The Second "Main Event"

Well, it has been a while now since the first main event. The second comes just in time for the winter solstice and the holidays. Since the first one, we've had a lot of clear skies, warm temperatures, then a handful of weak storms getting a bit colder with each one...until now.
The second event of the season will come in early Sunday morning, and last through Monday afternoon. It will feature a gradual warming trend, is associated with an Atmospheric River of moisture, and will be underscored by waves of dense snowfall, with increasing density (and rain at mid to low elevations), with the heaviest precip rates coinciding with the strongest winds (70+ mph on the ridges).

Below is a Meteogram from the GFS Model at SLC. Notice the thick grey line that says "FR"? That indicates the freezing level, to the left of the graph are heights in millibars. For reference 700mb is usually around 10,000ft. above sea level. The higher the mb, lower the elevation, and vice versa. Unfortunately, temperatures get quite warm.

 

Yesterday's National Weather Service Forecast Discussion was a classic..

"WARM AIR ADVECTION AT 700MB WILL STRENGTHEN SATURDAY NIGHT...ABOUT  
  THE SAME TIME DEEP MOISTURE STARTS TO ARRIVE OVER NORTHERN UTAH.  
  THIS WILL BE THE BEGINNING OF A PROLONGED WET PERIOD FOR NORTHERN  
  UTAH. HOWEVER...WARMING AIRMASS WILL LEAD TO HIGHER SNOW LEVELS WITH  
  MOST VALLEYS SEEING RAIN AND MOUNTAINS RECEIVING RELATIVELY HIGH- 
  DENSITY SNOW. 
   
  && 
   
  .LONG TERM (AFTER 00Z MONDAY)...IMPRESSIVE ATMOSPHERIC RIVER /AR/  
  EVENT PUNCHING INTO THE OREGON COAST WILL TRANSLATE SOUTHEAST AND  
  PEAK OVER THE NORTHERN THIRD OF THE FORECAST AREA SUNDAY INTO  
  MONDAY. ORIENTATION AND LOCATION OF THE AR SUCH THAT VAPOR TRANSPORT  
  WILL REMAIN QUITE SOLIDIFIED INTO THE INTERIOR MTN WEST 
  ...NAEFS MEAN INTEGRATED WATER VAPOR TRANSPORT VALUES CURRENTLY  
  FALLING WITHIN A 15 TO 30 YEAR RETURN INTERVAL LOCALLY FOR THIS TIME  
  OF YEAR. NOTEWORTHY. THE STRONG UPPER JET AXIS AIDING INFLUX REMAINS  
  NORTHEAST ACROSS WESTERN WYOMING IN GUIDANCE HOWEVER...BUT STILL A  
  SOLID 135 KNOTS ALOFT ACROSS NORTHERN UTAH. WAA PEAKING COINCIDENT  
  WITH PEAK PWAT...0.7 INCHES SUN NIGHT...A KEY TO THE PROLONGED  
  PERIOD OF ASCENT UPGLIDE AND PRECIP FORECAST ACROSS THE NORTH  
  THROUGH THAT TIME. VERTICAL PROFILES IN BUFR SOUNDINGS REMAIN QUITE  
  STABLE THROUGH THIS EVENT...BUT ANOMALOUS MOISTURE COMBINED WITH AT  
  LEAST SOME DYNAMIC LIFT WITHIN NORTHWESTERLY FLOW ALOFT IS OWING TO  
  INCREASED CONFIDENCE OF THE BULLISH STORM TOTAL QPF REFLECTED IN  
  GUIDANCE...IN EXCESS OF 2 INCHES SOME NORTHERN MOUNTAIN AREAS.  
  CONFIDENCE OF REALIZING OR EXCEEDING THIS REMAINS HIGHEST FOR THE  
  MTNS NORTH OF I-80 THOUGH SOME AREAS FAVORED BY NORTHWEST FLOW IN  
  THE CENTRAL WASATCH COULD SEE SIMILAR VALUES. WAA ADVECTION PEAKS H7  
  TEMPS TO NEAR -2 C...THUS RAIN/SNOW LINE THROUGHOUT THE BULK OF THE  
  EVENT SHOULD HOVER AROUND 7500 FEET. CONTINUED TO BUMP POPS IN THE  
  NORTHERN MTNS AND UPPED QPF IN REGARDS TO THIS THINKING. -CHENG/MERRILL"

Look at that, integrated water vapor transport values fall within a 15 to 30 year return interval!

Check out the 250mb wind speeds, getting into the 160-170kt range! It is like an arm of an outstretched  basketball player slam dunking right onto us! Like the current of a wide lazy river where the ripples form over the current, ripples of disturbances flow through this, and each one will provide enhanced dynamics for lifting air and therefore, precip.
 
Check out the water vapor transport forecast, very colorful indeed.

Total precip accumulation with the GFS looks to be 2 to 2.5" in the Central Wasatch. But remember, the GFS uses a large grid that doesn't take terrain into account very well at all, and this storm will be highly influenced by terrain lifting. My thoughts are that places with preferred orographics on a WNW flow could easily see twice that amount.

Avalanche danger will very likely reach High by Monday, and it will surely take some time to settle down afterwards. Our snowpack is generally thin, weak, and faceted, including numerous faceted layers (and just low density layers of big stellars) that could be a problem once a slab is stacked on top. Here comes our slab.

In the longer range, it looks like a fairly strong cold front will slide through on Xmas.