Wednesday, December 17, 2014

When No Snow Is a Good Thing

Smog over the Salt Lake Valley this morning as viewed from the Avenues
So far this year, the air quality in the Salt Lake Valley has been far better than the previous two years.  Although it is on the upper east bench and thus is a cleaner observing site compared to those on the valley floor, air quality sensors at our mountain meteorology lab near Red Butte Canyon show that PM2.5 concentrations over the past month have only briefly exceeded 30 ug/m3.

Source: MesoWest
Differences in the severity, frequency, and persistence of poor cool-season air quality episodes over the Salt Lake Valley aren't the result of changes in emissions, but instead meteorology.  Although we've had another dry start to the inversion season this year, we haven't had a big cold intrusion since mid November to establish a cold pool over the valley.  In addition, we've had enough weak systems come through to stir things up from time to time.  But, perhaps most importantly, we haven't had any significant snow cover on the valley floor.  Yes, I admit it.  This is one time when no snow is a good thing.

Compared to a snow-free land surface, snow has two major influences on the exchange of energy between the Earth's surface and the atmosphere.  First, it reflects more sunlight back to space.  Second, it has a very low thermal conductivity, so it effectively insulates the atmosphere from the ground (or alternatively from houses or buildings if you are in a residential or commercial area).  Collectively, these two effects result in stronger and more persistent inversions than would occur during non snow-covered periods.  

When the right conditions exist, we can still get inversions without valley snow cover, even some strong ones.  However, the lack of snow cover has probably helped some with the air quality so far this winter.  

Events and Announcements:

I'll be talking about Secrets of the Greatest Snow on Earth, snow, weather, and who knows what else tomorrow (Thursday Dec 18) on KUER's RadioWest show at 11 am.  Tune in to 90.1 FM or catch the live stream here.  

Tuesday, December 16, 2014

Secrets Gets Around

US Air Force Colonel Steve DeSordi made my day today when he sent me a photo of him reading Secrets of the Greatest Snow on Earth at the Qatari Minister of Defense's beach house on the Persian Gulf.

Monday, December 15, 2014

Looking Back at the World 24-Hour Snowfall Record

The world 24-hour snowfall record is 76 inches at Silver Lake, Colorado on April 14–15, 1921.  I've always been interested in this record for a few reasons.  One is that it's HUGE.  Another is that if I had a time machine, it would be one of the events I would surely target to experience, both to see the crazy snowfall records, but also to see if the record truly "holds water."

Silver Lake is just east of the Continental Divide and due west of Boulder, Colorado, at an elevation of a bit over 10,000 feet (see the P thumbnail below).

As is the case with many old snowfall records, the Silver Lake record has its quirks.  The veracity of the record was first examined in a paper by J. L. H. Paulhus of the U.S. Weather Bureau (now the National Weather Service) that appeared in Monthly Weather Review in 1953.  It turns out that the 24-hour total of 76 inches is an estimate based on the proration of a 27.5 hour accumulation of 87 inches.

Source Paulhus (1953)
The record was further examined by a team of meteorologists assembled by the National Weather Service to evaluate the potential of a new record of 77 inches on the Tug Hill Plateau in 1997 (report available here).  As summarized in their report:
"The Silver Lake observer recorded 6 feet of snow on the ground at the 6:00 p.m.
observation on April 14, of which 5.5 feet were aged winter snowpack already compacted. The next depth of snow-on-ground observation was taken at 6:00 p.m. on the 16th, 19 hours after the snow had ended. Thirteen feet of snow were reported to be on the ground at that time, which settled to a depth of 11 feet by the 19th. This increase in snow depth is exceptionally great and nearly equals the amount of snowfall reported for the 27.5 hour period. This is strong evidence for a truly remarkable snow event. Unfortunately, it is not possible to determine if a representative site for measuring snow depth had been selected by the observer."
In examining surrounding snowfall observations, it is clear that the area observed heavy snowfall during this period.  The team noted, for example, that several feet of snow with large amounts of water fell over the region.  On the other hand, they ultimately concluded:
"The lack of a specific 24-hour measurement of snowfall during the storm, suspicions of a less-than-ideal exposure for the Silver Lake measurements, and a tendency for larger monthly and seasonal snowfall totals for the Silver Lake station in comparison with nearby stations during the 3-year period from June 1920-June 1923, when one particular observer was responsible for Silver Lake observations, all cast some doubt on the published 76-inch, 24-hour total."
Nevertheless, they go on to accept 76 inches as a reasonable estimate of the 24-hour snowfall (in case you are wondering, the 77" total for the Tug Hill was not accepted because it was based on six observations in 24 hours instead of a maximum of four as is generally recommended).

Thanks to modern data assimilation and numerical modeling wizardry, we can now get a look at the synoptic setup for the Silver Lake record.  Shown below are the 850-mb and 500-mb analyses for 1200 UTC 15 April 1921 from the 20th Century Reanlysis (20CR).  It shows a well-known pattern for heavy precipitation along the eastern slopes of the Colorado Rockies (e.g., Poulos et al. 2002) with a vertically stacked cyclone centered over eastern New Mexico, high pressure over the northern plains, and deep geostrophic easterly flow extending from the surface to well above the crest of the Continental Divide.

Source: NOAA/ESRL 
In all likelihood, the Silver Lake event was exceptional.  Whether or not it produced 76 inches remains forever lost in history.  This is the case for nearly all older snow records (including the possibility of 78 inches in 24 hours in Alaska recently reviewed by Christopher Burt).  We really don't know what the 24-hour snowfall record is, although it's probably north of 60 inches and south of 80.  Get that time machine warmed up.

Sunday, December 14, 2014

The Endangered Stellar Dendrite Returns

It was great to see the return today of the unofficial state bird of Utah, the stellar dendrite.

They were plentiful on the landscape.

Looks like a storm total of 11 inches now on the Alta–Collins stake.  However, we need more.

Then again, as Dave Hanscom and Alexis Kelner say in Wasatch Tours Volume 2, true cross-country touring is, after all, a character developing and strengthening form of recreation.

Saturday, December 13, 2014

A Slow Snow Start Across the West

It's been a while since I took a broader look at the state of the snow across the west so I thought I'd have a look today.

Based on snowpack snow-water equivalent, things are fairly grim across much of the west including Colorado, Utah, Nevada, Oregon, and Washington.  The northwest interior fares a bit better, with some mountain basins sitting closer to average.  Relative to average, the healthiest snowpacks are in western Wyoming.  The "beefiest" snowpack relative to climatology is in the Albion Mountains of southeast Idaho.  Pomerelle ski area is there, but even at 145% of average, they are only reporting a 27-33" base.

Source NRCS
Another perspective is provided by automated snow-depth sensors.  If you are looking for 3 feet or greater, look for upright green, blue, or purple triangles (see scale at lower left).  If you look hard, you can find a few of the green ones (37–48 inches) in the North Cascades and Northern Rockies.  There looks to be one in the Park Range of Colorado too.  

Source NRCS
There are no SNOTELs in the southern Sierra, but Mammoth is reporting a 24-36 inch base this morning.

Things are most grim in the Pacific Northwest.  Here's a shot from Snoqualmie Pass this morning.

Near as I can tell, none of the Washington ski areas are open, including Mt. Baker.  Their most recent report is below and is one of the gnarliest I think I've seen.  10 inches of rain, 104 mph winds, and "on the positive side...we haven't lost any ski runs or had any damage any where in the area."  

Meanwhile, we can be grateful for the 4" of snow that fell overnight at Alta and whatever we get from the lingering snow showers today.  

Friday, December 12, 2014

Let's Try This Again!

Yesterday was one of the more frustrating days I can remember as a meteorologist.  Temperatures at the Salt Lake Airport simply refused to budge.  The meteogram below shows how the day went.  We were 55º before 11 am, and still could only top out at 57ºF.  Granted, that's still 19ºF above average, but it fell well short of my expectations.

Source: MesoWest
The development of fairly thick cloud cover didn't help any, but I'm still amazed that the strong flow couldn't scour out the shallow stable layer and lens of cool air that was camped over the Salt Lake Valley.  You can see both the strong flow and the shallow stable layer in the afternoon sounding below.  If we had scoured out that layer, we would have likely had a record high for the day.

Source: SPC
So, let's do this again today.  Once again we have everything in place.  Strong winds aloft (even stronger than yesterday is likely), a warm airmass, and an existing maximum temperature record for the day of 61ºF.  In addition, we're already 50ºF (warmer than yesterday, at least I got that right) and we have a well developed dry slot moving over us to give us at least a few hours of sun.

Therefore, I'm going to stick to my guns and say we will tie or break the record for today and we will see temperatures in the low 60s.  I'm stubborn if anything.

Forecasts for tonight and Saturday continue to suggest a modest snow event in the central Wasatch as the tremendous storm system that has been clobbering California has largely been shredded by the Sierra Nevada (as can be inferred by the image above), leaving us just a few scraps.  The models call for an upper-level front to pass tonight, giving us a burst of mountain snowfall and valley showers, and then a surface-based front to pass during the day tomorrow.  Total snowfall in the central Wasatch will largely depend on how productive these two features can be, especially the latter.  I'll stick with something in the 4-8 inch range for total snowfall through 5 PM Saturday at Alta.

Events and Announcements:

I've joined the dark side and you can now follow me on Twitter: @ProfessorPowder.  Tip of the hat to Backcountry Magazine for the clever handle.

I'll be giving a talk on my new book Secrets of the Greatest Snow on Earth at the Alta Lodge tonight at 5:30 PM, with a book signing to follow.  King's English will be there selling books if you need to pick one up.