Wednesday, March 21, 2018

Atmospheric River Precipitation Needs Forcing Too

By now, you've probably heard that an atmospheric river is coming to Utah tonight and tomorrow.

Atmospheric rivers are corridors of strong atmospheric water vapor transport.  Such transport is dependent on both the water vapor content of the atmosphere and the strength of the flow.  Values are typically highest when the water vapor content of the atmosphere is high and the flow is strong. 

Atmospheric rivers can be associated with heavy precipitation, but strong atmospheric water vapor transport, by itself, doesn't generate precipitation.  Some forcing is needed to lift the airmass, form clouds, and generate precipitation.

In the case of an atmospheric river, one possibility is to have it cross a mountain barrier, which yields strong upslope flow and precipitation enhancement.  This happens commonly in the mountains of California during atmospheric river landfall.

Another option is to have large-scale forcing, such as the ascent typically found near cold fronts or along warm fronts.

With this in mind, the plot below shows a time-height section (time in this case increasing to the right) at a location near Salt Lake Cit airport from the 0600 UTC initialize GFS.  The color fill is the water vapor transport, and you can see how it maximizes at around 0000 UTC 23 March (6 PM MDT Thursday).  Note, however, that at this location, although there is a peak in precipitation at that time, a greater peak is found later, when the vapor fluxes are lower, but when the surface-based cold front is moving through.
Source: CW3E
This indicates the importance of forcing.  Water vapor transport can be very important, but a mechanism for generating precipitation is also needed.

That forecast above, however, is from a model (GFS) with relatively flat terrain.   If one were to go to a places like Snowbasin or Sundance, the story could be different.  At these locations, southwesterly flow is oriented strongly across the local topography and and significant generation of precipitation can occur.

Although I hesitate to use the actual totals produced by the 3-km NAM, the forecast below valid 0300 UTC 23 March (9 PM MDT Thursday) illustrates this well.  Note the heavier precipitation in the areas around Mt. Timpanogos (Sundance) and the northern Wasatch (Snowbasin), as well as over the Uintas.  These are areas where the mountains are oriented across the crest-level (10,000 ft) flow.  You can also see the heavier precipitation over northwest Utah where there is forcing along the cold front.

Sadly, ahead of the cold front, this is a very warm storm.  Our NAM derived forecast for the upper Cottonwoods shows wet bulb zero levels (typically the snow level is about 1000 feet below this) reaching as high as 10000 feet Thursday before lowering late Thursday night/early Friday morning with the frontal passage.  Much of the precipitation ahead of the front will fall in the form of rain at elevations below 8000-9000 feet.

Keep an eye on official forecasts the next couple of days.  Ultimately, the timing, intensity, and amount of precipitation will depend on both the characteristics of the atmospheric river, the mountain effects, and the cold front.  I will also add that there is the potential for strong pre-frontal souhterly winds and thunderstorms late Thursday and Thursday night.  Too much to cover in a blog post from Seattle. 

Sunday, March 18, 2018

Warm Atmospheric River Event Possible Later This Week

Atmospheric rivers are narrow corridors of strong atmospheric moisture transport, often connected in some way to moisture exported into the midlatitudes from the tropics or the subtropics.  Although Utah sometimes sees the remnants of weakened atmospheric rivers, bonafide atmospheric river conditions in Utah happen only a few times each cool season.

However, we have a shot at one making it to the state Wednesday night and Thursday if model forecasts hold.  The average integrated water vapor transport in forecasts produced by the Global Ensemble Forecast System for 0600 UTC 22 March (0000 MDT Thursday) shows strong values extending from the Pacific Ocean northeastward into Utah.  Values exceeding 250 kg/m/s (outlined by red line) indicate atmospheric river conditions, and most of Utah is covered.

Source: NWS
Two ingredients favor atmospheric river penetration into Utah in these forecasts.  The first is that the atmospheric river is strong on the coast, with integrated water vapor transport values over 750 kg/m/s.  The second is that the flow moves up the lower Colorado River Basin, missing the southern Sierra Nevada and other high terrain features that typically generate precipitation and deplete the water vapor content of airmasses moving into the western interior.

It is still a bit early to discuss details of where, when, and how much precipitation will fall.  Not all atmospheric river events are big precipitation producers in Utah, but some are.  Much depends on the orientation of the atmospheric river, the orientation of the flow relative to major terrain features, and the efficiency of the precipitation processes embedded in the flow.  Also important is the duration of the event.  Right now, it looks like we'll see a warm, humid (by Utah standards) precipitation event developing first in southwest Utah late Wednesday and then in other parts of the state Wednesday night and Thursday.  Until then, enjoy the spring break and monitor forecasts.

Saturday, March 17, 2018

A Good Spring Break to Be Nimble

I've been surveying the forecasts for spring break and it is looking like a week where you can do just about anything if you are adaptive and pay attention to forecasts.

I think everyone is well aware that the weekend is for skiing in the Wasatch where the snow continues to pile up.  Alta-Collins got 10 inches overnight and has another 4" through 2 PM.  Between the cloud cover and the low temperatures, the snow should be holding up well on higher elevation north aspects.  Over three inches of water has fallen in the last three days, making this one of the more productive storm cycles of the period.  My advice is that you ski until you drop through the weekend.

Monday is a tougher call and one that you will need to make for yourself.  Good skiing may persist, or you might opt to head south for adventures in southern Utah.  It will be a cool day, but sunny statewide.  Tuesday looks spectacular, with upper-level ridging over the state. 

Wednesday looks to be the warmest day of the week for most of the state, but there's a threat of rain spreading into southwest Utah in the latter part of the day.  It's a bit too early to call that, so if you head south, pay attention to forecasts.

Depending on how things play out, it might be wise to be back in Salt Lake for skiing Thursday when we may see a warm storm.  Altitude will be your friend, as it almost always is so late in the season. 

Bottom line: Ski today and tomorrow.  Monday go with your instincts.  Southern Utah Tuesday, then check the forecast and adjust accordingly. 

Being nimble should allow you to maximize your hedonistic pursuits.  I, on the other hand, will be banished to a conference room in Seattle for the week.  Fortunately, there will be real, craft beer on tap each evening to dull the pain.

Friday, March 16, 2018

The Truth About Powder Skiing

"The best deep-powder skiing is not found in the lightest snow but rather in snow with enough 'body' to provide good flotation for the running ski."
- Ed LaChapelle, 1962

You can have your bottomless blower pow.  You can rave about pit deep 4%.  You can have it all.  The truth is, those deep, dry days don't provide the best powder skiing.  Give me some Cascade concrete and and put some cold smoke on top of it.  

And that's what we found on sheltered upper-elevation north aspects today. 

What we didn't find were people. We pulled into the White Pine lot at 8:15 and found only two cars in the lot.  Two!  While gearing up, I kept waiting for the yellow lights to start blinking and then the sound of incoming shells as there's no way that there can only be two cars in the lot on a powder day.  

Yet the dream was true.  

Then the day dawned clear, with a postcard view down Little Cottonwood on the climb up.  

After passing a snowshoer near the boundary for the Lone Peak Wilderness, we found no tracks.  None.  Just a hint of a skin track from a couple of souls from yesterday buried under the cold smoke to lead the way.   For hours we broke trail and did laps in perfect powder, not seeing a soul until about 2 PM.  It was like being on a hut trip in interior BC.  Nobody around.  Surfy hero snow with just the right body.  Zipping through well spaced trees as if there was no tomorrow.  Ed LaChapelle skiing. 

I need to get out more during the week.  So few people, so much enjoyment.  Add hero snow and my favorite touring partner, and you have a perfect day.

Thursday, March 15, 2018

After the Deluge...

Storms have produced some impressive precipitation amounts (water equivalent) for March from yesterday afternoon through this morning.  Some totals through 7-8 AM or so this morning include .78" at the Salt Lake City Airport, just over an inch in Olympus Cove, 1.25" at Spruces, and 1.54" at Alta Collins.  

Sadly, much of that water fell as liquid in the lower to middle elevations.  For example, 0.68 of the 1.54" that fell at Alta Collins fell at temperatures at or above 32ºF.  During that period, snow levels were initially at almost 9000 feet and lowered to about 8000 feet.  They have since dropped to the valley floor and as of about 8 AM, it is snowing at the University of Utah.

At Alta Collins, the automated interval snow-depth sensor suggests about 10 inches of snowfall.  Initially, that snow was probably a mixture of graupel and white sludge, but densities dropped with snow levels overnight, so what is there should be right-side-up.   

Radar imagery shows that the precipitation feature currently producing snow on campus is swinging through and that we will probably see things letting up soon.  

Expect some snow and rain showers today, and maybe even some thunder.  Similarly, the mountains will see periods of snow and don't be surprised if you hear a clap of thunder there too.  It won't be as active as yesterday and thankfully it is much colder.  Hit and miss snow showers, including the band moving through presently, will produce perhaps another 3-5 inches at Alta-Collins through 5 PM.  

It will be interesting to hear how the snow holds up today now that we're into mid-March.  Sunbreaks are welcome in January, but can be caustic this time of year.  South aspects won't last long.  A real challenge for backcountry skiers as we head deeper into spring is that monsters continue to live in the basement on high-north aspects that preserve powder so well.  

Wednesday, March 14, 2018

Expect the Unexpected from This Spring Storm

It's worth a look at the combined cloud/radar and NAM cloud/precipitation forecast loop below to get an idea of the lack of organization of precipitation systems forecast to impact the weather of northern Utah over the next 2-3 days.  Note their banded or "blobular" structures.  Blobular is of course a highly scientific word (ha ha) used here to describe cellular features produced by a model that due to it's sparse grid spacing (12-km) is incapable of producing convective storms that look like those of the real world. 

The chaotic nature of those precipitation features means if you are looking for a precise forecast of when and how much it is going to rain or snow over the next couple of days, you've come to the wrong place. 

Let's start with perhaps the easy part: Today.  A combination of instability, strong flow, and vertical wind shear means we will see some showers and thunderstorms this afternoon.  The NAM forecast sounding for 2200 UTC (4 PM MDT) shows 320 Joules/kg of surface Convective Available Potential Energy (CAPE), a measure of how much energy a surface parcel of air would gain if it were lifted vertically through the atmosphere.  Locally, values may be higher.  Although such CAPE values are pretty pathetic for those looking for midwest-type severe storms, but are enough to make things interesting for Utahns.  Strong flow and vertical shear is also indicated in the wind profile. 
As such, the Storm Prediction Center has us in marginal risk of severe thunderstorms in the mid to late afternoon when "thunderstorms will offer the potential for damaging gusts and hail near severe limits." 

Source: NWS
Beyond showers and thunderstorms, expect some gusty south winds today, with the possibility of some blowing dust.  There is no longer snow cover over valleys and basins to our south and west, so dust emissions are possible if the land-surface conditions are favorable and flows are sufficiently strong.  Temperatures today will remain mild, although snow levels may drop locally during stronger showers and thunderstorms and may include large graupel or hail. 

After today, the pattern might best be described as unsettled, which is a nice way of saying there will be precipitation, but where, when, and how much is unclear.  Snow levels will fall overnight and probably be near bench level early tomorrow morning. 

The now somewhat old 0300 UTC initialized SHREF shows a mean of about 1 inch of water total at Alta-Collins by 0000 UTC (6 PM MDT) tomorrow afternoon, but the range is colossal, spanning from about 0.1 to 2 inches. 

Everything will depend on the position and intensity of precipitation features accompanying the system as it swings through. 

Stuff that falls today will likely be of the wet, high-density variety at high elevations, possibly including some big graupel or hail.  A garbage bag might be required at times, especially at mid and lower elevations, which will probably see rain that could turn temporarily frozen precipitation of a "variety of forms" during periods with higher precipitation rates.  Snow levels and densities will drop later tonight. 

A reasonable guess for Alta-Collins would be 7-14 inches from today through 6 PM tomorrow, with more possible if they are lucky enough to get a pounding from one or more of these precipitation features.  Note that I use the scientific term "guess."  Expect the snow to come in fits and starts at times. 

Welcome to spring!