Thursday, January 29, 2015

The Snow Potential (or Lake Thereof) from Brush By Storms

If the forecast from the 0600 UTC GFS verifies, the central Wasatch will remain sandwiched between storms to our south or storms to our north over the next week, getting just what we can from brush-by precipitation events.  

First we have the system moving into the southwest today and giving them precipitation through Saturday.  We are expected to be on the northern edge of this system.  

Once that system moves downstream, the we get brushed by systems to our north.  Again, we're right on the edge of the action.  

In situations like this, a slight shift in storm position can make a difference and I like to consult forecast ensembles to get an idea of range of possibilities.  Let's start with the Short Range Ensemble Forecast System (SREF), from which we can examine the potential for precipitation as the system affecting the southwest brushes by us the next couple of days.  There are 22 members of this ensemble, of which only 9 produce any precipitation at all at the Salt Lake City grid point, with two generating over 0.15".  

A forecast like that suggests we may see a few valley showers and mountain snow showers tonight through early Saturday morning, but accumulations will probably be minimal.  The odds of a few inches during this period in the upper Cottonwoods aren't zero, but they're pretty low (less than 10%).

For the next week, we can look at our ensemble of downscaled forecasts from the North American Ensemble Forecast System (NAEFS) for the 7-day period beginning yesterday afternoon and running through next Wednesday afternoon.  As can be seen below, the probability of at least 6 inches of total snow accumulation in this period is fairly low, maxing out in the central Wasatch at about 60 or 70%.  The odds of more than 24 inches isn't zero, but it's less than 10%.  We would need one of those systems to shift southward for that to happen.  

Here's another way to look at it.  Below we've extracted the accumulated water equivalent (top) and snowfall (bottom) from the downscaled NAEFS ensemble members for Alta.  The vast majority of the members are producing 8 inches or less of snow for the entire period.  There is one member, from the Canadian Ensemble, that is very excited about a huge storm late in the period (this always seems to be the case with the Canadian Ensemble.  The Canadians clearly love snow!).    

So, looking at all these ensembles, the most likely forecast scenario for the next week (i.e., through Wednesday) in the central Wasatch is that we continue to see drier than average weather, with just a few periods of snow showers or snow as we are brushed by systems to the south or north.  Although not zero, the odds of a major storm are low and would require a more direct hit than forecast by most of the ensemble members.  

REI Talk Tonight

I'll be giving a talk on my book Secrets of the Greatest Snow on Earth, including a look at topics such as the real reasons why Utah snow is so great, where to find deep powder around the world, and the avalanche history of Little Cottonwood Canyon, at 7 PM tonight (Thursday) at the Salt Lake City REI (3285 E 3300 S).  The talk is free and open to the public, although REI does request that you register by clicking here.  My friends from Weller Book Works will be selling copies of the book if you haven't picked one up yet.  I'll be happy to sign any copies you bring or buy at the event.  

Wednesday, January 28, 2015

Time to Head South

The highest elevations of Arizona, New Mexico, southern Utah, and southern Colorado will be getting some late Thursday through Saturday thanks to moisture streaming into the region from the south and east as shown in the integrated water vapor and 850-mb wind forecast for 0600 UTC 30 Jan (late Thursday).

This leads to the development of wide-spread precipitation across the region. as illustrated by the forecast below for 1800 UTC (1100 MST) Friday 30 January.

The northern Utah mountains are skunked in the forecast for that time and that is largely the case through the weekend, although we may see a few lingering snow showers today and then perhaps a stray snow shower or two through Friday.  Although a few inches fell in the northern Wasatch last night (the central Wasatch go the shaft), it appears we will make it through January with only one major storm.  What a pity.

Those of you considering a southern road trip should keep an eye on the forecasts.  It's a complex system and the timing and intensity of snowfall will vary by location and I can't possibly hope to summarize that here!

Tuesday, January 27, 2015

An Impressive Record Aloft

Yesterday afternoon's sounding from the Salt Lake City International Airport recorded a 700-mb (~10,000 ft) temperature of 7.8ºC.  This is the highest 700 mb temperature ever recorded between December 17 and March 21 and in line with what we typically observe in mid June.  I've annotated the observation on the sounding climatology below to show how anomalous the temperature is for January.  The red trace represents the highest 700-mb temperatures at each of the twice-daily sounding times during the year.

Source: SPC
Perhaps not surprisingly, Alta set a record high for the day of 58ºF.  This beat their previous record of 43ºF for the date by 15ºF!

Source: NWS
That's a huge trouncing, but the previous 43ºF record for yesterday was the lowest maximum temperature record during January (tied with Jan 3rd).  The record for the entire month of January remains 59ªF, set on January 12, 1996 and January 20, 2005 (the latter noted above).  Thus, the max temperature at Alta yesterday was pretty damn impressive, but not an all-time record breaker for the month.  

Valley showers and mountain snowshowers are on tap for today and tonight and possibly a bit tomorrow.  This looks to be a sporadic, hit and miss event and the models continue to flounder around on accumulations, but continue to call for a modest event.  Through late tomorrow, the 6Z NAM is going for about 3 inches at Alta, whereas the 6Z GFS is in there for 7.5 inches, with most of this falling this afternoon and tonight.  I'll stick with 3–6 inches for a storm total as that seems to be in the heart of the distribution and I'm not sure how much to weight the new high-res GFS for mountain precipitation.  

Monday, January 26, 2015

Welcome to Bizzaro world.

Consider yourself transported to a Bizzaro world, one in which the upper-level flow is from the east and you can ski in short sleeves in January.

Here's the view of the Bizzaro world this morning.  The upper-level flow is what we call highly amplified, with a ridge over western North America and a deep trough over the eastern United States (Northeast skiers rejoice, dumpage is coming your way!).  There is also a closed upper-level low off of Baja California that will provide us with more Bizzaro weather in the near future.

Here's this morning's sounding from the Salt Lake City airport.  Note the deep easterly flow from 800–550 mb.  It will be an unusually short flight if you are coming to Salt Lake City from Chicago today.  Temperatures increase from 0ºC at the surface to 7.2ºC at 775 mb.  At 700 mb, roughly 10,000 feet, the free atmosphere temperature was 5.2ºC.

Source: University of Wyoming
That's quite steamy for late January.  The graph below shows the daily minimum (blue line), median (black line), and maximum (red line) 700-mb temperature in all available soundings taken at the Salt Lake City or Ogden airports since 1948.  I've annotated with the green line this morning's 700-mb temperature.  The record for the month of January is 7.2ºC (12Z Jan 20).  However, from January 21 to 10 March the highest on record is 5.8ºC (Feb 1).  So, we are very close to as hot as it gets at 700-mb for this time of year.  In fact, this morning's 700-mb temperature is about the median for June 1st.  If we had the June sun rather than the January sun, I'd be calling for a high at the airport of 73ºF.

Source: SPC
That closed low will move northward and bring a monsoon-like surge of moisture into the southwest that will spread into Utah tonight and tomorrow.  That sounds exciting, but we're missing the surface heating of summer and the models can't seem to get their act together and converge on a solution.  The 12Z NAM is going for only 0.11" of snow-water equivalent through 11 PM Wednesday, whereas the 6Z GFS is going for 0.8" (the 12Z run isn't available as I write this).  Your guess is as good as mine.  Hold a gun to my head and I'd go for 3-6" in upper Little Cottonwood by 11 PM Wed.

Sunday, January 25, 2015

Snow, Warmth, the "Monsoon Surge", and the Storm Killing Rex Block

Here are your four weekend updates from the Wasatch Range.

1. State of the Snow

Much of the snow in the Wasatch has been affected by wind, sun, or humans, but incredibly there is some good settled powder to be had in sheltered areas on the north side of the compass despite it being almost 2 weeks since the last storm.

This time of year you can really see the power of the sun as the south aspects are baking and in some cases have lost their snow, whereas you can find powder on the north aspects.

2. Warmth

I noted a couple of days ago about the building ridge and the potential for very warm temperatures at 700-mb (10,000 feet) today (Sunday) and Monday.  Well, the great warmup is underway and the GFS forecast 700-mb temperature is 3.5ºC for 5 PM this afternoon and 5.8ºC for 2 PM Monday afternoon. You can kiss more of that south aspect snow goodbye.

3. The January "Monsoon Surge"

Tropical moisture will be streaming into northern Utah and giving us some precipitation Tuesday and Wednesday.  It's an odd pattern for January and the model forecasts still lack consistency with regards to potential storm totals.  Brett called it a "refresher" in this morning's Utah Avalanche Center advisory and I'm good with that at this stage.  We'll see how it comes together.

4. The Storm Killing Rex Block

There's no end to the storm-killing Rex Block (a.k.a. high-over-low block).  Although we get a flirtation with tropical moisture Tuesday and Wednesday, it pops right back up at full strength thereafter.  Below is the Euro forecasts for next weekend (ensemble mean left, high-res forecast right) and we're high-and-dry if that forecast verifies.

Source: ECMWF
A fairly similar forecast exists in the GFS.  I've added one of our regional panels below for late Friday night to show the net impact of the Rex Block on precipitation.  High and dry over the northern portion of the western U.S., with precipitation confined to the southwest.

How long could this go on?  Well, let's just say that the overall pattern persists in both the 10-day GFS and ECMWF forecasts.  Of course, there's always hope that the models are off the mark at those long lead times...

Friday, January 23, 2015

Snow Climate Vulnerability and Resiliency

There is much to be learned about future climate change by looking at the present or the past and that certainly is the case for the current water year, which began on October 1st.

So far, it has been an unusually warm water year with an average temperature of 43.1ºF at the Salt Lake City airport through January 20.  Only four similar periods in the instrumented period have been warmer.

One way to isolate the influence of temperature on the mountain snowpack is to examine the fraction of water year precipitation that is retained in the snowpack.  This fraction will be near 100% if most of the precipitation falls as snow and there is little loss of snow on the ground to either melting or evaporation and it will be less than 100% (possibly far less) if some of the precipitation falls as rain and is not retained in the snowpack (for example, rain that falls before snow is on the ground or falls on a snowpack that is water saturated) or if there is melting or evaporation.

We can do this using data collected by USDA Natural Resources Conservation Service SNOTEL stations.  These remote, automated stations measure water year precipitation using a large storage precipitation gauge (liquid precipitation equivalent - i.e., the combined water from rainfall and frozen precipitation after melt) and the amount of water in the snowpack (i.e., snow water equivalent) using a pillow that rests on the ground and measures the weight of the snow.

Source: Secrets of the Greatest Snow on Earth
These observations are not perfect.  Sometimes the precipitation gauge doesn't collect all the snow that is falling due to strong winds.  Sometimes the snow depth on the pillow might be unrepresentative due to wind transport.  Nevertheless, they are quite useful.

We begin with a low-elevation SNOTEL station characteristic of what I'll call a vulnerable snow climate, the Ben Lomond Trail SNOTEL at 5829 ft at the base of Ben Lomond Peak in the North Ogden Valley.  This is the lowest SNOTEL in the Wasatch Mountains.  This site has observed 10.1 inches of precipitation (black line) of which only 6.5 inches (64%, blue line) is retained in the snowpack.

Source: NRCS
Most of the 36% not in the snowpack is precipitation that appears to have fallen as rain and was not retained in the snowpack.  There are only a couple of brief downturns where there is water loss due to melting (note: the snow on the surface can melt without net loss of snowpack if it refreezes as it percolates through the pack).  This site is well shaded from the afternoon sun and thus tends to retain snow well.  More sun exposed areas near this site likely saw greater losses at times.

Next, let's go up the mountain to an upper-elevation station characteristics of what I'll call a resilient snow climate, the Ben Lomond Peak SNOTEL at 8000 feet.  This site has observed 15.5 inches of precipitation (black line) of which 14.1 inches (91%, blue line) is retained in the snowpack.  Thus, despite the warmth of the water year, most of the precipitation that has fallen is still stored in the snowpack.

Source: NRCS
One can find similar results at other upper elevation sites, such as Thaynes Canyon at Park City Mountain Resort.

Source: NRCS
So, this has been a warm year and it has had an impact on the snowpack at lower elevations because a significant fraction of precipitation has fallen as rain.  At the lowest elevations and on aspects that receive afternoon sun, there have also been losses due to melting.  This is why the cross country center at Mountain Dell has sometimes looked less than desirable.

Mountain Dell, January 12, 2015
At upper elevations, however, most of the precipitation that has fallen (about 90% or more) is still retained in the snowpack on northerly aspects and other shady locations.  These areas have some insurance against warming because of their aspect and their altitude.  I emphasize northerly aspects and other shady locations because there clearly have been losses due to melt on other aspects this year (except perhaps at the highest elevations).  

Overall, this provides some glimpse of the future snow climate of the Wasatch Mountains.  It remains unclear how our average precipitation climate will shift in the future (the models have varying projections), but warming is coming.  It is likely that the fraction of precipitation retained in the snowpack at low elevations during winter and at the end of the snow accumulation season will decline.  Upper elevation northerly aspects have some insurance against the initial wave of global warming and their future will depend on just how high greenhouse gas concentrations get and how much the climate warms.