000
FOUS11 KWBC 131944
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
343 PM EDT FRI MAR 13 2009

VALID 00Z SAT MAR 14 2009 - 00Z TUE MAR 17 2009


DAY 1...

...PACIFIC NORTHWEST...
A MID/UPPER LEVEL TROF OVER THE NORTHEAST PACIFIC WILL MOVE
ONSHORE ON DAY 1.  A FRONT AT THE SFC ALSO MOVE INLAND IN
ASSOCIATION WITH THE UPPER TROF.  THE FRONT WILL PROVIDE A FOCUS
FOR SNOW IN THE WA CASCADES GIVEN LOW LEVEL CONVERGENCE...LOW-MID
LEVEL THETA-E ADVECTION...AND UPPER LEVEL DIFLUENCE/DIVERGENCE
COMBINED WITH PW VALUES INCREASING TO NEAR HALF AN INCH.  MANUAL
PROGS BLENDED THE ECMWF/GFS/SREF MEAN/GFS/UKMET/CANADIAN GLOBAL
FOR THE HIGHER AMOUNTS IN THE NORTHERN CASCADES.  AMOUNTS DECREASE
HEADING SOUTH DUE TO SHORTER DURATION OF SNOW IN THE OR CASCADES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT

DAY 2...

...PAC NW/NORTHERN ROCKIES...

THE ECMWF/GFS/UKMET/NAM MODELS ALL SHOW A STRONG 300 MB JET OF
130-140 KT COMING ONSHORE IN OR AND PROPAGATING INLAND INTO THE
NORTHERN ROCKIES.  AREAS OF HEAVY SNOW SPREAD INTO THE INTERIOR
RANGES OF ID AND THEN NORTHWEST WY AND MOISTURE FLUXES INCREASE IN
THE 850-500 MB LAYER.  THE GFS/UKMET/ECMWF MODELS SHOW A CLOSED
850 MB CIRCULATION SHOULD BE MAKING ITS WAY ONSHORE NEAR VANCOUVER
ISLAND...WITH THE STRONGEST LOW LEVEL JET COMING INTO THE WA
CASCADES. THE PRIMARY UNCERTAINTY ON DAY TWO REMAINS THE SNOW
LEVEL IN THE OR CASCADES...WHICH OSCILLATE UP AND DOWN WITH
CHANGES IN THE HEIGHT FIELD AND MOVEMENT OF THE INITIAL UPPER
TROUGH INLAND.  MANUAL PROGS BLENDED THE ECMWF/GFS/SREF
MEAN/GFS/UKMET/CANADIAN GLOBAL TO MITIGATE TYPICAL DIFFERENCES IN
QPF AMOUNTS.  ACCUMULATIONS ARE EXPECTED TO REACH A FOOT IN THE WA
CASCADES AND HIGHER ELEVATIONS OF THE OR CASCADES....WHERE THE
HIGHEST MOISTURE FLUX ANOMALIES DEVELOP.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT

DAY THREE...

...PAC NW/NORTHERN ROCKIES...

PERSISTENT DEEP LAYER WEST TO SOUTHWEST FLOW IS FORECAST TO
CONTINUE ONSHORE FROM THE NORTHEAST PACIFIC INTO THE PACIFIC
NORTHWEST AND THEN NORTHERN ROCKIES.
THE MODEL GUIDANCE IS IN BETTER THAN AVERAGE AGREEMENT...WITH THE
PEAK MOISTURE FLUXES CONTINUING ACRS THE OR CASCADES INTO THE
RANGES OF SOUTHERN ID AND THEN NORTHWEST WY.
THE GFS INDICATES THE 700 MB WIND PEAKS AT 50 KT FROM THE OR
CASCADES IN THE RANGES OF SOUTHERN ID...LEADING TO STRONG UPSLOPE
FLOW AND INCREASING SNOW AMOUNTS FROM DAY TWO IN THE SOUTHERN ID
RANGES.  THE PERSISTENT ASCENT AND MOISTURE FLUXES LEAD TO GOOD
CLUSTERING OF MODEL PRECIPITATION AND INDICATION OF AN INCH TO TWO
INCHES LIQUID EQUIVALENT QPF IN THE NAM/GFS/ECMWF/CANADIAN GLOBAL
MODELS/SREF MEAN.  THIS LED TO A MDT RISK OF A FOOT OF SNOW.  THE
12Z CANADIAN MODEL AND INCREASED AMOUNTS AND 12Z ECMWF DECREASED
AMOUNTS TO BETTER AGREE WITH THE NAM/GFS/SREFMEAN.   MANUAL PROGS
BLENDED THE 12Z ECMWF/GFS/NAM/09Z SREF MEAN/12Z UKMET.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN
$$