on, and the resorts are de facto backcountry.  Be careful out there.  

Sunday Forecast Update

Not much change from yesterday's forecasts except for a skiff of precip later today and a bit later cold-front arrival tomorrow evening.  Below his this morning's (Sunday) 12Z Sunday GFS-derived forecast for ltitle Cottonowood.  Temperatures on Mt. Baldy climb to > 35˚F through tonight and remain warm through tomorrow.  There's a bit of precipitation associated with a weak-warm-front-like feature moving through this afternoon.  The GFS generates about 0.16" of water at Alta Collins and just about a half inch of snow due to snow changing to rain during the passage of the feature, even at 9700 feet.  

The GFS continues to keep things mainly dry until the approach of the cold front Monday evening.  That's when temperatures and snow levels drop and precipitation gets going.  Expect snow levels to fall pretty rapidly and, perhaps after a bit of rain to start, the event to be mainly snow at upper elevations with snow levels eventually falling to below 6000 feet by Tuesday morning.  The GFS is still pretty excited for water totals (~2") and snowfall totals (~20") at Alta-Collins during and following the frontal passage.  Those totals are for late Monday through Wednesday morning and include the skiff of precipitation this afternoon and the cold frontal and post-frontal precipitation.

The GFS is very close to the downscaled SREF mean water equivalent and snowfall for Alta Collins.  The SREF has most members has a supermajority of members generating 1.5 to 2.5 inches of water (including a bit this afternoon) and 15 to 25 inches of snow.  Take a smidge off of those for the frontal and post-frontal period. 

If, however, you are like me and believe the key to a happy life is low expectations, then look to the ECMWF model which is on the lower end with about 1.5 inches. 

By and large, things continue to look good for a big dump.  With high wind and precipitation rates, rising avalanche danger is likely, including at the resorts which are still de facto backcountry.  I have been wondering if Alta might close to uphill to do some avy work, but nothing yet on their site suggesting this.  Please post below if you hear anything.  

The Coming Storm and Cautionary Words about Atmospheric Rivers in Utah

Everything remains on track for the big storm in California and eventually more October snow for the central Wasatch.  

Current forecasts show a very well-defined precipitation system moving across California and the Great Basin Sunday and Monday.  The GFS forecast valid 0600 UTC 25 October (0000 MDT Monday) shows heavy precipitation (>1" water equivalent in 3 h) in the Tahoe area, to the southeast along the Sierra Nevada, and then along the cold front in the northern Great Basin.   

By 1800 UTC 25 October (1200 MDT Monday), the action has shifted southeastward to the southern Sierra and Mammoth Mountain and to central Nevada.  It is impressive to see a continuous precipitation band extending across the Sierra and into the Great Basin.  We will see if that verifies, but often there's a break due to shadowing downstream of the Sierra.  

Much has been made of the "atmospheric river" conditions that will penetrate into the Great Basin and indeed at 1800 UTC (1200 MDT) Monday northern Utah is solidly in an atmospheric river ahead of the cold front with large values of integrated vapor transport (see below).  However, as can be seen in the GFS forecast above, there's virtually no precipitation over northern Utah. More on this in a minute.  

Source: Center for Western Weather and Water Extremes

Ultimately, the GFS brings the frontal band into northern Utah late on Monday.  Below is the forecast for 0300 UTC 26 October (2100 MDT Monday) with the frontal band over northern Utah.  

The time height section for Alta helps summarize the next few days and sheds some insights into the the atmospheric river forecast for Utah.  Recall that time increases to the left in this diagram.  First up is today's storm, which will bring 2-5" of wet-snow scraps to upper Little Cottonwood.  On Monday, warm, moist southwesterly flow develops aloft. Freezing levels (thick blue line) rise to over 11,000 feet, which sounds scary, but the low levels are drier with lower relative humidity levels.  This, and the lack of any well define warm-frontal feature to drive large scale lift, is why the GFS isn't generating much in the way of precipitation until the arrival of the cold front later in the day.  

The GFS terrain is not as "impressive" as reality, so perhaps we will get some showers at times before the approach of the front, but I suspect accumulations will be limited.  Thus, I'm not panicked about a big rain-on-snow event.  I think it is more likely we will see a few mountain showers on Monday ahead of the front, perhaps a bit of rain to high elevations as the front approaches, but then snow levels will drop abruptly as the cold front moves in.  

Supporting this narrative is our GFS-derived forecast for upper Little Cottonwood (time increases to the right in this graphic).  I've identified the front with a light blue line.  Ahead of the front, temperatures on Mt. Baldy exceed 35˚F and the wet bulb zero level (typically the snow level is a bit below this level) over 11,000 feet.  

However, during this period, the GFS is generating just a skiff or two of precipitation.  Temperatures and wet-bulb zero levels drop abruptly with the front.  At Alta Collins, the GFS-derived precipitation total is a bit over 1.5 inches and snowfall about 20 inches with and following the late Monday front.  If you are wondering, the ECMWF is just a bit behind that.  Total water equivalent from the GFS, including today's storm, is a bit over 2 inches.  In the Euro 1.83".  

Wednesday is still a long ways off to be talking about details, but right now I see the Monday to Tuesday storm as a 10-20 inch event at Alta Collins, with the possibility for more if the post-frontal period is especially productive.  It's too far off to have confidence about what that will bring.  

To wrap up, a few cautionary words about atmospheric rivers in Utah.  There is a strong correlation between integrated vapor transport (IVT) and precipitation in California, especially in the mountains.  This is shown really well by the left hand figure below from Rutz et al. (2014), which shows correlation coefficients between IVT and precipitation in the coastal range, southern Cascades, and most of the Sierra above 0.65.  IVT and atmospheric river intensity have a good deal of value there.  In Utah, however, the correlations are lower and the predictive value of IVT is lower and perhaps about 0.35 to 0.40 in our mountains.  This is an issue we've discussed in previous posts.   

Source: Rutz, Steenburgh, and Ralph (2014)

This doesn't mean that an atmospheric river can't bring heavy precipitation to our mountains, but it does mean that you shouldn't have AR-myopia.  Also needed is large-scale forcing for precipitation and/or lower-level vapor transport with higher relative humidities to drive orographic precipitation processes.  Right now, it doesn't look like we will have those on Monday until the front approaches.   

The Angry Pacific Storm Track

It's good to see the Pacific Storm Track angry again.  Below is the situation at 1200 UTC (0600 MDT) this morning.  The Pacific Jet is oriented predominantly zonally (i.e., from west to east) and quite intense with a maximum speed in the western Pacific of over 90 meters per second (180 knots, yellow to orange color fill).  Just ahead of and beneath the jet-speed maximum, called a jet streak by meteorologists, is a surface cyclone (circled in red) just south of the western Aleutians.  Ahead of that system are two cyclones and associated frontal systems (cold fronts identified with blue lines and triangles).  

The easternmost of those frontal systemswill give us a bit of valley rain and mountain snow tomorrow.  The one further west will decay and pass to the north.  However, all hell breaks lose further upstream as the upper-level pattern amplifies and the surface cyclone that is currently south of the western Aleutians rounds the base of the amplifying upper-level trough and undergoes explosive cyclogenesis off the Pacific coast.  

The result is a very powerful cyclone off the Pacific Northwest coast that will transport copious amounts of moisture into the western United States.  

Below is the forecast of integrated vapor transport, a measure of the total water vapor transport throughout the atmosphere, at 1800 UTC 24 October (Noon Sunday MDT).  Values > 250 kg/m/s (color shaded) are indicative of Atmospheric River Conditions.  Values in an elongated plume that intersects the Bay area are greater than 1400 kg/m/s.  Additionally, the transport vectors are oriented perpendicular to the coastal ranges and Sierra Nevada, which is optimal for precipitation enhancement.  

Source: Center for Western Weather and Water Extremes

Below is our downscaled SREF forecast plume for Kirkwood.  The ensemble mean accumulation from about 1200 UTC 24 October to 0000 UTC 26 October is a bit over 7 inches.  Snowfall is about 30 inches, although this is a fairly warm storm and a lot is going to depend on elevation, temperature, and precipitation intensity.  The "driest" ensemble members generate about 4 inches of water.  

What will happen in the Wasatch?  The GFS forecast currently calls for an extended period of Atmospheric River conditions from 2100 UTC 24 October (2 PM Sunday) through 2100 UTC 25 October (2 PM Monday).  Unlike California, Atmospheric River conditions here are a bit more fickle and the GFS keeps things dry until the cold front approaches on Monday.  One can see this general behavior in our downscaled NAEFS plumes for Alta.  After a bit of snowfall on Saturday, things hold off until after 1200 UTC 25 October, with most members bringing in the precipitation later on Monday (although there are some that start earlier).  

That ushers in a very heavy period of snowfall as the moisture associated with this system combines with the frontal forcing.  I suspect the big snowfall numbers being put out by the NAEFS are over done, so don't let those bias your enthusiasm.  

By and large, things look good for further building of base at the Cottonwood resorts.  A few inches Saturday, a break with wind on Sunday, and then a big burst on Monday/Monday night.  I'll try and do an update over the weekend as the details concerning this system come together.  

A Scary Pre-Halloween Forecast

The Pacific Storm Track is angry my friends and about to unleash some serious weather on the western United States.  

I'll focus on one component of the forecast, precipitation, and will use the downscaled NAEFS-derived forecast below to summarize.

Over the next 7 days, the average amount of precipitation (liquid equivalent) produced by the 52 NAEFS members in the mountains of northern California exceeds 10 inches in portions of the Coastal Mountains and Cascades/far northern Sierra.  At these "favored" locations the driest members produce over 5 inches and the wettest over 20 inches.  

The plume for the Mt. Shasta Ski Bowl at 8,000 ft elevation shows a mean of almost 8" with most of the NAEFS members clustered between 6 and 8.5", although the full range of all members is about 3.5 to 12".  Our snowfall algorithm is known to overestimate snow-to-liquid ratio in this part of the world so be cautious interpreting the snowfall plume.  Snow levels will be pretty high through late morning Friday time, but significant snowfall is likely at upper elevations thereafter.   

The scary aspect of this forecast for northern California is heavy rainfall on new burn scars.  The potential for flooding and debris flows is high.  

Meanwhile in Utah, the forecast is scary for meteorologists because of uncertainty.  At Alta Collins, it's likely to stay mainly dry through Friday and then things get interesting.  The spread in the NAEFS forecast for precipitation and snowfall amount through early next week is enormous and ranges from practically nothing to 6" of water and nearly 80" of snow through 0000 UTC 27 October (6 PM Tuesday).  

In the NAEFS plume, it is often a member from the Canadian model that is the high outlier, but in this instance, both the Canadian ensemble and the American GEFS ensemble are exhibiting huge spread.  Note that there are also differences in the timing of the heaviest precipitation periods.  

The GFS sits about in the middle of the downscaled NAEFS distribution, so I'll use our GFS-derived forecast here as one possible outcome.  The GFS produces two major storm periods.  One is from late Saturday to Sunday morning.  The other starts Monday night and extends into Tuesday night (recall that the NAEFS illustrates there are variations in timing depending on the ensemble member). Total water equivalent a bit over 2.5 inches for the entire period with total snowfall just over 25" at Alta Collins.  

If you do the math on the snowfall and precipitation, that's a mean snow-to-liquid ratio of about 10:1, which is lower than average, meaning heavier snow.  I see that as a good thing as paste is better than blower for base, but snow levels at times could be at or above 7000 feet depending on how everything comes together.  There is also a major warmup between the two storms, with 11,000 ft temperatures reaching almost 40˚F.  

If the storms come together, we will see a major increase in avalanche risk and I suspect that resorts like Alta may consider closing for a while to uphill to do avy control.  Monitor resort and Utah Avalanche Center web sites, respect snow-safety closures, and remember that if open to uphill skiing, the resorts are still de facto backcountry.  

A Blessed October

October is coming through in spades for skiers.  As of 7 am this morning, the latest storm has produced 0.85" of water and 9" of snow at Alta-Collins, bringing the snow depth up to a solid for mid-October 25". 

The HRRR analysis for 1200 UTC (6 AM MDT) this morning shows wrap around precipitation over the Wasatch and Wasatch Front, which is a pretty close match to reality.  

The forecast for 1500 UTC (9 AM MDT) shows the continuous precipitation shield breaking up into more scattered precipitation.  

Thus, I expect the snowfall in the central Wasatch to transition to snow showers that become widely scattered by later this morning.  Storm totals should be in the 12" range, give or take.  

It's been a mighty long time since we've had this much snow so early in the season.  Below are traces of the snowpack water equivalent at Snowbird every water year (beginning October 1) since the 2000 water year.  This year is in blue, showing about 4" as of about midnight last night.  The green line that surges to over 10 inches in late October is the 2005 water year and reflects the big October 2004 storm cycle.  However, that didn't start as early as this one, although it delivered big time when it did.  

The models are hinting at the possibility of another period of precipitation sometime this coming weekend to early next week.  

There's not a lot of agreement yet amongst the various ensemble model members concerning details, so stay tuned.  

Will the Trend Be Our Friend?

 If you asked me a week ago about the prospects for snow tonight and tomorrow, I would have said unlikely.  However, as discussed a few days ago in the Dropout Forecasts blog post, a closed low suddenly appeared in the model runs late last week and over the past few days the models have trended toward a snowier forecast for the central Wasatch.  Will the model trend be our friend?

The 0600 UTC GFS suggests this will be a two part storm. Part I arrives later this afternoon as cold, moist southerly to southwesterly flow ahead of the upper-level trough pushes into northern Utah.

Part II comes overnight as the trough moves downstream and "wraparound" precipitation in the westerly to northwesterly flow behind it gives another period of mountain snowfall.  

Our experimental GFS-derived forecast guidance product for Little Cottonwood Canyon shows temperatures dropping precipitously this afternoon (upper left panel).  The bottom panels show two periods of heaviest water equivalent and snowfall with the two-part storm, although periods of snowfall occur between those two parts.  

Total water equivalent through noon tomorrow is 1.05", with almost 11 inches of snow.  If we have a look at our downscaled SREF product, 20 of the 26 members generate 7 to 20 inches of snow.  Three are very excited, producing around 30 inches, and 2 are pessimistic with less than 7 inches.  

Finally, the ECMWF is going for 0.78" of water, so we see good alignment amongst most of the models for a decent storm.  I expect this snowfall to be relatively high density to start and at the end be closer to average snow-to-liquid ratios for Alta.  

I'm inclined to go for 7-14" at Alta-Collins through noon tomorrow.  I think the odds of less than 7 inches are pretty low.  More than 14" is possible, but we need one of the storm parts to come through in spades.  

I don't like to be greedy in mid October, but we really need this storm.  The warmth and sun over the weekend did a great deal of damage.  I ski toured up Collins gulch yesterday morning and took the photo below on my descent showing how the south aspects were baked even before another round of warmth that afternoon.  The snow on Mambo looks fine, but in reality it was mostly coral reef.  

Off piste, only the shadiest, highest, north aspects held soft snow without a melt-freeze crust.  We need a good dumpage from this storm for more base, a deeper snowpack, and to smooth things out.  Low-end totals will probably mean a bumpy ride Tuesday morning.  Upper-end totals will help smooth things out. Be aware that the ski resorts are de facto backcountry currently as avalanche hazard increases with new snow and wind.  

A Great Warming

Currently temperatures at upper elevations in the central Wasatch are in the teens, but that will be changing rapidly through tomorrow as temperatures climb with the approach of an upper-level ridge.  

Our GFS-derived forecast guidance for Little Cottonwood Canyon shows temperatures on Mt. Baldy (11,000') climbing from the low teens this morning to 40˚F by tomorrow (Saturday) afternoon!

Free atmosphere 700-mb (10,000 ft) temperatures reach 6˚C by late tomorrow as well.  Balmy.  

Be aware of this change if you will be ski touring today or tomorrow, especially on south aspects today and all but high north tomorrow. 

Dropout Forecasts

Every now and then, the forecast skill of a computer modeling system plummets, leading to a brief period of poor performance.  Such periods are sometimes referred to as dropout forecasts.

I've identified some examples in the plot below, which presents one measure of forecast skill, the standardized anomaly correlation of the 5-day (120-hour) sea level pressure forecast relative to the National Weather Service Weather Prediction Center sea level pressure analysis for the contiguous United States.  Without getting into details, the higher the standardized anomaly correlation, the better the forecast.  The graph presents the standardized anomaly correlation over the past 91 days from several models including the GFS (red circles), UK Met Office Model (brown diamonds), and ECMWF Integrated Forecast System (green Y).  

Source: NCEP

One can see that model skill varies daily, but there are times when one or more models suddenly produce much lower standardized anomaly correlations values.  These are the dropouts.  The UK Met Office Model seems to have done this several times during the period (I've identified five such forecasts in red circles).  Several model forecasts on the 19th of August were also quite poor and I've identified these with a purple circle.  

The causes of these dropouts is an important research topic.  They strongly impact forecast reliability.  If your forecast model gets it right 90% of the time, but it fails spectacularly the other 10%, it doesn't inspire confidence!

An example of a regional forecast dropout is about to unfold in the western U.S. over the next few days.  Below is the 180 hour GFS forecast from 12 UTC 11 October valid 0000 UTC 19 October (6 PM MDT Monday).  Ridging dominates the western U.S. with not a single drop of precipitation to be found in the western states (I've never considered Texas to be a western state, so ignore the little blob of green there).  There is evidence of a weak trough over the lower Colorado River Basin.    

A day later, only two of the 52 NAEFS members suggested any precipitation at Alta-Collins in the period around 0000-1200 UTC 19 October.  

Then the forecasts shifted.  The forecast from this morning, valid 0000 UTC 19 October, features a much deeper closed low centered right over Utah.  And there's precip in northwest Nevada, northern Utah, southeast Idaho, and western Wyoming!

If that verifies, we'll get a little mountains now out of the system, which would be a blessing, but for our area, this is a pretty big dropout.  No real hint of the closed low at 6-7 days lead time.  

An example of gremlins in the vast machine that forecasts our weather.  

The Next Storm

If you are in the mountains today consider yourself blessed.  It will surely be the most beautiful morning of the season so far with clearing skies and a fresh coat of new snow.   The view from Snowbird at sunrise is spectacular.

Observations from Alta-Collins indicate 2 inches of snow overnight and a storm total of 16 inches, bringing the total depth to 21 inches.  Snowpack water equivalent at the Snowbird SNOTEL sits very near 3 inches.  

High clouds will, however, be moving in this afternoon in advance of the next storm system.  The weak surface front associated with that system is forecast by the GFS to move into the Salt Lake Valley early Thursday morning and bring a period of unstable, moist, west-northwesterly flow.

Below is our GFS-derived forecast guidance for Little Cottonwood Canyon showing the bulk of the precipitation at Alta mainly from 5 AM Thursday morning to 6 PM Thursday afternoon.  During that period, the GFS produces about 0.4 inches of water and 6 inches of low density snow with snow-to-liquid ratios of around 15-17 to 1.  

Given the cold conditions and northwesterly flow, there is also potential for lake effect.  Odds are best Thursday morning when the moisture is deepest.  Given the west-northwesterly flow, our GFS-based algorithms give the best chance to the Bountiful area.  

Of course, the GFS is just one model.  The ECMWF has about 0.25" of water equivalent in the upper Cottonwoods and, consistent with our lake-effect product above puts the heaviest precipitation (just over 0.5") just north of the Salt Lake County/Davis County line in the Bountiful Area mountains.  

On the other hand, the 12Z HRRR has a bit of a different idea.  It generates about 0.2-0.25" of water equivalent in the central Wasatch, but a lake band further west extending into the northern Oquirrhs where precipitation is heaviest. 

These are the joys of forecasting post-frontal instability showers.  I'm going to go for 4-8" at Alta-Collins for tonight through Thursday afternoon and will be praying for more.  You should too.  

Something for Everyone, But Pray for Snow

Pretty impressive storm last night with something for everyone.  Here's what I've been able to piece together for accumulations based on automated measurements through 7 AM MDT this morning:

• Alta-Collins: 1.15" liquid precipitation equivalent/13" Snow
• Solitude Summit: 12-13" Snow
• Thaynes Canyon SNOTEL: 1.2-1.3"/12-13"
• Deer Valley Ontario: 1.14"/8-9"
• Mill D North SNOTEL: 1.2"/10"

You can see from those numbers that the water equivalents are remarkably even across the central Wasatch and I suspect you'll find at least 8" of new snow throughout the range at elevations near and above 9000 feet, with more in favored areas. 

Mountain snow showers will continue at times today and tonight, followed by a break Wednesday, with another trough moving in Wednesday night and giving the mountains snow showers at times through Thursday night.  The numbers from the 6Z GFS are below with 4.6" forecast for Alta Collins for late last night through early Wednesday morning and then another 4.4" with the next storm.

The downscaled SREF has a range of about 2 to 11 inches for the entire period.  That's a lot of spread, and it likely reflects the scattered nature of precipitation features expected during the period.

I'm inclined to go with another 3-6" for Alta-Collins through early Wednesday morning and then another 3-6" for the Thursday storm. 

Given the current 20" snow depth at Alta-Collins and 2.6" of snowpack water equivalent at the Snowbird SNOTEL (9177 ft), that will put upper Little Cottonwood near what I consider to be the lower limit for skiing in meadowy areas on a natural snowpack (24" base with about 3" of water).  

Based on those numbers and the forecast, NOW IS THE TIME TO PRAY FOR SNOW.  The extended forecast for the weekend and at least the first part of next week is not good, with precipitation odds scant.  We need one of both of these systems to be an overproducer to push us into deeper territory.