The RRFS Snow Ensemble

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

The RRFS Ensemble is a 6-member ensemble that is under development for future operational use by the National Weather Service.  Based on the FV3 dynamical core (i.e., the software that solves the atmospheric equations of motion), it is run at 3-km grid spacing and provides forecasts out to 60 hours.  The RRFS Ensemble is projected to go operational in 2024, although there have been a number of issues and challenges identified during testing that may affect that, in particular related to the forecasting of convective storms in the midwest.  The RRFS Ensemble has not been carefully evaluated over the western US, so one of the reasons we are producing this product is to evaluate its fidelity for orographic precipitation.  

We are also interested in testing our techniques for snow prediction.  Thus, what we call the RRFS-Snow Ensemble is basically an ensemble in which we plug in new techniques to predict snow-to-liquid ratio (SLR) and snow amount.  A summary of the data and methods and graphics is provided below. 

Precipitation Downscaling

Although we downscale the Utah Snow Ensemble from the lower resolution global ensemble grids, the RRFS is providing forecasts at 3-km grid spacing. Thus, we are currently doing no precipitation downscaling and just using the raw model grids.  There might be some advantage to downscaling the RRFS eventually, but for now, we're not doing it.  

Snow-to-Liquid Ratio (SLR)

Snow-to-liquid ratio (SLR) is based on a new random forest algorithm developed using SLR observations from more than 900 Community Collaborative Rain, Hail & Snow Network (CoCoRaHS) observing sites.  Thank you to all the volunteer observers and the CoCoRaHS team!  In particular we are using a subset of CoCoRaHS observing sites at which the observers are taking manual cores of the snowfall data, which we hope will reduce issues related to precipitation undercatch, which is a problem with the water equivalent measured by many gauges.  The random forest is trained using data from across the contiguous United States and in testing has performed better than existing operational techniques in both the western and eastern United States.  

Snow Level

Identifying snow level in the west or precipitation type in the east is a bit of a thorny issue.  The so-called "wet-bulb" technique that we use in the western United States works fairly well when the temperature decrease with height is close to what meteorologists call a wet-adiabatic lapse rate.  It doesn't work well if the atmosphere is stable and/or has a warm nose above freezing aloft.  

As a result, we decided to use a more physics-based approach to identify if snow is occurring.  At each model grid point, we calculate the melting energy in the model soundings.  This is the amount of energy available to melt snow in the sounding.  The technique is based on Bourgouin (2000), although we use wet-bulb temperature rather than dry-bulb temperature to calculate melting energy (special thanks to Kevin Birk of the National Weather Service for providing some of the initial code for this work).  Currently we are applying our random forest SLR without adjustment if the melting energy is ≤ 2 J/kg and assuming the precipitation is all rain if the melting energy is ≥ 9 J/kg.  If the melting energy is between those values, we reduce the SLR between the random forest value and 0 based on linear interpolation between the two thresholds.  

Those thresholds are based on published values, but admittedly, the data is not comprehensive.  It may require some modification over time.  However, they do give results similar to the wet-bulb method when the lapse rate is near moist adiabatic and can deal with more complicated temperature profiles.

Note that we are not attempting here to diagnose freezing rain or sleet.  The melting energy approach we are using will basically give us a SLR of 0 in those instances.  So, our plots only show forecasts of accumulated snow.  

We are working with another group to possibly incorporate a machine learning technique for precipitation type in the future, but it may be a while before we get to that.  

Four-Panel Plots

We provide loops of four-panel plots of the following variables for several regions, including over the central and eastern US: 

• 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
• 1-h precipitation (water equivalent)
• 1-h snow
• Wet-bulb 0.5°C height above ground level (based on the lowest wet-bulb 0.5°C level)
• SLR

SLR and snowfall are calculated in 1-h intervals, with the resulting 1-h accumulations summed to provide accumulations over longer periods.  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 RRFS control forecast at upper left, the ensemble mean at upper right, the ensemble minimum at lower left, and the ensemble maximum at lower right.  Below is an example of the total snowfall through 60-h over the Wasatch Front.

For some of the caveats of interpreting these plots, see my blog post on The Utah Snow Ensemble. 

Plume Plots

We are also providing forecast plumes and violin plots for many locations, including several in the northeast US, to provide more information about SLR uncertainty.  These are identical to those for the Utah Snow Ensemble, so refer to The Utah Snow Ensemble blog post for information on interpreting these plots.  The RRFS ensemble, however, only has 6 members, so there are not a lot of forecasts and the violins are going to be based in part on interpolation fro sparse data.  Some groups use time-lagging (i.e, using older forecasts) to increase the ensemble members (but also decreasing the forecast period), but we're not bothering with that for now. Perhaps at some point we will change the lower left panel from wet-bulb 0.5 level to melting energy, but for now we're keeping it consistent with the Utah Snow Ensemble.  The wet-bulb 0.5°C level is based on the lowest level in the sounding, so it will not tell you where the top of a warm nose is and may not be a useful variable in situations where there is a warm nose aloft.  

Caveats and Disclaimers

This is an experimental product.  In fact, it is an experimental SLR product post-processing experimental ensemble modeling system!  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 RRFS ensemble is based on data and products from the 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.

The Oquirrhs Got the Snow

If you are looking for snow, perhaps you should look to the west to the Oquirrh Range instead to the Cottonwoods. The latest snow water equivalents from SNOTEL sites show the highest amounts at the three SNOTEl stations in the Oquirrhs.  These numbers do not include the lake-effect snow that fell out there last night and this morning.   

Source: NRCS

Shall we have a closer look?  The fattest snowpack is at Rocky Basin Settlement (8704 ft) where the snow water equivalent sits at 2.7". 

That's well above median for that site, although median so early in the season isn't a very robust statistic.  

For comparison, the Snowbird SNOTEL (9177 ft) is at only 1.2 inches.

Why are the Oquirrhs so blessed?  A big chunk of the snowpack at Rocky Basin Settlement fell on October 29.  They got some snow in the southwest flow like the Cottonwoods, but they did very well in the north-northwesterly flow following trough passage.  For example, the radar image below for 1821 UTC 29 October shows strong echoes just to the west of the Oquirrh crest.  These echoes persisted for a good chunk of the afternoon. 

It's hard to say exactly what the role of the lake was during that period, but the echoes are suggestive that both lake-effect and orographic (mountain lifting) processes contributed to snow enhancement over the Oquirrhs.

Then, in the evening, some lake-effect snow developed. 

For a while, as midnight approached, a very localized band developed.  

So, the Oquirrhs have done much better in the post-trough north-northwesterly flow than the Cottonwoods.  

Is this unusual?  It depends on what you use as a baseline.  It is unusual for the Oquirrhs to have more snow than the Cottonwoods.  At Rocky Basin Settlement, peak median snow water equivalent is 23.9 inches compared to 42.9 inches at the Snowbird Snotel. 

On the other hand, lake-effect periods produce about the same amount of snow in the Oquirrhs as in the Cottonwoods.  The figure below shows the water-equivalent produced by lake-effect periods during the 1998-2009 water years (adapted from Yeager et al. 2013).  During those water years, lake-effect periods produced an average of 2.12 and 2.37 inches of precipitation water equivalent at the Rocky Basin Settlement and Dry Fork SNOTEL stations in the Oquirrhs, respectively, compared to 2.06" and 2.38" at the Mill D North and Snowbird SNOTEL stations in the Wastach, respectively (see left figure below).  

However, since the Oquirrhs are otherwise drier, lake-effect periods constitute a greater fraction of the cool-season precipitation there (right figure above), including 6.3% and 8.4% of the cool-season precipitation at the Rocky Basin Settlement and Dry Fork Snotels.  For comparison, lake-effect periods produce 5.9% and 5.1% of the cool-season precipitation at Mill D North and Snowbird.  

So, such lake-effect snow is not unusual in the Oquirrhs.  They are currently ahead of the Cottonwoods because they were favored given the north-northwesterly flow in the wake of the trough on 29 November.  A similar situation occurred last night and this morning.  

As the saying goes, it's better to be lucky than good.  

Another Digging Trough

 It's déjà vu all over again if you are a meteorologist with another digging trough (or if you prefer "splitting") forecast to move across the western US this weekend.

The last one gave us 8" at Alta-Collins, with another inch yesterday.  The current snow depth sitting at 8".  I don't think they've bothered firing up the guns yet so the base remains scant even on the Mamba-Corkscrew trails.  

The GFS forecasts the 700-mb trough to be right over Salt Lake City at 0600 UTC (0000 MDT) Sunday.  As I mentioned the trough is a digger and a splitter, so a the strongest "dynamics" or large-scale rising motion, is to our south over Arizona (the red contour indicates large-scale rising motion of 1.5 cm/s).   

Although not a direct hit, the models are generating precipitation over northern Utah as the front moves through (see upper-right panel above) and then a period of cold northwesterly flow on Sunday as the trough moves downstream (see below).  

Total water equivalent and snowfall generated by the GFS through 11 PM MDT Sunday at Alta-Collins are 0.99" and 14.3", respectively.  A look at the Utah Snow Ensemble shows that 06Z 4 Nov (just an hour later as the ensemble forecasts are only processed every 6 hours), the ENS Mean is about 0.6"/9" and GEFS mean about 0.8"/12".  

This reminds me a good deal of the last storm in that it is not all that well put together, but there is some potential if the band with the trough and the post-frontal northwesterlies come through.  I'm once again thinking something in the 8-14" range, although if I were a wagering man, I might say the odds of going more than that are bit greater than less than that.  This is also indicated by the plume diagrams above which show a lot of clustering between 8 and 12" of snow and then about 10 members that er in the 15-21" range by 06Z 4 Nov.   

There is some potential for another storm midweek next week.  

Scary Things for Halloween

 

It's been a while since we did a list on this blog, so let's create a list of 10 scary things for Halloween:

• Not only is this season a crappy one for snow, but we have several more in the rest of the 2020s, resulting in further decline of the Great Salt Lake.

• I wake up one day and all of the code used to create products for weather.utah.edu is gone.  Or worse yet, I just accidentally delete it.  Even with backups, it would take a huge amount of time to get everything running again.

• I abandon the Wasatch Weather Weenies due to the demands of new "workload policies" being implemented at the University of Utah  

• The 10% budget cuts being floated by legislative leaders for the University of Utah come to fruition and instead of cutting administration, most of the pain is passed down to instructional units that are already stressed by recent enrollment increases. 

• The legislature passes a revised version of last year's Senate Bill 226, the so-called School of General Education Act, establishing a new School of General Education at the University of Utah with all students required to take mandated courses taught by new faculty who are commited to "traditional" general education.   

• UDOT accelerates plans for the Little Cottonwood Canyon gondola. To pay for it, they propose a sales tax for Salt Lake City and Park City.  The cost for a round-trip ride will be $75, but for $60 you can get a one-way ticket and then ski down a new trail that they will construct along Little Cottonwood Creek. 

• The Wasatch Front continues to add 45,000 residents a year and every one of them is a digital nomad that skis 50+ days a year.  

• Powder Mountain goes fully private with a gated entry at the bottom of SR-158. 

• Deer Valley expands into the Mayflower area only to discover that it hardly snows there.  Oh wait, this one is already scary.

• Alta announces plans to build a high-speed six pack directly from the Wildcat base to the top of High Boy.  To deal with the added volumes of skiers, upper High Boy and Stonecrusher will be covered by extensive snowmaking.  Whippet poles will be required unless you ski Greeley Bowl.

• The current storm is a bust and produces only 2" at Alta.

Wait, that's 11.  Happy Halloween.  

The Storm Ahead

Change is on the way as a deep upper-level trough is expected to move into the western United States early this week.  The GFS forecast for 0000 UTC 29 October (6 PM MDT Monday) puts the surface front over northern Utah and the Salt Lake Valley in cooler, northwesterly, post-frontal flow.  

This storm reminds me a good deal of the last with the trough digging into California and the bulk of the "dynamics", or large-scale lift ahead of the trough, moving across southern Utah and Colorado.  However, there is potentially one important difference.  Instead of closing off over central Utah, the trough remains what we call an "open wave" which means the GFS ultimately puts us into a colder, moister, northwesterly flow on Tuesday, as illustrated below.  

Our GFS-derived forecast for Little Cottonwood Canyon ultimately generates just over an inch of water and 16 inches of snow through 6 AM MDT Wednesday.  This is a colder storm than the previous one two, with temperatures forecast to drop to 10°F at the top of Mt. Baldy and snow to liquid ratios ultimately incresing to about 20:1 at the end of the storm.  

All in all, the GFS is producing a pretty good right-side-up Goldilocks storm.  If only we had a base for it to fall on.  

That said, the GFS is a bit more excited than most members of the Utah Snow Ensemble.  Both the GEFS and ENS mean through 6 AM MDT Wednesday (12Z 30 Oct) are right around 0.7" of water and 10" of snow.  If you like to wishcast, there are 3 members (out of 82) that produce more than 1.5" of water and 20" of snow.  

We've been working internally to produce snow forecasts using the experimental Rapid Refresh Forecast System (RRFS) that is being developed by the National Weather Service.  It only has 6 members and only goes out to 00Z 30 Oct (6 PM MDT Tuesday), so it doesn't capture the entire storm period, but through that time, the members are putting out 4-12" of snow.  For comparison, the GFS is putting out 15". 

So, to summarize, most of the model guidance at the moment is giving us a modest storm, with the GFS and a relatively small percentage of ensemble members going for amounts above 15".  There really are two wildcards with this storm.  One is will the frontal precipitation with the system, which is advertise to be sort of disorganized over northern Utah, come together for a while over the central Wasatch.  The second is will we get a decent shot of northwesterly flow post-frontal magic.  I'm inclined to be thinking 8-14" for a storm total through Wednesday morning, but we will see how things come together over the next couple of days. 

Transition to a More Active Pattern?

Fall this year has been pretty quiet and warm.  September at the Salt Lake City Airport brought only 0.42" of precipitation compared to a normal of 1.06".  October only 0.61" so far, compared to a normal of 0.96" for the month to date.  

Average temperatures for September 1 - October 22 have also been remarkably high.  In fact, they are the highest on record, with a mean of 70.2°F, just ahead of the same period in 2022 (70.1°F). 

We had a mainly dry frontal passage in the Salt Lake Valley last night, so it will be cooler today before ridging returns, but the models are now advertising some major changes next week including a deep trough forecast by the GFS to be over the western US at 1200 UTC 29 October (6 AM Tuesday).  

Snowfall produced by this system will depend a lot on track as this one has some potential to "go south" literally and figuratively.  In the GFS forecast above, much of the flow and dynamics is going into southwest Colorado, for example.  As a result, the GFS forecast, while dropping temperatures substantially in upper Little Cottonwood early next week, is only generating 0.38" of water and 5.5" of snow through 12 PM MDT Thursday, better known as Halloween. This includes a bit from the trough on Tuesday and then another one that drops in on Wednesday.

A look at the Utah Snow Ensemble shows that the means of the downscaled GEFFS and ENS ensembles are about 0.6 and 0.7" water and 8" and 10" of snow, respectively, through 1200 UTC 31 October (0600 MDT Thursday).  The range though is quite large, from nothing to 22" for snow, reflecting uncertainty in the trough characteristics as it swings through the western US.  A few members generate some snow later in the forecast period in early November, although there are others that stay dry.  

My take is that we are starting to see the transition to a more active pattern as the jet stream strengthes with the approach of winter, although storm systems can come in fits and starts this time of year at Utah's latitude.  That said, right now the ensembles are not giving good odds for a big enough dump to ensure ski touring in grassy areas and meadows by the end of October.  Things will either need to come in above the snowier members or you will need to have extremely low standards for skiing quality and personal safety.  Concentrate on the hoping for the former and reevaluating the latter.