The Utah Snow Ensemble

We are excited to share that the Utah Snow Ensemble is now available on https://weather.utah.edu and has replaced the old NAEFS product.

The Utah Snow Ensemble is an 82-member ensemble for predicting snow over the contiguous western United States based on the ECMWF ensemble (ENS) and the US National Centers for Environmental Prediction Global Ensemble Forecast System (GEFS), with 51 members coming from the ENS and 31 from the GEFS.  The forecasts are provided in 6-h intervals out to 240 hours (10 days).  A summary of the data and methods and graphics is provided below. 

Precipitation Downscaling

Precipitation from all ENS and GEFS members is downloaded on a 0.25°x0.25° grid and downscaled to 800-m grid spacing to provide higher resolution guidance.  The downscaling assumes seasonally varying climatological precipitation-altitude relationships.  See Lewis et al. (2017) for more information.  Such a technique produces improved forecasts, but it does not account for periods during which the strength of orographic precipitation enhancement departs strongly from climatology.  

Snow-to-Liquid Ratio (SLR)

Snow-to-liquid ratio (SLR) is based on a new multiple linear regression algorithm developed using SLR observations from 14 western US snow study sites (thank you to everyone who provided that data!) and temperature and wind profiles from the ERA5 reanalysis.  Although we have developed more sophisticated machine learning algorithms, they were too slow for processing more than 3000 ensemble members and forecast hours, with the small accuracy loss and large speed gain of multiple linear regression a reasonable compromise. 

Snow Level

The multiple linear regression derived SLR is used where the high-resolution terrain used for downscaling is at or above the 0.5°C wet-bulb temperature level.  Below that level, we assume the SLR decreases linearly to 0 (i.e., becomes rain) over a distance of 200 m toward the ground, as illustrated schematically below.  Everything below that level is assumed to be rain.  

This approach is based on snow-level estimation techniques used by the National Weather Service Western Region (click here to access the Technical Report).  It is straightforward and fast.  The technique will struggle, however, when there are warm noses aloft (an issue at times in the Columbia Basin and Cascade Mountains, especially the Columbia Gorge and mountain passes), if the decrease in temperature with height is small or zero (e.g., isothermal), or if there are dense hydrometeors like graupel that can penetrate farther down through the transition zone.  We make no effort to identify such periods or the possible presence of freezing rain or sleet/ice pellets instead of snow when warm noses are present aloft. Improving this method over the west will probably require some sort of machine learning method since the the coarse vertical resolution of temperature data provided for the ENS and GEFS limits our ability to take a direct thermodynamic approach.  This is a subject for future work.  

Four-Panel Plots

We provide loops of four-panel plots of the following variables for several regions: 

• Total precipitation (water equivalent) since the beginning of the forecast period
• Total snow since the beginning of the forecast period
• 24-h precipitation (water equivalent)
• 24-h snow
• 6-h precipitation (water equivalent)
• 6-h snow
• Wet-bulb 0.5°C height above ground level
• SLR

SLR and snowfall are calculated in 6-h intervals, with the resulting 6-h accumulations summed to provide accumulations over longer periods.  This is similar to what you would get if you were measuring snow on a freshly wiped snowboard every six hours and adding it up.  Thus, the 24- and total snowfall should not be confused with the change in snow depth on the ground over long time periods, which would be affected by settlement.  

Each four panel plot includes the the downscaled ENS control forecast at upper left, the downscaled ensemble mean at upper right, the downscaled ensemble minimum at lower left, and the downscaled ensemble maximum at lower right.  Below is an example of the total snowfall through 240-h over northern Utah. 

The choice of the downscaled ECMWF ENS control for upper-left is somewhat arbitrary, but I often find it helpful to be able to look at what one member is doing. The ensemble min and max plots at the bottom are based on the lowest and highest values at each downscaled gridpoint, respectively.  Thus, it is very likely that many ensemble members contribute minima or maxima in these plots so do not use them to illustrate a plausible forecast outcome across a large region.   For instance, if you have one ensemble member that produces heavy snow over far northern Utah but none over central Utah and another that does the opposite, the ensemble max forecast is going to show heavy snow everywhere, even though neither member is calling for such a widespread event.  Additionally, the maximum at any grid point is sometimes a pretty big outlier compared to other forecasts at that point.  Please don't use the ensemble max forecast at lower right for click bait suggesting some sort of apocalyptic storm. 

Plume Plots

We have completely upgraded the popular forecast plumes to provide more information about snow level and SLR uncertainty.  As shown below, the plumes now include six panels, with accumulated precipitation and snow on top (including GEFS and ENS means), 6-h precipitation and snow in the middle, wet-bulb 0.5°C level at bottom left, and snow-to-liquid ratio at bottom right. 

The bottom four panels are violin plots, which are designed to provide detailed information about the distribution of forecasts produced by the 82 ensemble members. The violin itself (filled with blue) summarizes the distribution of all 82 forecasts.  The width is standardized, so the 82 forecasts are all similar when the violin is short.  More forecasts fall into areas where the violin is wide.   In contrast, the 82 forecasts produce a wider range of possibilities when the violin is tall and there are few forecasts in areas where there is just a line. 

An example for snow-to-liquid ratio is below, which is extracted from the plot above.  I highlight four violins.  The violin at 12Z 2 October is short and extends from an SLR of 7 to 10.  That's a small forecast range with all of the forecasts falling between 7 and 10.  At 6Z 4 October, the range is larger, going from 0 to 9 and the violin has two bulges, indicating that there is clustering of the forecasts near zero and then at about 7.  Such a distribution is called bimodal and suggests two possible outcomes, one with rain (i.e., SLRs at 0) and others with dense snow (SLRs near 7).  At 18Z 5 October, almost all of the forecasts are near zero, but there is a long whisker going to 9 suggesting there may be a one or two forecasts that call for snow.  Finally, at 0Z 7 Oct, all of the forecasts are zero or no snow.  

Some context for the above SLR forecasts is provided by the wet-bulb 0.5°C violins below.  The wet-bulb 0.5°C level is an approximation for the upper-part of the transition zone where we would expect wet snow.  At 12Z 2 October, when the SLR range is small, most of the forecasts show that the site is above the 0.5°C level.  That means the forecasts put the site near the top of the transition zone or above it.  Hence the SLR range is small.  At 6Z 4 October, most of the forecasts put the wet-bulb 0.5°C level above the site, some well above it, but there are a few that put it below the site.  This leads to the bimodal distribution, a clustering of SLR at 0, but some forecasts with an SLR near 7 produced by the colder members.  At 18Z 5 October, nearly all of the forecasts put the wet-bulb 0.5°C level well above the site, but there is one outlier that puts it below the site and possibly one or two others that put the site in the transition zone.  As a result most of the forecast SLRs are near zero, but there are a couple of outliers that have dense snow with SLRs in the high single digits. Finally, at 6Z 7 October, all of the forecasts put the wet-bulb 0.5°C more than 1000 feet above the site, and the SLRs are all zero.  

Note that we provide SLR distributions whether or not precipitation is forecast by the model.  Thus, an SLR of 0 doesn't mean it's raining, but it means if there is precipitation, we are diagnosing that it would be rain.  

The violins contain additional statistical information, summarized below. The thick horizontal red line indicates the median of the forecasts.  The thick vertical black line indicates what is known as the interquartile range (IQR) or the middle 50% of the forecasts.  The vertical red line indicates the middle 90% of the forecasts.  Finally, the blue horizontal lines or whiskers denote the minimum and maximum extrema. 

The top of the brown region is the station elevation.  Note that due to the limits of resolution, the model elevation can differ from this elevation.  We provide the model elevation in plume plot header so you can compare. Sometimes this is important.  For the plots above, the site is Washington Pass.  The site elevation is 5450 feet, but the model elevation is a bit higher, at 5713 feet.  

Caveats and Disclaimers

This is an experimental product.  Feedback is helpful to us as we are trying to find ways to better forecast snow and its characteristics and squeeze everything we can out of the operational model suite.  Tell us what works and what doesn't.  

The Utah Snow Ensemble is based on data and products of the European Center for Medium Range Weather Forecasting (ECMWF), National Centers for Environmental Prediction (NCEP), University of Utah, and other groups.  These groups do not accept any liability whatsoever for any error or omission in the data and their availability, or for any loss or damage arising from their use. 

This blog post may be updated as needed.

Crazy Late September Heat

Just outside my window I hear the late September dogs

And I understand their warning I understand their song

Since you left I feel the change in the air

And night after night I'm searching for mercy everywhere

So I wake in the street and I call out your name

I shout to the sky please

Come on let it rain

Let it rain down on me

Let the rain touch my hands

Let the rain set me free

- Melissa Etheridge

Plenty of insanity in the weather world currently with Hurricane Helene providing a sad reminder that it is water, not wind, that causes most tropical cyclone impacts.  That said, there is insanity here as well.  Yesterday, KSLC reached 96°F.  Not only is that a record for the day, but it is the highest temperature ever observed after 19 September.  Below is a listing of the 10 highest maximum temperature observed at KSLC from 16 September to 15 October.  The record is 97, but that was on September 19th, 1956, a full 10 days earlier in the calendar.  

About the only positive thing right now is the sun goes down early and the nights are long, so it's at least tolerable for sleeping.  

Looking elsewhere, pity the people in Phoenix.  Yesterday's high was 117°F.  LET ME SAY THAT AGAIN.  YESTERDAY'S HIGH WAS 117°F.  That is an utter and total obliteration of anything that has ever been observed previously this time of year.  Again, top-10 below for 16 September - 15 October.  Yesterday's 117 is a full 4 degrees ahead of the previous record, set the previous day, and #3 set two days before that.  Until this year, the highest temperature observed after September 15 was 110.  

Simply incredible.  

Congrats to Ron Perla

 Ron Perla, an pioneering avalanche researcher and Little Cottonwood legend, was recently named a distinguished alumnus of the University of Utah Department of Atmospheric Sciences. As described in this College of Science online article, Ron was an Alta Ski Patroller and US Forest Service Snow Ranger in the 1950s and early 1970s who researched slab properties, avalanche rescue methods, etc.  He commuted back and forth from Alta to the U to complete his classes in meteorology and conduct his research.  Ron was also a mountaineer who established first routes on Mt. Robson (Canadian Rockies), Grand Teton, and our own Lone Peak in the Wasatch.  

Ron Perla, US Forest Service, 1968

Alta skiers may be familiar with a run called "Perla's" on Mt. Baldy, which is named for Ron.  Ron tells the story of how he survived an avalanche on Mt. Baldy in 1967:

“In 1967, I was working as a USFS Snow Ranger near the top of Mt. Baldy,” Perla says. “The cornice broke off prematurely, and I fell into a Baldy chute. The cornice blocks triggered a large avalanche. I was tumbled around with no chance of 'swimming,' and somehow I missed all of the rocks. Just before I lost consciousness under the snow, I managed to thrust an arm up to the surface. I was found quickly.” 

Backcountry skiers might also be familiar with "Perla's Peak" on the south side of Little Cottonwood Canyon just up canyon from the head of Coalpit #4. I don't know if there's a good story behind that.  

Last year I spoke with Ron about the history of the J & J Quinney Alpine Meteorological Research Center at Alta, which for a time in the 1970s was "operated" by my department.  You can read about it here.  

Congratulations to Ron and many thanks for all he has done to keep us safer in the mountains. 

Sneak Peek

 We're getting closer to having a replacement for the NAEFS with a lot of big upgrades including replacing the Canadian with the ECMWF ensemble, a new snow-to-liquid ratio algorithm, more and better graphics, etc.  We're thinking of calling it the Utah SnowCast Experimental Ensemble (USCEE, or "You Ski").  If you have other ideas, let me know.  Naming rights available for a large enough donation.  

Below is our new time-series layout.  I'm plotting Berthoud Pass, CO since they hve a shot at some snow in the near future.  There's a huge amount of information in these plots.  Probably more than most people want to see, but this is an experimental product and we're trying to understand the capabilities and limitations.  

More soon.  This is one reason why blogging has been light lately.  

It's Snowing!

It's snowing, not in Utah, but in the Alps.  And probably enough to ski in some areas.

Today's snow depth analysis for western Austria shows significant snow depths for September at mid and upper elevations.  

Source: https://avalanche.report/weather/map/snow-height

There are a few questionable observations, mainly due to the need to recalibrate (i.e., some stations strated with no snow but a measured snow depth of say 10 cm) but here's one that looks legit:  76 cm (30 inches) at Seegrube (1921 m) above Innsbruck.  

Source: https://avalanche.report

Here's a corresponding web cam view confirming that it looks legit.

Source: https://www.foto-webcam.eu/

The GFS forecast for 0600 UTC 14 September shows a great setup for heavy precipitation in the northern Alps of Austria and the Alpine Foreland (i.e., the upstream plains) of Austria and Germany with a deep closed low centered over the Balkan States and strong integrated vapor transport wrapping cyclonically (counter-clocwise) from the western Mediterranean and across the Black Sea Basin, Ukraine, Poland, and Germany.  That is a favored moisture-transport corridor for the northern and western Alps.   

The situation though in eastern Austria is quite serious as heavy precipitation in that area is expected to continue for some time.  Below is the GFS forecast valid 1200 UTC tomorrow showing continued precipitation in the Lower and Upper Austrian States.  

Todays Tirol Daily News in western Austria highlights the flooding in the east as well as the heavy snowfall and avalanche situation in Tirol in western Austria.  

I took a look at the Avalanche danger article and if my translation is accurate, it was a 70 year old hiker who was buried on Saturday and is still not recovered.  The second person was on the rescue team and was partially buried and with a right leg injury.  The rescue was called of that afternoon.  

Enjoy the Lake Stink and Clean Air While They Last

Smoke primarily from fires in southern California invaded northern Utah yesterday resulting in hazardous air quality for the Salt Lake Valley.  

Modis imagery from yesterday afternoon showed a corridor of smoke extending from multiple fires in the mountains around the Los Angeles Basin, across the Las Vegas area, and through western Utah to northern Utah. 

Source:  https://worldview.earthdata.nasa.gov/

PM2.5 at our mountain meteorology lab on the University of Utah campus crested at 120 ug/m3, at the upper end of what is considered to be unhealthy levels.  

Source: https://mesowest.utah.edu

Thankfully, those values have dropped into the single digits this morning, so all you notice as you inhale might be the lake stink, which I noticed in the avenues as I rode to work.

Breath easy for now because we're in a gap between the smoke from SoCal and smoke from OreIda (Oregon and Idaho).  The latest HRRR smoke forecast has smoke from the latter moving into the area later this morning and afternoon.  The forecast valid 2100 UTC (3 PM MDT) this afternoon shows near-surface concentrations in the Salt Lake City area near 60 ug/m3, not as high as yesterday's peak, but still at unheathy levels.  

By 0000 UTC (6 PM MDT), concnetrations > 60 ug/m3 have spread over the entire Salt Lake Valley and extend in a narrow region into southwest Wyoming.  

There are areas, however, where concentrations are > 100 ug/m3, so this has the potential to be another bad round of unhealthy air quality depending on your location and how this plays out.  Exercise this morning if you can.  

It's a Front!

Exciting news in the weather department this week with a bonafide cold front coming late Wednesday.  The 0600 UTC GFS has the cold front draped over northern Utah at 0300 UTC 12 Sep (9 PM MDT Wednesday), so right now the frontal passage looks to happen in the evening in the Salt Lake Valley, although give that a +/- 3h window given the inherent uncertainties.  

The pre-frontal airmass is fairly dry, so the models aren't going for much in the way of precipitation prior to or during the frontal passage, although there may be some high-based convective clouds and wind.  Perhaps the biggest concern will be fire-weather conditions.  The National Weather Service has issued a red flag warning for most of the lowland regions of Utah for 11 AM Wednesday to midnight Wednesday night given the wind and low humidity.  

Thursday will seem fall like. High temperatures at KSLC are likely to be around 72F. Somewhat elevated areas like the east bench will struggle to hit 70.  Think of it as a fast-forward to the wonderful temperatures of early October.  

Mountain snow?  It's a possibility, but the moisture-starved nature of this system means that nothing to a skiff is probably most likely and that will be on Thursday when the coldest air arrives rather than with the cold frontal passage. Below is our downscaled and combined EPS+GEFS ensemble forecast of the total snowfall through 0000 UTC 13 Sep (6 PM Thursday) showing a mean from the 82 ensemble members of about a half an inch and a maximum of 3.4 inches at at Alta-Collins. 

We don't have plumes working for this product yet, but I suspect that most ensemble members are not going for snow. I won't be surprised to see a little of the white stuff, but I also won't be surprised if we see some flakes with little to no accumulation.  

I'm hoping to do a deeper dive into this newer ensemble product soon, but I'll share a few details here, with the caveat that we're still testing and tweaking:

• The ensemble will replace our current NAEFS product and is based on all 51 members of the ECMWF Ensemble Prediction System (EPS) and all 31 members of the NCEP (US) Global Ensemble Forecast System (GEFS).
• Model forecasts are downloaded on a 0.25 degree latitude-longitude grid and downscaled to 800 meter grid spacing.  
• Snow-to-liquid ratio is based on a new algorithm that we have developed using data from 14 snow-study stations in the western United States, including at least 3 sites in northern Utah.  This should be a big improvement over what was in the NAEFS.
• Due to the use of the EPS, which is not available (for free) until 8 hours and 40 minutes after the EPS initialization time, this product will be available a couple of hours later than the NAEFS, but that's a tradeoff I'm willing to make as the EPS is the best ensemble product out there.  
• We are anticipating providing more and better graphics, although we're still trying to figure out what we can provide in a reasonable amount of time.

First Snow?

For a couple of days now the models have been suggesting that the coldest trough of the season so far will push into the Pacific Northwest during the middle of next week.  Below is the GFS forecast valid 0000 UTC 13 September (6 PM MDT Thursday) with the 500-mb low center over central Colordo and northwest Utah in the cold air behind the accompanying cold front.  

Precipitation chances for northern Utah vary depending on the model run.  We've been working on a new ensemble for snowfall forecasting over Utah based on the European Ensemble Prediction System (EPS) and the American Global Ensemble Forecast System.  It will have 82 members, downscaling to 800-m grid spacing, and a new snow-to-liquid algorithm based on machine learning with observations from 14 western U.S. snow study stations.  The run from 0000 UTC shows that the EPS control is producing a bit under an inch of snow at the highest elevations in the central Wasatch, a mean of up to 0.4", and a max of 5.7 inches for the 24-h period ending 0000 UTC 13 September.  

So, chances are probably more likely than not that we won't get much, but it's something worth keeping an eye on and a welcome change from summer weather.  

Meteorological Summer is Over

Meteorological summer, consisting of the months of June, July, and August, is thankfully over.  

In case you haven't noticed, temperatures have been climbing in the Salt Lake area in recent decades.  This summer at KSLC featured an average temparture of 80.3F, good for the 5th warmest on record.  The top 9 years are all in the 21st century.  

Yes, I know some of you don't like the KSLC site, but these trends do also reflect global warming and urbanization.  Both of those are real, and they are reducing the comfort and livability of Salt Lake City and probably a good portion of the Wasatch Front.  

To that point, there are continuous records from the Bountiful Bench site since 1975 and this summer looks like it will squeak in as the warmest on record (there are still 2 days that haven't yet been received, but the current average temperature is 76.8F, 0.4F warmer than the next warmest summer (2021).  

If you are wondering, the year so far rates as the 2nd warmest on record at KSLC, behind only 2018.