Blog Break

 I'll be away from the blog for a couple of weeks.  Enjoy the snow and the March sun!

How About Something Different?

Many people are likely suffering from spring fever this weekend with sunshine and mild temperatures along the Wasatch Front.  Saturday's high temperature at the Salt Lake City International Airport was 56F and the National Weather Service is going for 61F for today (Sunday).

Source: NWS.  Downloaded at 8:41 AM MST

Nevertheless, the snow held up fairly well at Alta yesterday, although I only skied until 1:30.  If you are wondering, the record high for today is 68 and for the month is 69 (set on Feb 28).  We should fall short of these. 

A look at that forecast though does show something "different" on top for Monday night when a surface cold front and upper-level trough will move through northern Utah.  The GFS forecast valid 0600 UTC 27 February (11 PM Monday night) is shown below, showing the front draped over northern Utah.

Mountain snow and some valley rain look to develop Monday afternoon. Snow levels will likely fall to the valley floor by about 11 PM. Most of the action looks to be overnight on Monday, although we could see a few snow showers during the day on Tuesday.  

Most members of our downscaled SREF are putting out 1–4" of snow for the airport.  

Our HRRR-derived snowfall product, which goes through 1200 UTC 27 Feb (5 AM MST Tuesday), is putting out 2.2" for the airport and 3.6" for Cottonwood Heights.  

Right now, the National Weather Service has a winter weather advisory up for 1 to 3 inches in the Salt Lake and Utah Valleys.  

Source: NWS.  Downloaded 9:08 AM 25 Feb 2024

Forecasts evolve, so keep an eye on things and plan your Tuesday commute accordingly.  For larger accumulations in the valley, the front would need to be more productive or stall, or the post-frontal environment on Tuesday will need to be more favorable than currently advertised by the models.  

A Wet, Mild February

It's too early for a full monthly recap, but the numbers for the first 20 days of February are worth a look.

Total precipitation at the Salt Lake City International Airport through yesterday was 2.93" which rates as the 7th most on record *for the entire month.*

Source: https://xmacis.rcc-acis.org/

We can climb up a little higher before the end of the month (and will probably get some more today and tonight).

It has also been warm.  For the first 20 days of the month, the average temperature was 40.5F, which rates as the 9th warmest.  

Source: https://xmacis.rcc-acis.org/

Finally, despite all of that precipitation, we've only had 1.2" of snow for the month at the airport.  Snowfall has been scant at the lower elevations, even as the snowpack has really benefited at upper elevations.  

Speaking of upper elevations, the Snowbird snotel now sits at 33.8" of water equivalent (black line below).  This is 142% of median for the day.  That's a pretty healthy snowpack.  Last year on this day we were at 40.3", but we shouldn't make comparisons with last year.  That said, a look at the purple line below shows that it was about now that Mother Nature hit the boosters and things really took off last year.   

Source: NRCS

We'll see what happens this year.  If you are like me and you don't consider it over until it's over, we are at 81% of the median peak.  I'll feel better for runoff when we get to 100%, although one has to feel good about where we are right now.  

Stop Calling Everything an Atmospheric River

The phrase atmospheric river is a relatively new one.  The phrase "tropospheric river" was first used in a paper by Reginald Newell and coauthors in 1992 to describe narrow "filaments" of strong water vapor flux or transport in the atomphsere.  Then in a 1994 paper, Newell and Yong Zhu termed these features atmospheric rivers, a phrase that has caught on and is now widely used not only amongst scientists, but also with the general public.  Hereafter, I will abbreviate atmospheric river to AR for convenience.

In Utah, however, the phrase is being used to describe many situations that do no reach AR criteria, including today.  

ATMOSPHERIC RIVER: Another subtropical train of moisture will move in today. Brunt of the valley rain will be This afternoon into Wednesday morning. Rain totals look healthy and mountain snow totals will be around 1-2 FEET for the Cottonwoods. #utwx pic.twitter.com/XciepWzobZ

— Matthew Johnson (@KSL_Matt) February 20, 2024

ARs are defined and categorized using a variable called integrated vapor transport or IVT.  Think of it as a measure of how much water vapor is moving through a one meter wide curtain that extends from the Earth's surface to very high altitudes every second.  The larger the value, the more water vapor is moving through this curtain.  

Scientists have developed a categorization scale for ARs to help describe their strengths and impacts.  In this scale, the minimum IVT needed for an AR is 250 kg/m/s.  That would be the equivalent of 250 kg of water vapor moving through that curtain every second.  The scale uses both IVT and its duration to categorize atmospheric rivers.  At 250 kg/m/s, an AR would be categorized as either category 1 or category 2, the latter if it were to persist for more than 48 hours.  Impacts are expected to be primarily beneficial (e.g., soaking rains, rain or snow for water resources, etc.) but potentially become hazardous if long duration.  

Source: Ralph et al. (2019)

ARs can have much higher IVTs, in some cases exceeding 1250 kg/m/s.  At these higher levels, ARs are more hazardous, especially if they are long duration.  

ARs reaching northern Utah are much weaker than when they made landfall on the Pacific coast.  This is because they loose a good deal of water vapor as they move inland and produce precipitation, especially over mountain ranges like the Cascades or Sierra Nevada.  Most ARs we observe here are weak, category 1 ARs.  They primarily have beneficial impacts, but they can produce hazards related to flooding and avalanches.

On weather.utah.edu, our four-panel "synoptic" diagnostic includes an analyisis of IVT at lower right.  The GFS forecast for this morning showed weak AR1-level IVT of ≥ 250 kg/m/s on the SoCal coast and near Las Vegas, but elsewhere, IVT was below such levels.  Over northern Utah, in many areas we were below 100 kg/m/s. 

By 0000 UTC 21 Feb (5 PM MST Tuesday) we are still below 250 kg/m/s.  

What we are seeing are the remnants of what was a weak AR on the California coast (it may have reached category 2 for a time while it was moving across central California), but it is no longer an AR. 

My concern about calling these weak features ARs is that it will condition the public to not take them seriously.  Ideally, we should not be using the phrase AR to describe today's weather in northern Utah.  It just doesn't meet the criterion.  

Limitations of Alta–Zermatt Comparisons

 

Zermatt, Switzerland

It is not unusual to hear people make comparisons between Alta and Zermatt when it comes to transportation.  One often hears statements like "Alta should be like Zermatt" or "we should have a train like Zermatt."  This article is not to argue against transportation upgrades (I am likely supportive of a well designed mountain transportation system for the Wasatch), but to instead discuss some of the limitations of these comparisons and the unique challenges facing Little Cottonwood Canyon.  

Alta is a small town.  The population for the 2010 census was 383.  This dropped to 228 for the 2020 census.  I'm not sure how confident to be in either of these numbers, but we are talking about less than 400 permanent residents.  It has five lodges [Snowpine (58 rooms), Alta (57 rooms), Goldminer's Daughter (89 rooms), Alta Peruvian (80 rooms), Rustler (85 rooms)], some condos, and some additional rental units.  Down the road, Snowbird reports that they have 882 total rooms.  I'm not sure what the total lodging capacity is in upper Little Cottonwood (perhaps someone has it), but let's say it is around 2,500 people.  

At the base of Little Cottonwood, Salt Lake County has a population of almost 1.2 million people.  The Wasatch Front has a population of 2 million people.  

Zermatt is often called a village, but it is really a small city.  It has a modest population of 5,733, but also 106 hotels with 7,310 beds (https://www.zermatt.ch/en/content/download/5298/131222/version/62/file/Destination+infrastructure.pdf). There are also apartments which add several thousand additional beds.  In 2023, Zermat saw about 2.25 million overnight stays at hotels and rental apartments (https://www.zermatt.ch/en/content/download/73156/3346478/version/3/file/2023_Jahresbericht+ZT+-+Web+klein.pdf).  

At the base of the Matter Valley that leads to Zermatt is Visp, with a population of 6,777.  The entire Valais Canton, which includes Visp and Zermatt and extends from the southeast shore of Lake Geneva to the Furkapass in south central Switzerland has a population of 343,000.

So, consider the difference between big ski days at the two resorts.  At Alta (and Snowbird), most of the skiers have to get to the resort during an intense morning rush hour.  At Zermatt, most of the skiers are already at the resort.  They arrive in a less intense pulse the prior day and evening.  

My point here isn't to argue against a mountain transportation system, but to highlight important differences between Alta and Zermatt that should be recognized.

World Cup XC Returns to the US

Correction: Total screwup on my part.  The times for the races indicated below are Central European Time not Central US Time.  You would think that I would know that (duh!).  Thus the races aren't in the evening, but in the morning and early afternoon local time.  Apologies!

World cup cross-country skiing returns to the US for the first time in 23 years with racing this Saturday and Sunday.

Source: @FISCrossCountry

More than 30,000 fans are expected for the races, which will be held at Theodore Wirth Regional Park.  In fact, the event is sold out.

Snowfall in the Minneapolis area has been scant this season, but Mother Nature provided several inches of natural snow this week that will help beautify the event (which could have gone on with artificial snow).  Forecasts suggest a dry weekend although there will be some wind for the Saturday sprints.  Race-time temps should be  25-30F each day.

On the Women's side, the US team is having an amazing season. Jessie Diggins, who worked to bring the World Cup to Minnesota, is leading the overall standings and the weekend races are all in skate, which is her best technique.  Rosie Brennan is fourth in the overall standings and also threat to win or podium.  Sophia Laukli is also having a strong season.  

On the men's side, the Norwegians are better than dominating, but Ben Ogden and Gus Schumacher are having good seasons and will surely bring their best.  

It is my understanding that the races will be streamed live on Peacock, NBCSports.com and the NBC Sports app.  I'm currently laid up with COVID, so I'll be watching for sure.  

Snowfall Extremes at Alta, Part II

This is the second part of a blogstorm examining a new paper by Michael Wasserstein and I examining snowfall extremes at Alta.  The paper was just published in what is known as early online release and is available at https://journals.ametsoc.org/view/journals/mwre/aop/MWR-D-23-0206.1/MWR-D-23-0206.1.xml, but may be paywalled if you don't work at an institution with a license for American Meteorological Society journals or have an American Meteorological Society membership (apologies).  However, we are summarizing the key findings in this blogstorm. 

In Part I, we examined the relationships between flow direction and heavy snowfall at Alta, highlighting that heavy snowfall can occur from a wide range of flow directions, but most commonly for SSW or WNW flow for Liquid Precipitation Equivalent (LPE) and the latter for snowfall amount.  The bias toward WNW flow for snowfall amount reflects that such flows are typically colder and feature higher snow-to-liquid ratios, which means you get more snow out of a unit of water.  We also identified seven key synoptic patterns for generating heavy snowfall.  

Here we examine the relationship between Integrated Vapor Transport (IVT, a common metric used to identify atmospheric rivers) and heavy snowfall at Alta.  Unlike what one might find in the Sierra Nevada or Cascades, this relationship is actually quite complex or, as I like to call it, fickle.  

To illustrate this, we looked at the time-integrated IVT (or TIVT) during extreme precipitation periods.  IVT is an instantaneous measure of the horizontal transport of water vapor over a given location.  TIVT is the total horizontal transport of water vapor over a given period (in this case 12 hours).  

The figures below present the TIVT for the snow amount and LPE extremes, with direction consistent with the 700-mb (crest level) wind and vapor transport direction.  An IVT of 250 kg/m/s, generally used as the low-end threshold for an atmospheric river, yields a TIVT of 1.1x10e7 kg/m (Editors note: This value was updated from the original post to correct an incorrect value), which is near the outer circle of these figures. These figures show that true AR conditions for a 12-hour period at Alta are extremely rare.  In addition, many snow amount and LPE extremes occur for TIVTs well below those associated with ARs.   

Source: Wasserstein and Steenburgh (2024)

So we decided to take this a step further and look at how much of the vapor that is transported over Alta is converted into precipitation during the snowfall amount and LPE extremes.  We call this the Local Precipitation Efficiency.  Consistent with the analysis above, this efficiency is especially high during WNW or NW flow. 

Source: Wasserstein and Steenburgh (2024)

So those northwesterly flow storms get a lot out of a little.  

Finally, we decided to look at what happens when the IVT is high at Alta.  Given that true AR conditions with IVT ≥ 250 kg/m/s are extremely rare at Alta, we lowered the IVT threshold to 200 kg/m/s.  During the 23 cool seasons we examined, there were 112 periods (about 5 per year) in which the mid-point IVT was ≥ 200 kg/m/s.  Of these, only 19 produced an LPE extreme at Alta and 37 produced no precipitation at all.  These are the the highest local IVT events a year although some can go big, some are total busts.  

All of this indicates that one needs to avoid what I'll call AR-myopia at Alta.  Snowfall extremes, especially in NW flow, can occur with relatively low IVT.  Locally high IVT (≥ 200 kg/m/s) can sometimes produce a big snowfall amount or LPE event, but there are times when it produces little to no precipitation.  The correlation between IVT and precipitation at Alta simply is not high enough to justify using IVT in isolation for precipitation prediction.  

To that point, we also breakdown the differences between high IVT events that produce an LPE extreme and those that don't produce LPE.  The former typically are colder, feature higher relative humidities, and stronger large-scale ascent.  The latter is sometimes referred to by meteorologists as "forcing" and would be produced by, for example, and upper-level trough and/or surface front.  Essentially, you need an environment that favors precipitation generation.  During high IVT, forced ascent over the central Wasatch, by itself, is not sufficient to do the job if the airmass is not close to saturation and there is a lack of large scale forcing for precipitation.   

Thanks for reading!

Snowfall Extremes at Alta, Part I

Michael Wasserstein and I have a new paper out examining our favorite research topic: Snowfall extremes at Alta. The paper was just published in what is known as early online release and is available at https://journals.ametsoc.org/view/journals/mwre/aop/MWR-D-23-0206.1/MWR-D-23-0206.1.xml. 

It may be paywalled if you don't work at an institution with a license for American Meteorological Society journals or have an American Meteorological Society membership (apologies).  However, we'll hit some of the highlights in this special two part Wasatch Weather Weenies blogstorm, beginning here with Part I.  

The inspiration for this work was a paper by Larry Dunn, a long-time meteorologist and avid skier, that was published as a NOAA Technical Memorandum in 1983. 

Many of the relationships between flow (especially wind direction) and local snowfall enhancement at various locations in the Wasatch Range that are used today are based on this paper.  

Michael and I thought we would do an update, focused on Alta, and using modern atmospheric analyses, diagnostics, and observations. Using data collected and generously provided by the Alta Ski Patrol, we focused on large 12-hour snowfall events, defined in two ways.  The first was based on snowfall amount.  The second based on the liquid precipitation equivalent (LPE) of snowfall.  Large here means in the top 5% (known as the 95th percentile) with at least 2.54 cm (1 inch) of snow for snowfall or 2.54 mm (0.1 inch) of water for LPE. This 95th percentile works out to be 30.5 cm (12 inches) for snowfall amount and 27.9 mm (1.1 inches) of water for LPE.  

Source: Wasserstein and Steenburgh (2024)

Below is a classic figure from Dunn (1983) illustrating the 700-mb (about 10,000 ft) wind speed and direction during heavy precipitation events at Alta. He used 1" of water equivalent in 24 hours for the threshold for heavy events and presented his results in a hand-drawn diagram that meteorologists sometimes refer to as a wind rose.  This figure identified the high frequency of heavy events in northwesterly flow.  Note also that events peak at a speed of around 10 m/s (about 20 knots)

Source: Dunn (1983)

Below is our updated take using our 1.1" in 12-h threshold and enabling comparisons with climatology (all periods during the cool season), all snow events, all LPE events, and all of the extremes.  In figure d, you can see the high frequency of 700-mb flow from the WNW and NW during snow amount extremes peaking at near 10 m/s. If you look carefully though, you'll see a secondary maximum for flow from the SSW.  For LPE, there is are two clear maxima centered on the SSW and WNW.  You will also notice that snow and LPE extremes have occurred across a wide range of flow directions from SE to WNW (or even N for the former).  

Source: Wasserstein and Steenburgh (2024)

A few key points to take from this figure.  First, snowfall extremes can happen for a wide range of flow directions and wind speeds.  This is why I like to emphasize the great diversity of storms that affect Alta and not discount storms just because they don't have NW flow.  If they dynamics are right, Alta can get it.  Nevertheless, a high frequency of large LPE events occurs for flow form the SW and WNW.  This is known as a bimodal distribution.  For snow though, the WNW maximum is much stronger.  This reflects the influence of snow-to-liquid ratio.  The WNW flow storms tend to be colder, and produce more snow per unit of LPE, so that flow direction lights up more frequently for snowfall amount extremes.  

We also identified seven major synoptic storm types contributing to snowfall extremes at Alta.  Four were associated with enhanced integrated vapor transport (IVT) penetrating inland from the Pacific coast from the south (SIVT), southwest (SWIVT), west (WIVT), or northwest (NWIVT).  

Source: Wasserstein and Steenburgh (2024)

IVT is often used to identify atmospheric rivers, but this turns out to be a complex matter that we will talk about in greater depth in Part II.  

The other three patterns were northwesterly post frontal flow events (of course), frontal (associated with a stationary or cold front), and cold-core lows with southwesterly flow.  

Source: Wasserstein and Steenburgh (2024)

We did not distinguish between northwesterly post frontal flow events with and without lake effect as that would have been another major effort.  We'll let you sort through the radar data to do that!

That's enough for now.  We'll dig into this further in a forthcoming Part II post.  

Complicated Week Ahead

I suspect the skiing yesterday was very good.  I had to sit it out due to a balky back.  Hope you were more fortunate. 

The big weather story today is really from SoCal where some big rainfall numbers are coming in from the LA Basin and environs.  Below are 24-hour totals as of this morning with seven sites in the Santa Monica Mountains and Beverly Hills coming in with more than 10 inches of rain.  

The analysis below shows the situation at 0000 UTC 5 Feb (5 PM MST Sunday) with a deep cyclone just off the central California coast and strong integrated vapor transport directed toward the Santa Monica and San Gabriel Mountains.  It was a great recipe for heavy orographic precipitation development.  

Utah will be seeing bits and pieces of that storm as the trough moves through the western US.  As I looked at things this morning, I had no idea how to summarize the next few days.  The system is so broken up over the western interior that it's hard to summarize the timing and intensity of when things will happen with any confidence.  Perhaps the simplest description is to say we will see warm and windy conditions through tomorrow then a gradual transition to cooler weather (but not cold) midweek, with periods of precipitation at times.

The GFS forecast below provides one perspective. Note the forecast temperatures in the mid 20s at Alta-Collins with wet-bulb zero levels between about 6000 and 7000 feet through Tuesday night when temperatures begin to decline.  The GFS produces some precipitation today, late Tuesday, and then late Wednesday and Thursday totaling a bit over 1.2" of water and 17" of snow, which will be fairly high density today and tomorrow before transitioning to lower density on Wednesday.  

I don't have a lot of confidence in the timing and amounts for the bits and pieces as they come through. Timing and intensity are hard to anticipate with confidence, but the periods of snow will add up as the week goes on perhaps to 2 to 3" of water at the end of the work week.  The mean of the downscaled NAEFS is a bit over 3" with most members in the 2-4" range.  

Keep an eye on forecasts and expect updates as this is an evolving situation with a lot of complexity.  

Storm Update

 Precipitation moved into northern Utah last night producing lowland rain and upland snow.  As of 6 AM, the Alta-Collins automated sensor was up to 6" of moderately high density snow with .66" of water (11% water content).  High density at this stage is good to smooth things out and bury the hard underlying surface.  

Radar at 6:46 AM showed widespread precipitation over the Great Salt Lake and much of northwest Utah.  The Cottonwoods and the central Wasatch were seeing some snowshowers, but were in a bit of a lull.  

That lull should, however, be short lived as the trough moves in from the west.  The HRRR, for example, keeps the central Wasatch on the edge of the action through about 1500 UTC (8 AM).  

But then it swings the trough in with widespread precipitation over all of northern Utah by 1800 UTC (11 AM).

So, snow today, possibly heavy at times, especially with the trough passage.  It is fairly mild out there this morning.  It is 32F at Spruces campground, so the freezing level is around 7000-7500 feet (the snow level would be below this), but temperatures will gradually fall today.  

The HRRR-derived forecast guidance for upper Little Cottonwood summarizes the situation quite well.  The precipitation and snowfall time series on the bottom show the morning lull through about 8 or 9 AM, but then higher precipitation and snowfall rates during the day and into the evening, adding an additional 8" of snow or so to what has already fallen.  The wet-bulb zero (upper right), gradually lowers throughout the day reaching about 6500 feet by 2 PM and 5500 feet by 8 PM.  Thus, snow levels will be lowering to bench levels by late afternoon or evening.  Finally, snow-to-liquid ratios gradually increase, although they hang in there around "average" values (~13:1) for this afternoon.  

I like 6-12" more through 8PM tonight at which point we'll have to see if the post-frontal magic can kick in overnight.