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. 

An Exciting Forecast

I'm pretty excited about the latest forecasts as it looks like we are going to see a bonafide frontal system moving through Utah, spreading the goods around statewide.  

But before jumping ahead to the storm, it's worth talking about what is going to be happening today and tomorrow.  During the day today, a cold trough will be dropping down out of the northwest and through Utah.  It will generate a little snow, mainly in the mountains and drop temperatures at all elevations.  Below is the GFS forecast valid 0000 UTC 12 February (5 PM MST Tuesday) showing the cold northwesterly flow with 700-mb temperatures of as low as -20ºC grazing the Utah-Idaho border. 

My 20/20 rule is something I use to remember what the outlier 700-mb temperatures are in Salt Lake City and the northern Wasatch.  Anything above 20°C is unusually warm or below -20°C is unusually cold.  As shown in the sounding climatology below, 20ºC has historically been an upper-limit for 700-mb temperatures in the summer.  Below -20C is less rare, but the spike like nature of the periods below that temperature tell you that they are episodic and extreme cold surges.  Thus, when we start getting below -20°C we are getting into rare (but not extreme) air.  

Source: SPC

The GFS forecasts bottom out the 700-mb temperatures at -21°C on Wednesday morning.  If you are going to be skiing tomorrow morning, be prepared.  Our GFS-derived, machine-learned forecast product for Little Cottonwood Canyon forecasts 8 AM temperatures of -11°F on Mt. Baldy (11,000 ft) and -6ºF at Alta-Collins (9600 ft).  

After that cold incursion, the south winds return ahead of the frontal system that will affect us later in the week.  Temperatures climb on Thursday and by 0000 UTC 14 February (5 PM Thursday) we are at the tip of an atmospheric river that penetrates to northern Utah via the lower Colorado River Basin.  700-mb temperatures have climbed back to -7°C at that time, so crest-level temperatures will rebound some on Thursday, with some periods of snow developing in the afternoon.  

The GFS then goes hog wild overnight as the meat of the system moves in, with heavy snowfall developing in the mountains overnight.  Below is the forecast valid 1200 UTC 14 Feb (5 AM Friday).  Give your valentine a powder day this year instead of roses.  

Then, we get what is currently advertised as a bonafide post-frontal northwesterly flow period on Friday and Saturday.  Wouldn't that be sweet! Better than chocolate, although watch the calories as the added weight is surely going to ramp up the backcountry avalanche hazard and challenge patrollers at the resorts.  

If you are wondering, storm total at Alta-Collins in the GFS for the period from 11 AM Thursday through through 11 PM Saturday is 2.11" of water and 27.1" of snow.  

The Utah Snow Ensemble died a temporary death yesterday.  I'm going to try and resuscitate it this morning.  It's not an easy thing to keep alive.  Thus, we're missing the all too critical ensemble to give us an adea of the range of possibilities, but the Euro looks to be on track with the GFS and I'm inclined to think that this is a situation where I would be using the GFS as near the lower limit of the storm total range given the potential for snow in the northwesterly flow, which the models really struggle with.  

My view is that right now this storm looks pretty good, but details are difficult to forecast at longer range, especially the post-frontal crap shoot.  A small change in flow direction can make a big difference.  The GFS and Euro have me thinking 1.5-3" of water and 25-40" of snow for Alta-Collins for the period from 11 AM Thursday through through 11 PM Saturday, but let's see how the models evolve over the next couple of days.

Editors note: This post was edited to correct a incorrect date and phrase.  

Close, But Not Close Enough

Yesterday evening's weak disturbance tried to get us to Steenburgh Winter this season, but alas, like the Rams in Super Bowl XXXIV, we came up just short.  The obs from Alta-Collins show 5 quick inches falling, bringing the total snow depth to 99" for three hours.  

Close, but not close enough. The rules of Steenburgh Winter are quite clear and strictly enforced.  If the plane of the goal line is not broken, there is no touchdown.  If there is no 100" by Feb 10, there is no Steenburgh Winter.  

Yesterday's Graupel Fest

Hell of a storm yesterday in Little Cottonwood Canyon with copious amounts of graupel.  Observations from the Alta-Collins site showed a storm total of 19" of snow and 2.70" of water which fell over a period of 18 hours.  Water equivalent rates remained above 0.2" per hour from 9 AM to 3 PM, above 0.37" per hour from 11 AM to 2 PM, and peaked at 0.41" from 12 to 1 PM.  The frontal structure during this event was complex, but I'd say the heaviest precipitation period was prior to and possibly during the frontal passage.  

Watching the radar, it appeared that the Alpine Ridge between Little Cottwonwood Canyon was the locus for heavy precipitation development and that is confirmed by the MRMS Radar only precipitation estimate below which showed an elongated area of more than 1.25" of water and a maximum of up to 1.50" from just west of the American Fork Twin above Snowbird to Sunset Peak just east of Alta. 

Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/

We have been looking at extreme events like this in Utah and around the world in collaboration with several scientists including Baker Perry, a geographer and mountaineer who has installed high-altitude weather stations on Mt. Everest and in the tropical Andes.  In the case of the latter, they have observed hourly maximum water equivalent of snowfall rates of 20.3 mm (0.80") at Laguna Sibinacocha, Peru (4,895 m), 16.6 mm (0.65") at Chacaltaya, Bolivia (5,160 m), and 14.5 mm (0.57") at Quelccaya Ice Cap, Peru (5,650 m).  We've been investigating how extreme these are.

Peter Veals at the U combed through the Alta-Collins data we have stored in MesoWest (special thanks to the Alta Ski Patrol) and it appears that the "mother of all Alta graupel fests" occurred on 5 January 2008 when an incredible 3.99" of water equivalent fell in 21 hours.  Hourly water equivalent rates were at or above 0.35" per hour for 5 consecutive hours, totaling 2.18" during the heart of the storm.  Hourly water equivalent rates reached 0.54", which is the record at Alta-Collins.  

Source: MesoWest/Alta Ski Patrol

Radar imagery at the time of maximum water equivalent snowfall rate was very similar to yesterday with a reflectivity maximum parked right over the Alpine Ridge, again ahead of the approaching cold front which in the image below is producing precipitation to the northwest over the Great Salt Lake. 

Source: NCEI

So, yesterday's graupel fest was impressive, but there have been bigger.  At issue is what are the processes that contribute to such extremes?  It does appear that the interaction of the SW-WSW flow with the Alpine Ridge is probably critical as that ridge appears to serve as a locus for localized storm and graupel development in both cases, but why doesn't this happen more frequently?  I don't have answers to these questions, but it would be fun to investigate.  

Cuts to NOAA/NWS Budget and Personnel

Below is a transcript of the letter that I sent to Utah Senator John Curtis concerning the cuts being proposed for NOAA and the National Weather Service by President Trump and DOGE.  A slightly modified version was sent to Representative Blake Moore and submitted to the Salt Lake Tribune for consideration as Public Commentary.  If enacted, these cuts will put the safety of Utahn's at risk.  Media reports suggest that the cuts being considered are 30% to the budget and 50% to personnel. There is no private sector company today ready to take on the enormous responsibility of protecting life and property in Utah from high-impact weather and cuts to the National Weather Service will weaken their ability to provide critical decisions support services to emergency managers.  If you agree (or even if you disagree and want to share an alternative opinion), please consider writing your legislative leaders. 

Dear Senator Curtis:

I am a Professor of Atmospheric Sciences at the University of Utah who has worked for 30 years to improve weather prediction in Utah.  I lead research to improve the understanding and prediction of winter storms in Utah’s mountains and develop methods to improve snowfall forecasting across the continental United States using artificial intelligence. The forecast techniques my group has developed are used by the National Weather Service and private companies.  I am also proud to have served as the graduate advisor for several Air Force officers who are contributing to weather support for our Nation’s defense.  I write today as a private citizen.  The views expressed in this letter are mine and independent from the University of Utah.  

I am deeply concerned about the cuts being proposed by President Trump and the Department of Government Efficiency (DOGE) for the National Oceanic and Atmospheric Administration, especially the National Weather Service.  The National Weather Service is widely recognized in public surveys as one of our Nation’s most important government agencies.  The United States probably has the most diverse range of high-impact weather systems of any country in the world including hurricanes and tropical cyclones, floods and flash floods, tornadoes, heat waves, cold waves, winter storms, downslope windstorms, and wind-driven wildfires.  Many of these affect Utah, including flash floods in canyon country, winter storms statewide, and downslope windstorms (also known as canyon winds) along the northern Wasatch Front and other areas of Utah.  The National Weather Service provides essential weather forecasts to protect lives and property during these high-impact weather events.  They provide timely and accurate weather forecasts and have developed strong relationships with emergency managers and other partners to prepare for storms, respond to weather-related hazards, and provide decision support services.  

The National Weather Service Forecast Office in Salt Lake City provides forecasts and decision support services for most of the State of Utah.   The National Weather Service is currently understaffed due to chronically low hiring rates over the past several years.  Current hiring freezes and staffing reductions will further exacerbate this situation, placing Utah’s emergency response to high-impact weather at risk.  One need only look to the recent wildfires in California to see how vulnerable urban areas in downslope wind areas can be during drought.  Utah is not immune to such wildfire hazards, especially along the northern Wasatch Front, but also in many other regions. 

The National Weather Service is also critical for our Nation’s weather, water, and climate enterprise, which has benefited from the long-term synergy between the public, private, academic, and military sectors to improve weather observations and forecasts.  I have observed this throughout my career but also during the time I spent as an elected member of the Council of the American Meteorological Society, which serves all four of these sectors.  The National Weather Service and its partners in NOAA, the FAA, the Department of Defense, and NASA, develop and maintain foundational weather observing systems including geostationary and polar orbiting satellites, surveillance weather radars, and surface observing systems at airports and other weather sensitive locations.  The National Weather Service also runs a complex suite of computer forecast models that take all this weather data and provide forecasts for the Nation, as well as American interests around the world.  This data is freely available to the public, including private companies, who in turn produce value-added products for their customers.  For example, I have former students who use National Weather Service data to improve efficiency and profitability at companies such as Amazon Prime Air. 

The proposed cuts in the National Weather Service budget and staffing come at a time when we are entering our Nation’s must vulnerable period for high-impact weather: the spring tornado season; hurricane season (beginning June 1st); and wildfire season, which in some areas of the western US is now year round, but will expand across other areas of the west in the coming months.  

Historically, support for the National Weather Service and weather observations, forecasts, and research has been bipartisan.  Even this week, Senators Lisa Murkowski (R-AK) and Alex Padilla (D-CA) announced bipartisan legislation to improve the atmospheric river forecasting, an activity that would benefit Utah.  I ask that you work to ensure adequate funding and staffing of the National Weather Service so that they can fully meet their mission of protecting lives and property here in the State of Utah.

Thank you for your consideration.

Sincerely,

Dr. Jim Steenburgh