Huge Spring Dump, Avalanches Likely

After depriving us of quality powder for most of the winter, Mother Nature turned on the spigot big time last night.  Automated observations from the Alta-Collins observing station (9662 ft) show the total snow depth increasing from around 64 inches yesterday (multi-hour average) to 80 inches as of 7 am this morning.  I'm calling it a 16 inch storm total, although it looks like 14 inches of that fell after 6 PM yesterday.  Total water during this period was a whopping 2.41 inches.  

Radar images from last night show the remarkably juicy northwesterly flow and how Alta was fortuitously located in the narrowing band as the night dragged on.  

Source: NCAR/RAL

Source: NCAR/RAL

I've looked at some radar loops and there may have been some lake augmentation at times, but the large-scale precipitation band was an important component of this storm (note how it extends well upstream of the lake in the images above).  

The Utah Avalanche Center is closed for the season, but they did issue a tweet overnight highlighting that avalanches are likely, including terrain within closed ski areas, which is de facto backcountry right now.  

As much as I was hoping to get out this morning and followup on yesterday's creamy turns, I'm electing to stay home.  Today is the epitome of too much of a good thing. 

Creamy Goodness

More snow at Alta this afternoon.

Sorry kid, you have to earn your turns today.

The reward for hard work was more creamy goodness.

Want some?  Get up early tomorrow as the snow won't last long.  Be careful as the snowpack could be cranky especially if it keeps snowing and if it warms tomorrow.  Further, the snow is high density and any wet sloughs or slabs will pack a much bigger wallop than we Utahn's are used to.

May Freshies Forecast Lookin' Good!

Things are coming together for several inches of fresh today and tonight in the upper-elevations of the Wasatch for those of you who still feel the need to feed the habit (count me in).

As of 7:45 this morning, all was relatively quiet over the Salt Lake Valley and the Wasatch Mountains to the east.  Given the southwesterly pre-frontal flow, an orographic cloud hung over Lone Peak and the downstream central Wasatch Mountains.

However, a glance to the west shows the Oquirrhs shrouded in cloud cover and light snow.  A harbinger of things to come?

Indeed, the regional radar looks optimistic with returns to our west that will slowly spread over the area and into the central Wasatch this morning. 

Source: NCAR/RAL

The NAM calls for valley rain and high elevation snow this afternoon.  Below is the accumulated snow-water equivalent from 3PM to 6 PM MDT.  No yard work today!

And the grid point output for Alta shows about 0.75" of SWE and a bit over 6 inches of snow from late this morning through midnight tonight.  

Thus, things look good for creamy May freshies in the upper elevations late today and first thing tomorrow morning.  It won't be the Greatest Snow on Earth, but who cares.  It's May.  

Best Chances for Good Skiing

It may be deja vu all over again this weekend.  Alta-Collins has had 0.4" of water equivalent in the past 24 hours and will likely see some snow in fits and spurts today as well.  Then we have the potential for snow this weekend.

My approach when it comes to spring storms is to get out during or just following periods of significant snowfall.  If you wait too long, everything gets warm and manky, which not only results in poorer skiing, but also the potential for wet avalanches.

We spoke in the previous post (This NAM Forecast Is a Work of Art) about the dramatic comma-shaped character of the cloud and precipitation pattern for Saturday, as well as the pronounced dry slot.  Today's 1200 UTC NAM shows a similar pattern for 1500 UTC (0900 MDT) Saturday, with much of the area around Salt Lake City in the cyclonically wrapped dry slot, but precipitation just to our south.

Eventually the band of precipitation moves moves into northern Utah, but not until later in the day.

As such, my strategy for tomorrow may be one of patience.  If I get up and it's snowing and it looks like it will continue, I'll probably jump on it.  A bird in hand is worth two in the bush.  If it's not doing anything, I'll probably wait until late Saturday or early Sunday and hope the band comes through (the GFS is less optimistic about that band, but it's also been producing colossally terrible precipitation forecasts since it was upgraded this winter).    Given the convective nature of precipitation processes this time of year and the warm conditions, one needs to weigh the odds, but ultimately flexible.

One thing is for sure, this is likely to be another warm storm.  700 mb temperatures will be between -3 and 0ºC through noon Sunday (then they go even higher...all the more reason to get out early), which means wet snow and best accumulations above 9000 feet.  Total accumulations will probably be limited, in part by the high density.  The NAM is calling for about 0.6 inches of water at Alta-Collins from today at 6 AM through Sunday 6 AM, which translates to 5 inches of snow.  Something in the 4-8 inch range for that period is probably a best guess above 9000 feet.  Less below.

This NAM Forecast Is a Work of Art

We rarely see such a beautiful comma cloud and precipitation pattern over the Intermountain West as forecast by the NAM for 0900 UTC (0300 MDT) Saturday.  Simply spectacular!

0600 UTC 14 May 2015 NAM forecast of 500-mb wind (vectors), temperature (red contours every 1ºC)
3-h accumulated precipitation, and outgoing long-wave radiation valid 0900 UTC 16 May 2015

Patterns like this are associated with well developed upper-level troughs, but often such troughs have cloud and precipitation patterns that are broken up by the topography of the west. That's not the case in the NAM forecast above.

The beautiful comma cloud and hook-shaped precipitation pattern is associated with two major airstreams.  The first is sometimes called the TROWAL (TRough Of Warm air ALoft) airstream.  The TROWAL airstream is associated with warm advection and ascends along a tongue of warm air that wraps round the back side of the upper level low, forming the comma.  

The second airstream descends on the back side of the upper-level trough, wraps around the forward side of the upper-level trough, and forms the dry slot.  At upper-levels, this airstream is typically dry because it has descended from the upper-troposphere.  The interlacing of these two three-dimensional airstreams leads to the beautiful comma cloud.

The term dry slot can give the false impression that this is an area of benign weather.  Au contraire in some situations.  Although the dry slot may be cloud free and feature low relative humidity air at upper-levels, at low levels, the air beneath the dryslot can vapor laden.  The airmass beneath the dry slot often features large Convective Available Potential Energy (CAPE), which is jacked up further by solar heating during the day if the dry slot is cloud free.  Being ahead of the upper-level trough, the forward portion of the dry slot is also an area of rising motion.  As a result, convection frequently breaks out within or at the leading edge of the dry slot.  The vertical wind shear in the dry slot region is also sometimes favorable for the development of severe convection.  Even the low-resolution NAM is generating some dry-slot convection in the forecast above.  

Get High If You Want to Ski

Although Utah is home to the Greatest Snow on Earth during winter (except for perhaps this year), once spring rolls around, I'm more inclined to think about skiing elsewhere.  There are big lines and beautiful views to be had around the west, but this year, you might have to work a bit harder than usual to find some that are adequately snow covered.

Below is the basin-wide percent of average snowpack analysis for the western U.S. from the NRCS.  These numbers are simply heinous for most of the west.  Only portions of Colorado and SE Wyoming are near average (the high numbers in SE Utah are either spurious or the result of what we call the "statistics of small numbers" — there's very little snow there).

In the Sierra and Cascades, the snowpack is non-existent or <10% of average in most basins.  One plus is that with no low-elevation snow, access to higher elevation tours is much easier than it typically is in May.  For example, the road to the South Climb Trailhead on Mt. Adams is melted out.  The climber's report suggests you'll find snow starting at 6100 feet.

Averages can be deceiving.  Let's instead look at the actual water equivalent of the snowpack.  No matter where you are, given the warmth of this past winter, if you want to find snow, you need to get high.  Specifically, you need to look to the upper elevations of the North Cascades, Montana, Wyoming, and Colorado.  The Cascade Volcanoes will probably work too, although those aren't sampled by SNOTEL.

Source: NRCS

The highest SWE in the west is at Easy Pass (5270 feet) in the North Cascades, which currently sits at 45 inches. Moving east, Noisy Basin (6040 ft) and Flattop Mountain (6300 ft) near or in Glacier National Park sit at 33.7 and 31.1 inches, respectively.  Moving south Grand Targhee on the west slope of the Tetons (9260 ft) is at 31.7 inches and the MedBow Snotel in the Medicine Bow Range sits at 32.4 inches.

In Colorado, the snowpack SWEs are not as high as found in other areas, but they have a healthy snowpack near or above average at upper elevations along the Continental Divide from Independence Pass to Niwot Ridge.  The plot below shows only only SNOTELs above 10,000 ft, with green signifying 90-110% of average, light blue 110-125% of average, medium blue 125-150% of average, and dark blue 150-175% of average.

Source: CBRFC

The Grizzly Peak Snotel at Arapohoe Basin didn't quite crest at average, but the cool and intermittently snowy weather in the recent past puts it a bit above average at present.

So, all is not lost.  There's not a lot of snow to be had in Utah.  Upper elevations of the Cottonwoods are about your best bet.  Road tripping and getting high will get you turns elsewhere.  In some instances, access may actually be easier than it typically is this time of year.