18″ of Snow in 12 Hours for Montana Today:

18 inches of snow in 12 hours is a lot of snow in a short amount of time.  It adds up to over an inch an hour for 12 hours.  It’s going to be a good day in western Montana and Central Idaho.  Of course, there are no people, nor roads, nor ski resorts in Central Idaho.  One of the last unspoiled places in the lower 48.

NOAA:

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE MISSOULA MT
827 AM MST FRI DEC 20 2013

...WINTER STORM WARNING REMAINS IN EFFECT FROM 10 AM THIS MORNING
TO 10 AM PST SATURDAY...

* IMPACTS/TIMING: SNOW WILL BEGIN TO FALL AROUND NOON AND
  INTENSITIES WILL BECOME HEAVY BY 2 PM AND REMAIN MODERATE TO
  HEAVY OVERNIGHT. TRAVEL WILL BE VERY DIFFICULT TO DANGEROUS AT
  TIMES. SNOWFALL RATES COULD APPROACH 2 INCHES PER HOUR CREATING
  THE POTENTIAL FOR VEHICLES TO BECOME STUCK.

* SNOW: OVER A FOOT OF SNOW IS EXPECTED IN THE MOUNTAINS... WITH
  AMOUNTS UPWARDS OF 18 INCHES POSSIBLE WITHIN A 12 HOUR PERIOD.
  VALLEY LOCATIONS WILL SEE 8 TO 12 INCHES OF SNOW IN THE SAME
  PERIOD.

* LOCATIONS IMPACTED INCLUDE...ELK RIVER...HIGHWAY 11 PIERCE TO
  HEADQUARTERS...PIERCE...DIXIE...ELK CITY...HIGHWAY 12 LOWELL
  TO LOLO PASS