From the National Weather Service Saturday morning:
A SIGNIFICANT SNOW-PRODUCING SYSTEM WILL CAUSE WIDESPREAD
IMPACTS TO AREAS IN WESTERN MONTANA AND NORTH CENTRAL IDAHO
BEGINNING THIS AFTERNOON...
...WINTER WEATHER ADVISORY NOW IN EFFECT FROM 5 PM THIS AFTERNOON
TO 11 AM MST MONDAY...
* IMPACTS/TIMING: LIGHT SNOW WILL DEVELOP LATE THIS AFTERNOON
WITH MODERATE SNOW AND A BRIEF PERIOD OF HEAVY SNOW POSSIBLE
SUNDAY MORNING. ROADS WILL BECOME HAZARDOUS BY SUNDAY MORNING...
ESPECIALLY OVER MACDONALD PASS. GUSTY EAST WINDS WILL ALSO
DEVELOP SUNDAY AFTERNOON THAT WILL BRING THE THREAT FOR BLOWING
SNOW ON HIGHWAY 12 AND I-90 SUNDAY AFTERNOON.
* SNOW ACCUMULATIONS: GENERALLY 3 TO 6 INCHES WITH 5 TO 9 INCHES
IN GEORGETOWN LAKE AND OVER HOMESTAKE AND MACDONALD PASSES.