Just a quick post today following up on some themes from posts the past few days.
The graphic below lops through GFS 228, 204, 180, 156, 132, 108, and 84 hour forecasts valid at 0000 UTC 4 December (5 PM MST Sunday). Imagine trying to forecast for Sunday afternoon based on just this single forecast system. Good luck.
The loop also illustrates the folly of forecasting based on model trends, or what forecasters call "DModel/Dt" (i.e., the rate of change of the model forecast with respect to time). Clearly, there is no evidence of a consistent trend in those forecasts. The pattern is too chaotic, leading to a lack of run-to-run consistency, even in the more recent forecasts.
It's fun to talk about DModel/Dt, but studies have found that it has little forecast value for medium-range forecasts. Hamill (2003) examined the value of DModel/Dt and here's what they found:
"Extrapolation of forecast trends was shown to have little forecast value. Also, there was only a small amount
of information on forecast accuracy from the amount of discrepancy between short-term lagged forecasts. The
lack of validity of this rule of thumb suggest that others should also be carefully scrutinized before use.
The medium range models have been calling for the ridge from hell, which I'll officially name "Heartbreak Ridge", to develop over the western U.S. next week.
Just check out these GEFS forecasts from this morning.
Penn State E-wall
Blocking patterns don't come much uglier than that, and the ECMWF ensemble forecast (mean and standard deviation below) is no prettier.
I've been in denial about these forecasts for a few days, hoping that we might get something Sunday/Monday for storm chasing and to help the skiing some, but it hit me today that we could be hosed big time.
Let's put the situation into historical perspective. Records for the Snowbird SNOTEL cover 29 years. Time series of snowpack water equivalent from 1 October to 1 February are provided below. We currently sit at 3.1 inches, which I've circled below. There are only 2 water years with less snow, 2000 and 2010.
Source: NWS
That wouldn't be cause for panic, but that block scares the bejesus out of me. I stuck an arrow for the future on the graph, assuming we get perhaps 0.5" of water out of the Sunday/Monday storm (it could be more or less, but I lean toward less). We would go through the first week of December near the lowest snowpack in that 29 years. If instead we hold at the current snowpack, 3.1 inches would match the low for Dec 10 set in 2010.
Although there is some uncertainty in the SNOTEL data, we're near the bottom of the barrel. Note that the SNOTEL records don't go back to 1976/77, better known as the "drought year", when only 13.5" fell in November and 17" in December at Alta. That year may have been worse.
Bottom line: Burn skis, sacrifice a goat, or whatever else you can think of to conjure snow up Sunday/Monday. We could be facing a really ugly start to December.
About the only thing that keeps me going in times like these are thoughts of the 100 inch storm in November 2001. We'll need it if this continues.
Having cut my teeth in an era before ensembles, I confess some tendency to pull up the medium range forecasts produced by the Global Forecast System (GFS) when I arrive at the office during times of drought to scan for the next hope for flakes.
This is a colossal mistake, especially in the current pattern.
The predictability at 4-7 days seems to have been remarkably low the past few weeks, leading to individual GFS forecasts that might be described as "all over the place," although one might argue that in general they have trended to drier as the forecast lead-time decreases.
Here's an example of the types of changes that one sees. The 162 hour forecast from the GFS initialized at 0600 UTC 27 January November shows a major storm for all of the mountains of Utah on Sunday afternoon. Great hope right?
Two days later, the 114 hour forecast from the run initialized at 0600 UTC last night says NO SNOW FOR YOU!
Let's consult an ensemble forecast. We had a problem with our 0000 UTC NAEFS processing, so I'll use the plume forecast for Alta-Collins from yesterday. Look at that spread. There are some members generating over 3" of water and 30" of snow and others absolutely nothing.
I personally like plume diagrams, but the members producing the heaviest precipitation are the ones that attract the eye's attention and cause the heart to race. Close inspection of that diagram shows that most members (about 40%) are producing less than 0.5" of water. Thus, if one never bothered looking at the GFS, but instead the ensembles, the possibility that we're not going to see much is there. On the other hand, the possibility of a larger storm is small, but not zero.
Humans like single model runs. They are easy to look at and interpret, and they produce plausible forecasts.
However, for medium-range forecasts, they can be deceptive. They do not provide estimates of the range of possibilities, and in a pattern like this, that's a problem.
I go to bed at night in a pattern like this expecting the worst (dust on dirt) and hoping for the best (major dumpage.
Yesterday's frontal passage was a bummer for skiers, providing little in the way of the white stuff in the Cottonwood Canyons, but provided us with plenty of excitement thanks to the Doppler on Wheels.
We deployed that morning to a rattlesnake speedway in the Utah desert where a dark cloud rose from the desert floor and we headed straight into the storm. If you have no idea what that means, watch the video below.
More accurately, we set up along the side of the causeway to Stansbury Island, just north of the Tooele Valley. When I drove out to meet the team, the surface front had already pushed into the northern Tooele Valley, with low level "fractus clouds" seen in the photo below at levels just abouve the ground, near its leading nose. At this time, the front was quite shallow.
The rattlesnake speedway in the Utah desert was actually the Stansbury Island Causeway, ideal for surveying the frontal structure over the Tooele Valley and precipitation processes over the Oquirrh Mountains. We could also can over the Stansbury Mountains (background below).
The shallow nature of the front was very apparent in the radar data we collected. A Doppler radar is capable of measuring how fast scatterers in the atmosphere, in this case snow and rain, are moving toward or away from the radar. This allows us to use radar scans, known as PPIs, which are oriented at a slight angle to the horizon, to infer changes in the wind across the area and in the vertical. In the plot below, cool colors represent flow toward the radar, warm away, with the radar in the middle of the image. There is a clear indication of flow from the northwest near the radar and south-southwest at ranges more removed from the radar site. Since the radar scan is tilted at a slight angle to the horizon, this is an indication of strong vertical wind shear in the frontal zone not far above the Earth's surface.
We can also configure the DOW to scan in vertical slices, known as RHIs. The RHI below is oriented to the east and scans over the southern Great Salt Lake, eventually hitting the lower slope of the Oquirrh Mountains near Point of the Mountain where they rise above the south lake shore. Doppler velocities in this image are primarily away from the radar, consistent with northwesterly flow, except near the ground just to the west of Point of the Mountain. This reflects the splitting of flow around the north end of the Oquirrh Mountains, with the flow there having perhaps a slight NNE component, which results in a weak flow component toward the radar (green).
The DOW is also a polarimetric radar, which means it sends out and receives radar energy in two planes, one horizontal and one vertical. The shape of the raindrops or snowflakes can be inferred using this information. One product we use to do this is known as "differential reflectivity," with reflectivity the amount of radar energy is scattered by back to the radar. If the horizontal and vertical radar energy is similar, the differential reflectivity is zero, and the precipitation is likely circular. If on the other hand, the horizontal radar energy is larger, then the precipitation is wider than it is high, and the differential reflectivity is positive. Dendritic snow, those wonderful flakes with six arms that produce blower powder, often produces high differential reflectivity, because the flakes tend to fall "flat."
The RHI below shows two layers of high differential reflectivity (indicated by yellows). One is near the ground and reflects the melting layer in which snowflakes are sticking together and falling relatively flat. The other is farther aloft and likely reflects a layer in which dendrites are growing and falling. The temperatures in this layer were likely between -12ºC and -18ºC, which favors the formation of dendrites.
An interesting aspect of the storm was a near-complete lack of any enhancement of precipitation over the mountains. It was a frontally forced event, rather than a mountain forced event. In fact, it clearly precipitated more over the Tooele Valley than over the Oquirrh Mountains. Only in the late stages of the event did the front finally decide to move over the Oquirrhs. An example is the RHI of horizontal radar reflectivity below, taken looking east-south-east across the northern Oquirrhs. The yellows are ground clutter from the radar energy bouncing off the Earth's surface, with the sloping area representing the western slope of the Oquirrh Mountains. Above the ground clutter, the transition from light to dark blue reflects increasing precipitation toward the mountains, but this doesn't reflect an orographic effect, but is the frontal band as it moved into the Oquirrh Mountains.
It's such a pity that the storm didn't produce much in the Wasatch. However, thanks to the mobile capabilities of the DOW, we were able to go to the storm and get a wonderful dataset.
Looking west from the University of Utah Campus at about 7:15 AM this morning
It was a Mordor-like sunrise over the Salt Lake Valley this morning (e.g., above), simultaneously spectacular and apocalyptic. With the overnight southerly flow, there is a decent plume of dust over the western valley (evident below the fire-lit clouds above), but clear skies to the east.
The weather over the past 24 hours has been quite remarkable if you know where to look. While it was quite mild across much of northern Utah during the holiday weekend, a thin lens of cold air remained over the relatively cold waters of the Great Salt Lake. Our Hat Island observing site, for example, hasn't eclipsed 55ºF over the past five days.
Source: MesoWest
Yesterday, that airmass made its presence known by penetrating into the northern Salt Lake Valley. At 4 PM (2300 UTC), a sharp lake-breeze front extended across the central part of the valley with southerly flow to the south and northwesterly flow to the north. Temperatures dropped from 70ºF in the Sandy "banana belt" to near 50 (or lower) near the Great Salt Lake. Almost climatology!
Source: MesoWest
The sounding from the Salt Lake City airport at about that time showed the shallow nature of the cold lens, with a surface temperature of just under 10ºC (49ºF), but temperatures of almost 17ºC at 827 mb (about 5600 feet).
Source: SPC
Forecasters predicting a record high for the Salt Lake Airport were likely flummoxed!
Fortunately, the south winds saved the "day", blowing the lake breeze north and causing temperatures to rise rapidly around 8 PM.
Source: MesoWest
The transition from cold, damp lake-modified air to warmer, drier air was remarkably abrupt, and led to a high temperature for the calendar day of 69ºF. That is the 2nd latest 69ºF temperature recorded at the airport on record (December 1st is the latest), and it happened in the dark. In addition, a temperature of 68ºF was recorded after midnight, which sets a record high for today. This is not your grandparents (or even parents) climate that we are living in.
Moving on to the weather this morning, we are still in the warm southerlies with some wind-borne dust over the western Salt Lake Valley. However, cooler air remains over the Great Salt Lake with northwesterly flow developing over northwest Utah.
The HRRR brings the surface front into the northern Salt Lake Valley at 1900 UTC (1 PM MST) this afternoon. It will be a dry frontal passage, with the precipitation trailing the surface front.
We will be "storm chasing" with the DOW, which we plan to deploy just south of Stansbury Island to examine the front penetrating into the Tooele Valley and across the Great Salt Lake. Although a dry frontal passage, we're hoping there is enough dust in the air to be able to examine some fine-scale aspects of the frontal passage. We'll then work on whatever precipitation comes through, focusing on the frontal-band interaction with the Stansbury and Oquirrh Mountains.
For skiers, this looks like a pretty pathetic event. The models have the band fall apart as it moves in. Snowshowers are possible, but it won't add up to much, as we discussed over the weekend. Sad!
Tree skiing anyone? Source: Park City Mountain Resort
It's not too often that I hop on my bike at 10 AM on November 25th, but that's what I did today and I even wore fingerless gloves. Whatever.
The plot below shows the range of temperatures each day this month (dark blue bars) compared to the normal range (green region) and the maximum (top of red) and minimum (bottom of blue) temperatures.
Source: NWS
Little wonder why the past month has pretty much been a downer for skiing. The past four days have had minimum temperatures close to the average maximum for this time of year.
I am asked two things in patterns like this. The first is will the next storm deliver. The second is when will the pattern change.
If forced to pick between a yes and a no, I'll have to go with the latter for the first question, at least for the central Wasatch. If you bought into the medium range forecasts from the GFS a few days ago, your hopes may have been raised, but with the exception of the storm on Nov 16–17, the GFS has disappointed for most of the fall. Now that we sit 48 hours from Monday, all but three of the SREF members are generating less than 0.4" of water for Alta.
Source: weather.utah.edu
There's still some hope if a lower probability, snowier outcome, but chances are we'll get just enough snow to cover the rocks.
Now let's talk pattern change. Looking at the large scale, I don't see much in the way of major changes over the next 10 days. The active storm track remains to our north. In a pattern like this, a decent dump is not impossible, but it needs something to come through and tickle northern Utah in just the right way. For example, a slow moving front that is quite productive, similar to the one that affected us on Nov 16–17 (but colder), would do the job. Hope.
On the plus side, it is warm in the valley and the trails are in good shape for mountain biking, hiking, and trail running. So much better than this scenario with a valley inversion, which would be truly miserable.
The weather this Thanksgiving weekend has been and will be completely outrageous based on climatological standards.
The mean temperature at the Salt Lake City Airport on Wednesday and Thursday was 55.8ºF. If that number holds for through Sunday, it would be more than 5ºF warmer than any Nov 22–26 period on record.
Source: NOAA Regional Climate Centers
With three days to go, you could say I'm cherry picking, but the low last night was about 44 (won't know officially until 11 AM), the National Weather Service forecast high for today is 60, and the numbers for Saturday and Sunday are 40/63 and 44/67. Those would give an average temperature for the period of 54.1, still blowing away any comparable period on record and likely representing the warmest Thanksgiving in Salt Lake history (I haven't bothered doing calculations for the variable thanksgiving dates, so that's really only a guess).
With this recent string of warm weather, I thought we might be back on pace for hottest year on record, but we still lag behind 2015, 2016, and 2012.
Source: NOAA Regional Climate Centers
After the big storm last week, I believe it's been a net loss of snowpack in the mountains on all aspects below 8500 feet and those facing south or west even above that altitude.
I've always wondered what the worst possible conditions that a ski area would operate under are in Utah and Snowbird may be setting the mark this weekend [Addendum at 4:50 PM Friday 24 Nov: Snowbird has suspended skiing operations until cold weather returns]. I've always considered Big Emma, even when covered adequately with snow, tho be the most terrifying run at Snowbird due to its frequently icy conditions and the density and diversity of skiers. I'd rather ski Pipeline any day of the week. Yesterday, it must have been truly terrifying with a narrow white ribbon of death bisecting it.
Thankfully, Alta showed constraint and even opened for uphill skiing.
We did a quick ascent to burn off calories for Thanksgiving turkey.
The snowpack is thin, but solid. They had groomed from a bit below the angle station down through corkscrew.
For the most part, it was bone rattlers rather than chin ticklers, but it was skiing. Those going up after us would probably have gotten some softening corn.
Our next hope for snow is Monday and Monday night. Still seeing a lot of spread in the NAEFS ensemble.
We need a storm not just for skiing, but also for our storm-chasing efforts with the Doppler on Wheels. Please come through Mother Nature!
For nearly three years, the National Center for Atmospheric Research (NCAR) has produced a daily, 10-member, cloud permitting ensemble at 3-km grid spacing known as the "NCAR Ensemble".
For those of us in the western U.S., the NCAR Ensemble forecasts, available from web sites hosted by NCAR and the University of Utah, attempted to do something that no current operational forecast system could three years ago — capture the extreme spatial contrasts and quantify the inherent uncertainty of precipitation over the western United States.
Last night's forecast, for example, shows the major deluge expected to affect the Pacific Northwest through Thanksgiving. At 3-km grid spacing, the NCAR ensemble accounts for many regional and local topographic influences and, with 10-members, one can derive statistics related to the range of possible forecast outcomes and the likelihood of precipitation above certain thresholds (our standard 1" and 2" thresholds work well for the Wasatch, but not the Cascades!).
Plume diagrams allow one to examine precipitation at various locations, including Mt. Baker Ski Area below. Such a pity that nearly all of that water will fall in the form of rain.
These products are popular with readers of this blog, friends in the snow-safety community, and operational forecasters.
Recently, NCAR announced that the NCAR Ensemble will sunset at the end of the calendar year. More information is below.
Although I'm sad to see it go, I believe this move makes sense. NCAR is a research lab, not an operational center. They need to be unshackled from routine forecasting and free to explore creative ideas and pursue modeling breakthroughs. The NCAR Ensemble did this for three years. It has allowed us to learn a great deal about cloud-permitting ensembles. For example, we have a paper examining the performance of the NCAR Ensemble that may be the subject of a future post.
Given that tomorrow is Thanksgiving, it seems fitting to toast the NCAR Ensemble team that includes Kate Fossell, Glen Romine, Craig Schwartz, and Ryan Sobash. Thanks so much! We look forward to a few more weeks of NCAR Ensemble forecasts, and hope that Mother Nature shifts this damn pattern so that we can actually use them for powder hunting in Utah!
It's not quite like an east-coast snow-eater event when temperature are near 50, fog, and rain, but for Utah, this is as close as it gets.
Current (7-8 AM) temperatures in the central Wasatch are 37 at Spruces, 39 at the base of Park City, 38 at the base of Alta, and 33 at Alta-Collins, and 31 at Germania Pass. That puts the freezing level at about 10,000 feet.
In addition, dense, mid-level overcast is draped over the mountains, with some west snow at upper elevations, rain at mid elevations, and the transition zone in between. The Alta webcam below summarizes the dreary conditions quite well.
For the most part, snow on north facing aspects will survive just fine this time of year under clear, dry conditions, even when temperatures are above 32ºF. Without direct sunlight, you simply don't have enough energy to melt snow.
However, if you can add humidity and cloud cover to the mix, things change. You lose the cooling effect of snow sublimation and gain the energy input of infrared radiation from the clouds.
The eastern U.S. gets these conditions in spades at times, with fog and rain doing it's number on snow frequently during the cool season.
We don't really get such a snow-eating extreme in Utah. Today is about as close as it gets. Above 9000 feet, everything will be fine. Below 8000 feet, we're going to see snow losses. In between there may be a net loss, but it probably not a huge one.
Fall continues it's grip on the Wasatch, with no desire to let go and let winter take control....
High resolution forecast models are not necessarily better forecast models and precipitation forecasts produced by the NAM Conus Nest (hereafter the NAM-3km) are a prime example of this.
The NAM-3km covers the continental US at a grid spacing of 3 km, four times the resolution of the 12-km NAM in which it is nested. With such high resolution, you would think the NAM-3km would be especially useful for precipitation forecasting over the complex terrain of the western US, but it isn't, because it has a major overprediction problem.
Tom Gowan, a graduate student in my research group, recently led a study examining the performance of several forecast models at mountain locations across the western U.S. during last winter. I have been holding off on publicly sharing these results broadly since the paper describing this work is still in review, but the results are too pertinent to forecast needs right now not to share at this juncture. In the case of the NAM-3km, we used pre-operational test runs from last winter that were kindly provided by NCEP.
The plot below shows the ratio of mean-daily precipitation produced by the NAM-3km to that observed at SNOTEL stations. Overprediction is evident at the majority of sites, with on average the NAM-3km producing 1.3 times as much precipitation as observed.
Source: Gowan et al., in review.
A major reason for this is that the NAM-3km produces far more major precipitation events than observed, especially over the interior western US. In the plot below, the frequency bias is the ratio of the number of forecast events to the number of observed events in each event size bin. The NAM-3km has by far the largest overprediction problem. Note that the NCAR ensemble also produces too many large events, although the magnitude of the problem is not as acute.
Source: Gowan et al. in review.
I bring up these issues today because the NAM-3km is going batsh-t crazy for the storm later today and tonight. For Alta-Collins, the 12-km NAM is producing .08" of precipitation through 10 AM tomorrow. In contrast, the 3-km NAM is producing 2.04"!
The loop below shows steady, drippy precipitation over the Wasatch and nearby ranges during the overnight period.
Now, it is always dangerous to say a model is wrong before the forecast verifies, but I'm going to say it anyway. This forecast is wrong. There's little evidence to support such huge precipitation totals. Even in the NCAR ensemble, 7 of the 10 members produce less than 0.2" of precipitation, and the wettest goes for about .57".
This issue plagued the NAM Conus Nest when it was run at 4-km grid spacing and it appears to carry over to the higher resolution upgrade.
The bottom line is this. If you want great deep powder skiing, consider using the NAM-3km for your holodeck experience. However, if you live in the real world, avoid using the NAM-3km precipitation forecasts unless you want to be severely disappointed on a regular basis.
I debated for a while whether or not to ski this weekend. I'm not a fan of low snowpack, bony conditions and often stick to skiing the main run along Alta's Collins lift under such conditions. With Alta closed to uphill, we opted to take a couple of laps at Brighton in the Great Western area since rumors were that they were asking tourers to avoid other parts of the mountain. I pulled out my oldest sticks for the day, a pair of Karhu Jak BCs that are probably 10-12 years old.
However, I was pleasantly surprised to survive both runs without harming a single rock. I was also glad to rediscover that the Jak BC really was a great ski, even if you didn't see them much in Utah.
We stuck to grassy runs that were heavily traveled. The touring and skimo crowd cut up the area pretty good.
Some sections of untracked could be found in some areas and we went home satisfied, without injury, which is the main goal of any first day.
2. The snowpack isn't really all that meager
It's worth remembering that it is only November 19. Thanksgiving is pretty early this year. Our snowpack seems pretty meager, but really it isn't. The Snowbird SNOTEL is measuring 3.1 inches of snow water equivalent (SWE). Median is 4.1 inches. An inch of water equivalent is basically one modest storm. So, we're one storm below median. Yes, it hurts to look at the snowpack in the Tetons and Sawtooths (or should I say Sawteeth?), but we're not really doing all that bad.........yet.
3. The next week may suck
The model forecasts for Thanksgiving week don't look so great for Wasatch skiers. The NAM forecast for 5 PM MST tomorrow (monday) shows a classic "dirty ridge" scenario with moisture spilling over a low-amplitude ridge and into Utah. 700-mb temperatures are a balmy 0ºC. This is a recipe for riming and perhaps some wet, rimed snow at times in the mountains. It probably won't add up to much for the snowpack and we'll probably see a net loss at elevations below 7000 feet (not that there's much there currently).
Eventually, a high amplitude ridge builds over for Thanksgiving Day. Beautiful weather across Utah for driving over the river and through the woods to grandmother's house. Good valley mountain biking. Good canyoneering.
However, 700-mb temperatures are a whopping +6ºC, which would be a record for the last week of November (although we have observed 700-mb temperatures of +8ºC in mid December). Brighton was making snow today where they could, with the low-angle sun and dry conditions allowing for it in shady spots. The resorts will probably need to continue to selectively pick spots and times over the next few days for making snow.
Antelope Island has been very good to me the past two days. On Friday, we had a great IOP basing near the Fielding Garr Ranch. U grad student Trey Alvey took the shot below at the end of the IOP. Talk about a great vantage point for orographic precipitation research.
Driving around the island in the rain yesterday morning reminded me that I needed to return, so we took a quick trip there this morning to survey the views and the bison. The photo below is from Buffalo Point. Hard to believe this is a short drive from metropolitan Utah. There was an interesting cumulus streamer reminiscent of a banner cloud forming on the lee side of Farnsworth Peak (Oquirrh Mountains).
One has to wonder what it was like for the pioneers to gaze upon this body of water after traveling for weeks across the plains and mountains.
A highlight of the day was watching this beast lumbering through the grass.
Medium range forecasts suggest a below average snow week ahead with above average temperatures. Not good for storm chasing or skiing. Perhaps I'll return to Antelope Island with my bike over the holiday weekend.
I like to joke that "storm chasing" Utah style involves sitting in one place and scanning storms repeatedly, which is exactly what we've been doing today for OREO IOP2.
After our overnight team returned to Salt Lake late last night, today's day team deployed to Antelope Island near the Fielding Garr Ranch.
Antelope Island is a great place to operate the DOW as there are unblocked vistas of much of the northern Wasatch and even the Salt Lake Valley. It's a bit farther from the Cottonwoods than we like, but we can do a great deal looking at other parts of the Wasatch.
Much of the day we scanned a relatively broad frontal band. Pretty boring by our standards, but it might still yield some interesting data. However, during the afternoon, the flow shifted to WNW and the atmosphere destabilized, yielding some shallow convective showers.
These showers produced a bit of graupel in downtown Salt Lake City and the Avenues (and perhapse elsewhere).
One thing we can do with the DOW is take vertical scans through storms. The orange and yellow stuff at the bottom of the vertical scans below are ground clutter produced by mountains, but the purples are some of the convective showers, which you can see are shearing off downstream with height.
Given a relatively pessimistic storm chasing forecast after Monday night, we'll probably work this shallow stuff to the last gasp.
