Wednesday, November 22, 2017

Giving Thanks for the NCAR Ensemble

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.

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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!

Tuesday, November 21, 2017

Mini Snow Eater Conditions

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....

Monday, November 20, 2017

Precipitation Overprediction Problems with the NAM Conus Nest

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.  

Sunday, November 19, 2017

Three Thoughts on This Sunday

1. Yes Virginia, there is skiing

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.  

Saturday, November 18, 2017

The Beauty of Antelope Island

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. 

Friday, November 17, 2017

Update on Storm Chasing Efforts during IOP2

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.