Problems I Want to Work On

Last night's storm period was one that I really enjoyed simply because it shows how remarkably variable snowfall can be in the central Wasatch.  As suggested in the prior post, the low-level ESE flow favored Deer Valley, which received more snow than Alta-Collins.  

Deer Valley/Ontario: 1.41"/14+" (snow depth sensor got flaky at 7:35 AM; 14 was the total at that time)
Alta-Collins: 1.02"/10"

It seems that a lot of data is not flowing into MesoWest from there resorts right now so I couldn't dig much deeper than that, but the Utah Avalanche Center report included storm totals as of 5 am also showing the decrease in snowfall from east to west. 

Source: https://utahavalanchecenter.org/

This is not something that is unusual for such a flow pattern or that experienced meteorologists wouldn't anticipate, as illustrated by yesterday's blog post.  However, we don't have a good understanding of the physical processes operating during such storm periods, nor do we have models that reliably anticipate such fine-scale snowfall variations.  By reliable, I mean not getting it right every now and then but instead being able to do it consistently. 

This has been the most challenging academic year of my career.  I'm not getting any younger and it has me thinking about what I want to do in my last few years as a researcher.  During my career, I have always been interested in snow and I'm especially interested in understanding and predicting microscale variations of snowfall in areas of complex terrain.  

Last night's case is a good example.  What I wouldn't give to have had a portable radar in the Heber Valley to see what is happening to the cloud and precipitation system on the south side of the Deer Valley Ridgeline and over the Snake Creek area southeast of Brighton.  It would have been so exciting.  More importantly, I'd like to compare this storms to others, as we all like to generalize, but the reality is that there are a lot of variations that we can't anticipate.  

Additionally, what I wouldn't give to have the time and horsepower to improve fine-scale forecasts of these storms using traditional numerical weather prediction or newer Artificial Intelligence/Machine Learning prediction systems.  

The reality is that we do not currently have a high-resolution ensemble that can reliably predict these fine-scale snowfall variations.  If you think the HRRR can do it, think again.  Here's the forecast from yesterday morning's HRR through 1200 UTC 6 March (5 AM today).  The numbers for Deer Valley and Alta-Collins aren't bad, but note how the snowfall maximizes not on the Deer Valley ridgeline but instead on the Alpine Ridgeline near Lone Peak.  We don't have observations up there, but I think that's an unrealistic spatial pattern.  

In part, this may be due to the resolution, or the grid spacing of the HRRR, which is about 3 km, or possibly due to how it deals with cloud processes, including the growth and transport of snow in storms.  The swiss aren't running models at 3 km grid spacing.  They are running them at 1 km grid spacing and trying to get even finer.  That might help.  However, one also has to be able to handle the snow growth, transport, and fallout processes right and this is where observations and clever minds are needed to incorporate such effects into our modeling systems.  

AI/ML is pretty exciting and is going to become increasingly important moving forward, but it's unclear how to do it for such fine scale precipitation patterns in which training datasets are limited.  There are some proposed approaches, but it's going to take careful testing and evaluation to advance AI/ML capabilities for situations like this.  

I guess in the end things haven't changed much throughout my career.  I love snow and winter storms in complex terrain and these are the kinds of problems that I want to work on.  Beyond my own personal interests, advances in these areas would likely help with forecasting for the 2034 Olympic Winter Games, road weather and avalanche mitigation in Little Cottonwood Canyon, and other weather and climate applicatios over northenr Utah.  

A Big Spring Storm

The model runs are pretty spicy and exciting this morning with a significant storm moving later today.  The latest GFS suggest precipitation beginning in the central Wasatch this afternoon or evening and continuing into Friday.  Let's have a look.  

The GFS forecast valid 0600 UTC 6 March (11 PM Wednesday) shows that the early part of the storm is characterized by an deep upper-level trough (upper left panel below) with a lower-tropospheric cyclone (red L in images below) over Nevada.  Associated with this system is an inland penetrating atmospheric river  atmospheric river that approaches northern Utah via the lower Colorado River Basin.  The crest-level 700-mb flow (lower left panel) is southerly or even south-southeasterly in the vicinity of the central Wasatch with 700-mb temperatures near -5°C.

Thus, the overnight storm period has all the hallmarks of a warm, high-density storm period.  Through 9 AM tomorrow morning, the GFS generates 0.77" of water and 8.2" of 10% water content snow. The HRRR is even more excited with water with 1.65" of water and 13.8" of 12% water content snow.  And that's just the start.  For the Wasatch Back crowd, that little bit of easterly flow gets my attention and suggests this could be a period where  you do pretty well.  This could be a period where snowfall is greater at Deer Valley and less at Snowbird, but we'll see if that pans out.  

The GFS drags the main cyclone very slowly across our area, resulting in a prolongued period of moist southerly to southwesterly flow that continues through the day tomorrow,  By 0000 UTC 7 March (5 PM Thursday), the storm is so wrapped up that we actually have cooler air moving into northern Utah from the southwest, as illustrated by the 700-mb (crest-level forecast at lower left).  By this time, the AR has moved downstream, but there's fairly deep instability and it's March, so things might get a bit convective tomorrow afternoon.  

I'm not sure if we might get a break in there for a bit as sometimes there can be a dry slot that sets up in a pattern like this, but by and large I suspect we'll see periods of snow tomorrow that will continue to stack up.  

Eventually the system moves through and we get into colder, unstable, northwest flow for Friday.

I've summarized the storm phases in the time-height section below.  Time increases to the left.  First there is the warm and juicy AR period Thursday night.  Note the low-level southeast flow that might favor the eastern side of the central Wasatch.  Then on Thursday, colder air wraps around the system and moves in aloft from the south and southwest.  Call it an upper cold front if you like, but it will destabilize things.  Temperatures drop further on Friday evening with a transition to colder, post-frontal flow.  

For Alta-Collins, storm totals in the GFS through 7 PM Friday are 1.77" of water and 24.1" of snow.  Expect that to be high density snow to start and lower density Thursday night and Friday.  The HRRR only runs through 5 AM Friday but it has 2.22" of water and 20.2" of snow.  It's less bullish on the post-frontal period whereas the GFS produces snow more continuously through the period.  Such model inconsistencies are one of the joys of being a meteorologist!

I'm inclined to go for a storm total of 1.5-3" of water and 20-36" of snow for Alta-Collins through Friday evening.  There are uncertainties in timing and details, but this looks like a pretty good storm.  The early part could be a big producer on the Wasatch Back.  It's a warm storm and perhaps snow levels will flirt with the lower Jordanelle/Mayflower area during the atmospheric river phase tonight and early tomorrow.  At upper elevations, I suspect the snow will be fairly dense everywhere before trending drier late Thursday and Friday. 

Buckle up and monitor forecasts.  There's a lot of moving parts in this storm. 

Meteorological Winter Ends

Meteorological winter, which includes December, January, and February, ended yesterday.  In the Salt Lake Valley, it wasn't much of a winter.  The average temperature at the Salt Lake City International Airport was about 36.4°F, making it about the 7th warmest on record*.  The asterisk is because there appear to be two days missing from the records I can access, so those numbers could change slightly.  

Snowfall was scant with only 10.4".  A day is missing for the snowfall records too, although I believe the total is probably right (the missing day was likely snow free).  This is the third lowest on record behind 2014/15 (6.0") and 1962/63 (8.8").  

Source: https://xmacis.rcc-acis.org/

The SNOTEL data in the central Wasatch has been a bit squirrelly this year, but for end of meteorological winter snowpack, most sites are a bit below average including Snowbird (90%), Brighton (88%), Thaynes (73%), and Parley's Summit (95%).  Mill D North sits at 118% and is the high outlier.  

All of the data discussed above is collected by the NOAA/National Weather Service and National Resources Conservation Service.  Media reports suggest firings occurred in both of these agencies prior to and during this past week.  Additional employees in both agencies opted for the "Fork in the Road" buyout.  I haven't seen official numbers, but I've seen some estimates suggesting that the NWS lost about 10% of their workforce.  It is my impression that nearly everyone in the weather, water, and climate enterprise, which spans the government, private, and academic sectors, believes this will reduce the Nation's capacity to anticipate, prepare, and respond to weather and related hazards as we enter severe weather, hurricane, and wildfire season.  It will also affect the Nation's ability to advance the prediction of weather and water related hazards in the long term. 

Slim Pickings

It's looking like the last week of February and the first two days of March will largely be snowfree in the central Wasatch.  We have a weak cold front moving through today, but it's a dry one.  If it wasn't for the wind and the lower temperatures this afternoon, you probably wouldn't have noticed it.

The Utah Snow Ensemble is flatlined until 2 March when a couple plucky members produce a little snow.  Most hold off until 0000 UTC 3 March or later.  After that, the various ensemble members are throwing spaghetti on the wall.  Some do nothing, others up to 40" of snow.  The spread in the runs is pretty consistent from 0 to 40.  Your guess is as good as mine for what's going to happen.  

The reason for that spread is the highly split flow forecast to pervade over the west.  The latest Euro forecast valid 0000 UTC Monday 3 March shows the pronounced split over the eastern Pacific with one branch of the jet moving northward through Canada and the other southward across Baja, northern Mexico, and the southwest US. 

There is a closed low embedded in the southern branch of the jet, and that's the main hope for later in the forecast period.  

These are the times that try water manager's souls.  We had just gotten the mid- and upper-elevation snowpack up near average after the last storm cycle, but a long dry patch now would not be helpful.  It will make it more difficult to get to a near average snowpack at the end of the snow accumulation season and could spread out the runoff period.  It's perhaps a little early to worry about that, but now that March is a approaching, it's something to ponder.  Perhaps the wetter forecasts will verify and we'll get a more active mid March to prevent that from happening.

A Scrappy Pattern

Our recent storm cycle has pushed the mountain snowpack to about as close to median as you can get in northern Utah.  Per the latest SNOTEL observations, northern Utah basins are sitting anywhere from 90 to 107% of median snowpack water equivalent. 

Source: NRCS

The situation becomes grimmer over central and southern Utah, but I have cut that off in the plot above because who wants bad news these days. 

We should add a bit more to the northern Utah mountain snowpack over the next few days as we're in what I'll call a "scrappy" west to northwest flow through Tuesday, meaning the mountains will squeeze out a bit of snow from time to time, and then another storm system rolls in later in the week.  As is often the case, the US GEFS ensemble is a bit wetter than the European Ensemble (ENS), but through 0000 UTC 22 February (5 PM Friday), the middle 50% of the Utah Snow Ensemble is generating 1.04–1.66" of water and 14-24" of snow for Alta-Collins.  Spread is greatest for the storm later in the week when the US GEFS ensemble is more bullish than the European.  Take a wait and see attitude for that one. 

Challenges of Snow Level and Precipitation-Type Forecasting

Forecasts remain very much on track with the expectations from a couple of days ago (see An Exciting Forecast).  For the central Wasatch and Alta-Collins specifically, the models are calling for snowfall to being today in southwesterly flow ahead of the approaching trough and continuing for the most part through Saturday afternoon when we are in the colder, post-frontal storm phase.  

I wouldn't be surprised to see a break or two in the snowfall at times in there, but totals look to be impressive.  Through 11 PM MST Saturday, the GFS is coming in with 1.90" of water and 24" of snow.  The HRRR doesn't go out that far, but just through 11 PM MST Friday it's at 2.4" of water and 24" of snow, so it's quite excited about the warmer part of the storm.   Most members of the 82-member Utah Snow Ensemble are in the 1.5–3" of water and 20–40" of snow range through 0600 UTC 16 Feb (11 PM MST Saturday).  

This is also a statewide storm, not some localized miracle for Alta (at least until we're in the post frontal stage), so everyone should get some.  Good news for all. 

But the fly in the ointment for the forecast is what is going to happen in the Salt Lake Valley later today and tonight, with the forecasts providing a "teaching" opportunity for me with regards to how we forecast precipitation type using model soudings. 

We will begin with this morning's observed sounding from the Salt Lake City International Airport as it sets the stage for the changes that are coming over the next 24 hours or so.  Thanks to the cold surge earlier this week, the temperatures in this sounding (red line) are below 0ºC everywhere. It is also dry in the lower to mid levels.  Between the surface and 700-mb, the dewpoint depression (the difference between the temperature and the dewpoint) is more than 10ºC except right at the surface.  

As a result, the wet-bulb temperature, indicated by the thin blue line, is generally 2-3°C colder than the actual temperature.  Meteorologists use wet-bulb temperature to forecast precipitation type because it the temperature the air will cool to if you evaporate water into the atmosphere, as happens for example when precipitation begins to fall.  

So, this mornings sounding is cold and could get colder with precipitation.

However, that assumes that the sounding doesn't change and in reality it is going to change and change a lot today and tonight due to the transport of warm air and moisture in advance of the approaching system.  So the next step for the meteorologist is to use the computer models to try to get a handle on what those changes will be.  Typically this is done by looking initially at maps like the ones below which are from the GFS and provide the large-scale context for tonight's forecast.  Valid at 0600 UTC 14 February (11 PM MST Thursday), they snow northern Utah in warm, moist southerly flow with widespread precipitation over northern Utah except in the lower elevations of western Utah.  

After that, a meteorologist might look at forecast soundings from the model.  This is typically done using what are known as "BUFR" soundings.  BUFR is short for Binary Universal Form for the Representation of meteorological data.  That's a mouthful, but the long and short of it is that BUFR is a binary data format maintained by the World Meteorological Organization and widely used to store high-resolution forecast profiles from computer models, including those for the Salt Lake City International Airport.  

The GFS BUFR sounding for the Salt Lake City International Airport valid at 0600 UTC 14 February (11 PM MST Thursday) shows considerable warming at all levels as we would expect based on the maps above, but if you look at the temperature trace (red line), it is very different from the surface to about 800 mb (7000 ft) than aloft.  In that low-level layer, temperatures stay very close to 0°C (I've indicated the 0°C line with a dark grey line; it is skewed because of the design of this plot which is called a "Skew-T"...that's a story for another day).  

We sometimes call such a layer "isothermal" because the temperature is nearly constant with height.  An isothermal layer near 0°C is not unusual to find in winter storms because in heavy snow, the melting of the snow tends to lower the temperature to 0°C, but no lower, similar to adding ice to a cold drink.  

In that sounding, there is only a very shallow layer near 800 mb that is above 0°C, not enough to fully melt falling snow, so we would expect the precipitation to fall as snow all the way down to the valley floor if it were to verify.  

However, other models have different ideas.  The HRRR for example also warms things up aloft, but it has a very different profile below about 700-mb.  There is a larger dewpoint depression (meaning the relative humidity is lower) and it is much warmer near and below 800 mb.  In fact the surface temperature in this sounding is about 5°C (41°F) and even the dewpoint is about +1°C.  

In a sounding like that, we would expect rain or maybe mixed rain and snow.  At the airport, the snow would turn to rain much earlier in the HRRR (and precipitation would probably be lighter too). 

These differences reflect differences in the resolution and parameterization of physical processes in the two models.  The GFS is lower resolution and doesn't resolve the terrain as well as the HRRR.  It also deals with mixing due to the friction experienced by the atmosphere near the Earth's surface different than the HRRR.  As a result, during this period, the GFS produces more precipitation at the airport and the cooling effects of that precipitation combined with the difference in mixing leads to a stable layer over the valley and snow persisiting longer.  In contrast, the HRRR produces less precipitation at the airport and mixes out the atmosphere more readily resulting in warmer low-level temperatures and an earlier transition to rain. 

Neither the GFS or the HRRR is perfect, so the devil is in the details concerning what unfolds tonight on the valley floor and the amount of snow that falls before things change to rain could vary a lot across the valley simply because precipitation rates tend to vary a lot across the valley in patterns like this (as do elevations).    

For the airport, the National Weather Service Forecast calls for snow today, but "rain, possibly mixed with snow becoming all rain after 2 am" tonight.  

Screenshot taken at 8:03 AM MST Thursday 13 February

So, their expection is that we will see the snow turn into rain overnight.  

This is a time worth monitoring the forecasts, especially if you have to get around the valley tonight or in the early morning tomorrow as I do.  I have to get my wife to the airport at o'dark 30 and am keeping a close eye on this and planning an earlier awakening to see what happens and be prepared for winter driving conditions if the snow hangs overnight, especially on the east bench.