PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
220 AM EST SAT NOV 07 2009
 
VALID 12Z SAT NOV 07 2009 - 12Z TUE NOV 10 2009
 
DAYS 1 TO 3...

...PACIFIC NORTHWEST AND NORTHERN ROCKIES...

AN UPPER LEVEL JET CROSSING THE PACIFIC NORTHWEST INTO THE
NORTHERN ROCKIES BUCKLES WITH UPPER DIVERGENCE MAXIMA SUPPORTING
ASCENT IN THE WA CASCADES...AND THEN TO INLAND RANGES IN NORTHERN
ID/NORTHWEST OVERNIGHT. THE COMBINATION OF THE DYNAMICS AND
PERSISTENT ONSHORE COMPONENT... SHOULD RESULT IN HVY SNOWFALL FROM
THE WA CASCADES INTO THE NRN ROCKIES. ON SUN... HEIGHTS ARE FCST
TO INCREASE WITH THE UPPER JET DEPARTING AND A  MID-UPPER LEVEL
RIDGE BUILDING.  THE SHIFTING LOW-MID LEVEL FLOW BACKS TO THE
SOUTHWEST.  THIS RESULTS IN THE AXIS OF HEAVY SNOW MOVING NORTHER
INTO VANCOUVER ISLAND/BRITISH COLUMBIA.  THE PRIMARY SNOW IS
EXPECTED OVER THE NORTHERN WA CASCADES TO THE MOUNTAINS IN FAR
NORTHERN ID AND NORTHWEST MT...BUT THE RISKS ARE RELATIVELY LOW
GIVEN HIGHER QPF IS FURTHER NORTH IN CANADA...WITH A RANGE OF A
TENTH TO QUARTER INCH AMONG THE 00Z MODEL RUNS.  ALSO...THE
SOUTHERN WA CASCADES SEE THE RISK FALLING ON DAY TWO AS WARM
ADVECTION RAISES SNOW ELEVATIONS AND THE HIGHER QPF IS FURTHER
NORTH ACROSS THE SUITE OF MODEL/ENSEMBLE SOLUTIONS.  
   
ON MON/MON NIGHT... A MORE AMPLIFIED UPPER TROUGH WILL APPROACH
THE COAST AND TAP INTO MORE AVAILABLE PAC MOISTURE. OVERALL MANUAL
PROGS FOLLOWED A BLEND OF THE GFS/ECMWF AND SREF MEAN QPF/THERMAL
PROFILES FOR THE NRN/CENTRAL WA CASCADES INTO THE NRN ROCKIES.
TYPICAL MODEL TO MODEL AND RUN TO RUN DIFFERENCES LED TO
PREFERENCE FOR A CONSENSUS SOLUTION.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN