PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
305 PM EST FRI NOV 20 2009
 
VALID 00Z SAT NOV 21 2009 - 00Z TUE NOV 24 2009
 

DAY 1...

NORTHERN CALIFORNIA/SOUTHERN OREGON...  
DYNAMICS/SNOW-MAKING POTENTIAL ASSOCIATED WITH SPLITTING PACIFIC
TROUGH CONTAINS A PORTION OF THE COLD POOL ALOFT MIGRATING ESEWD
BEHIND THE COLD FRONT TONIGHT. WITH EMBEDDED SHORTWAVE
VORTICITY...MIGRATING THROUGH THE BASE OF THE H5 TROUGH AND ACROSS
THE COLD/MOIST LOW-LEVEL ENVIRONMENT IN PLACE...MANUAL GRAPHICS
CARRY A SLIGHT RISK OF 4+ INCHES ALONG THE COASTAL RANGES OF NW
CALIF AT/ABOVE 2500FT MSL WITH A SLIGHT RISK OF 4-8 ABOVE 3000FT
MSL ALONG THE WESTERN FOOTHILLS OF THE SIERRA...TRINITY ALPS AND
SWRN SISKIYOUS. WITH HPC DAY1 QPF USED FOR A BASELINE TO DERIVE
SNOW TOTALS IN THE SIERRA...THERE WAS INSUFFICIENT QPF TO CARRY A
RISK OF 12 INCHES INVOF LAKE TAHOE. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

DAYS 1-2..

PACIFIC NORTHWEST...
DEEP SURFACE LOW MOVING THROUGH THE CENTRAL BC COAST AND DEEPENING
COLD POOL IN THE VERTICAL BRINGS ANOTHER ROUND OF LOW-ELEVATION
SNOWFALL TO THE PUGET SOUND AREA... OLYMPIC PENINSULA AND WESTERN
SLOPES OF THE WASHINGTON AND NORTHERN OREGON CASCADES TONIGHT
THROUGH SATURDAY EVENING. 4-8 INCHES OF NEW SNOWFALL IS EXPECTED
ABOVE 3000 FT MSL WITH A SLIGHT RISK OF 12 INCHES ABOVE 5000 FT
MSL.  

NORTHERN ROCKIES...
LEADING EDGE OF PACIFIC COLD FRONT AND UPPER-LEVEL TROUGH MIGRATES
INTO THE CONTINENTAL DIVIDE OF ID/MT AND NORTHWEST WYOMING TONIGHT
AND SATURDAY. SOUTHERN THROUGH WESTERN EXPOSURES ABOVE 5000 FT MSL
WILL CAPTURE THE BEST DYNAMICS AND OROGRAPHICS...WITH ISOLATED 4-8
INCH SNOWS MATCHED UP WITH THE HPC PRELIM DAY 1 QPFS. DAY 2
SNOWFALL AMOUNTS ARE ANTICIPATED AT LOWER ELEVATIONS WITH THE
UPPER-LEVEL TROUGH AXIS AND LOW-LEVEL PACIFIC AIRMASS GENERATING
SCATTERED CONVECTION. 4-8 INCHES OF NEW SNOWFALL ARE EXPECTED
ABOVE 4000 FT MSL WITH MINOR ACCUMULATIONS BELOW 3000 FT MSL...IN
THE STRONGER AREAS OF SHOWERS.    

THE PROBABILITY OF SIGNIFICANT ICING AND/OR SNOW IS LESS THAN 10
PERCENT.

DAY 3...

NORTHERN AND CENTRAL ROCKIES...
PACIFIC TROUGH BEGINS TO EXIT OUT ACROSS THE CENTRAL US ON DAY
WITH THE HIGHER SNOWFALL AMOUNTS FOCUSED ALONG THE WEST AND
SOUTHWEST EXPOSURES OF EASTERN UTAH AND WESTERN
COLORADO...INCLUDING THE DIVIDE FROM WEST CENTRAL MONTANA TO SOUTH
CENTRAL WYOMING. USED A BLEND OF THE SREF/NAM PROFILES WITH THE
QPF FORECAST OF HPC AND ECMWF FOR THE DETAILS...WITH 4-8 INCHES
ALONG THE NORTHERN COLORADO DIVIDE...PORTIONS OF THE
WASATCH...WEST CENTRAL TETONS ABOVE 6500 FT MSL AND ISOLATED AREAS
OF NORTHWESTERN MONTANA ABOVE 5500 FT MSL.

THE PROBABILITY OF SIGNIFICANT ICING AND/OR SNOW IS LESS THAN 10
PERCENT.

VOJTESAK