Deja Vu All Over Again

That's my favorite post title when we are in a stormy pattern and it still appears like we are on track for another pasting related to the inland penetration of another potent atmospheric river, as discussed in previous posts.  As I write this at 10 am MST, snow is falling at upper elevations in the mountains and we have a classic "cloud storm" scenario in the Salt Lake Valley in which the sky is full of virga, but just a few raindrops are surviving to reach the ground.

This morning's "cloud storm" from the Salt Lake City International Airport

This cloud storm scenario occurs when the moisture and dynamics for precipitation development are elevated and we have dry air at low levels, as can be seen in this morning's sounding.  

Source: NCAR/RAL

Forecasts are largely on track from yesterday with atmospheric-river-related precipitation spreading into northern Utah today and persisting through tomorrow.  In the upper Cottonwoods, the model forecasts are actually a bit wetter than yesterday's, with this morning's 12-km NAM generating 1.75 inches of water by 5 PM tomorrow (Thursday).  Such water totals might be on the high side, but something in the 1-1.75 inch range for Alta-Collins seems like a good bet.  Most of this will fall as high-density snow at upper elevations with snow levels rising to perhaps 8000 feet.   

Source: http://weather.utah.edu

Yup, another Cascadian experience, but it skied well last weekend.  Ski it if it's white.  

The Next Atmospheric River

Our second atmospheric river event in a week's time is setting up over the Pacific Ocean with a tremendous export of tropical moisture from the area near Hawaii streaming northward toward the Pacific Northwest.

1800 UTC 9 Feb – 1800 UTC 11 Feb 2014 GFS analysis of 925 mb wind (vectors) and integrated water vapor (mm, color contours with warmer colors indicating higher values).  

Over the next 36 hours, the atmospheric river will move eastward and sag southward.  By 5 PM tomorrow (Wednesday) afternoon, the axis of strongest water vapor transport will be draped across southern Idaho and Wyoming.  As was the case with the atmospheric river event this past weekend, the strength of the water vapor transport over the interior will be unusually high for this time of year with a frequency of occurrence during this 3 week period of about once every 5 years or rarer.

It certainly looks like the upper-elevation mountains of southern Idaho and western Wyoming are going to get another pasting, as indicated by the GFS accumulated precipitation through Saturday morning.

As you might deduce from the plot above, the mountains of northern Utah sit on the southern periphery of the heaviest 4-day accumulated precipitation in the current model forecasts.  Nevertheless, this does look like another significant event for the northern Wasatch and the Bear River Range (although not as big as the previous storm), with the Cottonwoods and southern Wasatch likely seeing their most significant precipitation late Wednesday and Thursday when the AR dips southward.  

This is also going to be another warm event thanks to the Hawaiian connection.  Direct output from the 12-km NAM (below) suggests rising ridge-top temperatures on Wednesday with the snow-to-liquid ratio dropping to less than 10 (i.e., more than 10% water content).  Snow levels will also rise to close to 8000 feet in the Cottonwoods.  Provided we only get a brief visit by the AR, as presently forecast, water totals in the Cottonwoods will be much lower than we saw during the last event, although snow and snow water equivalent rates could be strong at times late Wednesday and Thursday.  

If you don't like these Cascadian conditions, wait a couple of days and head to Zion on Friday to enjoy some 70 degree sun.  

Welcome to Seattle!

A drippy, foggy Monday morning in Salt Lake City

This morning dawned with Seattle-like rain and fog in the Salt Lake Valley and Cascade-like conditions in the Wasatch Mountains where high-density snow continues to pile up in the upper elevations.  Water and snow storm totals reported on the Utah Avalanche Center web site this morning range from 2.87"/30" at Alta to 46"/6.94" at Sundance.  Logan and American Fork Canyons remain closed and tragically, two deaths have occurred in backcountry avalanches.  

The storm should wind down today, with a break in the action tomorrow.  Beyond that, I hope you like the Northwest weather as we're going to get more of it.  A broad, low amplitude ridge remains over the region throughout the week, with another push of moisture into the northern mountains likely late Wednesday and Thursday.  Yes, it will be another atmospheric river event, with the tongue of tropical moisture that will be coming our way beginning to surge northeastward near Hawaii (westernmost arrow), just upstream of the remnants of the atmospheric river that gave us the deluge the past few days (easternmost arrow).  

GFS Integrated Water Vapor (mm, warmer colors indicate higher values) at 1200 UTC 10 Feb 2013.

If things come together as currently projected by the models, that will mean another round of rain for the valleys and high-density snow for the mountains.  The biggest uncertainty in this forecast concerns the position of the atmospheric river as a slight shift to the north or south can make a big difference for water/snow totals.  After that, the models call for the ridge to amplify somewhat with dry conditions but very warm temperatures on Friday.  

The GFS forecast time series shown below summarizes the situation.  Note in particular the heavy precipitation late Wednesday into Thursday (bottom left panel) and then the big warmup thereafter with temperatures on Mt. Baldy (11,000 ft) going into solidly into the 30s (upper left panel).  The GFS typically overestimates snow levels so don't panic about them going to almost 10,000 feet on Wedensday (upper right), but it is going to be a warm storm, so expect snow levels to be pushing 8000 feet.

Source: http://weather.utah.edu

Looks like an investment in Gore-Tex or Hefty bags might be in order.  A pair of ultra-fat fully rockered skis would be nice too.  

Positively Cascadian

Before discussing the balmy nature of the weather this morning, here's a very quick rundown of the water totals produced by the storm since 5 AM Thursday based on my eyeball estimates:

Tony Grove Lake (8474 ft): 6.6 inches (Logan Area Mountains)
Ben Lomond Peak (8000 ft): 6.2 inches (Ogden Area Mountains)
Snowbasin Middle Bowl (7400 ft): 3.5 inches (Ogden Area Mountains)
Alta-Collins (9662 ft): 2.3 inches (Salt Lake Area Mountains)

I don't have much faith in any of the automated stations in Big Cottonwood Canyon that measure snow water equivalent, but the Utah Avalanche Center reports 3.21 inches for whatever site they are using (probably manual measurements from one of the resorts), so we will go with that.

The Snowbasin-Middle Bowl and Alta-Collins observing sites are two of the most reliable in the Wasatch and they pretty much tell the tale for this storm.  Temperatures have risen dramatically through the period and are now a Cascade-like 35ºF at Snowbasin Middle Bowl and 27ºF at Alta-Collins.

Thus, we're probably seeing a snow level somewhere near 8000 feet in the central and northern Wasatch and a freezing level near about 8500 feet.  That huge temperature increase means a transition from lower to higher density snow as well as from snow to rain at below 8000 feet during the storm.  Ski conditions today should be "interesting."  Perhaps nobody will care about the Cascadian conditions since beggars can't be choosers.  

One of the interesting aspects of this event is that it has come in waves, despite the fact that the models called for relatively steady precipitation rates.  You can see this in the data from both Snowbasin-Middle Bowl and Alta-Collins.

Clearly we have some work to do to improve our modeling and prediction of these variations in precipitation rate as they are critical for avalanche mitigation efforts along highways at at the resorts and for people like me who are simply hoping to ski when it is snowing the hardest. 

A Good Day with More to Come

Finally!

It was great to get in some storm skiing this afternoon.  Snowfall rates at Alta were pretty much pegged at about an inch an hour from 9 am to 4 pm.

Source: MesoWest. 

Looks like Alta-Collins is now up to 1.5 inches of water 17 inches of snow for the storm total and climbing.  

I heard the lift lines were long in the morning, but I skied in the afternoon when the crowds were tolerable and the steady snow made for fun turns.  I even ran into a few members of the University of Utah Snow Study Team who having returned from the Tug Hill Plateau are trying to rack up as much vertical as possible.    

Of course, the best way to finish a ski day is to come home and see the radar lit up upstream.  Voila!

Source: NCAR/RAL

Yup, things are looking good for more snow, courtesy of this latest atmospheric river.  Enjoy!

Some Early Numbers

As reported by the Utah Avalanche Center at 7am this morning, the biggest winners so far in this storm are those areas typically favored in southwesterly to westerly flow including the northern Wasatch (storm totals ~16"), Upper Big Cottonwood (storm totals ~16-18"), and the Provo Area Mountains (storm totals ~16").  Upper Little Cottonwood lags a bit behind (storm totals ~8-10").

Time series of precipitation from Snowbasin-Middle Bowl and Alta-Collins show some interesting aspects of the early part of the storm.  First, it has come in pieces, with the bulk of accumulation coming from 0000-0900 MST 7 Feb (late Thursday night and Friday morning) and then from about 1600 MST 7 Feb - 0300 MST 8 Feb (Friday afternoon to very early Saturday morning).   Snow water equivalent (see blue line and accumulated precipitation scale at right) was nearly double at Snowbasin compared to Alta-Collins.  However, we had a trough passage early this morning and precipitation rates have picked up at Alta-Collins in the last couple of hours.

Source: MesoWest

Source: MesoWest

The storm total snow water equivalent at Alta-Collins since 0000 MST 7 Feb is about 0.7 inches.  For comparison, the 0000 UTC 7 Feb (1700 MST 6 Feb) initialized 12-km NAM called for about 1.2 inches, so at least so far the 12-km NAM has overforecast the event at Alta-Collins, as we suggested in earlier posts.  We will have to see, however, how things play out today and tomorrow as a series of troughs moves through.  These troughs will cause the crest-level flow to fluctuate back and forth from SW to NW and this, along with shifts in the area of maximum water vapor transport and dynamical forcing, means there will be a lot of variations in storm characteristics.  In the end, however, everyone will get some and we're still on track for the biggest storm of the season so far.  I'll also stick with my forecast from the two previous posts, which called for a storm total by 5 PM Sunday afternoon of 2-3.5 inches of water and 25-40 inches of snow at Alta-Collins.  The high end numbers there might be a bit optimistic at this stage, but I think we'll be able to get to at least 2 inches of SWE/25 inches of snow by then.

Here We Go!

The obscured Wasatch Range from the University of Utah, 8:30 AM MST 7 Feb 2014

Our multi day snow event is now underway.  Overnight and early morning snow totals as of 8 am in the mountains based on automated sensors include 6 inches at Snowbasin-Middle Bowl, 7 inches at Park City-Jupiter, 7 inches at Solitude-Powderhorn, 9 inches at Solitude-Summit, and 5 inches at Alta-Collins.

Although the mountain snows today may be heavy at times, they will likely come in fits and starts with some breaks at times.  Things get more interesting overnight tonight and tomorrow as an atmospheric river, a narrow corridor of strong moisture transport, makes landfall in northern California and penetrates into northern Nevada and Utah.  

Atmospheric rivers feature strong values of integrated water vapor transport (IVT), a measure of the amount of water vapor flowing over a given location every second.  By 1800 UTC (1100 MST) tomorrow, very high values of IVT associated with the atmospheric river extend over northern California and across northern Nevada and Utah. For late January and early February, such high IVT values have a climatological frequency of occurrence of about once every 5-10 years over northern Utah. 

Source: NWS

The productivity of this storm depends on how much of that water vapor we can convert to precipitation.  The 12-km NAM remains fairly excited for the Wasatch, generating almost 3 inches of snow-water equivalent and 40 inches of snow at Alta by 5 PM Sunday (just a shade less than the 12-km NAM forecast from yesterday morning as discussed in the previous post) with some additional accumulations Sunday night.  

Source: weather.utah.edu

I still think the NAM is in the upper portion of possibilities for this storm at Alta and will stick with my forecast from yesterday which calls for a total by 5 PM Sunday afternoon of 2-3.5 inches of water and 25-40 inches of snow at Alta-Collins.  Some bigger water totals are possible in the northern Wasatch.