Update on the Approaching Storm

Strong warm fronts of the type we are experiencing today are less common in Utah than they are in the eastern United States.  Today in the Salt Lake Valley reminded me a good deal of the type of weather I experienced in my youth in upstate New York.  Damp, grey, and somewhat raw.

Meanwhile, friends have reported a fun graupel-filled day in the mountains.  As of 4 PM this afternoon, the water equivalent at Alta-Collins since midnight is up to 0.38" and Snowbasin Middle Bowl 0.78".  Warm fronts often feature high stability and that is indeed the case this afternoon with a temperature of 36ºF at the Salt Lake City Airport, 36ºF at the Big Cottonwood S-curves, 34ºF at Spruces, and 31ºF at the base of Solitude.

There's very little change in the forecast for the big event tomorrow night and Friday.  As shown in the 1800 UTC NAM forecast below, temperatures and snow levels (typically about 1000 feet below the wet-bulb zero level at upper right) are expected to remain quite high through Thursday night, lower gradually late Thursday night and early Friday, and then drop more rapidly with the approach and passage of the front Friday afternoon.  At their highest, snow levels will probably peak at about 8000 feet in the central Wasatch early Thursday night.  Juicy atmospheric river conditions with bring high precipitation rates through Friday morning, afterwhich a strong cold front will keep things going Friday afternoon .  Water totals being produced by the NAMfrom Thursday afternoon through Friday evening remain impressive and exceed 2 inches at the Alta-Collins grid point.

Thus, this looks like another good base builder for the upper elevations.  The mid elevations are going to see some rain, but should recover with the frontal passage.  Lower temperatures should yield a transition to drier snow Friday afternoon.

And speaking of lower temperatures, Saturday looks brutally cold with a 700-mb temperature of about -20ºC (even colder in the NAM), which is about as cold as it gets around here.

A Stronger, Juicier, and Warmer Atmospheric River

I've been busy writing a proposal and not paying attention to the weather today, so when I finally got around to looking at the maps, the first word that came to mind was wow.

In particular, what caught my attention was the forecast for Thursday night and Friday.  Yes, there is a warm front moving through tomorrow, but that's small potatoes compared to the event being advertised for Thursday night and Friday.

The 1200 UTC GFS forecast shows an extremely broad atmospheric river extending into the Intermountain West at 0600 UTC Friday (11 PM Thursday) with very large values of integrated water vapor transport (over 400 kg/m/s) sneaking around the southern and northern ends of the Sierra Nevada and high values even downstream of the southern high Sierra.

Those values are quite a bit higher than observed during the previous atmospheric river event and are unusually high for December.

Compared to the previous atmospheric river event, the forecast event also features higher peak temperatures, larger 24-hour precipitation totals at Alta (and probably other sites), a stronger upper-level trough, and flow trajectories from the southwest.

As a result, the 12-km NAM, which is typically a relatively conservative forecast model, is pumping out some big numbers with 2.82 inches at the Alta grid point from 5 PM Thursday to 5 PM Friday (with a bit more thereafter).  That is a very robust 24-hour precipitation total for Alta.

Unfortunately, this looks to be a very warm storm until cooler air moves in on Friday.  From 5 PM Thursday to 5 PM Friday, NAM forecast 700-mb temperatures are between -3.3 and -0.6 C, with the highest temperatures Thursday from 8 to 11 PM.  Although one can't pin point snow levels at such lead times, these numbers are consistent with snow levels reaching about 8000 feet during that period.  Basically, this period of the storm looks like one where the rich (i.e., high elevations) will get richer (i.e., more snow) and the poor (lower elevations) will get poorer (snow loss) until temperatures and snow levels fall on Friday.

Of course, it can be wise to temper one's meteorological excitement with a does of ensemble reality.  The NAM is relatively wet (in the upper third) of precipitation forecasts for Alta produced by


All of that is impressive, but compared to the various members of the downscaled short-range ensemble forecast system (SREF), the NAM is probably in the upper 20% of precip forecasts for that period and this is still a fairly long-lead forecast.  Nevertheless, it does appear atmospheric river #2 is on its way and it will be bringing low-elevation rain and high-elevation Cascade Concrete before temperatures and snow levels fall on Friday.

Sitting Right at Average

Observations from NRCS SNOTEL stations shows the snowpack at upper-elevations in the central Wasatch sits pretty much right at average, whereas some sites in the northern Wasatch are doing just a bit better than that.

Source: NRCS

As of today, the snowpack water equivalent at Snowbird sits at 8.2 inches and Mill-D North at 6.2 inches.  The big winner so far is Ben Lomond Peak in the northern Wasatch with a snowpack water equivalent of 9.5 inches.  I always say that Ben Lomond is "pound for pound" the snowiest place in the Wasatch Range.  For a given elevation, the snowpack there is usually deeper than anywhere else around here.

Count your blessings and lets hope we get some more.  Warm, sloppy storminess is being advertised for later this week by the GFS.  We'll see how things come together and take a look in a couple of days.

Storm Update

Alta looking like Alta should

Today provided my favorite conditions for lift served skiing.  Heavy snow and wind that starts in the morning, enabling great skiing, especially in wind buffed areas, in the afternoon.  I know it might sound heretical to rank that ahead of a blue-bird powder day, but those days have simply gotten too crowded at the resorts.  I'd rather go ski touring.  

Today was quite warm.  Snow levels at about 4:30 PM were just below the base of Snowbird.  It was so strange to drive down the canyon in the rain in December.

Owing to the warmth, the snow today consisted almost entirely of graupel, ice pellets, and heavily rimed crystals.  At times, if your face was exposed, it felt like a bad shave.  If you look at some of the crystals below, you can see evidence of a hexagonal shape, but they are so heavily rimed that the original snowflake is impossible to discern.  

Snowfall totals are essentially meaningless in a storm like this.  From Thursday afternoon to Friday morning, Alta-Collins recorded only 7 inches of snow, but that snow contained 0.89" of water.  Then, from early this morning until 5 PM, Alta-Collins recorded only 5 inches of snow, but that snow contained 0.78" of water.  We're talking high-density base builder, which is just what we need.

The water equivalent so far for this atmospheric river event is thus up to 1.67" of water.  As I type this, the radar looks BEAUTIFUL.  Solid echoes upstream.  

Source: RAL

With several hours of snowfall to come, we will probably be adding another 0.5 to 0.75 inches of water to the upper elevation snowpack before morning.  Snow levels will also be lowering (thankfully).  

Good Overnight Returns, Break Today, Then More

Although it's only 7 inches of snow, we got good returns from yesterday afternoon through 7 am this morning in terms of water equivalent with 0.82" falling at Alta-Collins.  That will make for creamy turns and good base building, and base is what we need.  Consistent with the high-density snow, temperatures at 7 am were mild, including 40ºF in downtown Salt Lake City, 30ºF at the base of Park City, 32ºF at The Spruces, and 22ºF at the Top of Collins.

Today will remain mild and, although we will see some snow at times, I expect a break will develop during the day as the HRRR and NAM call for the precipitation shield to shift northward.  By later (0000 UTC/5 PM MST) today, most of the action is expected to be to our north.

That break could extend through the night before the main course arrives on Saturday when a strong frontal system pushes through northern Utah to put an exclamation point on this atmospheric river event.

The NAM generates an impressive total of 1.79" of water at Alta-Collins with the frontal system from 5 AM Saturday until 5 AM Sunday, which would be a wonderful add to our current totals.

These Are the Times That Try a Meteorologist's Soul

There can be little doubt that meteorologists live for patterns like the one we are about to experience, but that doesn't make weather prediction any easier.  Yes, these are the times that try a meteorologist's soul.  If anyone can send me a sixer of Pineapple Express for the weekend, do it immediately!

Here's how I'm going to get through the next few days.

Step 1: Take a Deep Breath and Channel Mr. Spock

I think the term atmospheric river is a useful one, but it incites hype and panic that may or may not be justified.

In addition, some of the models, as we will discuss below, are prone to put out huge numbers that could, but are not very likely to verify.  Channeling Mr. Spock and thinking logically is essential.  This is a significant event, but as is always the case, there is a range of possible outcomes.

Step 2: View the GFS Precipitation with Skepticism

A couple of years ago, when they upgraded the GFS to 13-km grid spacing, it started putting out HUGE water-equivalent numbers for Alta.  The basic problem is this.  The GFS has a very simply "parameterization" (i.e., technique) for simulating cloud processes and it is far too efficient over high mountain areas of the western U.S., leading to excessive precipitation over high-elevation areas (despite the fact that the model terrain is relatively smooth).  An example is shown below.

Now you might say it is supposed to precipitation more in the mountains and that is generally true (especially in a pattern like this).  The problem is it simply precipitates too much and most of the time the GFS produces far too much precipitation in most areas.  Ditto for the NAM 4-km.  It's not that it's impossible that we get the totals advertised by the GFS, but it is on the outer limits of what might happen.  Only hypesters grab the GFS precip and run with it.

Step 3: Embrace Uncertainty

The models are in reasonable agreement on what will happen through tonight with the passage of a warm front and mid-level trough across the state, as depicted below.

After that, the differences are subtle, but very important for orographic precipitation.  Check out, for example, the plume diagram from the NCAR ensemble at Alta.  Fairly tight agreement through about 04Z when the warm front pushes through, after which the precipitation forecasts diverge rapidly with a wide range thereafter.

Much simply depends on the position of the moisture plume, the strength and timing of embedded disturbances, and the local flow direction.

The 0600 UTC GFS, for example, keeps us in predominantly zonal (westerly) flow with precipitation persisting late tomorrow and tomorrow evening in the Cottonwoods, whereas the 0600 UTC NAM shunts the precipitation northward and gives the Cottonwoods a break.  The two models also differ in subtle but important ways over the weekend.

Step 4: Putting It All Together

We are looking at an active period for the Wasatch Range beginning late today.  There will probably be breaks in the action and variations in precipitation intensity that are difficult to anticipate at this juncture.  My best guess for Alta would be a total water equivalent through 11 PM Saturday of 1.5 to 3 inches.  This brackets the 2" produced by the NAM, but is skewed slightly to higher values.  One can certainly find models and ensemble members going bigger than this, and I would not be surprised if that happened, but I tend to be conservative and consider that to be a lower probability possibility.

Snow levels will rise today and tonight and could flirt with the base of Park City Mountain Resort tomorrow and tomorrow night before falling late Friday night.  The higher temperatures and wind will mean higher water content snow through Friday night before we see drier snow on Saturday.