Thursday, December 18, 2014

Things Are Brewing in the Pacific

Over the past couple of days, the medium-range forecast models have been hinting at the potential for an inland penetrating atmospheric river (AR) and a significant precipitation event for the mountains of northern Utah beginning on Sunday.

We begin yesterday in the western Pacific where strong moisture convergence along a cold front east of Japan contributed to the formation of the AR, which was characterized by high integrated water vapor values (contours, with warmer colors indicating higher values).

Those of you longing for powder might take a gander at the satellite image above and see numerous cloud bands extending from the coast of Asia to Japan where it has been puking powder for the better part of the past week or two.  My facebook feed from Hakuba has simultaneously invoked joy and jealousy as it's been day after day of photos like the ones below.

But I digress.  The loop below shows the observed and forecast IR satellite image and integrated water vapor through 1200 UTC 21 December.  Note how the AR moves across the Pacific and then becomes directed toward the Pacific Northwest.

Although the images above present integrated water vapor, a better diagnostic for tracking ARs into the western interior is integrated water vapor transport, which measures the flow of water vapor over a given location and correlates better with mountain precipitation during the cool season.  The mean forecast of integrated water vapor transport by all members of the Global Ensemble Forecast System (GEFS, top panel) and by the GFS (bottom panel) for 1200 UTC Monday 22 December show the AR spilling over the long-wave ridge centered off the coast of California and then extending east southeastward across southern Idaho, northern Utah and western Wyoming, and Western Colorado.

Source: NWS
This is an ideal track for moisture penetration into the interior western United States because the AR moves across the lower, narrower Cascade Mountains of Oregon and over Snake River Plain.  Thus, the loss of water vapor to mountain precipitation is smaller than when an AR intersects higher and wider mountain barriers like the southern Sierra Nevada.

The GFS accumulated precipitation forecast through 1200 UTC (0600 MST) Tuesday shows precipitation maxima over the Wasatch and mountains of western Wyoming, as well as the western Colorado Rockies.

Source: NWS
Over the Wasatch, most of this precipitation falls beginning early Sunday.  As a result, I need to point out that we're looking at a 72-126 hour forecast and thus there are important issues to consider with regards to forecast confidence.  First, much depends on the position of the AR.  A shift to the north would be bad.  A shift to the south would be good (for central Wasatch snow).

As a result, when we look at an ensemble of our experimental downscaled forecasts from the North American Ensemble Forecast System for Alta-Collins we see some that produce a great deal of precipitation (direct AR hit) and others that are fairly dry (miss to the north).  We think that our downscaling is overforecasting, so don't take the absolute values too literally.   They key point is that this has the potential to be a significant event, but there remains some uncertainty.

The second concern is the temperatures.  This will be a warm event.  The 700-mb temperatures on Sunday and Monday are currently forecast to be between -2ºC and 0ºC.   That's very warm and it will likely yield wet, high-density snow.  That's good for the upper elevations where we need a good pasting for base, but this could be another lower elevation rain event.  It's too soon to speculate on exactly where snow levels will be.

So, the bottom line is that we have some potential for a significant mountain precipitation event beginning on Sunday.  The gory details depend on how things come together the next few days.  Keep your fingers crossed and stay tuned to the forecast.

Events and Announcements:

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

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.