Thursday, December 8, 2016

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.

Wednesday, December 7, 2016

A Primer on Atmospheric Rivers

Since the first atmospheric river event of the season will be affecting northern Utah later this week, it seems appropriate that we spend a few minutes today discussing just what the hell an atmospheric river is, how they penetrate into the western U.S. interior, and what sorts of impacts that they have on snow and skiing in the Wasatch Range.

What is an atmospheric river?

Atmospheric rivers are narrow corridors of strong vertically integrated water vapor transport.   Vertically integrated means that we are interested in the total water vapor transport through the troposphere, the lower weather producing portion of the atmosphere that extends to about 15 km above sea level (give or take).  The phrase was first coined by MIT scientists Reginald Newell and Yong Zhu in a 1994 research paper, although they used the term tropospheric river for the same phenomenon in an earlier paper.  Their major contribution was recognizing the importance of these features for understanding the global water budget, the export of moisture from the tropics to the mid latitudes, and the generation of precipitation in the mid and high latitudes (their 1994 paper was particularly interested in understanding Greenland ice-core records).

Prior to the 1990s, atmospheric rivers were known by various names.  For example, western US meteorologists used the term Pineapple Express to describe moisture plumes originating from near Hawaii.  There's even an IPA brewed by Drought Works in Missoula, MT with the name.

Most atmospheric rivers originate in from the tropics or subtropics and are often found ahead of cold or occluded fronts associated with midlatitude cyclones.  Their high water vapor content reflects their origin in the tropics, but also the convergence of mid-latitude moisture, which can offset water vapor losses to precipitation.

Ideally, atmospheric rivers are identified using a diagnostic quantity known as integrated vapor transport, or IVT, since it is the transport of water vapor that is most important and also correlates best with mountain precipitation.  In practice, integrated water vapor, or IWV, is often used since it is more readily measured by some satellites and a common field in model output grids.  We have found IVT (figure b below) to be a better diagnostic than IWV (figure a below) for tracing atmospheric rivers inland over the western US.  It also has a stronger correlation with precipitation, especially in the mountains.

Source: Rutz et al. (2014)
Strong objections to the term atmospheric river have been made by some scientists, in part because their behavior is so different from water rivers.   I have mixed views, but feel the term is here to stay and think it has drawn the attention of the public in a way that would not have been possible without such a catchy phrase (although this has downsides too).

Inland Penetration

Atmospheric rivers typically weaken as they move inland into the interior of the western United States.  Precipitation, especially over mountain barriers like the Cascades and the Sierra Nevada, removes water vapor from the river, weakening the integrated vapor transport.  As a result, inland penetration more likely if the river is strong when it makes landfall and if it traverses areas with lower topography.

The high Sierra Nevada south of Lake Tahoe is an atmospheric river graveyard as it is the highest barrier in the Pacific states.  If one looks at low-level trajectories launched within atmospheric rivers when they make landfall, those that make it into the interior within an atmospheric river nearly always circumscribe or avoid the southern high Sierra.  Atmospheric river conditions are extremely rare downstream of that high barrier, especially over central Nevada and western Utah.  Northern Utah and the Wasatch Range are fortunate to be downstream just enough to benefit from river that circumscribe the range.

Source: Rutz et al. (2015)
Thus, the preferred pathways for atmospheric river penetration into northern Utah are: (1) a westerly, west-northwesterly, or weakly clockwise curving trajectory crossing the lower terrain of the Sierra Nevada and Cascades of northern California and Oregon or (2) a southwesterly or weakly counter-clockwise turning trajectory across Southern California or the Baja Peninsula and following the lower Colorado River Valley.

Impacts on snow and skiing in Utah

On average, bonafide atmospheric river conditions exist over northern Utah on only a few days each cool season (October to March), although atmospheric river remnants certainly contribute to precipitation more frequently than that.  Not all atmospheric river events are huge snow producers.  Much depends on the flow, instability, large-scale characteristics, and duration of the event.  However, some produce prolific amounts of snow and water equivalent.  Others can feature high snow levels (bummer!).   I tend to look forward to major, long-lived atmospheric river events as they are nearly always noteworthy, often provide memorable skiing (sometimes good, sometimes challenging), and sometimes memorable avalanche cycles.  For example, in my book Secrets of the Greatest Snow on Earth, I discuss the 17–23 December, 2010 atmospheric river event that produced 75 inches of snow at Sundance Ski Area, snow levels that reached as high as 7500 feet, and massive avalanches on Mount Timpanogos.

The 17–23 December 2010 atmospheric river event.  Photos courtesy Bill Nalli.
Later this week

Forecasts from the Global Ensemble Forecast System (mean IVT top figure below) and GFS (bottom figure) suggest we will see atmospheric-river or near-atmospheric-river conditions pushing into northern Utah late Thursday and persisting through Saturday morning.  Meteorologists use a threshold (red line below) for identifying atmospheric rivers (red line below) and the forecast for 0000 UTC Saturday (5 PM MST Friday) shows how the atmospheric river is able to penetrate inland across the lower northern Sierra Nevada and Southern Cascades.  Note also how the water vapor transport declines rapidly across the southern High Sierra.

Water totals being spit out by the models and ensembles range from 1 to 3 or more inches through Saturday, depending on location.  If time permits, I hope to take a closer look at the forecasts tomorrow.  

Addendum @ 9:20 AM

I forgot to include the all important summary figure showing the preferred regimes and pathways for atmospheric river penetration into the western U.S. interior.  Much thanks to U alum Jon Rutz for leading this work.

Source: Rutz et al. (2015)

Tuesday, December 6, 2016

Snow Prospects in the Short, Medium, and Extended Forecast

Plenty to discuss today as there's all sorts of action in the short-, medium-, and extended-range forecast periods.


When it comes to snow, sometimes the most difficult forecasts are for light accumulations.  

The 1300 UTC initialized HRRR simulated reflectivity forecast below shows the challenges for today.  A relatively weak weather system over Oregon and Idaho is forecast to strengthen some as it moves into the Wasatch Front later today.  

For Salt Lake City, most ensemble members produce less than 0.1" of water equivalent through 0200 UTC (7 PM MST), although there are a couple that to for around 0.15".  
NCAR Ensemble forecast for the Salt Lake City International Airport

Thus, this could be everything from a few snow showers to an inch or two of snow.  On a meteorological scale of 0 to 10, that's about a 1, but it's coming in around rush hour.  If precipitation is light and intermittent, it probably won't be a big deal, but a brief burst of heavier precipitation, while a lower probability possibility, could cause some snafus.  Best to keep an eye on this as the day evolves.

A couple of members go for some significant snow overnight due to lake effect.  As usual, that's a tough forecast, so consider yourself fortunate if you get some.  

Best guess is perhaps 2-4" of cold smoke in the Cottonwoods later this afternoon and tonight.  Emphasis on guess as the models have been erratic and the lake is always a dicey proposition.  These amounts are a little lower than one might infer from the NCAR ensemble or overnight NCEP model runs (not shown), but reflect the fact that the system seems pretty weak right now.  Thus, hope we do better.  

Medium Range

We've already discussed the big cool down for tomorrow (Wednesday) and how it will be short lived.  The models bring in a pretty juicy pattern beginning late Thursday through Saturday night.  The NAEFS plume for Alta shows the weak system passing through later today (~0000 UTC 7 December), a break for tomorrow and early Thursday, and then the action picking up.  By 1200 UTC 11 December (5 am MST Sunday), our downscaled accumulation estimates fringe from about in inch of water equivalent to just over 4 inches of water equivalent.  You can further excite yourself by looking at the snowfall plume.  

NAEFS forecast for Alta
   Personally, I think it's too early to be talking about accumulations, but the pattern does suggest we will be adding to our base at upper elevations.  There will, however, be some yo-yoing of the snow levels, which could reach or even exceed 7000 feet Friday night.  

Extended Range

Most ensemble members suggest an active, stormy pattern through next week.  The Utah Avalanche Center and snow-safety teams at the resorts and along the highways will have their work cut out for him if the advertised pattern verifies.  I've already said more than I should as I put little faith in 5-10 day forecasts, but lets keep our fingers crossed. 

Monday, December 5, 2016

Dramatic Swings in Weather This Week

If the computer models hold, we are in for quite a roller coaster this week as far as weather is concerned, especially in the mountains.

First, we have two troughs moving through the area, one that went through last night, the other scheduled for tomorrow (Tuesday).  Temperatures will really bottom out in the wake of the 2nd trough and by Wednesday we are looking at 700-mb (10,000 ft) temperatures near -18ºC over the Wasatch Front.  That means temperatures below zero fahrenheit above 10,000 ft.

GFS 700-mb temperature forecast valid 1800 UTC (11 AM Wednesday MST) 7 December
If you look at the forecast above, however, you will also see that it is quite warm over the north Pacific off the coast of California, where 700-mb temperatures are above 0ºC.

If we look at the precipitable water forecast for that time, we also see a tongue of vapor-rich air just upstream of the California coast.  

GFS precipitable water forecast valid 1800 UTC (11 AM Wednesday MST) 7 December
That airmass pushes inland very rapidly, and by 0000 UTC (5 PM Thursday MST) we've rebounded to -5ºC.

GFS 700-mb temperature forecast valid 0000 UTC (5 PM Thursday MST) 9 December
So, our flirtation with brutally cold air should be brief and it appears we'll be in for milder weather later in the week through the weekend.

How much moisture will get into northern Utah will depend, however, on the inland track of the atmospheric river and the impacts of the Sierra Nevada.  A direct track across the southern high Sierra is usually bad for moisture transport into the Intermountain West due to the blocking of low-level moisture and the loss of water vapor on the western (windward) side of the range.  As is often the case, and suggested in the precipitable water and precipitation forecast for Thursday afternoon (below), there is a major decrease in precipitable water across the Sierra due to these effects.

GFS precipitable water forecast valid 0000 UTC (5 PM Thursday MST) 9 December
The NAEFS plume diagram for Alta is, however, fairly optimistic for some base-building water-laden storms late Thursday through the weekend.

That would be good news for the upper elevations of the Wasatch, but let's take a closer look in a couple of days as this is still too far out for me to get excited about details.

Sunday, December 4, 2016

Damn Cold by Wednesday

Although some snow is on tap for tonight and tomorrow, the big story this week is going to be the cold.

The coldest air of the season is scheduled to push into Utah mid week and by 1800 UTC (11 AM MST) Wednesday, we are looking at 700-mb temperatures near -18C at 700 mb (10000 ft).

That means ridge-top temperatures below zero fahrenheit.

Get the rum and hot chocolate ready!

Friday, December 2, 2016

Shameless, Self-Serving, Christmas Gift Recommendation

Like this blog?  Be sure to pick up my book Secrets of the Greatest Snow on Earth.

It's an ideal Christmas gift for lovers of Utah snow and skiing, or even skiers in other regions (yes, I cover that).  Available online from Amazon and Barnes and Noble, and local booksellers Weller Book Works, Kings English, and Dolly's Bookstore (call ahead for availability).