Tuesday, January 23, 2018

The More Things Change, The More They Stay The Same

"Pattern change" is currently on my list of banned words and phrases for good reason as it was being thrown around repeatedly for weeks despite the fact that nobody really knows what it means. 

As I've been watching the large-scale pattern for the last several weeks, there has certainly been variability.  Ridges and troughs have formed and dissipated.  There have been some major cyclogenesis events.  However, one thing has remained constant.  The large-scale pattern has been very high amplitude, meaning a wavy jet, especially from the Pacific Ocean to Europe. 

That situation looks to continue for the foreseeable future.  Below is the 10-day GFS forecast for the northern hemisphere dynamic tropopause.  The dynamic tropopause separates the troposphere, or the lower atmosphere in which we all live and reside, from the stratosphere and basically sits at jet-stream level.  Note in particular the high-amplitude, wavy nature of the pattern, with strong ridges and anticyclones (high-pressure systems) forming in several areas including over the Behring Sea and North Atlantic.  Basically, the large-scale flow is highly disrupted. 

This is essentially what we have seen now for weeks.  High amplitude patterns like this can be good for snow if you are in the right place (check out the Alps), but we haven't.  The tendency in Utah has been for us to be under the influence of high-amplitude ridging or just to the south of the storm track. 

So, the more things change, the more they stay the same.  We get the occasional storm, but a real storm cycle is hard to come by.  Unless the storm track shifts southward more than currently predicted, that looks to be the case over the next seven days.  Our best bet for snow, as indicated by the NAEFS forecast plume below is Thursday and maybe Thursday night, with maybe a bit here or there thereafter. 

The NAEFS plume above may even be a bit optimistic for Thursday and Thursday night as the water equivalents advertised by the NAM, GFS, and Euro fall in the low end.  Most members of our downscaled SREF are generating only .1 to .5 inches of water equivalent.  The optimistic Canadian model (CMCE members in the plot above) appears to be an outlier. 

As usual, keep expectations low and hope for the best.  A small storm will be appreciated, but recognize that I have yet to see the whites of the eyes of a real pattern change.  However, my crystal ball only sees out about 7 to 10 days.  Let's hope February is better. 

Monday, January 22, 2018

So Much Snow, So Little Water

It was wonderful to see the white stuff return to the valley and the Wasatch Front this weekend.  Everybody got some.  Here are some of the bigger totals as reported to the National Weather Service (water equivalent in parentheses).

Bountiful Bench (through 5 PM Saturday): 13.5" (0.87")
Bountiful (5 PM Saturday): 10.8" (not reported)
Cottonwood Heights (7 AM Sunday): 17.5" (1.64")
Holladay (7 AM Sunday): 15.5" (0.91")
Summit Park (12 PM Sunday): 20" (not reported)
Alta Collins (4 PM Sunday): 23" (1.16")
Canyons 8800 ft (3 PM Sunday): 20" (0.90")

It was a complicated storm featuring a frontal phase Friday night, a period Saturday and Saturday evening with strong enhancement along the benches, and a period Saturday night when the Little Cottonwood magic kicked in. 

There are, however, a couple of key observations to be made.  The first is that the water-equivalents were largest along the east bench of the Salt Lake Valley, not in the highest elevations of the Wasatch Mountains.  Alta-Collins, Snowbird, and Alta-Guard were the three wettest sites, each coming in just over an inch of water.  In contrast, two sites in Cottonwood Heights reported over 1.5 inches of water. 

A big reason for this is the first two storm phases, which featured strong frontal forcing and then a period with weak low-level northwesterly flow in the Salt Lake Valley, but light flow aloft with strongly stable conditions.  One can't always assume the climatological increase of precipitation (water equivalent) with elevation, and Friday night and Saturday provided a good example of that. 

The second key observation is that this storm really didn't add much to the snowpack.  The 23 inches of snow that fell at Alta-Collins sounds like a lot, but it had an average water content of only 5%.  The 10 inches that fell Saturday night had a water content of only 3%. 

So much snow, so little water!

Basically, the storm put smiles on skiers faces, but water managers still have heartburn.  The Snowbird SNOTEL snowpack water equivalent didn't go up as much as one might expect from the precipitation gauge observations (this is not uncommon), but even if one jacks up the snowpack water equivalent to account for this, we're still losing ground to average.  At the end of December, we were 6.1" below average snowpack water equivalent.  Today, even with the weekend snow, we're 9.1" of water below average. 

Source: NWS
This time of year, we should be adding about 1.5 inches of water to the upper-elevation snowpack in Little Cottonwood Canyon every week.  In the first 3 weeks of January, we added about 1 inch per week.   We're still losing ground. 

This trend looks to continue for the next week.  Other than some snow showers today, our next change of a storm is in the Thursday-Friday time range.  The downscaled NAEFS ensemble generates anywhere from about 0.25 to 0.9" of water with that storm at Alta Collins.  Some members produce a bit more after that event, but not much.  Only one NAEFS member reaches the coveted 1.5" water mark for the week.   

The end of January roughly marks the midpoint of the winter snow accumulation season at upper elevations.  At Snowbird, for example, the snowpack water equivalent averages 22.1" on February 1, roughly half the average peak of 44 inches on April 27.  With 9.9" currently on the ground, we look to be about 50% of average at the end of the month, with somewhere between 10 and 11.5 inches of snowpack water equivalent.  

We need a real storm cycle and we need it now.  

Sunday, January 21, 2018

About Last Night...

If you want blower pow, last night was your storm.  From 5 PM yesterday to 6 AM this morning, the Alta-Collins observing site picked up a paltry .32" of water equivalent, but 10" of snow.  That's a mean water content of only 3.2%!   That's a snow-to-liquid ratio of 31-to-1.  If you want a big dump, but don't have a lot of water to play with, that's how to do it. 

Really, conditions overnight were only marginally better for orographic precipitation generation than they were yesterday.  The 0000 UTC (5 PM MST Saturday) sounding still showed stable conditions at mid level with strong wind shear from 700 to 600 mb. 

The morning sounding is a bit better with northerly flow through depths and colder temperatures aloft, although a weak stable layer remained just above 700 mb. 

But the secret to the overnight dumpage wasn't large orographic precipitation enhancement.  There was some enhancement, but .32" of total water and a maximum water equivalent rate of 0.05" per hour isn't much.  The secret was the huge snow-to-liquid ratio.  If the snow-to-liquid ratio for this storm was Alta's average of 13-to-1, the snowfall would have been 4 inches.  But at 31-to-1 you get 10 inches and the stuff that Kodak moments are made of.  Yup, this was a snowfall that will bring smiles to skiers, but continue to give water managers, who need to see higher water content dumpages, heartburn. 

Saturday, January 20, 2018

A Good Front Followed by Bad Orographics

A cold front moved across northern Utah last night bringing much needed snow from the valley floor to the highest peaks.  Powder panic brought gridlock to most (probably all) routes to the Cottonwoods, providing an all-too-frequent reminder that we are loving our canyons to death. 

Those who braved the traffic were rewarded with knee deep powder.  I would describe the skiing as good, not great.  A spongy layer of high-density snow to start would have helped reduce the bottom feeding, but this year, beggars can't be choosers.  

Snowfall produced by the front went largely as advertised by the models, at least in the upper Cottonwoods.  Alta-Collins had .59" of water and 7 inches of snow through 8 AM this morning.  This compares very well with the NAM forecast we discussed on Tuesday, which put out .64" of water and 8 inches of snow (see Frontal Snowfall Event on Tap for Late Tomorrow and Tomorrow Night).  The observed water totals are also near the middle of what was advertised by the University of Utah downscaled SREF ensembles.  The model wizards can be happy about this period.

I suspect that those hoping for a true storm ski day were a bit disappointed, however, with today's offering.  Light snow fell for much of the day, but since 8 am it added up to only .15" and 2 inches of snow at Alta Collins, which is probably a bit more than what we saw where we were ski touring in Big Cottonwood Canyon.  

Quite frankly, the orographic forcing today simply sucked, which we discussed as a possibility yesterday (see Probabilistic Snowfall Forecasting).  The morning sounding shows the situation quite well.  The atmosphere was quite moist, but also generally stable below 700 mb (10,000 ft), with a strong stable just above, which is associated with the front aloft.  The flow at low levels was northerly, but swung to southwesterly in the strong stable layer.  This is simply not a recipe for orographic enhancement.  

Source: NCAR/RAL
During the afternoon, the flow on Alta-Mt. Baldy slowly shifted to northwesterly to westerly, but remained weak and with high atmospheric stability, wasn't generating much at upper elevations.  

Instead, the radar loop below shows the development of the strongest echoes along the east bench and within the lower canyons through 0028 UTC (5:28 PM MST).  

Driving down Big Cottonwood late this afternoon, it was clearly snowing harder in the very bottom of the canyon and along the east bench than it was in the upper canyon.  Note that you can also see this effect in the Oquirrh Mountains where the radar returns, especially later in the loop are stronger on the lower and mid elevation western slopes.  

The devil is in the details.  

Friday, January 19, 2018

Probabilistic Snowfall Forecasting

For decades, snowfall forecasts have typically involved the issuance of a range of accumulation amounts, typically (but not always) based on a factor of two.  For example, 3-6 inches, 4-8 inches, etc.

I have no idea why.  Perhaps it is a convenience thing.  Maybe people like it that way.  I don't know what that range even means.  Does it represent the middle 50% of possible outcomes, with a 25% chance of more and a 25% chance of less?  Does it represent the middle 80%?  Why always use a factor of two?  Sometimes the range needs to be bigger, especially in longer range forecasts. 

And then there is my favorite, "higher amounts in favored locations."  What the hell does that really mean and how do you verify it? 

There was a time when snowfall forecasting was truly guesswork, but things are changing.  Computer models are now capable or will soon be capable of simulating smaller storm details, what meteorologists call "cloud scale."  Ensembles can be used to better estimate the future outcomes.  There remains much work to do, but there is great potential to dramatically improve snowfall forecasting. 

The National Weather Service is now producing experimental probabilistic snowfall forecasts, and they are available at https://www.weather.gov/slc/winter.  They provide much richer information about storm potential than a simple range.  For example, ,aps are provided showing the likelihood of snowfall above several thresholds, an example of which is the probability of 6" of snow or more for the period from 5 AM today to 5 PM Sunday, shown below. 

Source: NWS
They also provide a table with snow amount potentials and probabilities of snow within certain ranges, shown below, as well as above certain thresholds. 

Source: NWS
Readers of this blog are snow lovers.  Start perusing these forecasts and provide feedback through the links on the page. 

Now, to clarify some of my scattered comments to yesterday's post about the situation on Saturday.  Although we have a front pushing through tonight, it is a very slow mover.  As a result, this is not a frontal passage in which we quickly get into deep, unstable, northwesterly flow on Saturday morning. 

This is evident in the NAM time-height section for Alta below.  The front at Alta is a late arriver (light blue line), in this case moving through at or just after 0600 UTC (11 PM MST tonight).  Then, look at the winds behind the front on Saturday (circled).  They are NNW at low levels, but NNE near 700 mb (10,000 ft) and then SSW at 600 mb. 

This reflects the slow movement of the front through the area. 

If we look at the sounding for 1800 UTC (11 AM MST) Saturday morning, we see the low level northerly flow, but note how the winds shift to NNE and then SSW with height.  The temperature and dewpoint traces show a sharp inversion just above 700-mb, or 10,000 feet.  
This is not a recipe for our classic northwesterly instability snow showers over Alta for two reasons.  First, the flow direction isn't right.  Second, the instability is too shallow.  

However, if you look at the sounding for 0000 UTC (5 PM MST) tomorrow afternoon, the low level flow is NNW through a deeper layer, although a capping inversion remains based just below 600 mb.  This is closer to what is needed for the NW instability showers, but the capping inversion height is right on the edge of what I would like to see.  Tough to say if it's high enough that Alta can benefit, or just a bit too low so that the mid and lower canyons and east bench do better.  
And that's just one model run.  There are variations in the timing of these changes, wind directions with height, etc., if one looks at other models.  

All of this illustrates what a complex mess this is for Saturday and why probabilistic forecasting is necessary.  The good news is there's enough going on that in the end, this will be a decent storm for the mountains and even the mountain valleys after snow levels lower today and this evening.  

Thursday, January 18, 2018

Frontal Snowfall Event On Tap for Late Tomorrow and Tomorrow Night

After another 10-day or so stretch with limited to no accumulations, our next storm will be served up late tomorrow and tomorrow night. 

Most of the precipitation for the central Wasatch looks to be primarily frontally forced.  The large scale setup is shown below and features an upper level trough that is initially tilted from southwest to northeast (referred to as "positively" tilted by meteorologists) that closes off and becomes more north-south oriented as it moves inland across the western US.  

This has both pluses and minuses for snowfall prospects in the central Wasatch.  The plus is that the front may slow as it drags through northern Utah, extending the period of frontal snowfall, as depicted below in the 1200 UTC NAM forecast.  At 000 UTC 20 January (5 PM MST Friday), the surface front is over Utah County with precipitation over the northern Wasatch.  

Frontal precipitation fills in, however, as the front phases with moisture sneaking around the southern end of the Sierra Nevada over the next 3 hours. 

That precipitation continues for another 3 hours as the front makes slow progress into southern and eastern Utah. 

By 0900 UTC 20 March (2 AM MST Saturday) the main frontal band is just downstream of the central Wasatch, with some post-frontal snow showers persisting. 

The minus for snowfall prospects is with the low closing off, the post-frontal winds shift very quickly to northerly, when we would prefer a period of northwesterly flow for better orographic forcing.  Note in the Salt Lake City time height section below that the post frontal flow is predominantly northerly and deepens gradually from about 0Z Saturday through 6Z Sunday.  

Actual numbers derived from the 12Z NAM show the wet bulb zero dropping during the day Friday (snow level is usually about 1000 ft below this level), with values low enough that most of the precipitation produced during this event should fall as snow in the mountain valleys.  Perhaps Mountain Dell might see a bit of rain to start, but then turn over to snow.  Total water equivalent at Alta is 0.64" through Saturday at 8 AM, with snow densities decreasing during the storm for a right-side up snowfall.  

Looking more broadly at the ensembles shows that the NAM is roughly in the upper half of the SREF plume for Alta.  Through 18Z 20 January (11 AM) the SREF members put out anywhere from 0.3 to 0.8 inches of water, the former being a slightly better than dust on crust event adding up to perhaps 4 inches of snow, the latter representing a lower end deep powder day with perhaps 10-12 inches of snow.  

That spread represents variations in the strength and speed of the front.  Increases in precipitation after 18Z 20 January occur in some model runs that are more bullish on the post-frontal precipitation.  

I continue to keep expectations low and hope for the best.